chemical admixtures essential components of quality concrete
DESCRIPTION
Chemical admixtures for concreteTRANSCRIPT
1
Chemical admixtures:
Essential Components of
Quality Concrete
C. Jolicoeur, N. Mikanovic, M.-A. Simard and J. Sharman
Modern-day concrete frequently incorporates one or more
chemical admixtures to achieve specified material
properties. In a context where binder systems have become
increasingly complex, either due to addition of pozzolans
(for example, silica fume), or partial cement replacement by
supplementary cementitious materials (fly ash, blast furnace
slag, etc), or addition of fillers, and where concrete
performance requirements are increasingly demanding,
chemical admixtures are rapidly gaining in importance and
in diversity. For concrete practitioners, the variety of
concrete admixture types, and the diversity of admixtures
within each type, create a rather complex environment. This
paper attempts to present an overview the field of chemical
admixtures and provide some perspective on the need for
these admixtures, their function and benefits in application.
A particular emphasis is given to those admixtures — water
reducing and colloidal — which influence the rheological
properties of fresh concrete.
In recent years, the use of chemical additives in concrete has
grown considerably, for virtually all types of concrete and
their applications. These additives commonly referred to as
'concrete chemical admixtures' now comprise a wide variety
of chemicals which are usually introduced during the
batching of concrete (occasionally after the batching
process, or before, in the dry cementitious binder) to
enhance specific properties of the fresh or hardened
concrete material. This rapidly expanding field has already
been the subject of several monographs and it is consistently
monitored by various standards and regulatory agencies
(ASTM, ACI, RILEM, etc), which overview testing
methods and codes of practice1-8
.
Concrete chemical admixtures are usually classified
according to the specific functions they are intended to
perform; hence, the following groups of admixtures are
commonly designated:
water-reducing admixtures
set modifiers (retarders, accelerators)
air-entraining agents
anti-bleeding/segregation admixtures
corrosion inhibitors
curing and shrinkage (drying) reducing admixtures
water-proofing admixtures
anti-freezing admixtures
admixtures controlling alkali-aggregate reaction
(AAR)
Each of these families of concrete admixtures may comprise
sub-classes, for example: low-range and high-range water-
reducing admixtures (WR and HRWR). While the general
function of each admixture group is obvious, some
admixtures may exert more than one function, for example,
water-reducing and set-retarding. Their ultimate role and
impact on concrete materials and technologies can be best
appreciated through an overview of the main requirements
for durable concrete and the current trends in construction
practices.
Indian Concr. J. 76 (2002) 537-549.
2
Need for admixtures
The pressing need for higher quality concrete (in terms of
strength and durability) inevitably points towards low
porosity materials; these imply minimum water content and
the use of ultra-fine supplementary materials (fillers or
pozzolans). In order to meet these requirements, while
retaining adequate concrete workability, high-range water
reducers must be employed. The material requirement is
exacerbated by the evolution of concrete handling and
placing technologies, which now largely favour the use of
flowing (pumpable) concrete.
In other recent developments, the requirements on the
properties of flowing concrete have been raised to achieve
'self-placing' and 'self-levelling' abilities. The latter are to
ensure that fresh concrete can flow with near-zero yield
stress in order to reach all areas, regardless of the degree of
congestion, and yield level surfaces requiring minimum
finishing. The self-placing requirement implies concrete
with a high fluidity, which will then be prone to bleeding
and segregation. To minimise the latter effects, admixtures
have been designed, the main function of which is to
increase the bulk viscosity of the cement paste at low shear
rate, in order to minimize water migration and segregation
effects, that is anti-bleed/segregation, or colloidal
admixtures.
As emphasized in recent literature, shrinkage-related effects
represent another significant limitation to concrete strength
and durability 9. Whether the shrinkage is autogeneous, or
the result of capillary pressure as the solution leaves the
micropores (drying shrinkage), the phenomenon increases
the microcracking tendency of the concrete, particularly at
early ages. To minimise these effects, an optimal control of
water migration in the capillary pores of the cement paste
during setting and hardening is essential. This may be
achieved through a combination of in-situ 'curing'
admixtures incorporated in the cement paste, and a proper
moisture supply during the drying and hardening phases.
Numerous other admixtures find use in concrete, exerting
specific functions in certain types of applications such as:
air-entraining admixtures to protect concrete from
internal stress due to volume changes during the
freezing and thawing of the concrete pore solution;
corrosion inhibitors to minimise degradation of steel
reinforcement in elements exposed to harsh
environments;
AAR control admixtures to minimise expansion
from the reaction of siliceous aggregate with lime,
leading to bulk deterioration of concrete elements;
water-proofing admixtures to minimise water
permeation and uptake by the hardened concrete.
