a thesis presented to the faculty of natural and applied
TRANSCRIPT
SOL-GEL SYNTHESIS OF SILICA NANOPARTICLES AND THIER ROLE IN
PREDICTING CEMENT MORTAR STRENGTH AT EARLY AGES
_________________________________________________
A Thesis
presented to
the Faculty of Natural and Applied Sciences
at Notre Dame University-Louaize
_________________________________________________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
_______________________________________________________________
by
RAYYAN HAMAD BOUKARROUM
NOVEMBER 2020
ii
© COPYRIGHT
By
Notre Dame University - Louaize
2020
All Rights Reserved
iii
Notre Dame University - Louaize
Faculty of Natural and Applied Sciences
Department of Sciences
We hereby approve the thesis of
Rayyan Hamad Bou Karroum
Candidate for the degree of Master of Science in Industrial Chemistry
____________________________________________________________________
Dr. Kamil Rahme Supervisor, Chair
_____________________________________________________________________
Dr. Layla Badr Committee Member
______________________________________________________________________
Dr. Robert Dib Committee Member
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ACKNOWLEDGEMENTS
First of all, I would like to express my sincere gratitude to my supervisor, Chairperson
Dr. Kamil Rahme, for his continuous support and guidance during the running of this
project.
I would also like to thank committee members, Dr. Layla Badr and Dr. Robert Dib, for
their time dedicated to review my work and for providing their constructive feedbacks
and comments.
Needless to say, valuable materials and information provided by FNAS staff during my
master’s program helped to broaden my knowledge and shaped my way of thinking and
planning for the testing procedure.
Last but not least, I would like to thank my fellow colleagues, lab staff at NDU and lab
colleagues at “Sibline Cement” for their help and support.
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Table of Contents
Abstract ........................................................................................................................................... 1
1. Introduction ............................................................................................................................ 3
1.1. Sibline Cement Factory Overview: .............................................................................. 4
1.2. Cement Definition: ......................................................................................................... 7
2. Literature Review: ................................................................................................................. 9
2.1. Brief overview of the main clinker phases ................................................................... 9
Alite .......................................................................................................................................... 9
Belite ...................................................................................................................................... 10
Aluminate ............................................................................................................................... 10
Ferrite .................................................................................................................................... 10
2.2. Hydration Reaction: .................................................................................................... 11
2.3. Nanotechnology and Cement Industry: ..................................................................... 14
2.3.1. NanoScience and Nano-Engineering: .................................................................... 16
2.4. A brief review on the inclusion of nanoparticles in cement matrix: ........................ 18
2.4.1. Nano-Inclusions in cement and concrete: .............................................................. 19
2.4.1.1. Carbon-Based Nano-inclusions: ........................................................................ 20
2.4.1.2. Non-Carbon-Based Nanoinclusions: ................................................................. 23
2.5. Effect of Curing Temperature on the Microstructure of Cement Paste:................ 27
2.6. Effect of nanosilica addition on mortars mechanical properties: ............................ 29
2.7. Nucleation and Pozzolanic reactions: ......................................................................... 32
2.8. Nano Silica Synthesis: .................................................................................................. 35
3. Research Purpose: ............................................................................................................... 37
3.1. Sol-Gel Synthesis of Silica Nanoparticles: ................................................................. 39
3.2. Hypothesis: ................................................................................................................... 42
3.3. Objectives: .................................................................................................................... 42
4. Sampling and Testing Plans: ............................................................................................... 43
4.1. Sampling plan ............................................................................................................... 43
4.2. Mortar preparation planning: .................................................................................... 44
4.2.1. Mortar Preparation: .............................................................................................. 45
4.2.2. Compressive Strength Testing: .............................................................................. 45
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5. Materials and Methods: ...................................................................................................... 47
5.1. Fineness (Blaine): ......................................................................................................... 48
5.2. X-Ray Diffraction (XRD): ........................................................................................... 49
5.3. X-Ray Fluorescence: .................................................................................................... 50
5.4. Malvern Panalytical - Zetasizer Nano ZSP: .............................................................. 52
6. Results and Discussion: ....................................................................................................... 53
6.1. Fourier Transform InfraRed (FTIR): ........................................................................ 53
6.2. X-Ray Diffraction (XRD): ........................................................................................... 55
6.3. X-Ray Fluorescence (XRF): ........................................................................................ 59
6.4. Size of pozzolanic SCM: .............................................................................................. 60
6.5. Effect of curing temperature: ..................................................................................... 61
6.6. Effect of Nano Silica Addition: ................................................................................... 63
6.7. Concluding Optimum Fast Curing Regime ............................................................... 65
7. Data Correlation and Strength Prediction ........................................................................ 66
8. Conclusion: ........................................................................................................................... 69
References ..................................................................................................................................... 71
List of Figures: ............................................................................................................................. 81
List of Tables: ............................................................................................................................... 82
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List of Abbreviations
Abbreviation Definition
ASTM American Society for Testing and Material
C2S Dicalcium Silicate (Belite)
C3A TriCalcium Aluminate
C3S TriCalcium Silicate (Alite)
C4AF TetraCalcium Aluminoferrite
CB Carbon Black
CH Calcium Hydroxide (Portlandite Ca(OH)2
CNF Carbon Nano Fiber
CNS Commercial Nano Silica
CNT Carbon Nano Tube
COV Coefficient of Variation
CSH Calcium Silicate Hydrate
DLS Dynamic Light Scattering
EN European Norm
GNP Graphene Nano Platelets
GO Graphene Oxide
ISO International Organization for Standardization
LSF Lime Saturation Factor
MLR Multiple Linear Regression
MS Micro Silica
MWCNT Multiple Walled Carbon Nano Tube
NL Lebanese Norm
NS Nano Silica
OHSAS Occupational Health and Safety Assessment Series
OPC Ordinary Portland Cement
PDI Poly Dispersity Index
R2 Coefficient of Determination - Goodness of Fit
RH Relative Humidity
RMSE Root Mean Square Error
SCM Supplementary Cementitious Material
SNS Synthesized Nano Silica
SWCNT Single Walled Carbon Nano Tube
TEOS TetraEthyl OrthoSilicate
XRD X-Ray Diffraction
XRF X-Ray Fluorescence
1
Abstract
Cement is one of the most fundamentally used material all over the world. Among the
many different properties of cement, compressive strength conventionally determined at
the age of 28 days remains the most important parameter to assess cement quality.
Waiting for 28 days to obtain a test result urges cement manufacturers to set quality
parameters higher than the minimum accepted by norms, which means higher
unnecessary manufacturing costs. The aim of this study is to assess the performance of
cement mortar when nano silica (Nano-SiO2) is used as supplementary cementitious
material (SCMs) and to find a fast curing regime that allow us to predict the 28 days
compressive strength of cement mortar at an early age. A simplified Sol-Gel method for
the synthesis of Nano-SiO2 from sodium silicate was established, using sulfuric acid as
hydrolyzing agent. This method allows a high yield (about 95%) production of Nano-
SiO2 particles of 99% purity. A series of tests were conducted to study the effect of
curing temperature and nanosilica addition on the strength development. X-Ray
Fluorescence Spectroscopy (XRF) was used to check the chemical composition of the
cement used, and the purity of synthesized Nano-SiO2. X-Ray Diffraction analysis
(XRD) allowed us to monitor the variation of Alite, Portlandite and amorphous phase
during hydration period. XRD was also used to study the nature of the synthesized nano
silica. The produced silica particles are of amorphous nature and have an average
hydrodynamic diameter of ~135 nm as determined by Dynamic Light scattering (DLS)
analysis. Pozzolanic reactivity of the synthesized nano silica was found to be higher than
that of microsilica, and lower than that of commercial nano silica having a diameter
of~20 nm. Optimum conditions for strength enhancement (at early ages) were found to be
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a curing temperature of 70 °C and nanosilica/cement substitution of 3% respectively.
Combining both optimums allowed us to estimate the ultimate 28 days compressive
strength at 72 hours with a Coefficient of Variation (COV) less than 2.5%.
3
1. Introduction
Cementitious materials are the most commonly and widely used among construction
materials, in the form of concrete, and in different types of infrastructures. In the last few
decades, the advances in technologies allowed to a massive production of cementitious
materials. Today, cement industry became one of the most fundamental blocks of our
modern world. For instance, the average production of cement in Lebanon is above 6.0
million tons per year.
Cement would gain its properties after we mix it with water. Calcium silicate minerals
will react with water molecules to produce a gel like structure which then would sets and
hardens to give concrete its strength. There are different parameters that define superior
quality of cement, and the most important one is the compressive strength determined for
cement mortars at the age of 28 days. This means that a cement sample would undergo a
strict laboratory procedure to be casted and then cured for 28 days in a water bath
controlled at specified temperature. Specimens are then taken out from water and tested
for compressive strength on a compression machine.
Most of the studies performed nowadays in the field of construction are trying to provide
high performance concretes with novel properties. This opened the door to explore the
fascinating properties that nanomaterials can provide to concrete. Researches on the
inclusion of various types of nanomaterials have proved the positive effect of these
materials on cement performance. Perhaps, the most interesting ones are nanoparticles
that may possess “pozzolanic reactivity”. In fact, these particles will react with the
hydration reaction byproduct to produce more of the glue mineral responsible for the
strength of cement. Nano-Silica, among other particles, may act as pozzolanic reactive
4
material in the cement matrix. Therefore, our aim in this study will be to synthesis silica
nanoparticles using a well described Sol-Gel method. The obtained SiO2 nanoparticles
will be further used in cement pastes as replacement to cement, in order to study their
pozzolanic reactivity. Moreover, the size effect of silica particles will be examined by
comparing the performance of cement including our synthesized silica particles with a
commercial nano-SiO2 of finer size, and a commercial microsilica of coarser size.
Finally, we will try to formulate a fast curing regime, with the help of nanosilica, in an
attempt to predict the normal 28 days compressive strength at earlier ages. All testing and
experiments were performed at NDU’s chemistry lab and “Sibline Cement” quality lab.
1.1. Sibline Cement Factory Overview:
SIBLINE is one of the major clinker and cement producers in the Republic of Lebanon.
Sibline currently produces 3,400 tons of clinker per day, with an annual production of
cement exceeding 1.35 Million tons.
Figure 1 - Sibline Cement Factory
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SIBLINE (Ciment de Sibline s.a.l.), is located in Mount Lebanon district, in Iklim El
Kharoub, Shouf area that is situated at the entrance of the southern part of the country.
Nevertheless, the company headquarters are located in Beirut.
SIBLINE currently provides 530 job positions and puts forward indirect employment for
more than 800 people in the domain of logistics, transportation and distribution of cement
and other products.
