microstructure development of calcium aluminate

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MICROSTRUCTURE DEVELOPMENT OF CALCIUM ALUMINATE CEMENTS ACCELERATED BY LITHIUM SULPHATE C. GOSSELIN and K. L. SCRIVENER Laboratory of Construction Materials, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland

SUMMARY: A multi-technique approach including the use of SEM, TGA, XRD and MIP has been used to study the microstructural development of accelerated CAC pastes within the first day of hydration under different isothermal conditions and realistic temperature curing, likely to occur in concrete. The formation of the hydrates assemblage is correlated with the distribution of porosity. Keywords: CAC, early age behaviour, lithium sulfate, microstructure, MIP, porosity, SEM, TGA, XRD.

INTRODUCTION The hydration and microstructural development of CAC are controlled by time and temperature. In particular, at low temperatures the formation of the stable hydrates is preceded by the formation of metastable phases, which convert into the thermodynamically more stable phases over time. This conversion and its effects on mechanical properties have been widely described and discussed [1]. The main reactive anhydrous compound calcium aluminate cements is monocalcium aluminate CaO.Al2O3 (CA). The hydration of CA at temperatures lower than about 20°C is dominated by the formation of CAH10 while, as the temperature increases up to 50°C, C2AH8 progressively predominates. Metastable phases nucleate and grow easily due to their anisotropic crystalline structures. CAH10 is poorly crystallised [2]. C2AH8 is more crystalline, with hexagonal plate morphology. There is a high activation energy for the formation of cubic C3AH6 due to its symmetric crystal structure. As CAH10 and C2AH8 are thermodynamically metastable they have a higher solubility than the stable phases: hydrogarnet C3AH6 and AH3. Once stable phases start to nucleate, the metastable phases dissolve. The stable phases have higher density than the metastable phases, therefore if the degree of reaction stays the same there will be an increase in porosity. However conversion releases water in the system. This water can react with remaining anhydrous CA to form new hydrates and fill this porosity created. At ambient temperatures conversion is very slow (several years). Higher curing temperatures accelerate the conversion and C3AH6 may precipitate after few hours. As the rate of hydration of calcium aluminate cements is very fast after setting, considerable Calcium Aluminate Cements: Proceedings of the Centenary Conference, Avignon, 30 June–2 July 2008. Fentiman CH, Mangabhai RJ and Scrivener KL. (Editors). IHS BRE Press, 2008, EP94. ISBN 978-1-84806-045-6. www.ihsbrepress.com.

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heat is generated. This means that in concrete elements of any significant size, conversion is often initiated during initial hardening. Due to the rapid reaction rate it is difficult to follow the microstructural development and the effect of temperature on this. Even in small plastic vials of cement paste, self heating may affect the hydrate assemblage formed. A further difficulty is that the time before setting may be highly variable in pastes, in contrast to mortars and concretes, where the presence of aggregate promotes nucleation and leads to reproducible setting times. This paper focuses on the identification and the assemblage of hydrated phases within the first 24 hours using small metal moulds, which enabled the temperature of the sample to be well controlled. The setting time is controlled by the addition of lithium sulphate which is often used in the field to control setting time.

EXPERIMENTAL PROCEDURES Materials The cement was Secar 51®, supplied by Kerneos and characterized by the chemical and mineralogical data in Table 1. Lithium sulfate was added (0.3% by weight of cement of a 5% Li2SO4 solution) to control the set time of the paste. Table 1. Chemical and mineralogical components of CAC. Oxides CaO 36.08 wt%