Another important development in the cement and concrete
industry is a major current drive to minimise the amount of
cement used in concrete mix, largely through the
replacement by supplementary cementitious materials,
mostly fly ash and blast furnace slag. This effort produces
several significant benefits: enhancement of concrete
durability; reduction of CO2 emission associated with the
production of portland cement; beneficiation of secondary
industrial materials. This widely expanding practice
involves a significant increase in the chemical complexity of
the binder system, which affects all properties of concrete:
rheological, bleeding and segregation behaviour, setting,
strength development, etc. This further enhances the need
for chemical admixtures, in order to accommodate changes
in the binder system and to optimise concrete mix design.
Critical admixtures for high performance
materials
The above overview on the need for concrete chemical
admixtures may be further focussed by examining the
concrete features which are most critical for achieving high
performance, that is, high strength and durability. This, in
turn, will identify corresponding ‗critical admixtures‘.
As is now widely realised, the properties of concrete which
determine high performance (as defined above) include:
rheology of fresh concrete; homogeneity in the plastic and
hardened states10
; porosity; dimensional stability through
the setting and hardening stages. Beyond variations in
concrete mix design parameters, these properties may be
controlled and optimised by various combinations of
admixtures typically:
rheology: HRWR and colloidal admixtures
homogeneity: HRWR and colloidal admixtures
(viscosity enhancing admixtures)
porosity: WR and HRWR
dimensional stability: shrinkage-reducing
admixtures (in-situ curing agents)
This promptly suggests that water-reducing admixtures,
colloidal agents and shrinkage control admixtures are three
‗critical‘ groups of admixtures for the design and production
of high quality concrete. For concrete in cold weather
environments, air-entraining agents must also be considered
as a critical admixture.
The present paper focuses on two of these critical
admixtures: water reducers and colloidal admixtures. As
noted above, these admixtures jointly determine the
rheological properties and the stability of fresh cement-
based systems, as well as the porosity of the hardened
materials. It is interesting that these two types of admixtures
are, in some ways, antagonists: water-reducers act to
fluidise the system, while colloidal admixtures have
thickening properties (viscosity enhancement). A brief
overview of their respective chemistry, mode of action,
3
performance evaluation methods and performance in
application is given below.
Water-reducing admixtures (WR and
HRWR)
Function
As is now universally accepted, the mechanical properties of
concrete are directly related to the porosity (total porosity,
pore structure) of the binder matrix. Since concrete porosity
is highly connected, the permeation of water and other
substances into the concrete matrix is also dependent on
porosity, with obvious consequences on concrete durability
(air voids generated intentionally with air-entraining agents
are not interconnected).
The complete hydration of portland cement requires
approximately 30 percent water. Any water added beyond
this level (say, 40-60 percent), will leave a corresponding
level of capillary porosity. Since, in these systems, the
volume fraction of water is approximately three times its
weight fraction, the pore volume due to excess water can be
very significant. Hence, minimisation of the excess water
through the use of water-reducing chemical additives is
understandably an important issue in concrete technology.
Many types of chemicals, mostly organic compounds, can
reduce the water requirement for a given concrete
workability. In broad terms, these admixtures enhance the
deflocculation and dispersion of cement particles, reducing
the apparent viscosity of the mix; often, such admixtures
temporarily repress the hydration reactions (retardation),
providing an extended control of the concrete workability 4.
Chemical nature and classification
Chemical additives which can perform as concrete water-
reducing admixtures have been classified by ASTM into
two broad categories according to their effectiveness
(ASTM C-494): water-reducers (WR, type A), from 5
percent water reduction to high-range water reducers:
(HRWR, type F) water reduction of more than 12 percent.
Currently employed HRWR exhibit much higher water
reduction, typically up to 30 percent; these are now
commonly referred to as 'superplasticisers'. Water-reducing
admixtures may be further subdivided into specific classes
according to their influence on setting times, for example
type D (WR and retarding), type E (WR and accelerating)
and type G (HRWR and retarding).