As of March 1st, 2007, “Secil” became a major shareholder in SIBLINE. Secil is a
Portuguese joint-stock company that owns and operates three cement plans in Portugal,
with an annual production capacity of 4 million tons of cement. In addition to Portugal
and Lebanon, Secil maintains partnerships with cement companies in Tunisia, Angola,
and Brazil.
In 1987, Sibline completed the construction and commissioning of its first production
line with a capacity of 800 tons of clinker per day.
In 1995, the company started a project for expanding its production capacity by adding a
new line of 2,400 tons of clinker per day. This new line was commissioned during late
1998 and enabled the company to be a major player in the Lebanese cement market. The
expansion increased the company’s growth subsequently through tripling its production
ability and optimizing its costs.
In 2012, SIBLINE completed an upgrade project on its clinker production line 1 which
consisted of preheater cyclone modifications and improvements including the installation
of the Gooseneck. This enhancement allows for more than 40% utilization of alternative
fuel (ex: petroleum coke and RDF). This project also permitted the installation of a
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completely new clinker cooler with a production capacity of 1000 metric tons per day. As
part of this upgrade and for a better compliance with the environmental requirements,
SIBLINE installed a bag house for final dedusting with a capacity of 1000 metric tons per
day.
A new filter on the second line was installed in 2016, cost about 6,000,000 USD. The
new filter uses a high technology based on the use of bags and helps to reduce emissions.
The installation of the filters and applying the new techniques earned SIBLINE OHSAS
18001 certificates; this is in addition to ISO 14001 and ISO 9001. The Industry Minister;
Mr. Hussein Haj Hassan; said, “It is a sign of the commitment of the national industry to
safety and health standards”.
Figure 2 - The New Bag Filter on Line II
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1.2. Cement Definition:
As defined by EN 197-1, cement is a hydraulic binder. It is a product made by blending,
grinding and firing different types of raw materials in order to obtain a special chemical
proportion of Calcium, Silica, Alumina and Iron oxides in the semi-finished product -
Clinker – which is the essence of cement. When mixed with water, cement undergoes a
series of hydration reactions which give it the properties of setting and hardening.
Hydraulic hardening mainly results from the reaction between calcium silicates
components and water, knowing that other chemical compounds such as aluminates may
also participate in the hardening process, but their contribution is considered negligible
compared to calcium silicates [1].
There are so many different types of cement available in the market. The American
Society for Testing and Material (ASTM) defines five major types of cements based on
their specifications and desired construction application [2]. On the other hand, the
European Norm defines and gives specification of 27 types of cement (grouped into five
categories) based on the type of the main constituents that make the cement, aside than
clinker and gypsum (ex: limestone, Pozzolana, etc..) [3]. The Lebanese norm “NL-53” is
similar to EN in classifying cement types according to its constituent (Table 1).
8
Table 1 - Cement types as per Lebanese Norm NL-53
Where “P” stands for Portland cement, “V” for fly ash, “S” for slag, “Z” for
pozzolana, and “L” for limestone. So, PA-L for examples is Portland cement with
Additive Limestone.
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2. Literature Review:
2.1. Brief overview of the main clinker phases
Cement manufacturing process is not a continuous process. It could be divided into
several separate stages. Cement chemistry begins with the decomposition of calcium
carbonate (CaCO3) at about 900 °C to leave calcium oxide (CaO, lime) and liberated
carbon dioxide (CO2); this process is known as calcination. This is followed by the
clinkerizing process, inside a high temperature Kiln (about 1450 °C), in which the
calcium oxide reacts with silica, alumina, and ferrous oxides to form the
CalciumSilicates, CalciumAluminates, and ferrites which are the clinker main
constituents and active ingredients. Clinker (the semi-finished product) is then ground or
milled together with gypsum and other additives to produce cement.
There are four main minerals in clinker that react with water to give cement its
properties:
Alite (Ca3SiO5) or C3S: is the most important constituent of all normal Portland cement
clinkers, it comprises about 50 to 70% of its constituents. It is tricalcium silicate
(Ca3SiO5) modified in composition and crystal structure by ionic substitution. It reacts
relatively quickly with water, and in normal Portland cements is the most important of
the constituent phases for strength development at early ages. This highly reactive clinker
mineral is an impure, polymorph calcium silicate that can occur in seven different crystal
modifications: three triclinic, three monoclinic and one rhombohedral structure [4]. Phase
transition among these forms occurs at different temperatures [5]:
10
In Portland cement clinker, alite occurs mainly in M1 & M3 polymorphs containing
about 4% impurities or substituent oxides [6]
Belite (Ca2SiO4) or C2S: The second major phase constituting about 15 to 30% of normal
Portland cement clinkers. It is dicalcium silicate (Ca2SiO4) modified in composition also
by ionic substitution (Mg2+, Na+, etc.). It reacts slowly with water, thus contributing a
little to the strength during early ages. Belite is also a polymorph with four known crystal
structures [7].
Aluminate (Ca3Al2O6) or C3A: constitutes about 5 to 10% of most normal Portland
cement clinkers. It reacts rapidly with water and can cause undesirable rapid setting,
unless a set-regulator is added (usually gypsum). The pure tricalcium aluminate
(Ca3Al2O6) is cubic and does not exhibit polymorphs, however, a number of ions can be
incorporated in Ca2+ or Al3+ site to result in many different structures [8]. The main
crystal structures of TriCalcium Aluminate (C3A) in cement clinker are cubic and
orthorhombic.
Ferrite (Ca4Al2Fe2O10) or C4AF: constitutes about 5 to 15% of normal Portland cement
clinker. It is tetra-calcium aluminoferrite (Ca4Al2Fe2O10). The rate at which it reacts with
water appears to be somehow variable. It has to do with the color of clinker, the presence
of iron gives clinker a darker color [4, 9].
T1 T2 T3 M1 M2 M3 R
1050 °C 1060 °C 990 °C 980 °C 920 °C 600 °C
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2.2. Hydration Reaction:
The hydration of Portland cement is a series of chemical reactions occurring
simultaneously between clinker minerals and water.
For a better understanding of Portland cement hydration process, it is preferred to discuss
the hydration process of each individual clinker mineral alone.
This section gives a brief description of the basic hydration reactions of clinker minerals
as individual phases, which is fundamental for the hydration of Portland cement in
general. However, we shall focus only on the hydration of calcium silicates as they
produce the compounds necessary for strength development (C-S-H).
Tricalciumisilicate (C3S) is the major cementitious component of Portland cement. Its
hydration reaction is represented by the following approximate chemical equation [10].
2Ca3SiO5 + 6H2O → Ca3Si2O7.3H2O + 3Ca(OH)2 (eq.1)
or in cement nomenclature
2C3S + 6 H → C3S2H3 + 3CH
The products formed are a calcium silicate hydrate (Ca3Si2O7.3H2O) and calcium
hydroxide (Ca(OH)2) known as C-S-H and portlandite (CH) respectively. The formula
given for C-S-H is a general one (or an estimate) as there are several forms of CSH
generated during hydration reaction.
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Dicalciumsilicate (C2S) hydrates much slower than C3S:
2Ca2SiO4 + 4H2O → Ca3Si2O7. 3H2O + Ca(OH)2 (eq.2)
Both C3S and C2S hydration reactions yield the same products, C-S-H and CH. C-S-H is
the glue that binds the cement paste, mortar and concrete ingredients together, and is
responsible for strength development in Portland cement based systems. Ca(OH)2 or
portlandite is a byproduct of the hydration reaction that has a negligible contribution to
strength.
As it is noticed from equations 1 and 2, C3S produces more portlandite than C2S.
Moreover, the fact that alite (C3S) reacts much more quickly than belite (C2S) means
that the former produces much more C-S-H at early ages than belite. Hence, alite
contributes more to the early age strength (up to 28 days) of cement mortars, while belite
contributes more to the strength at later ages [11].
CH (Calcium Hydroxide) is a solid mineral that might reduce the porosity, making the
structure denser, and therefore improve strength [12]. CH also reacts with reactive
silicates of supplementary cementitious materials (such as fly ash and pozzolana) to
produce additional C-S-H through what is called the pozzolanic reaction [13] (to be
further discussed in section 2.7).
A research was performed by CSHub (Concrete Sustainability Hub) on atomic scale
models to study the reason why alite is more reactive than belite. It has suggested that the
more “defective” crystals of alite provide it with more reactivity. In fact, it was observed
13
that some sites in alite that should be occupied by oxygen atoms are unfilled, which allow
water to start reacting or dissolving alite more readily than belite [11].
The hydration of TriCalcium Aluminate (C3A) and TetraCalcium Aluminoferrite
(C4AF) produce C3AH6 (3CaO.1Al2O3.6H2O), which is basically a sheet-like structure,
which in contrast to what happens with the fiber-like C-S-H, would have a negligible
contribution to concrete strength [14].
Figure 3- Schematic drawing of cement hydration [119]
14
Figure 4- A pictorial representation of a cross section of a cement grain. Adapted from Cement Microscopy;
Halliburton Services: Duncan, OK.
2.3. Nanotechnology and Cement Industry:
Concrete is mainly made of cement, water, sand, aggregates, and other additives. It is a
nano-structured, multi-phase, composite material that ages over time [15]. The
amorphous phase, calcium-silicate-hydrate (C-S-H), is the glue that holds concrete
ingredients together and is by itself a nanomaterial [16]. The properties, processes and
reactions occurring at the nanoscale in concrete, define the interactions that occur
between particles and phases at the microscale, as well as the effects of working loads
and the surrounding environment at the macroscale.
Since nanotechnology was introduced by Richard P. Feynman in his famous lecture at the
annual American Physical Society meeting at Caltech on December 29, 1959 “There’s
Plenty of Room at the Bottom”, there have been many revolutionary developments in
15
several fields to demonstrate Feynman’s idea of manipulating matter at the nanoscale
[17]. In the last two decades, research on the study and manipulation of matter at the
nanoscale has been expanding exponentially, supported by the advances achieved by
high-resolution technologies such as Electron Microscopy, Atomic Force Microscopy,
Scanning Tunneling Microscopy, and others. However, investments seem to partially
neglect the construction sector [18].
In general, infiltration of new materials and technologies to the construction sector faces
some obstacles, as the construction works are long-term processes, and since construction
companies usually tend to avoid risks of using materials that are not listed in official
construction codes [19]. Despite that, Figure 5 reveals that the scientific literature on
nanotechnology in cement or concrete, has significantly increased in the last decade.
Figure 5- Number of published papers, that, according to Scopus, include the terms cement and nanotechnology
or nanomaterials in the title, abstract or keywords, limited to the fields of engineering and materials science. [19]
16
Studies were performed to explore various properties of cement mortar and concrete
(such as mechanical performance, durability, electric resistance, permeability, etc.), by
inclusion of different types of nanomaterials, such as Carbon nanotubes (CNT) [20],
nanosilica [21], nano-Fe2O3 [22], nano-Al2O3 [23], etc.. Moreover, there are researches
that deal with the manufacturing of nanosized cement particles and the development of
nano-binders [24]. Nanosized particles have a high surface area to volume ratio, which
provides a high potential for tremendous chemical activities. It can act as: i) a nucleation
site for different cement phases (Alite, Belite, Aluminate and Alumino-Ferrite) enhancing
the hydration reaction due to their high reactivity, ii) as Nano-reinforcement, and as filler
densifying the cement matrix microstructure, hence, leading to less porosity and
permeability [15].