SiO2 5.05

Al2O3 Fe2O3 SO3 51.69 1.96 0.07

Crystal phases

CA

C2AS

C12A7

CT

wt%

63.5

21.5

0.04

5

MgO 0.84

K2O 0.43

α-A

C2S

Others

3.0

2.1

4.2

TiO2 2.44

Na2O 0.12

P2O5 Mn2O3 Others 0.21 0.05 1.06

Methods Cement paste was prepared by mixing for 1 minute with a paddle at a speed of 1200 rpm. The fresh paste was cast in copper moulds (internal diameter 38 mm, height 5 mm) and the set of moulds was placed on a frame of copper sheets. The frame was then immersed in a temperature controlled water bath. For each set of experiments, one mould was equipped with a thermocouple, embedded in the paste, in order to monitor its temperature. To stop the hydration process at each age, a mould was taken from the bath and cooled in liquid nitrogen for 5 minutes. A paste slice was removed from the frozen mould and stored in a Freeze-Dryer (Telstar Cryodos 50, -50 °C, 0.170 mbar) for 24 hours to sublimate the ice. Part of the slice was ground, sieved (< 100 μm) and reserved for XRD (X’Pert Pro PANalytical, CuKα, λ=1.54Å) and TGA (Metler Toledo, 10°/min under N2). The rest of the slice was used for SEM examination (FEI Quanta 200) to study the microstructural development. SEM Image Analysis was performed to quantify the volumetric fraction of solid phases and porosity. A set of 300 backscattered electrons images was acquired for each sample. The images were segmented by grey level corresponding to each phase present in the polished sections. A magnification of x3000 was chosen in order to well differentiate the phases. The accuracy of the method depends on the number and the spatial distribution of phases present in each image. In the case of CAC,

Microstructure development of calcium aluminate cements

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several phases can be simultaneously present in a micrograph, according to the degree of reaction and the rate of hydrates conversion. Moreover the morphology and the assemblage of the anhydrous phases (perovskite, C2AS and CA) as well that of the hydrated products (progressive C2AH8 formation, intermixed C3AH6 and AH3…) constitute a potential source of variation between two images of the same sample. Consequently careful and optimized segmentation of grey levels is required. A standard error of measurement (SEm) is calculated for every sample in order to evaluate the accuracy of the method. Mercury intrusion porosimetry, MIP (Porotec, bulk sample, Pmax=400 MPa, contact angle 145°) was also used to characterize the pore size distribution. At 20°C, the hydration was studied by isothermal calorimetry (TAM AIR calorimeter) in order to determine the sampling times. The calorimeter cannot be used at 70°C so the sampling times were estimated from the strength data of concrete cubes cured at 70°c published by French et al [3]. For the realistic temperature history, the paste was preliminary cured 1 hour at 20°C, corresponding to the induction time seen in the calorimeter. Afterwards a ramp of 10°C/h was applied until the paste temperature reached 70°C, and kept constant until 24 h. This temperature history is intended to simulate a realistic temperature evolution in a large section of CAC concrete.

ISOTHERMAL CALORIMETRY Figure 1 shows the calorimetric curve of CAC cured at 20°C, with and without Li2SO4. The peak which occurs within the first day of hydration is much higher than with Portland cement and rises much more sharply over only hour. This peak corresponds to the dissolution of CA [4] and the precipitation of hydrates. In the absence of Li2SO4 there is a long induction period 9h, which is reduced to one hour with lithium sulfate. Lithium sulfate is known to accelerate the reaction of CAC hydrates as lithium aluminate hydrates acting as heterogeneous nucleation sites [1]. A mineralogical study throughout such a rapid reaction presents experimental difficulties to stop hydration at discrete points and justifies the choice of copper moulds.

Fig. 1: 20°C calorimetric curve of CAC, with or without Li2SO4.

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MICROSTRUCTURAL DEVELOPMENT Isothermal 20°C Figure 2 shows the evolution of the intensity of the main diffraction peaks for anhydrous (30.05° and 31.41°2θ for CA and C2AS) and hydrated (12.43° and 8.71° 2θ for CAH10, C2AH8) phases. The evolution of peak intensity is compared with the heat evolved by isothermal calorimetry, illustrated by the dashed line. From Figure 2 it can be seen that the consumption of CA ceases as soon as the main calorimetry peak ends and, from this point on, the signal related to the hydrated compounds becomes steady. There is also a small, but significant drop in the main peak of C2AS, but this could be a dilution effect as the mass of the solid phases increases due to incorporation of water. TGA results, presented in Figure 3, confirm the nature of hydrates seen by XRD. The first broad peak, starting at 50°C and continuing up to 230°C, includes CAH10 and C2AH8 dehydration [5, 6]. The second smaller peak, with a maximum at 250°C, characterizes amorphous AH3 decomposition. For this curing temperature, further interpretation of TGA is difficult because of the drying process preceding the analyses. The high vacuum (0.170 mbar) applied during the drying process could lead to damage of the weakly structured CAH10, even if the latter is detected by XRD. The weight loss peak is broad, with no clear distinction between CAH10 and C2AH8, then a broad loss, corresponding to AH3 around 250°C. The total bound water deduced from mass loss up to 500°C in TGA is shown in Figure 4. Figure 5 presents SEM micrographs at the discrete ages from 1.3h to 24h. Each age is illustrated with a general view of the matrix on the right and details on hydrates on the left and a brief comment in legend. From Figure 5, the precipitation of CAH10 starts with nucleation inside and on the surface of the CAC grains. Outside the grains the plates of C2AH8 are apparent at the centre of the masses, this supports the hypothesis that Li2SO4 leads to the formation of AFm hydrates [7] which promote the nucleation of CAH10. The heat evolved and measured by calorimetry increases rapidly due to the multiplication of nucleation and precipitation sites within the matrix. At 3.9 hours, at the end of the heat evolution, the microstructure is already mostly filled with hydration products and there is little further evolution up to 24 hours. The microcracks seen at 24hrs are probably from the freeze drying process.