Commonly used WR admixtures comprises mainly sugars
and sugar derivatives (gluconates, hydroxy acids), often by-
products recovered from the agricultural and food
processing industries. Lignosulfonates (sodium or calcium
salts), a by-product of the bisulphite wood pulping process
is also widely used as concrete water-reducer 11
. The latter
consists of sulphonated polymers of substituted phenyl
groups connected by short alkyl, or ether linkages. Due to
the inherent molecular complexity of natural lignin, and
because of the non-specific character of the lignin
sulphonation / de-polymerisation / solubilisation reactions,
the admixture consists of a complex mixture of molecular
species. Fig 1 schematically illustrates the chemical
structure of gluconates and lignosulfonates
The class of HRWR, or superplasticisers, comprises a
variety of synthetic water-soluble organic polymers bearing
ionisable groups, mostly sulfonates (SO3) and / or
carboxylates (COO). The most widely used of such
polymers is polynaphthalene sulphonate (PNS) (sodium, or
calcium salt), the average chemical structure of which is
illustrated in Fig 2(a), together with a schematic
representation of the polymer as used in later illustrations. A
second important type of sulphonated superplasticiser
polymer (not shown) is polymelamine sulphonate (PMS). In
recent years, a wide range of polyacrylate co-polymers has
been proposed as HRWR. A major subgroup of the latter
comprises a variety of polyacrylate molecules partly
esterified with poly-ethyleneglycol side chains; hence their
designation as polyacrylate esters (PAE). Their chemical
structure and schematic representation is shown in Fig 2(b).
The water-reducing ability of PNS superplasticisers depends
on their molar mass — the higher the mass, the better their
performance. The weight average molar mass of
commercial PNS has been determined to be between 10 and
65 kDa 12
. For PAE-type superplasticisers, the performance
is not directly related to the molar mass, since many other
molecular parameters can influence the performance. For
various PAE, weight average molar masses between 20 and
100 kDa have been reported 13
O
H3CO
O
OH
H3CO
SO3Na
HO
nLignosulfonate
CH2
H
OH
H
OH
OH
H
H
OH
C ONa
O
HO
Gluconate
Fig 1 Schematic illustration of gluconates and
lignosulfonates water reducers
4
Mode of action
As noted above, WR and HRWR act to deflocculate and
disperse cement particles, thus enhancing the fluidity of the
cement paste. The molecular mechanisms through which
WR and particularly HRWR can disperse cement particles
in the paste are illustrated in Fig 3. The WR molecules
adsorb onto the surface of the hydrating cement grains,
conveying to these surfaces a negative electrical charge
(potential). The latter generates an electrostatic repulsion
between neighbouring cement particles, promoting
deflocculation and dispersion of these particles; the
phenomenon is depicted schematically in Fig 3(a) for PNS
molecules for which electrostatic dispersion is important.
In addition to the electrostatic forces, the dispersion of
particles is further assisted by repulsive forces originating in
'steric effects': the adsorbed polymer (neutral or charged)
constitutes a physical barrier to particle-particle contact, that
is, when the dangling chains of polymer adsorbed on two
adjacent surfaces begin to entangle, the resulting loss in
their entropy is highly unfavourable 14
. This is illustrated
schematically in Fig 3(b) with PAE molecules for which the
steric repulsion is likely to be dominant.
The physical effects underlying the mode of action of
HRWR, as described above, are complemented by 'chemical
effects', as illustrated in Fig 3(c). The chemical specificity
in HRWR is first manifested in the adsorption process; for
example, sulphonate-based polymers interact preferentially
with the aluminate phases of the cement, that is, by analogy
with the sulphate ions. In addition, several types of WR and
HRWR molecules are known to inhibit the nucleation and
growth of hydration products (gypsum, ettringite, or other
hydrates). While this may not directly improve dispersion of
cement particle, it prevents their reagglomeration and thus,
preserves the rheological properties of the cement paste.
Through specific chemical interactions, water-reducers may
exert set retardation effects depending on the nature of the
admixtures and the dosage. Lignosuphfonates and PAE-type
admixtures are inherently more retarding than PNS or PMS;
in practice, the retardation must be compensated through
addition of accelerating admixtures.
Water-reducing admixtures, by virtue of their combined
hydrophilic-hydrophobic character, can frequently act as
surfactants (soap) and induce excessive air entrainment in
concrete. Again, lignosulphonates and PAE admixtures are
inherently more active to entrain air than PNS- and PMS-
type compounds. This undesirable effect can usually be
minimised by the use of defoaming admixtures.
Performance evaluation
The performance of water-reducing admixtures is evaluated
from observations of their influence on the rheological
properties of the fresh cementitious system, grout, paste,
mortar, or concrete. For the complete description of
systems, as warranted in the development or optimisation
work, the detailed rheological behaviour (that is, shear stress
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(a) (b)
(c)
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---
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- ----
- ----=
>
CH2
SO3Na
n
=
>
- - - - - - - - - - - -
CH2 C
R1
COONa
CH2 C
R1
CO
O
n m
R
CH2 C
R1
X o
(a)
(b)
Fig 2 Schematic illustration of commonly used
superplasticizers; (a) polynaphthalene sulphonate (PNS)
(b) polyacrylate ester (PAE)
Fig 3 Mode of action of HRWR; (a) electrostatic
repulsion illustrated with PNS (b) steric repulsion
illustrated with PAE (c) inhibition of nucleation and
growth processes, either from a reacting surface or from
solution.