2.3.1. NanoScience and Nano-Engineering:
The nanoscience and nano-engineering of concrete are terms that are now commonly
used in the application of nanotechnology on cement-based materials. Nanoscience deals
with the measurement and characterization of the nano and micro-structures of cement-
based materials, to better understand the relationship between the reactions at nanoscale
and the performance of concrete at the macroscale. It is a modern discipline concerned
with the novel properties of materials when size is reduced to the nanometric scale [15].
Given the very small size of nanomaterials (between 1 and 100 nm), their high specific
surface to volume ratio, and self-assembly characteristics, the properties of material at
this scale may differ drastically from the bulk material. For example, the melting point of
nanoparticles decrease as they reach a critical size in the order of 50 nm, the material may
17
become soluble and offer particular electronic/optical properties such as a better electric
conductor, among other aspects [18].
Nano-engineering is the study of techniques to manipulate the concrete nanostructure in
order to exploit novel properties of the material such as electric resistivity, self-cleaning,
self-healing, high ductility and self-control of cracks [15].
Advances in the characterization tools of the nanoscale structure of cement-based
materials have provided scientists and engineers a better understanding of concrete
structure which help to improve its performance. As mentioned above, the important
features of C-S-H and other cement phases occurs at the nanometer scale. Figure 6 shows
the structure of Tobermorite layer (a calcium silicate mineral, an analogous to C-S-H).
Figure 6 - The schematic molecular structure of a single sheet of C-S-H. Circles represent calcium atoms located
at the center of Ca-O octahedra; Triangles show silicate tetrahedra ; OH-attachments are not shown. [26]
18
Better understanding of the concrete structure at nano-level helps to influence important
processes related to the production and the use of construction materials. These advanced
tools enabled to enhance properties such as strength, fracture control, corrosion and even
enabled altering of the desired properties [25].
2.4. A brief review on the inclusion of nanoparticles in cement matrix:
Although nanoparticles have a much higher unitary cost compared to cement or other
cementitious material, studies have shown that a better efficiency of concrete can be
achieved with only small volumes of nanoparticles [26].
When nanoparticles are mixed in the aqueous solution, they tend to form agglomerates
due to their high reactivity dictated by their high surface area (Van der Waals attraction
forces). Hence, a major obstacle to prepare a cement matrix composite including
nanoparticles lies in the difficulty to obtain a mix with uniformly dispersed inclusions
[27]. Techniques to achieve the homogeneous dispersion of nano-admixtures (admixtures
containing dispersed nanoparticles) are often required, and these are classified in three
main groups: chemical surface modification, physical surface modification (through
adsorption of surfactants or polymer wrappings), and mechanical methods of
ultrasonication, stirring, ball milling and shear mixing [18]. Ultrasonication has been
commonly used to attain uniform dispersion in the cement matrix. For example, when
using CNTs as inclusion, an ultrasonic probe imparts excitation energy to break up the
nanotube clusters at the expense of achieving decreased aspect ratios (length-to-diameter
ratio) [28]. Moreover, for the dispersion of CNTs, chemical modification often referred to
as “functionalization” is commonly used [18]. This is achieved by applying acid
mixtures, which oxidize CNTs and add carboxylic (COOH) or hydroxyl groups (OH) that
19
enhance the solubility of CNTs in the aqueous matrix. On the other hand, oxidizing CNTs
will make them tightly wrapped by the C-S-H phase due to the attraction created by
covalent bonds of oxide groups.
2.4.1. Nano-Inclusions in cement and concrete:
Nano inclusions in the cement matrix can be classified into carbon-based and non-
carbon-based. Most non-carbon-based inclusions consist of particles, primarily
pozzolanic, and oxide nanoparticles [28]. Pozzolanic materials reinforce the cement
matrix by means of their pore-filler effect and their contribution to the generation of the
C–S–H gel by the interaction of the highly reactive amorphous silica with the calcium
hydroxide phase Ca(OH)2. The strengthening effect of the pozzolanic group
proportionally increases as their size decreases, given the densification and chemical
nature of their reinforcing mechanism [29]. Oxide nanoparticles provide in addition to the
conventional mechanical improvements, modified phenomena in terms of electrical [30],
thermal [31], or chemical properties to the matrix, among other aspects.
The promising special properties present in nanophysics are still more fascinating in the
case of carbon-based nanomaterials, which have an unusual and complicated behavior at
the molecular level [32]. Carbon is the only element that has stable allotropes from the
zero to the third dimension. The most studied carbon-based nano-inclusions in the cement
matrix are primarily CNTs, carbon nanofibers (CNFs), graphene oxide (GO), graphite
nanoplatelets (GNPs), and carbon black (CB) [18].
20
2.4.1.1.Carbon-Based Nano-inclusions:
A. Carbon Nanotubes (CNT):
CNTs are usually classified as single-walled (SWCNT) and multi-walled (MWCNT). A
SWCNT is made of a graphene sheet rolled up into a cylinder. MWCNT are made up of
more than one graphene cylinder nested into one another [33].
CNTs exhibit extraordinary strength, with the elastic modulus in the order of TPa (above
1200 GPa) and tensile strength in the range of GPa [34]. In the mesoporous environment
of concrete, these nanotubes act as crack inhibitors (Figure 7). Moreover, they act as
filler, making the C-S-H net becoming denser and enhancing the quality of cement paste-
aggregate interface.
Figure 7-Example of crack bridging in a SWCNT/hydrated OPC composite. Bridging structures are SWCNT
bundles. [26]
21
It has been found in several studies that the inclusion of CNTs in cement, lead to a
remarkable increase in compressive and tensile strengths [35].
Another application of CNTs is linked to the electromagnetic shielding, which would
protect both devices and human health [36]. However, CNTs are not currently being used
for this purpose on a global scale, as there are various low-cost materials that are able to
perform this task.
B. Carbon Nanofibers (CNF):
CNFs are cylindrical nanostructures with graphene layers that can be stacked according
to three different patterns: in the shape of cups, cones, or plates. They have a tensile
strength of about 8 GPa, slightly lower than that of CNTs [37].
In their study, Yazdani et al. [37], compared the reinforcement obtained in mortar with
CNTs and with CNFs. They have noticed a remarkable increase in both compressive and
flexural strength by adding 0.1 weight-to cement percentage. The compressive and
flexural strengths increased by 54% and 14% respectively for 0.1% w/w of CNT, in
comparison to 68% and 8% for 0.1% w/w of CNF.
Another novel property on CNF that can be used in concrete construction is its high
electric conductivity. Galao et al. [38] reported that a CNF content of 2% w/w of cement
and a fixed voltage of 20V were able to prevent the freezing of concrete specimen. This
would be of potential application to protect concrete structures in regions having high
freeze/thaw effect.
22
C. Graphene Oxide (GO):
Graphene oxide consists of graphite that has been oxidized to intersperse the carbon
layers with oxygen molecules, and then, has been reduced to separate the carbon layers
completely into individual or few-layer graphene. The functional groups (carboxyl,
hydroxyl, and epoxy) provide GO with high water solubility. When compared to CNTs,
graphene oxide is easier to produce, possesses higher solubility in the aqueous cement
matrix, and its sheets increase the nucleation area for C-S-H gel [39].
Studies have shown that with small percentages of GO, an increase in compressive
strength of cement paste by 15% to 33 %, and in flexural strength by 41% to 59 % could
be achieved [18].
D. Carbon Black Nanoparticles (CB):
CB, which is produced by an incomplete combustion, is essentially composed of carbon
atoms in the form of an amorphous molecular structure. The reduced size of CB and its
electrical conductivity makes it an economical method for protecting steel rebars from
corrosion. Wen et al. [40] studied the electric conductivity of cement pastes prepared
with the same content ratio of CB on one hand, and CNF in the other hand. Carbon fibers
were found to be more effective than CB at increasing the electrical conductivity.
However, a partial replacement of up to 50% of carbon fibers by CB maintained the
conductivity at lower cost. In addition, it has been found that as the CB content increase,
corrosion of steel rebar in concrete structure decrease. This is due to the lower
permeability of chloride ions into the specimens.
23
2.4.1.2.Non-Carbon-Based Nanoinclusions:
Non-carbon-based materials are commonly referred to as nanoparticles. The application
of nanoparticles in cement matrix has become a key object research in the last decade.
Some of the most studied are: pozzolanic nano-SiO2, nano-Fe2O3, nano-Al2O3, nano-
TiO2, nano-MgO and nano-CaCO3.
Nanoparticles act as pore fillers and make C-S-H net more rigid because of their high
chemical reactivity. Some of the common positive effect of nanoparticle inclusion in
cement matrix include: improving mechanical performance, enhancing corrosion
resistance, reducing shrinkage and permeability and enhancing durability [41, 42].
In contrast to the carbon nanoallotropes, the production efficiency of nanoparticles has
improved, leading to cost reduction in the use of volumetric admixtures.
A. Nano-SiO2:
Nanosilica has been found to improve concrete workability and strength [43], to enhance
permeability and to help control of calcium leaching [44, 45]. It has also been shown that
nanosilica helps in facilitating the use of recycled materials in cement matrix. For
example, an improvement in the compressive strength of concrete was found when using
rubber from waste tires and nanosilica, thus allowing a structural use of concrete with
high rubber content [46]. A mortar with compressive strength and permeability
characteristics equivalent to that of high-performance concrete was achieved when using
recycled glass nanoparticles with cement paste [47]. Nano-silica behaves as a filler to
improve cement microstructure, and also acts as an activator to promote pozzolanic
reactions. The effect of Nano-Silica is further discussed in section 2.2.
24
B. Nano-TiO2:
In addition to its role in improving the compressive and flexural strength, nano Titania
provides a different characteristic known as the photocatalytic effect [26]. Nano-TiO2 has
been used in the self-cleaning concrete that contributes to destroy organic pollutants [48].
The removal of NOx by photocatalytic reaction is illustrated in Figure 8.
Figure 8- Schematic representation of photocatalytic effect of Nano-TiO2 particles in concrete [19]
The self-cleaning feature provided by titanium oxide embedded in cement matrix has
already been put into practice, example, the Jubilee Church in Rome, Italy.
25
C. Other Nanoparticles:
Many research studies have been made on different types of nanoparticles. In general,
most of the NPs showed an enhancement in concrete strength, better durability, less
permeability, and more dense structure.
