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APPLICATION OF X RAY DIFFRACTION (XRD) AND SCANNING ELECTRON MICROSCOPY (SEM) METHODS TO THE PORTLAND

CEMENT HYDRATION PROCESSES

JUMATE Elena*, MANEA Daniela Lucia, Tehnical University of Cluj-Napoca, *e-mail: elena.jumate@ccm.utcluj.ro (corresponding adress)

A B S T R A C T This paper presents a study performed on type I Portland cement with respect to the cement hydration processes performed at various time intervals. The methods used concern X-ray diffraction and electronic microscopy applied to define materials and to understand the changes occurring in mineral compounds (alite, belite, celite and brownmillerite) during their modification into hydrated mineral compounds (tobermorite, portlandite and etringite).

Keywords: tobermorite, portlandite, ettringite, mineral component

Received: March 2012 Accepted: March 2012 Revised: April 2012 Available online: May 2012

INTRODUCTION Knowing the chemical composition of the raw materials, of intermediate products and of the

final product represents an aspect of major importance in the fabrication and use of a product with expected specifications. The reactions occurring in the cement hydration process have, consequently, been of high interest for the researchers who studied them, among others, by means of the X ray diffraction (XRD) and scanning electron microscopy (SEM) methods.

The use of the mentioned methods allows more accurate information regarding the behaviour of the Portland cement paste during hydration, and a more realistic knowledge of the mechanisms that generate new properties such as strength and durability, which are among the most important in the selection of cement for a specific application.

The Portland cement represents a mixture of clinker and finely ground gypsum, where the clinker is made up of four main mineral components [1] (Table 1), at a maximum temperature of up to 1450°C. In the clinker, the calcium silicates represent 75 - 80 %, hence the name of silicatic cements, while calcium aluminates and calcium aluminoferrite form only 20 – 25 % [2].

Table 1. Main mineral components in Portland cement

Name of the mineral component

Chemical name Oxidic composition Abbreviated formula

Alite Tricalcium silicate 3 CaO • SiO2 C3S Belite Dicalcium silicate 2 CaO • SiO2 C2S

Celite I Tricalcium aluminate 3 CaO • Al2O3 C3A Celite II or Brownmillerite Tetracalcium aluminoferrite 4 CaO • Al2O3 • Fe2O3 C4AF

The Portland cement mixed with water forms a plastic paste or slurry that stiffens as time

goes on and then hardens into a resistant stone. When water is present, the mineral compounds undergo hydration and hydrolysis reactions, which are followed by the appearance of new hydrates. The hydro derivatives exhibit a colloidal structure, which, in time, is concentrated as a gel, and makes the cement paste more and more consistent. The gels in question and the mineral

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components hydro derivatives respectively, start losing hydration water and crystallise. As a result, the cement paste turns into a rigid body with very high strengths.

The cement hydration products are poorly soluble in water [3], as practically the hydration of cement particles is never completed, whatever the precipitation system of the hydration products as shown by the hardened cement stability in water. The hydration speed gradually diminishes. It was found out that, at 28 days of contact with water, the cement grains were hydrated only in a depth of 4µm, and after one year, the hydration reached a depth of 8µm [3, 4].

Alite (C3S) is the mineral component to be found in cement in the largest ratio (50%), under the form of colourless and equisized grains [3]. This is the calcium silicate with the highest hydrolysis reaction which very easily reacts to water. C3S hydration defines to a large extent the behaviour of the cement, though its rate is not constant and not even the rate changes are constant. C3S rapidly hydrates and hardens the cement slurry and enforces high initial (1-3 days) and final mechanical strengths [5].

Belite (C2S) is the mineral component that exhibits three or even four polymorphous forms [3], easily reacts with water and turns into a hydrated dicalcium silicate. This slowly hydrates and hardens the cement slurry and improves the cement mechanical strengths after 7 days. After 28 days, this mineral compound hardens and its mechanical strength will be very close to that of the calcium silicate hydrate originating in C3S [5].

The hydration and hydrolysis reactions of the two mineral compounds above also produce hydrosilicates that initially have a gel structure similar to that of the natural mineral called tobermorite. The calcium silicate hydrates form the majority of the hydration products, present a gel structure, where the solid phase is made up of a lattice of microcrystals, initially of angstrom size with eyes filled with a saturated composition of components: in a later stage, the crystals develop, age and strengthen, leading to the increase of the mechanical strengths [2, 4, 5].