5
versus shear rate) can be determined through appropriate
viscometers. With pastes and grouts, various commercial
rheometers are adequate for studies on small samples;
however, relevant measurements on mortars and concrete
require specially-designed rheometers 15-20
. For comparative
evaluation purposes, numerous methods have been designed
which rely on the measurement of one or more parameter
related to flow, in ways more or less related to the universal
concrete slump test (ASTM C 143).
With grouts, the Marsh flow cone method (flow time
through a funnel) provides a reliable means to compare
admixtures and determine their optimal dosage (that is,
saturation dosage) 10
. The method reflects the grout viscosity
at intermediate shear rates.
For pastes, the most commonly-used test for relative
evaluation of admixtures is the ‗mini-slump‘ test — a
miniature version of the standard concrete slump test 10,21
. In
the mini-slump test, the flow-spread area (or diameter) is
measured, which relates to viscosity under very low shear
rates. A series of results on Type-10 and silica fume cement
pastes with a PNS superplasticiser, obtained as function of
concentration and temperature, is illustrated in Fig 4 22
.
For the relative evaluation of water-reducing admixtures in
mortars, an impact flow table is widely used (ASTM C
230); in the latter, the flow spread from a normalised cone is
measured following a fixed number of impacts onto the
table. This method reflects mortar flow starting from zero
shear rate, thus emphasising yield stress.
For comparative, or screening purposes, the performance of
HRWR is readily evaluated through the mini-slump test on
cement pastes. Remarkably, the results of the latter,
obtained as function of dosage and time, accurately predict
the trends in the slump values and the slump retention of
fresh concrete.
Performance in application
Typical mini-slump data comparing three superplasticisers
in pastes having w/c of 0.30 and 0.20 respectively are
shown in Fig 5 23a
. The results demonstrate two important
features of these admixtures:
(i) PAE-type superplasticizers are more effective than
the reference PNS (or PMS) at the same dosage;
(ii) the benefit of PAE-type admixtures is particularly
evident at very low w/c (and relatively low
admixture dosages). These trends are generally
observed in concrete data as well, the differences
being again most noticeable at very low w/c.
Selected results on ultra-high strength concrete ( >150 MPa)
are illustrated in Fig 6 23b
: again, at comparable dosages (in
this case very high, 2 percent), PAE-type admixtures
provide higher concrete fluidity and slump retention than
the PNS and PMS-type admixtures. Other concrete data
0
50
100
150
200
0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%
PNS Dosage (Wt% dry basis)
Min
i-sl
um
p a
rea (
cm
2)
OPC
OPC + SF
40°C
20°C
Fig 4 Mini-slump spread in T-10 portland cement and
HSF cement pastes shown at 10 minutes, as a function of
temperature and superplasticiser (PNS) dosage, w/c =
0.35 (Adapted from 22)
0
50
100
150
200
250
300
350
400
0 0.2 0.4 0.6 0.8 1
SP dosage (Wt%, dry basis)
Past
e f
low
(m
m) PE W/C 0.3
PE W/C 0.2
PC W/C 0.3
PC W/C 0.2
NS W/C 0.3
NS W/C 0.2
Fig 5 Paste flow data obtained on the same cement paste
with several different HRWR as a function of dosage
and w/c; NS: PNS, PC: polycarboxylate, PE: polyether.
(Adapted from reference 23a)
0
5
10
15
20
25
30
0 30 60 90 120 150 180 210 240
Time (min)
Slu
mp (
cm
)
CAE
NSF
MSF
Fig 6 Ultra-high strength concrete slump in the presence
of various superplasticisers at 2 percent dosage, w/c =
0.225; NSF= PNS, MSF= PMS, CAE= PAE (Adapted
from 23b)
6
have been reported comparing different HRWR at their
respective saturation dosage (determined from Marsh cone
fluidity), rather than at equal dosages. As the results show in
Fig 7, under these conditions, all the superplasticisers tested
showed similar performances 24
.
The actual performance of different types of HRWR in
specific cementitious systems is generally found to depend
on numerous variables, either inherent to the admixture
itself, or to the properties of the binder system (for example,
cement type, fineness, chemical composition, etc.). This
combined variability may lead to incompatibility situations
(cement with HRWR, or HRWR with other admixtures),
such as that due to soluble sulphates; through competing
interactions, the latter plays an important role in the
adsorption equilibrium of PNS, or of PAE-type admixtures
25,26.