For example, the addition of 1% w/w of nanosilica, nano-Al2O3 and nano-Fe2O3 showed
an improvement in compressive strength by 16.4%, 15.4% and 10.5% respectively [49].
Nano-Fe2O3 has been found to provide concrete with self-sensing capabilities. The
volume electric resistance of cement mortar with nano-Fe2O3 was found to change with
the applied load, demonstrating that the mortar could sense its own compressive strength
[50].
Nano-Al2O3 has been shown to significantly increase the elastic modulus (up to 143% at
a dosage of 5% w/w) of concrete [23].
Nano-CaCO3 and nano-MgO have also shown to be compressive and flexural strength
enhancers, as well as to decrease shrinkage [51, 52]. A brief summary on the effect of
nano-additive on cement matrix is given in table (2):
26
Table 2 - Nanomaterials used in cement and their effect on its properties (retrieved from
nanowerk.com)
27
2.5. Effect of Curing Temperature on the Microstructure of Cement
Paste:
The reaction between cement and water is similar to any other chemical reaction, being
proceeding at a faster rate with increasing temperature [53]. The degree of hydration of
cement phases is found to be faster with higher temperature at the early ages, but at late
ages, the situation is found to be reversed [54]. The same effect was found for
compressive strength [55]. At elevated temperatures, the rapidly forming initial hydration
products are distributed randomly and heterogeneously in the cement paste. This leads to
higher porosity and lower strength. In contrast, at lower temperatures, the rate of
hydration is reduced, which gives the dissolved ions more time to diffuse into the mortar
solid phase before the precipitation of the hydrates may occur [56].
The solubility and diffusion rate of hydration product (dissolved ions) is generally low
[57]. At normal curing temperatures (20 ± 1 °C), the hydration products are allowed
sufficient time to diffuse and precipitate, relatively uniformly, through the interstitial
space within the cement matrix. However, at high temperatures, hydration ions do not
have sufficient time to diffuse away from the hydrated cement grains into cement matrix
voids. Hence, a high concentration of hydration products precipitates in the zone
surrounding cement grains. This phenomenon would have two fundamental effects: 1- it
leads to heterogeneous distribution of the hydration product in the paste matrix, with the
bulk concentration just near the cement grains. 2- creation of an impermeable rim around
cement grains, retarding water to further reach the unreacted cement and leading to a
subsequent decrease in the overall hydration [58].
28
A number of studies later confirmed these effects of fast curing. Goto and Roy [59]
examined the microstructure of hydrated cement pastes cured at 60 °C in its early age.
They found much higher volume of pores with diameter larger than 150 nm, compared to
cement paste cured at 27 °C. These pores could easily allow harmful substances to
circulate through the cement matrix, rendering the concrete structure susceptible to
deterioration. Another study done by Kjelsen et al [60] inspected the microstructure of
cement paste cured at temperatures ranging from 5 to 50 °C. Utilizing backscattered
imaging, they found that lower curing temperatures resulted in a more uniform
distribution of hydration product. On the other hand, when curing at elevated
temperatures, not only the distribution of hydration products was not uniform, but also
coarse interconnected pores were observed, which influence the strength and durability of
concrete.
Wang P. et al, [61] found that curing at 60 °C will change the microstructure of the
produced hydrates which would impact the compressive strength at late ages. In their
study they revealed that raising the curing temperature increases strength at low ages,
however, it decreases the late strength. On the other hand, mortar cured at low
temperature (below 40 °C), would develop higher strength at the late ages, even though it
was lower at early ages. Similar results were shown by Gallucci E et al, [62] whom found
that when cured at high temperatures for one year, the microstructure of mortar hydrates
was much coarser and less homogeneously distributed which in turn lead to more
capillary pores, that explains the deterioration in strength development at late stages.
29
2.6. Effect of nanosilica addition on mortars mechanical properties:
Due to its unique physical and chemical properties, nanomaterials have been a main
research topic in the last decade in an attempt to fabricate building material with novel
functions. One of the most studied nanomaterials in construction is nanosilica [63].
The filler effect of micro and nano-scaled silica helps in filling up the voids in cement
pastes, densifying the structure, which leads to enhancement is the mixture strength [64].
On the other hand, nanosilica particles possess a pozzolanic activity which means that it
reacts with Ca(OH)2 generated by cement hydration reaction to produce C-S-H gel,
hence, contributing more to the final strength [65]. The rate of pozzolanic reaction is
directly proportional to the fineness of the particles [66]. H. Li et al found that the higher
the surface area of nanosilica (the finer), the higher is the strength of cement composites
[67].
Previous studies done by Horszczaruk E. et al showed that the presence of nanosilica has
improved the flexural and compressive strength of cement by 9% and 4% respectively
when Nano-SiO2 is added as suspension in acetone [68] and by 8% and 26% respectively
when Nano-SiO2is added in water suspension [69].
When ultrafine material such as nanomaterial is added into the cement matrix, mortar or
concrete with different properties and characteristics from conventional ones are obtained
[27]. Nanosilica for instance, can increase the density of the mortar structure by filling
the space between C-S-H gel particles, as well as via its pozzolanic activity with calcium
hydroxide which increase the amount of C-S-H by acting as a nucleus for hydration
30
products, binding C-S-H particles together [70], hence, improving cement strength and
durability [71].
As previously mentioned, nanoparticles have a wide range of applications in the
concrete field, modifying various aspects of its properties such as strength, permeability,
durability, among others. However, a common problem regarding nano-modification of
cement properties is the effective dispersion of nanomaterial in the cement paste matrix
[72]. When nanoparticles are mixed in the aqueous solution, they tend to form
agglomerates due to their high reactivity dictated by their high surface area at the origin
of Van der Waals attraction forces. Nanoparticles supplied as dry powder, even though
produced in sizes below 100nm, usually tend to agglomerate, which reduces its total
specific surface area [73, 74, 75]. This in turn hinders the full exploitation of
nanomaterials since a remarkable portion is not exposed to the reaction media. Moreover,
the formation of these agglomerates hampers the uniform or proper dispersion of
nanomaterials within the cement matrix, which leads to the formation of the so-called
“weak zones” in concrete [72].
To achieve good dispersion level, a number of mechanical and chemical methods
(physical and chemical surface modification) can be applied, either separately or in
conjunction. Mechanical methods include high shear stirring [76], mechanical stirring
[77], ultra-sonication [78], and ball milling [79]. Physical surface modification is usually
carried out using surfactants; an amphiphilic organic compound possessing a hydrophilic
head and a hydrophobic tail [80]. Chemical surface modification is achieved by addition
of functional groups to nanoparticle surfaces [81], for example, applying an acid mixture
31
(nitric and sulfuric acids) to carbon nanotubes adds carboxylic (COOH) and hydroxyl
(OH) groups to its surface (Figure9).
Figure 9 - Surface Modification of Carbon Nanotubes [86]
The main purpose of mechanical methods is to break down nanomaterials into small sizes
that facilitate their dispersion. In contrast, physical surface modifications aim at
stabilizing the dispersion and preventing re-agglomeration of dispersed material through
steric hindrance and electrostatic repulsion.
There are already different methods applied in literature to add nanosilica into cement
matrices. A well common method is to mix a suspension of nanoparticles with water for
few minutes and then adding this mixture to other dry ingredients of mortar or concrete.
This usually requires the addition of a superplasticizer, a water reducing
chemical admixture (ex: Sodium poly(naphthalene formaldehyde) sulfonate
C21H14Na2O6S2) used with concrete to increase its strength. [66, 82, 83, 84]. Another
method is based on the addition of the powder nanosilica (NS) to mixing water and then
32
adding the suspension to other ingredients, or even adding NS directly as a powder to the
dry cement [69, 85]. A third method commonly used recently is to add nanosilica to
cement as suspension in acetone and then drying the mixture until complete evaporation
[86]. However, despite the fact that utilizing acetone as dispersion medium helps to avoid
the non-uniform dispersion of nanosilica, it has been found that the positive effect of
nanosilica addition is reduced due to the increase of the air content in fresh mortars [68].
The optimum amount of Nanosilica to enhance cement strength is also debatable. Some
authors prefer to use a relatively small percentage (0.25 to 5 wt% of cement) to avoid
agglomeration caused by difficulties to disperse nano-SiO2 when mixing [87], while
others indicate that improved performance could be also achieved using higher
percentages up to 10 wt% if proper adjustments and dispersion methods are used [78]. L.
Wang et al. [88]demonstrated that the higher the content of nSiO2, the better is the early
strength of cement. However, they also believed that too high content will lead to slow
development of strength at late ages. This might be due to the fact that the quantity of NS
present is more than that needed to react with the CH liberated during the hydration
reaction, which would cause deficiency in strength, since the excess quantity of NS is
replacing part of the cementitious material without contributing to strength [77].
2.7. Nucleation and Pozzolanic reactions:
The addition of micro or nano silica particles enhances the physical and mechanical
properties by filling the voids between cement grains, reducing porosity and permeability
(Figure 10).
33
Figure 10 - Microstructure Densification of Traditional, High Performance and Nano-Engineered Concrete [64]
Beside this effect, Nano-SiO2 has a pozzolanic reactivity with CH produced during
hydration reaction and produces more C-S-H. Two possible reaction mechanisms take
place during hydration of Portland cement in the presence of Nano-SiO2. First, C3S and
C2S in cement grains react with water to produce C-S-H. Second, the presence of Nano-
SiO2 in cement paste leads to the formation of H2SiO42-, which reacts with the excess
(unreacted) of Ca2+ to form additional C-S-H that spread in the water space between
cement grains and in the interstitial zone between cement grains and aggregates. These
“additional” C-S-H acts as nucleation seeds for the formation of more C-S-H phase.
Therefore, C-S-H formation is no longer limited to cement grain surface, but also may
take place in the pore spaces of the cement matrix. Thus, the formation of large number
of nucleation seeds leads to acceleration of hydration reaction [64].