Celite (C3A) is the mineral component that has the form of crystals in the clinker when slowly cooled down or as vitreous mass, when the clinker is cooled down fast. In this case it fills in the voids between the alite and belite crystals. Pure celite has a violent reaction with water; the slurry hardens instantaneously, which requires the addition of gypsum (CaSO42H2O) when the cement clinker is ground [3]. The amount of gypsum added to the cement clinker should be carefully controlled as a too large an amount leads to expansion and hence, to the damage of the hardened cement paste. The optimal gypsum amount shall be defined by observing the hydration heat release. The gypsum also reacts with brownmillerite (C4AF), forming calcium sulphoferrite hydrate and calcium sulphoaluminate hydrate, whose presence can accelerate the hydration of calcium silicates. The two calcium aluminate hydrates act as flux and diminish the clinker burning temperature [3, 5].

MATERIALS AND METHODS 1. State-of-the-art methods used to define cement hydration 1.1. X Ray Diffraction (XRD)

Diffraction is a physical phenomenon that consists in electromagnetic waves avoiding obstacles if the size of the obstacles compares to the wavelength. This phenomenon can be applied to the analysis of materials as the atom plans are placed at comparable distances to X ray lengths. X rays are electromagnetic waves similar to light, but whose wavelength is much shorter (� = 0,2 - 200 Å ).

XRD is produced as a reflexion at well defined angles. Every crystalline phase has its own diffraction image. The diffraction image contains a small number of maximum points that is not all

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the families of crystallographic planes give maximum diffraction points; all the crystalline phases with the same type of elementary cell will exhibit the same succession of Miller indices for the crystalline planes families giving a diffraction maximum points. For the XRD analysis we use diffraction devices (diffractometers), mainly according to the Bragg–Brentano system (Figure 1) (the sample rotates at a diffraction angle ”θ”, while the detector rotates at the angle ”2θ”. In Figure 2, the X ray diffractometer (Bruker) is shown.

Fig.1. The basic layout of an X ray diffractometer [5] Fig.2. X ray diffractometer (Bruker) [5] The diffractogram is made up of a succession of diffraction maximum points, showing the

intensity of the diffracted X radiation on the ordinate measured in pulses/second, and the angle ”2θ” on the abscissa, where ”θ” is the Bragg angle, measured in degrees. The diffraction image depends upon the material structure.

The diffraction methods allow for the performance of the following studies: the determination of the crystalline structures, the phase quantitative and qualitative analysis, the study of phase transformations, the study of the crystallographic texture, the size of the crystallites, the internal stresses in the sample, etc.

The identification of the crystalline phases can be carried out with the X ray diffraction method if the respective phase represents more than 3 - 4% mass. The identification can be made by calculation with Bragg’s relationship or computer-based, using Match, XpertScore software, in the PDF (Powder Diffraction File) database, where identification files for about 200,000 metal crystalline phases, alloys, oxides, salts, etc. are found [5, 6, 7].

1.2. Scanning electron microscopy (SEM)

SEM represents a high performance method used to investigate the structure of the materials. It is defined by: easiness to prepare samples to be tested, large diversity of information reached, good resolution associated with high field depth, large and continuous range of magnifying, etc. The examination of microstructures with SEM offers two benefits as compared to optical microscopy (OM): much more resolution and magnification, as well as very large field depth giving the impression that images obtained are outstanding. Thus, the field depth in OM when magnified 1200 times is 0,08 µm, while in SEM at 10.000 times magnifying, the field depth is 10 µm.

The scanning electron microscope SEM of type Jeol 5600 LV (Figure 3) presents the specifications:

- resolution 3,5 nm (35 Angstroms), with secondary electrons;

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- 300,000 times magnification; - the examination of nonconductive samples (ceramic, biological, medical, etc.) can be

made in reduced vacuum (up to 130 Pa) with backscattered electrons [8]; local quantitative chemical analyses can be made, based on the characteristic spectrum of X rays for the elements contained between Boron and Uranium, at a detection limit of 0,01 %.