In broad terms, the potential of HRWR admixtures can be
exploited along several practical schemes 11, 27
. The
schematic diagram reproduced in Fig 8 first illustrates how
the water reduction induced by the HRWR can be applied
either, to increase the slump of the concrete mix to obtain a
more fluid concrete, or to reduce the water content at
constant slump to achieve a higher quality concrete. A more
detailed inventory of the possibilities for exploiting the
beneficial effects of HRWR was outlined by Collepardi and
may be summarized as 11
:
Improving the workability of a given mix for ease
of placing, particularly in congested areas
Improving strength and durability at a given
workability by reducing water content
Reducing cement content for fixed strength and
workability, or for producing ‗low heat‘ concrete
Increase cement content at low w/c, while adding
other pozzolans (that is, silica fume) to produce
high performance concrete.
Considerable literature is now available on the use of WR,
particularly superplasticisers in many different types of
concrete applications.
Prospective developments
WR, especially HRWR/superplasticisers, have enabled a
quantum leap in the development of concrete technologies.
The search for new, more cost effective, admixtures in this
group will obviously continue, but the influence of new
admixtures is likely to be less dramatic than the impact of
the few key products which supported the early
development, namely, PNS and PMS. Future work in this
field is likely to yield more incremental benefits to cope
with shortcomings of currently available admixtures, as well
as the requirements from development in cement and
concrete technology; such products would typically:
have prolonged influence on concrete rheology
exert minimal effect on the cement hydration
reactions and set
be tolerant towards variations in the binder
composition (fly ash, slag, others)
be compatible with a maximum variety of other
chemical admixtures
be applicable in a broad range of different types of
concrete and applications.
Given the rapid evolution of binder systems, the inherent
composition variability of these systems, and the increasing
variety of concrete applications, it can readily be predicted
that a key requirement of future chemical admixtures will be
their ability to perform under the widest possible range of
parameters (binder composition, temperature, other
admixtures), that is, tolerance, robustness.
100
150
200
250
300
0 10 20 30 40 50 60 70
Time (min)
Slu
mp (
mm
)
N
M1
M2
CAE
Fig 7 HPC concrete slump for different superplasticiser
at saturation dosage, w/c 0.30; N= PNS, M= PMS, CAE=
PAE. (Adapted from reference 24)
30
40
50
60
120 140 160 180 200 220 240
Water content (kg/m3)
Flo
w t
able
spre
ad (
cm
)
with SP
without SPIncreased
strength
Increased
workability
Fig 8 Illustration of the use of WR admixtures: diagram
mapping the range of slump-strength (water content)
conditions achievable (Adapted from reference 27)
7
Colloidal admixtures
Function
As noted above, the increasing use of flowing, self-placing
(self-compacting, self-consolidating) concrete calls for
highly fluid mixes, which are inherently more prone to
separation effects: bleeding, surface segregation,
sedimentation of aggregate, segregation of aggregate
according to size and density, etc. This situation can be
alleviated through the use of additives commonly referred to
as ―colloidal admixtures‖. The latter include a variety of
products capable of enhancing the viscosity of concrete —
hence their alternate designation as viscosity-enhancing
admixtures (VEA). Their function is to maximise concrete
viscosity, in order to oppose any separation/segregation,
while retaining the desirable features of the free-flowing,
self-placing concrete. Since colloidal admixtures increase
the cohesion of cement-based systems in general, they can
also function as anti-washout admixtures (for underwater
concreting), or to reduce rebound and sagging in shotcrete
applications.
Chemical nature and classification
The type of admixtures which can impart the desired
rheological properties to self-placing concrete are currently
hydrophilic, water-soluble polymers having high molecular
weight. Under appropriate conditions, such polymers can
form a network of large molecules extending throughout the
mass, analogous to that found in a gel. Very small
interacting particles (in the nano-micrometer range) can also
generate gel-type behaviour. The dimensions of the
polymers or particles suitable for this type of application are
in the colloidal range, hence their designation as ‗colloidal
admixtures‖.
In comparison to water reducing-admixtures, colloidal
admixtures (viscosity-enhancing, anti-washout) are
relatively new; the main groups of polymers currently
proposed or used in such application are listed below 28
:
(i) Natural polymers including starch, welan gum,
xanthan gum and other natural gums, as well as
plant proteins.
(ii) Semi-synthetic polymers that include modified
starch and its derivatives, cellulose-ether
derivatives, such as hydroxypropyl methyl
cellulose (HPMC) and carboxy methyl cellulose
(CMC), sodium alginate and propylene glycol
alginate.
(iii) Synthetic polymers such as polyethylene oxide,
polyacrylamide, polyacrylate, and polyvinyl
alcohol.
As noted earlier, the stability of highly fluid cementitious
systems may also be improved by incorporating materials
which contain colloidal size particles such as fly ash, finely
divided calcium carbonate, blast furnace slag, diatomaceous
earth, etc, or materials having high surface area, or unusual
surface properties such as very fine clays, silica fume,
colloidal silica, milled asbestos 29
.