34
-
Pozzolanic reactivity can be described by the following reactions:
• Alite (C3S) hydration reaction
2Ca3SiO5 + 6H2O 6Ca2+ + 2H2SiO42- + 8OH- Ca3Si2O7.3H2O + 3Ca(OH)2 (eq.3)
Ca(OH)2 Ca2+ + 2OH- (eq.4)
• Silica particles in aqueous solution:
nSiO2 + 2H2O H4SiO4 (eq.5)
At this stage, pH of the paste matrix is above 12 (12.5 to 13.5) due to the presence of
portlandite (Ca(OH)2) [95]. Therefore, H4SiO4 dissociates to form the conjugated base
H3SiO4- at pH values above 9. Similarly, H3SiO4
- further dissociates into H2SiO4
2- at pH
values above 11 [96] via the following reactions:
H4SiO4 H3SiO4- + H+ (This occurs when pH >9)
H3SiO4 H2SiO42- + H+ (This occurs when pH >11) (eq.6)
The obtained H2SiO42- from pozzolanic reaction (eq.6) can now react with the unreacted
Ca2+ from C3S hydration (eq.4) to give additional C-S-H through this reaction:
3Ca2+ + 2 OH- + 2H2SiO42- Ca3Si2O7.3H2O (eq.7)
The Overall Pozzolanic reaction can be summarized as follows:
C3S + H2O + nano-SiO2 OH- + Ca2+ + H2SiO42- + H2SiO4
2-
Ca(OH)2 C-S-H C-S-H (additional)
Unreacted calcium from
C3S hydration
From pozzolanic reactivity
Additional C-S-H
From C3S From NS
35
2.8. Nano Silica Synthesis:
Silica powder is considered as competent material that could be used in a wide range of
applications such as catalysts [89], thermal and electrical insulators [90], filters [91],
fillers in engineering composites [92] and so on. They acquire this importance from their
unique properties such as low density, low thermal conductivity, and high specific
surface area [93].
Various methods have been used to produce silica particles, which can be categorized
into two main approaches: Top-Down and Bottom-Up. The Top-Down or physical
approach is characterized by fragmentation of microcrystalline material to yield Nano
sized crystals of this material [94]. This is usually done by utilizing some mechanical
means such as vibratory or planetary mills. On the other hand, the Bottom-Up or the
chemical approach involves a common route in which individual atoms and molecules
are brought together or self-assembled to form Nano structural materials [95].
The conventional method for nanosilica production is the Stober method, in which
TetraEthyl OrthoSilicate (TEOS), which acts as a silica precursor, is hydrolyzed in
alcohol (typically ethanol) in the presence of ammonia (NH3) as a catalyst [96]. This is
considered as an excellent method to produce monodispersed nanosilica particles using
the sol-gel process; however, it is believed that the production of nanosilica particles with
sizes lower than 50 nm is somehow difficult using Stober method [97] . Modified Stober
methods were needed to produce nanosilica with sizes about 10 nm using for example
basic amino acids such as L-Lysine [98] . N. Hassan et al [99] synthesized SiO2
nanostructures via sol-gel process using TEOS as silica precursor, acetic acid as catalyst
and distilled water as hydrolyzing agent. L.P, Singh et al [100] prepared spherical and
36
amorphous nanosilica particles, to study their effect in cement paste, by hydrolyzing
TEOS in a mixture of ethanol, water, ammonia, and surfactants. Recent studies on silica
production is directed towards extraction of silica particles from cheap raw materials such
as natural perlite and pumice [101]. Nanosilica could be also produced based on low
temperature alkali extraction from agricultural and industrial wastes like rise-husk [102]
and fly ashes [103].
37
3. Research Purpose:
In the last few decades, the advances in technologies allowed to a massive production of
cementitious materials. Today, cement industry became one of the most fundamental
blocks of our modern world. For instance, the average production of cement in Lebanon
is above 6.0 million tons per year.
Many studies have been done, mainly on concrete, in order to assess the effect of adding
nanoparticle in cement matrix. Few studies address the direct effect of nanoparticles on
cement paste and mortar. A number of specifications for cement have been placed all
over the world. The main purpose of these specifications is to set fixed values of a certain
readily determined properties of cement, which were found to be satisfactory in practice.
This would help manufacturers and consumers to detect inferior quality material by
comparing deviation to such standards [104].
As a matter of fact, cement compressive strength is an indirect measurement of the
cement hydration reaction. When mixed with water, cement reacts with water molecules
to produce a gel like structure which then sets and hardens to give concrete its strength.
The main constituents that are responsible for strength development in Portland cement
are the calcium silicates (C3S and C2S). The main product of the reaction of these
minerals with water is calcium silicate hydrate gel (C-S-H), which gives cement its
strength upon hardening.
Cement compressive strength at the age of 28 days is perhaps the most important quality
parameter on which cement sales are based. However, to obtain this result, a
representative cement sample should undergo a series of laboratory procedures in order to
38
be tested for strength determination later after 28 days. Most of the times, after 28 days of
production, the cement would have already been sold and incorporated into building
structures before the result is available. In fact, this represents a problem for cement
manufactures since they do not know the strength of cement that is being produced at any
time. To overcome this problem, quality control over the whole process of production is
done on hourly basis and quality parameters are set conservatively high to avoid falling
below the minimum strength requirements. Therefore, in order to have a safety margin,
cement plants are going higher with their conservative production costs.
Generating a model that can predict compressive strength at early ages would help
cement manufacturers to save time and expenses.
Extensive studies for predicting the ultimate 28 days compressive strength at early ages
have been carried out for concrete, but not for cement. And when available, very
complicated tools based mainly on cement specifications and microstructure were
generated. However, these tools require highly specialized people and sophisticated
equipment, which cost a lot of money. For example, Knofel D. [105] established a
formula to predict strength at 28 days using the quantities of alite, belite, aluminates and
ferrites that are determined by microscopy [106].
We will attempt in our work to formulate a simple procedure, which can be followed by
any laboratory, for estimating the 28 days compressive strength of cement at early ages.
This study will aim at predicting the 28 days cement compressive strength within 72
hours by using Nano-SiO2 and by elevating the curing temperature of cement mortars.
We will follow the same procedure required by the European Norm EN 196-1 [107] for
mixing, compacting, and curing the mortar specimens for the first 24 hours.
39
3.1. Sol-Gel Synthesis of Silica Nanoparticles:
Aiming at studying the effect of nanosilica on the hydration of cement paste, NS particles
were synthesized in our lab from sodium silicate powder. Na2SiO3 is dissolved in hot
distilled water (about 80 °C) while stirring at 450 rpm. pH of the solution is about 11.5.
After obtaining the clear homogenous solution, titrate with sulfuric acid (2N), drop by
drop, until pH 7.0. Silica starts to precipitate when pH of the mixture falls to less than 10.
Keep the mixture on magnetic stirrer at 350 rpm and allow standing at room temperature
overnight. The purpose of stirring overnight is to have dispersed silica particles and not a
network. Transfer the colloidal gel and apply centrifugation for 8 minutes at 8000 rpm.
Then wash with a mixture of distilled water and ethanol (80:20) three times while
centrifuging at the same speed. Collect the washed silica gel and transfer into a porcelain
crucible in order to dry overnight at 80 °C. After drying transfer the dried silica particles
to a muffle furnace controlled at 700 °C for a couple of hours. Nano Silica synthesis steps
are summarized in the set of pictures combined in figure 11.
40
Figure 11 - A. Colloidal Silica Gel at the end of titration; B. Gel after centrifugation and rinsing; C. Silica Gel
while drying; D. Silica Gel after drying; E. Calcining in muffle furnace; F. Calcined Silica Particles.
The above-mentioned procedure allows the synthesis of 99% pure SiO2 particles with a
high yield of 95%.
A B
B
C
D E F
41
The chemical reaction involved in this process is the following [101] :
Na2SiO3 + H2SO4 SiO2 + Na2SO4 + H2O (eq. 8)
It is a Sol-Gel process, in which the precursor (Na2SiO3) is hydrolyzed by H2SO4, until
pH 7, to yield a SOL (Si(OH)4). Condensation reaction causes the polymerization of
Si(OH)4 which leads to the transition of the sol into a Gel (Si-O-Si) (Figure 12).
Figure 12 - Schematic representation of Sol-Gel synthesis reaction of SiO2 nanoparticles.
42
3.2. Hypothesis:
Based on the information gathered from reviewing the literature, it is expected that:
A. High temperature curing would accelerate strength development at early age (2
Days), until certain optimum.
B. The addition of Nano-SiO2 would enhance cement compressive strength at early
and late age, until certain optimum.
C. By combining the effect of nano-SiO2 addition as cementitious material, and
increasing the curing temperature of cement mortars, a cumulative enhancement
of early strength might be observed.
3.3. Objectives:
1. Assessing the effect of changing curing temperature on cement mortar strength
development and finding the optimum.
2. Studying the effect of adding Nano-SiO2 on strength development of cement
mortar and finding the optimum.
3. Studying the effect of combining optimal conditions of objectives 1 & 2 on the
strength development of cement mortars.
4. Correlating the compressive strength results of accelerated hydration regime at 72
hours with the conventional 28 days strength of neat cement samples, aiming to
formulate a simple procedure for prediction of cement 28 days compressive
strength.
43
4. Sampling and Testing Plans:
4.1. Sampling plan
Five samples were collected during the working days of December 2019. 50 Kg
individual samples were collected while loading trucks for dispatch at random time
during the day shift. Samples were collected from loaded trucks as indicated by EN 196-
7. For each mortar preparation, only 450 grams is needed. Hence, the same sample could
be used for preparing different mortars to be tested at different conditions and with
different mixtures, provided that each original sample is well homogenized.
For this reason, each sample was homogenized and divided, in accordance with EN 196-
7, into five 10 Kgs samples to be used as follows:
i. First portion is used to test mortars under normal standard conditions.
ii. Second portion to study effect of curing temperature.
iii. Third portion to study effect of Nano-SiO2 addition
iv. Fourth portion to study the combine effect.
v. Fifth portion kept as a reserve.
X-Ray fluorescence was used to test all samples in order to ensure that they comply with
type P cement as required by Lebanese Norm NL-53. All samples specifications were
found to fall within requirements.
On the other hand, to ensure that the samples are very well mixed and homogenized, a
Blaine test was done on the five portions of each sample. As indicated by EN 196-6
44
[108], samples are to be considered belonging to the same population if the difference in
Blaine is less than 50 cm2/g. Results showed that all samples are well homogenized and
divided.
4.2. Mortar preparation planning:
Mortar mixtures from the five collected cement samples, with different ingredients were
prepared, all with water to cementitious material ratio of 0.5, as following:
i. Reference Mortar mixture (cement + sand + water), with a water to
cement ratio of 0.5. These are to be cured at standard condition (20 ± 1
°C) and to be tested for compressive strength at 2 and 28 days.
ii. Mortars from the five samples were prepared to be cured at different
temperatures of 20, 40, 50, 60, 70, 80 and 90 °C, in order to study the
effect of curing temperature on strength development for additional 24
hours.
iii. The same five samples were also studied for the effect of adding Nano-
SiO2 with different wt % of 1, 3, 5 and 7% wt/wt in substitution of cement.
iv. A final set would be prepared using optimum wt % of Nano-SiO2 to be
cured at the optimum temperature, to study their combined effect.
45
4.2.1. Mortar Preparation:
For mixing of mortar samples, distilled water and normalized standard sand is used for all
mixes. Mixing is done in accordance with EN 196-1. Cement and other constituents are
weighed using electronic balance. A quantity of 450 g of cementitious material is mixed
with 1350 g of sand 225 grams of water.