Fig.3. The scanning electron microscope SEM of type Jeol 5600 LV

1. The present stage of the Portland cements hydration processes studied with XRD and SEM

The evolution of the Portland cement in the hydration process was investigated by Manuel A.M. Giraldo, Jorge I. Tobon, and other researchers as well, with the help of the materials definition approaches that use XRD and SEM. They identified the modifications that occur in the mineral compounds (alite, belite, celite I and brownmillerite) during the hydration processes, wherefrom calcium silicate hydrates and calcium aluminate hydrates appear (tobermorite, portlandite and ettringite).

2.1. X Ray Diffraction (XRD)

The conclusions of the studies carried out by the researchers mentioned above pointed out the following results, with reference to the hydration processes investigated through X ray diffraction:

After three days, the largest peaks of the of the diffractograms correspond to the tobermorite gels, the second peak corresponds to portlandite, while ettringite exhibits the smallest values. The most abundant phase at three days age is that of tobermorite.

After seven days, the model resembles to the model after three days. The gels of tobermorite and the gels of portlandite have higher values that the ettringite phase.

After 28 days, tobermorite forms a mass which is more dense, more compact and continuous, but where still non-hydrated belite grains can be met and ettringite is difficult to recognise.

As time goes on, in the hydration process there occurs a diminution of values between the tobermorite and portlandite phase, between days three and seven, as well as between days seven and 28. This development can be explained by the fact that during the first three days of hydration tobermorites originating in the alite are more predominant, alite is present in larger amounts than belite which is the source of portlandite that is developed more slowly. As time goes on, a larger amount of portlandite is produced, so that at seven days, the difference between tobermorite and portlandite diminishes. After 28 days, the difference between the two compounds increases as

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tobermorite develops very much. While hydration continues, ettringite becomes more and more evident [9].

2.2. Scanning electron microscopy (SEM)

In the case of research performed with SEM, we present the conclusions of the studies performed by the researchers mentioned above, as follows.

After three days, while alite is hydrated, tobermorite develops to a higher extent and forms a continuous matrix of layers joined together. Portlandite in its hexagonal shape is also identified. Grains of still non-hydrated alite and belite can also be found. Ettringite starts appearing in small amounts. Due to hydration heat, cracks visible at the scanning electron microscope can also be seen.

After seven days, the hydration process has occurred to a higher degree, tobermorite forms and goes on developing the continuous matrix which had appeared at the age of three days. Portlandite can be identified in its characteristic hexagonal shape and the needle shaped ettringite can be seen quite well.

After 28 days, tobermorite forms a mass that exhibits more density, more compactness and continuity and where belite grains that have not yet hydrated can be identified. 2. Personal contributions regarding the study of the hydration processes of the Portland

cement by means of XRD and SEM The Portland cement slurry was prepared in the laboratory of the Faculty of Civil Engineering

of Cluj-Napoca. Tests were performed on the slurry, according to SR EN 196-3/A1:2009, the final formula being: 300g Portland cement and 96ml water. Cakes were made from the normal consistency slurry and were kept in relative humidity of 55 % at a temperature of 20°C, for 28 days. Samples were taken from the cakes at 1, 3, 7, 14 and 28 days, to be examined by means of the two methods mentioned that is XRD and SEM.

RESULTS 1. X Ray Diffraction (XRD)

The analyses made by XRD used a DRON 3 diffractometer, with an angular range of 2θ = 10 – 70 degrees, at a radiation of λ=1,54182 Å, voltage of 25 kV and intensity of 25 mA, in a Bragg – Brentano scheme. The diffraction samples were either powder (found by pestle milling) or cakes.

The diffraction spectra show that alite is found mostly in the sample, that part of it is hydrated and the calcium silicate hydrate is produced. Ettringite and portlandite are present in all hydration stages. Changes in the mineral compounds during hydration were highlighted where from silicate hydrates and aluminate hydrates appeared (tobermorite, portlandite and ettringite), Figure 4.

After three days, the highest peaks in the diffratograms corresponded to alite and tobermorite gels. After seven days, the spectrum is similar to the spectrum after three days. During the hydration process, values between tobermorite and portlandite phases are seen to diminish both between three and seven days and seven and 28 days.