Colloidal admixtures commonly used in cement-based
systems include derivatised cellulose (ethers) and
polysaccharides from microbial sources, such as welan gum.
The latter is an anionic, high molecular weight (~2.x10
6
g/mol) polysaccharide. The molecular structure of welan
gum is illustrated in Fig 9(b). Cellulose derivatives have
molecular weights between 105 – 10
6 g/mol. The structure
of non-ionic cellulose ether, which is a principal component
in cellulose-based colloidal admixtures, is shown in Fig
9(a).
An important feature of these polymeric admixtures is their
variable stability and performance under high pH conditions
and in the presence of calcium ions, or at high temperature.
In some cases, auxiliary agents are added which react with
the polymer molecule to increase its molecular weight,
thereby improving its cohesion-inducing properties.
Mode of action
Polymeric colloidal admixtures increase the cohesiveness of
cementitious-based systems through a combination of
several effects, depending on the type and concentration of
the polymer used; three contributing mechanisms, illustrated
in Fig 10, have been proposed, which may be summarised
as 30, 31
:
(a)
(b)
R= -CH3 : Methyl Cellulose
R= -CH2 – CH2 – OH : Hydroxyethyl Cellulose
R= H : Cellulose
Fig 9 Molecular structure of common colloidal
admixtures. (a) cellulose ethers; (b) Welan gum.
8
(i) Water adsorption: The hydrophilic polymer
molecules adsorb free water molecules; in doing
so, they tie down a part of the mixing water and
their apparent volume increases by swelling.
(ii) Association: Adjacent polymer chains can
develop attractive forces, resulting in the
formation of a gel-like network, thus further
blocking the motion of water, and increasing the
viscosity of the whole system.
(iii) Entanglement: At low shear rates, and especially
at high concentrations, the polymer molecules can
entangle, resulting in an increase in the apparent
viscosity.
In addition to viscosity enhancement effects, long-chain
viscosity agents such as cellulose derivatives may also
behave as flocculating agents, that is, adsorbing
simultaneously onto neighbouring cement particles forming
a bridged structure 32, 33
. This effect results in enhanced
particle-particle interactions and the formation of flocs in
which free water may be entrapped. Since this flocculation
effect opposes the dispersion function of HRWR used in
flowing concrete, the use of non-adsorptive colloidal agents
is generally preferable.
As with water-reducers, colloidal admixtures may exert
secondary effects on the behaviour of the cementitious
system. Due to their sugar-based structures, polysaccharide-
or cellulose-type admixtures can delay cement hydration
and increase the concrete setting time. The effect depends
on the type and concentration of the colloidal admixture, the
type and dosage of HRWR, as well as on the cement
composition and w/c 28-30, 34-36
.
Also, by analogy to water reducers, some cellulose
derivatives, for example, hydroxy-propyl methyl cellulose
and some synthetic polymers such as polyethylene oxide
may entrain substantial volumes of air in fresh concrete.
This effect can be counteracted with an appropriate de-
foaming agent 28
. Concrete containing colloidal admixture
generally requires higher dosages of air-entraining
admixtures to achieve a given air content 30, 34
.
Performance evaluation
Colloidal admixtures are used to control the rheological
properties of concrete in order to achieve self-levelling, self-
placing properties, and to ensure its stability towards
segregation effects. The performance assessment of
colloidal admixtures therefore requires various methods
which aim to measure the following properties: free flow,
constrained flow, dynamic stability and static stability. As
with water-reducing admixtures discussed above, the
detailed rheological influence of colloidal admixtures can be
described from studies of their shear stress versus shear rate
curve (or stress amplitude versus rate) using specially-
designed rheometers 18-20
. For relative practical
comparisons, numerous other methods have been proposed.
The free flow behaviour of concrete can be determined
through Abram‘s cone (ASTM C 143) or through a L-
shaped container, while measuring both the spread area and
the spreading time 37
. To measure constrained flow, several
devices have been reported, which comprises various types
of geometries and obstacles intending to create conditions
relevant to adverse field situations, for example, high re-bar
congestion 38,39
. The flow time, and filling capacity of the
concrete in these devices provide a measure of their ability
to perform as self-compacting concrete.
To evaluate the dynamic stability of concrete, and the
influence of colloidal admixtures on this property, a simple
test has been proposed which involves evaluation of the
paste-aggregate adhesion through aggregate sieving. The
static stability of concrete, particularly segregation effects,
is more difficult to quantify in the fresh state. The extent of
compaction of the concrete can be evaluated through a test
which relates to the behaviour of the fines in the
cementitious system 40
. The overall static stability in terms
of bleeding, compaction and segregation can also be
solvation
and swelling
gel
formation
molecular
entanglement
Fig 10 Schematic illustration of mechanisms
contributing to the function of colloidal admixtures (a)
water adsorption and retention (b) gel formation
(c) molecular entanglement.