After mixing, mortars are casted in three ganged molds to be compacted on a jolting
table. Casted molds are then transferred to a climatic curing chamber controlled for
temperature at 20 ± 1 °C and for Humidity at above 90% RH. Samples were demolded
after 24 hours and cured in a water bath controlled at the desired temperature, with a
maximum variation of ± 2 °C for additional 24 or 48 hours. To eliminate the effect of
changing characteristics of tap water, distilled water was used for curing of all samples.
Prisms that are tested for standard conditions were transferred to a control room where
they are immersed in water bath controlled at 20 ± 1 °C, until the age of testing.
4.2.2. Compressive Strength Testing:
Hardened mortar prisms were then tested for compressive strength on ToniZEM from
Tonitechnik (Figure13), a 300 KN press machine. Mortar was crushed on a 16 cm2
platens with a pace rate of 2.4 KN/s. The percentage of error for the load is less than
1.0%.
The reference samples were tested at age of 2 and 28 days after casting. Accelerated
mortar specimens were tested at age of 48 and 72 hours.
Equipment for mortar preparation and testing are represented by figures 13 to 16.
46
Figure 14 - Cement Mortar Mixer
Figure 16 - Curing Chamber Figure 15 - Jolting Table
Figure 13 - Compression Machine
47
5. Materials and Methods:
1. Type I or CEM I ordinary Portland cement (type P in accordance to NL-53).
2. Standard normalized sand that complies with EN 196-1.
3. Micro silica (Grace Force 10,000)
4. Commercial Nano-Silica (SkySpring Nanomaterials; Product# ssnano 6807)
5. Synthesized Nano-SiO2 particles
6. Automatic Mortar Mixer for mortar preparation.
7. Jolting table in order to compact the mortar mixes in a three-gang mold.
8. Curing chamber controlled for temperature at 20 ± 1 ˚C and humidity above 90%.
9. Water bath for controlled temperature curing.
10. Cement Compression Machine for the compressive strength determination.
11. Blaine Apparatus. (for fineness)
12. XRD (main phase constituent determination)
13. XRF (for qualitative and quantitative Oxide analysis)
14. Malvern-Zetasizer Nano ZSP Series.
15. Fourier Transform InfraRed (FTIR) Spectroscopy
There are numerous factors that may influence the rate of hydration and the strength
development of Portland cement such as cement chemical composition, fineness, and
humidity of mixing and curing conditions, etc. [15].
To eliminate as much as possible any confounding variables that may affect our study,
only one type of cement (OPC type P) was tested. A set of experiments were performed
48
on the cement samples to examine the ranges of fineness and mineral content of calcium
silicate phases that are responsible for the hydration products and thus the hardening of
cement.
5.1. Fineness (Blaine):
Automatic Blaine apparatus from “Ibertest” (Figure17) was used to determine the
fineness of cement samples. This is an apparatus for determining the specific surface of
cement, with a unit of measurement in cm2/g.
In the Blaine permeability test, an amount of cement is introduced into a cylindrical cell,
where it is compressed to a fixed volume, followed by passing a defined volume of air
through the compacted cement. The resistance to this air flow is inversely proportional to
particle size. The larger the particles will cause more gaps to be created through which air
can flow. Therefore, finer cement exerts more resistance to air flow than coarse cement.
Through an experimental formula, Blaine method can make an estimate of the surface
area, which will be greater as the finer the cement is.
Results revealed that the fineness of cement samples falls between 3150and 3300 cm2/g.
49
Figure 17 - Auto Blaine Permeability Apparatus – IBERTEST
5.2. X-Ray Diffraction (XRD):
Pressed cement samples were prepared in order to be analyzed by XRD. Bruker D4
ENDEAVOR (Figure18) with a Cu anode X-ray source, a tube voltage of 50 kV and
current of 50 mA. The scan field applied is 2θ = 5 to 65°. Results are integrated
automatically by TOPAS software provided by “Bruker” which uses Rietveld analysis.
Minerals of interest are the main cement clinker phases: Alite (C3S), Belite (C2S) and the
hydration product portlandite (CH).
On the other hand, XRD would be used to monitor the change in C3S, CH and C-S-H
peaks over hydration time. C-S-H phase is often described as amorphous since its XRD
pattern exhibits only few broad and weak diffraction maxima [109] . However, from
literature data, C-S-H may have XRD diffraction maxima at several approximate 2θ
angles: 7.4°, 16.7°, 29.1°, 32.0°, 49.8°, 55.0° and 66.8° [110]. Conversely, CH and C3S
have well-defined XRD peaks at 2θ angles of 18.0°, 34.10°, 47.12° and 50.81° for CH,
50
and at 2θ range of 29° to 35° for C3S [111].On the other hand, XRD was also utilized in
this work to determine the nature of silica particles synthesized by the Sol-Gel method of
choice.
5.3. X-Ray Fluorescence:
Thermo Fisher Scientific ARL 9900 IntelliPower Series X-Ray spectrometer (Figure 19)
is utilized in order to enable the qualitative and quantitative determination of oxides. Rh
Anode X-Ray tube with a 2.5 KW power is installed in the machine that allows excitation
of elements range from B to U.
This machine offers a simultaneous detection of six elements (CaO, SiO2, Al2O3, Fe2O3,
MgO & SO3) via six monochromators installed in it for each element. In addition, XRF
allows sequential detection through a smart goniometer containing one fixed collimator,
three crystals (AX06, InSb and LiF200) allowing measurement of elements range O to U,
and two types of detectors (one for light elements and one for heavy metals).
51
Figure 18 - XRD: Bruker D4 ENDEAVOR
Figure 19 - XRF: Thermo Fisher Scientific ARL9900
52
5.4. Malvern Panalytical - Zetasizer Nano ZSP:
The Zetasizer Nano ZSP series performs size measurements using a process called
Dynamic Light Scattering (DLS). If a small particle is illuminated by a laser beam, the
particle will scatter the light in all directions. Dynamic Light Scattering measures
Brownian motion of particles in suspension and relates this to the size of the particles. It
does this by illuminating the particles with a laser and analyzing the intensity fluctuations
in the scattered light.
The relation between the size of the particle and its motion is defined by stokes-Einstein
equation:
Rh=𝑘𝑇/6𝜋𝜂D (Stokes-Einstein equation)
where:
Rh: Hydrodynamic radius [m]
D: Diffusion coefficient [m²/s] – “speed of the particles”
K: Boltzmann constant [m²kg/Ks²]
T: Temperature [K]
𝜂: Viscosity of the solvent [Pa.s]
Diffusion rate is inversely proportional to particle radius. The smaller the particle the
faster it moves in liquid suspension.
53
6. Results and Discussion:
6.1. Fourier Transform InfraRed (FTIR):
Fourier Transform Infra-Red spectroscopy is used to obtain an infrared spectrum of a
sample in the solid, liquid or gas state. A sample is irradiated with IR source; the
absorption of the infrared light’s energy by the sample at different wavelengths is
measured, allowing the determination of the molecular composition of a material in the
sample. In fact, the resulting spectrum of each molecule in FTIR is unique like a
fingerprint; no two different molecules can have the same infrared spectrum.
FTIR was performed for both, the commercial Nano-SiO2 and the synthesized sample.
Figure (20) shows an overlap of spectra with minor difference, indicating identical
material. The peaks can be identified as follows: (i) ∼ 470 cm-1 (Si-O asymmetric
bending) (ii) ∼ 800 cm-1 (Si-O symmetric stretching) (iii) ∼ 950 cm-1 (Si-OH bond
stretching) (iv) ∼ 1100 cm-1 (Si-O-Si bond stretching) (v) ∼ 1630 cm-1 (O-H bending)
(vi) ∼ 3450 cm-1 (Si-OH stretching vibration, hydrogen bonding) [112, 113, 114]
54
Figure 20- FTIR spectra of commercial nano silica particles (ss nano in blue) and of the synthesized nano silica
particles (SNS in orange)
Examining the synthesized nano-SiO2 (SNS) spectrum, the absence of peaks at 950, 1630
and 3450 cm-1 (the ones corresponding to the presence of OH groups) is attributed to the
final step in its preparation, the calcination. Moreover, our Nano-SiO2 particles are
freshly prepared, while the commercial ones might contain some humidity verified by the
presence of large band (O-H stretching at 3000-3500 cm-1) clearly absent for CNS.
45
55
65
75
85
95
105
400900140019002400290034003900
% T
cm-1
CNSSNS
55
6.2. X-Ray Diffraction (XRD):
Testing the synthesized nano-SiO2 on XRD showed that the particles are of amorphous
structure. XRD diffractogram of the commercial nano-SiO2 (Figure 21 A) shows a hump
at around angle 2Ɵ = 22°.The same pattern was detected for synthesized NS (Figure 21
B). This reveals that the produced particles are of amorphous nature especially that the
pattern contains no peaks related to crystalline structure.
Figure 21- XRD pattern for commercial NS (A); and synthesized NS (B)
It should be noted here that K. Teixeira et al. [115] studied the effect of TiO2
nanoparticles on cement paste at elevated temperatures. XRD analysis of their neat
cement paste and the cement paste with 0.8% nano-TiO2, both cured at room temperature
and at 60 °C, showed similar peak intensities for CH, indicating that strength contribution
of nano-TiO2 particles is mainly due to its filler and nucleation site effect. On the other
hand, P. Sikora et al [63] noticed that CH peak intensities decreased by adding nanosilica,
which reflects the consumption of CH by pozzolanic activity of nano-SiO2.
A B
2Ɵ degrees 2Ɵ degrees
56
Our results from XRD analysis, represented in the diffractograms (Figures 22 and 23)
clearly show lower intensity CH peaks for cement containing 3% of synthesized nano-
silica (SNS) hydrated for 28 days compared with the neat cement paste hydrated for the
same period. There is a 28.9 % decrease in CH concentration. On the other hand,
amorphous phase increased with the addition of SNS, indicating an increase in the
hydration product (C-S-H).
Hence, pozzolanic reactivity of NS is well revealed by XRD data, implying that Nano-
SiO2 reacted with CH produced during the hydration reaction to produce more C-S-H
(represented by amorphous phase). Such increasing yield in C-S-H production is
expected to have a positive effect on the mechanical properties of cement mortar.
Figure 22- XRD pattern for Neat cement paste hydrated at 28 days
57
Figure 23 - XRD pattern for cement paste with 3% of synthesized NS hydrated at 28 days
On the other hand, if we compare the reactivity after 28 days of Neat cement, MS, NS
and synthesized Nano-SiO2 pastes (Figure 24), we can surprisingly see that CH peaks
have the following order: Neat Cement > MS > CNS > SNS
Figure 24 - XRD CH peaks comparison of the four pastes at age 28 days: Neat Cement Paste (blue); Cement
paste with MS (pink); Cement paste with CNS (black); and cement paste with SNS (green).