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Fig.4. X-rays pattern of Portland cement (cs-alite; csh- hydrated calcium silicate (tobermorite); p-portlandite; e-ettringite, b-belite; c-celite; bw-browmillerit) [5]

The phenomenon can be explained by the fact that in the first three days of hydration

tobermorites originating from alite are predominant, that alite is present in larger quantity than belite which is the source of portlandite that, in turn, is formed more slowly. More portlandite is produced in time, so that at seven days, the difference between tobermorite and portlandite reduces. At 28 days, the difference between the two compounds increases as tobermorite develops very much [5].

2. Scanning electron microscopy (SEM)

To analyse the morphological evolution of the Portland cement samples at various time intervals (after 3, 7 and 28 days) we used the scanning electron microscope SEM of type JEOL JSM5600LV, equipped with a EDX Oxford instruments spectrometer, INCA 200 Soft. As the microscopic analysis requires samples that are electrically conductive, all the samples were covered by a layer of gold, deposited by spraying. The samples were then broken and the analysis concerned the breaking surface. After three days, while alite is hydrated, tobermorite develops more and more, forming a continuous matrix in the form of layers joined together (Figure 5). Portlandite is also identified, in a hexagonal shape. Alite and belite grains not hydrated yet are still present. A more magnified image shows varied areas in the sample. These areas correspond to the various constituents of the cement, which appear after three days following the mixing of the cement. Thus, one of the constituents in these samples is ettringite (Figure 6), platelike structure is visible almost everywhere.

Fig.5. The tobermorite at 3 days Fig.6. The ettringite at 3 days

If the cement slurry is kept up to seven days, its structure will be more modified, so that its surface, under a small magnification, shows to be more compact (Figure 7). On the other hand,

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cracks were found (Figure 7) as a result of the hydration heat, produced by the exothermal hydration reactions. After seven days, the slurry exhibits a high degree of hydration, tobermorite forms and develops its continuous matrix evident after the third day. Portlandite is also identifiable in its characteristic hexagonal shape and ettringite in the forms of needles can also be seen at this age (Figure 8).

Fig.7. The morphology of cement surfaces at 7 days

Fig.8. The ettringite at 7 days

After 28 days, tobermorite forms a more dense, compact and continuous mass, with identifiable though still non-hydrated belite grains (Figure 9).

Fig.9. The morphology of cement surfaces at 28 days

In the case of portlandite, no notable change was found after seven days. At this age, the crystals are well developed, with well-defined sides and the characteristic hexagonal plates. Filaments of ettringite can also be seen among the particles (Figure 10).

Fig.10. The ettringite at 28 days

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CONCLUSIONS By means of the two methods DRX and SEM, the presence of hydration compounds as well

as of mineral compounds that are to hydrate in time was highlighted. The interpretation of the results with the DRX enables a more precise identification of the mineral phases, in all samples under investigation. Alite, belite and brownmillerite were well maintained after 28 days inclusively, while the calcium aluminate was not identified after the first day. There are three main hydrated compounds: tobermorite, ettringite and portlandite. The phases occur during the test. The study of cement hydration by means of SEM identifies the modifications that appear in the cement mineral compounds as follows. After 3 days the most abundant phase is that of tobermorite. Portlandite and ettringite are also present. After 7 days, the tobermorite gels and the portlandite gels present a higher value that that of ettringite. After 28 days, tobermite forms a much more compact and continuous mass where still non-hydrated belite grains can be found, while ettringite is difficult to recognise.

In the course of the time, during the hydration process, the values between the tobermorite and portlandite phase diminished. After 28 days, the difference between the two compounds becomes larger as tobermorite increases very much. Both methods applied in the cements hydration processes highlighted the same order for the appearance of the hydration compounds, namely fist appears tobermorite, followed by portlandite and ettringite. The advantage of using SEM lies in the possibility of pointing out other aspects, such as cracks, while the advantage of using DRX lies in the possibility of showing other hydration compounds that the ones mentioned. However, the cement hydration process is never fully completed. REFERENCES 1. IONESCU, I. and ISPAS, T. (2008), Propriet��ile �i tehnologia betoanelor (Concrete properties and

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