(a)
(b)
(c)
9
determined through a multiple-electrode conductivity
method develop recently 41
.
Performance in application
The main function of colloidal admixtures is to enhance the
cohesion and viscosity of fluid concrete at low shear rate,
while maintaining a relatively low resistance to flow at high
shear rates, such as encountered during mixing, pumping
and casting. In rheological studies, the colloidal admixtures
were found to raise the yield stress value, and increase the
apparent viscosity at all shear rates 34,35,42
. However, systems
modified with a colloidal agent exhibit a shear thinning (or
pseudoplastic) behaviour, that is, the apparent viscosity
decreases with increasing shear rate. The importance of this
effect depends on the type and dosage of the polymer; it can
be seen in more pronounced manner for mixes containing
welan gum, than for those containing cellulose derivatives,
under the same conditions 34,43
. Pseudoplastic behaviour is
believed to be the result of disentanglement of polymer
molecules and their alignment in the direction of flow at
higher shear rates.
In practice, highly flowable cement-based materials with
adequate resistance to bleeding, segregation, settlement,
etc., may only be obtained by proper combinations of
colloidal admixtures and HRWR. In view of the partially
opposing effects of these admixtures, the required dosage of
the latter may increase with increasing dosage of the
colloidal admixture 35
.
With respect to HRWR / colloidal admixture combinations,
the cellulose-based colloidal admixtures yielded erratic
cohesion and flowability behaviours with PNS-type HRWR,
but were compatible with PMS-type admixtures 29,44-46
.
Similar studies carried out with welan gum showed no
apparent incompatibility with either melamine-based or
naphthalene-based HRWR 46
.
Fig 11 illustrates the function and performance of colloidal
admixtures, used jointly with HRWR 42
. The apparent
viscosity measured at two different shear rates (8 and 256
rpm) are illustrated for the pure cement paste, the paste with
the HRWR or colloidal admixture added separately, and the
paste containing both admixtures. As is readily evident, the
influence of the admixtures is most pronounced at low shear
rate, the pastes exhibiting considerable shear thinning. It is
also apparent that, in this case, the combination of
admixtures exhibits synergy with respect to the increase in
apparent viscosity, that is, the viscosity increment for the
combination is higher than the sum of the individual effects.
A second illustration of the application of colloidal
admixtures on high fluidity concrete is taken from
investigations by Khayat et al. on bleeding, segregation and
overall stability of self-compacting concrete 41
. Fig 12
shows the variations of external bleeding (ASTM C-232)
and the segregation of the coarse aggregate of two
traditional flowable concretes (C1 and C2) and two highly
flowable self-compacting concretes (C3 and C4) containing
welan gum as colloidal admixture (C4 is typical of materials
used for underwater concreting).
The C1 and C2 mixtures had similar compositions, as well
as the C3 and C4 mixtures, except for w/c values and the
concentrations of chemical admixtures; the latter were
0
500
1000
1500
2000
8 rpm 256 rpm
Appare
nt
Vis
cosi
ty (
cP
)
C
CS
CG
CGS
C CS CG CGS
Fig 11 Apparent viscosity of pastes in the presence of
HRWR and of a colloidal admixture; C: cement, S: PNS,
G: polysaccharide gum (Adapted from reference 42)
0
0.01
0.02
0.03
0.04
20 220 420 620
C2 : W/C = 0.55 Slump = 220 mm
C4 : W/C = 0.48 Slump flow = 650 mm
Exte
rna
l bee
din
g (
ml/
cm2)
Elapsed time (min.)
C1 : W/C = 0.41 Slump = 220 mm
C3 : W/C = 0.42 Slump flow = 650 mm
C2 : W/C=0.55; 0.15% HRWR
Slump =220 mm
; 0.5% HRWR
C4 : W/C=0.48; 0.39% HRWR +
0.14% welan gum
Slump flow = 650 mm
C3 : W/C=0.42; 0.50% HRWR + 0.14%
welan gum; Slump flow = 650 mm
20 25 30 35 40
0
10
20
30
C3 (1.9%)
C4 (3.1%)
C1 (segregation coeff. = 4.3%)
C2 (6.0%)
5
15
25
Percentage of aggregate • 5 mm
Dis
tan
ce f
rom
bott
om
(c
m)
Fig 12 Relative stability of fluid concrete mixes. C1-2:
flowing concrete, C3-4: self levelling concrete. Top:
External bleeding versus time, Bottom: Seggregation of
aggregate. (Adapted from reference 41)
10
adjusted to allow comparisons between two flowable (250
mm slump) and two highly flowable (650 mm slump flow)
concretes. It can be seen that the external bleeding is much
lower and it occurs significantly later for self-compacting
concretes containing welan gum. Also, the mixtures with
greater w/c exhibit higher bleeding. The relative segregation
behaviour of these materials followed the same pattern as
the bleeding rates; mixtures containing welan gum were
found to be more stable overall.