Neat Cement Paste Hydrated at 28 Days
(Cement + MS) Paste Hydrated at 28 Days
(Cement + SNS) Paste Hydrated at 28 Days
(Cement + CNS) Paste Hydrated at 28 Days
58
At the first glance, one might think that since CH peak intensity for SNS-paste is lower
than that of Commercial NS, means that the synthesized NS have better reactivity with
cement grains. However, checking the Alite content for both pastes at 28 days shows a
lower value for commercial NS paste, which means that much more alite is consumed,
and hence, more CH is produced. This also conforms with the higher values of
amorphous phase generated by commercial NS paste (12.12%) compared to synthesized
NS paste (10.87%) as shown in Table (3).
Table 3 - XRD data of the four tested pastes
Alite (C3S) Portlandite (CH) Amorphous
% % %
Neat Cement Paste at 28 days 13.415 21.597 4.294
Cement Paste + 5% MS at 28 days 11.938 18.494 7.988
Cement Paste + 3% synthesized NS at 28 days 11.225 15.347 10.874
Cement Paste + 3% commercial NS at 28 days 8.368 16.839 12.121
59
6.3. X-Ray Fluorescence (XRF):
Cement samples were analyzed on XRF to confirm that they all comply with the
Lebanese national standard. Typical cement analysis is shown in table (4).
Table 4 - Typical XRF data for a type (P) cement with required specs by NL-53 and EN
197-1
Moreover, XRF analysis is done on the synthesized NanoSilica particles to check for
SiO2 purity. Results obtained for synthesized NS are shown in table (5).
Table 5- XRF Analysis Results of synthesized nanosilica
SiO2 20.97
Al2O3 4.41
Fe2O3 4.55
CaO 60.5
SO3 2.98 max 3.5 max 3.5
K2O 0.4
Na2O 0.06
TiO2 0.37
MgO 2.27 max 4 NA
P2O5 0.44
Cr2O3 0.001
CL- 0.043 max 0.1 max 0.1
Loss on Ignition 2.77 max 5 max 5
Sum 99.764
Na-Equivalent 0.32 max 1 NA
Insolubel Residue 0.78 max 4 max 5
Elements PercentagesLebanese Norm
Requirements
European Norm
Requirements
Oxide % Oxide %
SiO2 99.03 Na2O 0.31
Al2O3 0.02 TiO2 0.03
Fe2O3 0.03 MnO 0
CaO 0.06 P2O5 0
MgO 0.03 Cr2O3 0.006
SO3 0.05 Cl 0
K2O 0 LOI 0
SUM = 99.6
60
All collected cement samples are shown to fall within the specifications required by NL-
53 for type “P” cement.
Synthesized nanosilica particles are 99% pure silica with minor amounts of other oxides.
6.4. Size of pozzolanic SCM:
The following data were provided by suppliers of MS and commercial NS
Figure 25- Specs for: A. commercial NanoSilica; B. commercial Microsilica
It is worth mentioning that sieve analysis of microsilica showed a percentage of material
retained of 10% on the 45-micron sieve (or mesh # 325). From data shown in Figure 25,
we may deduce that nanosilica is finer than microsilica by about 30 times.
To test the size of synthesized nano-SiO2 particles, DLS was used on samples suspended
in deionized water.
DLS measurement shows an average particle size of about 135nm (size distribution by
Intensity), with a polydispersity index (PDI) of 0.3.
Result is shown in Figure 26.
Silicon Dioxide Nanoparticles/ Nanopowder
Product #: 6807NM
Porous White Powder
CAS Number: 7631-86-9
Empirical Formula: SiO2
Purity: 99.5+% APS: 15-20 nm
SSA: 640m2/g
Bulk Density: 0.08~0.10g/cm3
True Density: 2.648 g/cm3
Typical Properties
Appearance: Ultra-fine amorphous Light to dark grey.
Specific Gravity: 2.25±15 % at 20°C.
Bulk Density: ≥650 kg/m3.
SiO2 Content: 90 % min
SSA: 15 – 25 m2/g
A B
b
61
Figure 26- DLS of Synthesized nano-SiO2
Our results from DLS analysis demonstrate that the synthesized nanoparticles are nearly
monodisperse with diameter range that fit well in between of the commercially used
nano-SiO2 (~20 nm) and micro-SiO2 (~350 nm). Therefore, the synthesized nanoparticles
will be further used in this study and compared to commercial samples.
6.5. Effect of curing temperature:
Five different samples, collected at random on different dates were used to prepare
standard mortar prisms, to be tested for compressive strength at different curing
temperatures. In addition to the standard curing condition of 20 ± 1 °C, other curing
temperatures were applied: 40, 50, 60, 70, 80 and 90 °C respectively.
All samples were allowed first to settle in a curing chamber at standard conditions (T= 20
°C & RH > 90%) for 24 hours. After this initial curing time, samples were de-molded,
immersed in water and subjected to different accelerated curing regimes for another 24
hours, each at a different controlled temperature, using a water bath.
0
5
10
15
20
1 10 100 1000 10000
Inte
nsi
ty (
%)
Size (d.nm)
Size Distribution by Intensity
62
Table (6) shows compressive strength data for the five samples at different curing
temperatures:
Table 6 - Compressive strength of mortar specimens at different curing temperatures
Figure 27- Average compressive strength of mortar specimens cured at different temperatures.
Results shown in Table (6) disclose that as we raise the curing temperature, compressive
strength increases, until we reach a temperature of 70 °C, where the strength starts to
decrease again. This could be the effect of non-uniform distribution, or possible
deterioration of hydration product in the cement paste microstructure at high curing
temperature.
Temperature (°C) 20 40 50 60 70 80 90
Sample-1 15.5 19.1 22.8 29.1 32.9 30.1 28.2
Sample-2 14.1 17.8 21.1 26.3 30.1 27.4 25.7
Sample-3 15.3 19.3 22.5 28.6 32.6 30.1 29.4
Sample-4 17.9 28.5 33.1 40.9 46.1 42.9 44.1
Sample-5 16.2 20.3 25.8 31.1 33.0 31.8 30.6
Compressive Strength in MPa
15.821.0
25.1
31.234.9
32.5 31.6
0
5
10
15
20
25
30
35
20 40 50 60 70 80 90Co
mp
ress
ive
Str
en
gth
(M
pa)
Temperature (°C)
63
Figure (27) reflects the average determinations of all five samples at each curing
temperature. Therefore, our results demonstrate that curing at 70 °C is considered to be
the optimum curing temperature for the accelerated curing regime intended to be used.
6.6. Effect of Nano Silica Addition:
Utilizing the same set of samples used in determining the optimal curing temperature,
another series of mortar specimens were prepared to be tested for different percentages of
wt/wt NS to cement substitution. Nano silica used here is commercial nano silica (ssnano
# 6087NM).
From each sample, five different mortars having different wt/wt percentages of
NS/cement were prepared as follows:
• Reference mortar specimen with 0% NS.
• Mortar specimen with 1% NS in substitution to cement.
• Mortar specimen with 3% NS in substitution to cement.
• Mortar specimen with 5% NS in substitution to cement.
• Mortar specimen with 7% NS in substitution to cement.
Compressive strength was determined for the different samples at age of 2 and 28 days.
Results are shown in Table (7).
64
Table 7 - Compressive Strength results at age 2 and 28 days of the five samples with
different percentages of CNS as substitution wt/wt of cement
Data from table (7) clearly demonstrate that the best results were achieved using 3%
wt/wt of NS at both 2 and 28 days. Hence, this was considered as the optimum
substitution percentage. On the other hand, the same procedure was applied for micro
silica (MS) addition, for which 5% was determined to be the optimum (data not shown).
Table (8) includes a comparison of the strength enhancement at 28 days between MS,
commercial NS and synthesized NS.
Table 8 - Compressive Strength results at 28 days of Neat cement mortar and mortars with
5% MS, 3% CNS and 3% SNS respectively
% NS 0 1 3 5 7
2 Days 15.5 16.2 17.8 16.6 8.7
28 Days 48.5 50.0 56.7 51.9 29.8
2 Days 14.1 15.3 16.8 16.2 8.3
28 Days 47.9 50.4 56.9 52.4 28.6
2 Days 15.3 16.1 17.5 16.2 8.5
28 Days 49.4 51.3 57.5 53.3 33.5
2 Days 17.9 18.1 19.9 18.9 9.8
28 Days 49.6 53.1 57.8 52.8 30.3
2 Days 16.2 16.4 17.4 17.2 9.3
28 Days 48.1 50.3 57.0 51.1 29.5
Compressive Strength in MPa
Sample-1
Sample-2
Sample-3
Sample-4
Sample-5
Neat Cement
Cement + 5% MS
Cement + 3% Commercial NS
Cement + 3% Synthesized NS
47.9
50.2
56.9
53.8
Compressive Strength in MPa (28 days)
65
Optimum amount for synthesized NS was considered to be the same as that of
commercial NS due to scarce in material. Actually, in order to conduct the test to find
optimum quantity, we need about 100 grams of nano-SiO2, which is a big quantity to be
synthesized in a short time. That is why I used the same optimum of the commercial nano
silica to monitor the pozzolanic reaction by XRD. Moreover, I synthesized about 20
grams of silica nanoparticles in order to study its effect on mortar compressive strength.
It is clear that commercial NS provides better enhancement in ultimate 28 days strength
than synthesized NS, which in turn provides better strength enhancement than MS. This
conforms with both conclusions of, XRD data of pozzolanic reactivity, as well as with the
size effect of pozzolanic material on accelerating the hydration reaction.
6.7. Concluding Optimum Fast Curing Regime
Now, to ensure that the combined parameters would serve as optimum conditions, two
samples were chosen randomly and tested for 3% commercial NS wt/wt substitution at
three different curing temperatures of 60, 70 and 80 °C respectively.
Table 9 - Strength results of mortars with 3% CNS cured at high temperature
Results shown in Table (9) clearly ensure optimum conditions for both temperature and
NS addition with good repeatability.
Ensuring optimums
% NS 70
Sample-2 31.0
Sample-3 36.5
Compressive Strength in MPa
(24 hours at high °C)
60 80
28.8
33.9
30.1
34.6
66
7. Data Correlation and Strength Prediction
Our next step would be to correlate strength enhancement at early age (up to 72 hours)
with standard strength at 28 days, in an attempt to predict the 28 days strength using this
accelerated curing regime.
For this purpose, 25 different samples were collected (including the five samples
mentioned earlier) and tested for both the standard regime (standard mortar at T= 20±1
°C) at 28 days, and the accelerated one (3% NS substitution at 70 °C) after 48 and 72
hours.
Compressive strength data from both testing procedures were collected in order to be
fitted in an appropriate correlation method.
Based on the mineralogy (XRD) content of calcium silicates, and on the accelerated
regime strength results, a multiple linear regression model was developed to predict the
28 days compressive strength at earlier age of maximum 72 hours.
The data generated by “XLSTAT” best fits the Multiple Linear Regression equation (eq.
9) with a multiple coefficient of correlation R2 = 0.9, and a Root Mean Square Error
(RMSE) of 0.8. This result surely depicts a very good model as there exists a good
correlation between the actual 28 days strength and the predicted one, based on the
chosen parameters.
67
The 28 days compressive strength could be predicted from the values obtained by the
accelerated hydration regime using the following Multiple Linear Regression (MLR)
equation:
Sp= -41.886 + 0.874*SAcc48 + 1.046*SAcc72 + 0.135*A + 0.439*B (eq. 9)
Where, Sp is the predicted strength,
SAcc48 is the strength obtained by accelerated regime at 48 hours,
SAcc72 is the strength obtained by accelerated regime at 72 hours,
A is the Alite content
B is the Belite content.
Table 10 - Predicted strength when applying the MLR formula (eq. 9) and the optained
COV
RESIDUAL OUTPUT
Observation Predicted 28days Actual 28days Residuals avg std COV
1 48.03 48.50 0.47 48.27 0.33 0.69
2 47.58 47.90 0.32 47.74 0.22 0.47
3 51.70 50.70 -1.00 51.20 0.71 1.38
4 52.99 53.70 0.71 53.34 0.50 0.94
5 52.20 51.80 -0.40 52.00 0.28 0.54
6 53.68 54.30 0.62 53.99 0.44 0.81
7 51.89 51.10 -0.79 51.49 0.56 1.08
8 49.95 49.60 -0.35 49.77 0.25 0.49
9 50.43 50.60 0.17 50.51 0.12 0.24
10 49.74 48.10 -1.64 48.92 1.16 2.38
11 47.50 47.30 -0.20 47.40 0.14 0.29
12 50.53 50.30 -0.23 50.42 0.17 0.33
13 49.87 49.90 0.03 49.89 0.02 0.04
14 51.07 51.40 0.33 51.24 0.23 0.45
15 50.10 50.60 0.50 50.35 0.35 0.70
16 50.00 50.20 0.20 50.10 0.14 0.28
17 51.71 50.90 -0.81 51.30 0.57 1.11
18 52.16 51.10 -1.06 51.63 0.75 1.45
19 51.86 53.10 1.24 52.48 0.88 1.67
20 50.89 51.60 0.71 51.25 0.50 0.97
21 50.27 49.40 -0.87 49.84 0.62 1.24
22 50.32 51.60 1.28 50.96 0.90 1.77
23 51.91 52.10 0.19 52.00 0.14 0.26
24 50.73 51.00 0.27 50.86 0.19 0.38
25 50.09 50.40 0.31 50.25 0.22 0.44
68
Calculating the Coefficient of variation (COV), the results show a very good estimation
of the 28 days strength (Table 10). In fact, COV results fall within the repeatability limits
of the compressive strength testing method required by the European norm (EN 196-1),
which gives the closeness of agreement between two test results obtained to be less than
3.5 % when expressed as the coefficient of variation. Actually, as seen from Figure (28)
(left plot), all standardized residuals fall below 2.5% as an absolute value. The right plot
of the same figure clearly shows that all points are reasonably close to the straight line,
with no obvious outliers.
Figure 28 - Plot of predicted values against residuals (left); Plot of measured values against predicted (right)
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
47 48 49 50 51 52 53 54
Sta
ndard
ized r
esid
uals
Pred(28days)
Pred(28days) / Standardized residuals
35
40
45
50
55
60
65
70
35 45 55 65
Actu
al 28days S
trength
(M
Pa)
Predicted 28days Strength (MPa)
Pred(28days) - Actual 28days
69
8. Conclusion:
The hydration of Portland cement involves the reaction of calcium silicates phases with
water to form C-S-H. This is a solid structure that will fill the space between mortar or
concrete particles and results in an interlocking mass whose porosity is a function of
water to cement ratio. The lower the water to cement ratio, the higher the compressive
strength provided that there is sufficient water for mortar plasticity.
The well-known effect of temperature on the early strength development was observed
with the set of specimens tested at different curing temperatures. Strength development is
faster, the higher the temperature is. Highest values were achieved at 70 °C. Above this
temperature, strength begins to drop as an effect of the deterioration of hydrate
microstructure.
Synthesis of amorphous nanosilica particles could be applied in a cement lab through a
simple procedure. Sol-Gel process, in which sodium silicate solution is titrated by
sulfuric acid to produce a colloidal suspension of silica particles in aqueous sodium
sulfate solution. Silica particles are then collected by centrifugation and calcined in a
muffle furnace to get the amorphous pure NS with a yield of 95%.
Strength enhancement with addition of nanosilica, as a percentage wt/wt substitution of
cement, was observed for small quantities (1% to 5 %), with the optimum quantity
showed to be 3%. Nano-silica behaves as a filler to improve cement microstructure, and
acts also as an activator to promote pozzolanic reactions. NS reacts with calcium
hydroxide produced during the hydration reaction to yield more C-S-H, hence, improving
70
concrete behavior. Adding 3% NS as substitution of cement showed an enhancement of
14% at 2 days and 18% at 28 days.
It was obvious that the combined effect of the optimum temperature (70 °C) with that of
optimum NS percentage (3%) is cumulative. Results of this set of specimens were higher
than both of that cured with at 70 °C or with 3% NS tested separately.
A very good correlation was found between the values obtained by the applied fast curing
regime and the ultimate 28 days compressive strength. In fact, the obtained accelerated
strength result could be used to predict the 28 days strength with a coefficient of variation
of less than 2.5%.
This simple procedure can be used by cement manufacturers to estimate the 28 days
compressive strength of cement mortars at an early age of 72 hours. However, this
remains as an internal procedure and not a reference. Moreover, it would apply only to
OPC cement type (P). For other types of cement, the effect of additives such as
limestone, fly ash, or other pozzolanic material should be addressed and tested.
In this current work no superplasticizers were used and the water to cementitious material
ratio was maintained constant in order to make sure that the resulting strength
enhancement is solely obtained from the effect of nanosilica or increased temperature.
However, using superplasticizers could have a positive effect on strength through
reducing the water to cement ratio, and by improving nanoparticles dispersion in the
mortar matrix. This would in turn allow for more reduction in the cement quantity used in
concrete without affecting its performance.
71
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List of Figures:
Figure 1 - Sibline Cement Factory .................................................................................................... 4
Figure 2 - The New Bag Filter on Line II ........................................................................................... 6
Figure 3- Schematic drawing of cement hydration [119] .............................................................. 13
Figure 4- A pictorial representation of a cross section of a cement grain. Adapted from Cement
Microscopy; Halliburton Services: Duncan, OK. ............................................................................ 14
Figure 5- Number of published papers, that, according to Scopus, include the terms cement and
nanotechnology or nanomaterials in the title, abstract or keywords, limited to the fields of
engineering and materials science. [19] ........................................................................................ 15
Figure 6 - The schematic molecular structure of a single sheet of C-S-H. Circles represent calcium
atoms located at the center of Ca-O octahedra; Triangles show silicate tetrahedra ; OH-
attachments are not shown. [26] .................................................................................................. 17
Figure 7-Example of crack bridging in a SWCNT/hydrated OPC composite. Bridging structures are
SWCNT bundles. [26] ..................................................................................................................... 20
Figure 8- Schematic representation of photocatalytic effect of Nano-TiO2 particles in concrete
[19] ................................................................................................................................................. 24
Figure 9 - Surface Modification of Carbon Nanotubes [86] ........................................................... 31
Figure 10 - Microstructure Densification of Traditional, High Performance and Nano-Engineered
Concrete [64] ................................................................................................................................. 33
Figure 11 - A. Colloidal Silica Gel at the end of titration; B. Gel after centrifugation and rinsing; C.
Silica Gel while drying; D. Silica Gel after drying; E. Calcining in muffle furnace; F. Calcined Silica
Particles. ......................................................................................................................................... 40
Figure 12 - Schematic representation of Sol-Gel synthesis reaction of SiO2 nanoparticles.......... 41
Figure 13 - Compression Machine ................................................................................................. 46
Figure 14 - Cement Mortar Mixer .................................................................................................. 46
Figure 15 - Jolting Table ................................................................................................................. 46
Figure 16 - Curing Chamber ........................................................................................................... 46
Figure 17 - Auto Blaine Permeability Apparatus – IBERTEST ......................................................... 49
Figure 18 - XRD: Bruker D4 ENDEAVOR ......................................................................................... 51
Figure 19 - XRF: Thermo Fisher Scientific ARL9900 ....................................................................... 51
Figure 20- FTIR spectra of commercial nano silica particles (ss nano in blue) and of the
synthesized nano silica particles (SNS in orange) .......................................................................... 54
Figure 21- XRD pattern for commercial NS (A); and synthesized NS (B) ....................................... 55
Figure 22- XRD pattern for Neat cement paste hydrated at 28 days ............................................ 56
Figure 23 - XRD pattern for cement paste with 3% of synthesized NS hydrated at 28 days ......... 57
Figure 24 - XRD CH peaks comparison of the four pastes at age 28 days: Neat Cement Paste
(blue); Cement paste with MS (pink); Cement paste with CNS (black); and cement paste with SNS
(green). ........................................................................................................................................... 57
Figure 25- Specs for: A. commercial NanoSilica; B. commercial Microsilica ................................ 60
Figure 26- DLS of Synthesized nano-SiO2 ...................................................................................... 61
Figure 27- Average compressive strength of mortar specimens cured at different temperatures.
....................................................................................................................................................... 62
82
Figure 28 - Plot of predicted values against residuals (left); Plot of measured values against
predicted (right) ............................................................................................................................. 68
List of Tables:
Table 1 - Cement types as per Lebanese Norm NL-53 ..................................................................... 8
Table 2 - Nanomaterials used in cement and their effect on its properties (retrieved from
nanowerk.com) .............................................................................................................................. 26
Table 3 - XRD data of the four tested pastes ................................................................................. 58
Table 4 - Typical XRF data for a type (P) cement with required specs by NL-53 and EN 197-1 ..... 59
Table 5- XRF Analysis Results of synthesized nanosilica ................................................................ 59
Table 6 - Compressive strength of mortar specimens at different curing temperatures .............. 62
Table 7 - Compressive Strength results at age 2 and 28 days of the five samples with different
percentages of CNS as substitution wt/wt of cement ................................................................... 64
Table 8 - Compressive Strength results at 28 days of Neat cement mortar and mortars with 5%
MS, 3% CNS and 3% SNS respectively ............................................................................................ 64
Table 9 - Strength results of mortars with 3% CNS cured at high temperature ............................ 65
Table 10 - Predicted strength when applying the MLR formula (eq. 9) and the optained COV .... 67