In the same study, the vertical heterogeneity resulting from
bleeding and segregation phenomena was monitored
continuously in the fresh concrete samples through an in-
situ conductivity method developed recently 41
. From
conductivity measurements taken at different levels and as
function of time, the stability of each system is reflected by
the standard deviation of the conductivity values, ,
relative to the average conductivity; an apparent stability
index can thus be defined as Is= (1- / av). Through this
approach, the stability of cementitious systems can thus be
assessed quantitatively in real time and in a non-disruptive
manner.
Prospective developments
The beneficial impact of colloidal agents on the flowing,
pumping, filling capacity, bleeding, segregation, and other
properties of self-placing concrete are now widely
acknowledged. The main obstacle to their broader
acceptance appears to be their cost 47,48
. However, savings
from increased productivity, improvement of working
environment, improved service life and aesthetic are also
recognised and may actually override the increased initial
material costs for production of self-compacting concrete. In
this context, new low-cost colloidal agents such as modified
starch, precipitated silica and new biopolymer molecules are
actively being developed and introduced to the concrete
industry 49-53
.
A complementary approach to cost reduction of these
systems is through incorporation of large volume of fine
powdered materials such as fly ash, silica fume, limestone,
blast furnace slag, etc. to decrease cement content and
admixture demand 36,54-56
. It has been shown, however, that
highly flowable concretes formulated without colloidal
admixtures can be more sensitive to variability to its
formulation, raw materials and other factors such as
aggregate moisture content, cement fineness, temperature,
etc 57
.
A more technical limitation to the development of new
chemical admixtures and formulations of highly flowable
cement-based systems, as well as the general adoption of
these materials as mainstream construction materials, is the
absence of normalised test methods, especially for assessing
their resistance to segregation 47,58
. Considerable efforts are
thus required to develop testing/monitoring methods, as well
as guidelines or standards, to ensure adequate
characterisation of these materials and support their
optimum application. The multiple-probe conductivity
method referred to above for in-situ monitoring of bleeding,
segregation and strength development seems a promising
development 41,59,60
.
Conclusion
Given their important functional properties and their rapidly
increasing acceptance, chemical admixtures may rightly be
considered an integral, even an essential part of modern-day
concrete. This is particularly evident for some types of
admixtures such as HRWR and colloidal admixtures; used
in conjunction, these two families of admixtures allow ―fine
tuning‖ of the rheological properties and stability of flowing
concrete, even at the low water contents required for
optimum performance and durability.
The diversity of cementitious systems currently in use or
development, as well as the rapidly growing variety of
chemical admixtures proposed in each category of
admixtures, lead to a broad range of physical and chemical
interactions, some of which have been shown to be
deleterious (for example, incompatibility of cement-
admixture pairs, or between admixtures). Hence, to reap
adequate benefit of these ongoing developments, and
minimise any negative impact, researchers must
continuously seek to elucidate the phenomena in which the
various admixtures take part in cementitious systems, while
developing more efficient diagnostic tools. Concrete
practitioners should, on the other hand, resort to extensive
QC of the concrete components and performance testing on
reference mixes.
Acknowledgements
The authors gratefully acknowledge the financial support of
the Natural Sciences and Engineering Research Council of
Canada, Handy Chemicals Ltd and the Institute for
Intelligent Materials and Systems of the Université de
Sherbrooke. Numerous fruitful discussions with Prof Kamal
Khayat and Dr Monique Pagé are also gratefully
acknowledged.
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12
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59. Jolicoeur, C., Khayat, K.H., Pavate, T. and Pagé, M. Evaluation of effect of chemical admixture and supplementary cementitious
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60. Pavate, T., Mikanovic, N., Khayat, K.H. and Jolicoeur, C. Non-
published results.
Dr Carmel Jolicoeur, is professor in the
department of chemistry, Université de
Sherbrooke, Québec, Canada. His research
interests include the solution and colloid
chemistry of cementitious systems and chemical
admixtures for cement-based materials.
Mr Nikola Mikanovic, is a Ph.D. student in the
department of chemistry at the Université de
Sherbrooke, Quebec.
Mr Marc-André Simard, is a research chemist
in the department of chemistry at the Université
de Sherbrooke, Quebec.
Mr Jeff Sharman, is a research chemist in the
department of chemistry at the Université de
Sherbrooke, Quebec.