Fig. 2: XRD results of plain CAC cured at isotherm 20°C


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Fig. 3: TGA results of plain CAC cured at isotherm 20°C.

Fig. 4: Total bound water of plain CAC cured at 20, 70 and increasing temperature.

Isothermal 70°C The same approach and methodology described above were applied for paste cured at 70°C. Figures 6 and 7 display XRD intensity (30.05, 31.41, 8.71, 18.40 and 17.38 °2θ respectively for CA, C2AS, C2AH8, AH3 and C3AH6 peaks) and TGA results of plain CAC immediately cured at 70°C. Figure 8 presents the sequence of hydration observed under SEM. The three observation techniques are coherent and show the primarily precipitation of metastable hydrates for the first hour of hydration. C2AH8 is the major phase observed by XRD and SEM and after 30 Minutes there is already a strong peak for AH3. However, the TGA indicates two peaks, at low temperature with more weight loss related to the range usually associated with CAH10. Nevertheless there was no indication of this phase in the XRD. The pastes have been freeze dried and this peak disappears later, which indicate that it cannot be associated with free water. From 3 hours of hydration (70°C), clusters of small C3AH6 crystals are intermixed with AH3 giving a very different microstructure from the earlier one dominated by C2AH8.

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1.3h

The weak cohesion of the paste makes it difficult to impregnate with resin. The CAC grains are mostly surrounded with epoxy resin. However, some CAH10 can be seen precipitating inside the grain on the left and in a few large masses in the low magnification view.

1.5h

On the left, CAH10 precipitates inside and close to the surface of the CAC grains. CAH10 is partially converted into sheet shaped and brighter C2AH8. On the right, nucleation sites are increasing.

2.1h

On the left, the morphology of CAH10 (isolated microcrystalline) and C2AH8 (denser and brighter sheets clusters) are more distinct. On the right, the matrix densification is progressing.

3.9h

After the calorimetric peak, the matrix is almost filled with CAH10 partially converted into C2AH8. The dense matrix still contains microporosity.

Fig. 5: SEM observations of CAC hydrated at 20°C (continues).


24h

After 24h, the matrix is almost completely filled with hydrates. Microcracks arise from the freeze drying.

Fig. 5: SEM observations of CAC hydrated at 20°C (continued).

Fig. 6: XRD results of plain CAC cured at isotherm 70°C

Fig. 7: TGA results of plain CAC cured at isotherm 70°C

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0.5h


On the left, CAC grains show a high degree of reaction and C2AH8 is already crystallized with a plate like morphology. On the right, the matrix is significantly dense.

3h

On the left, C2AH8 (brighter sheets) is intermixed with AH3 (darker), while C3AH6 has not yet precipitated. On the right, the matrix is denser.

8h

On the left, C3AH3 has largely replaced C2AH8, presenting bright clusters intermixed with AH3 (darker hydrate). On the right, the matrix is brighter due to the atomic contrast of denser products (C3AH6) but the porosity seems similar.

24h

Intermixed C3AH6 and AH3 constitute the hydrated phases surrounding CAC grains. Hollow of CAC grain skeleton (made of non reacted ferrite and C2AS) is filled of C3AH6. The converted microstructure has significant porosity.

Fig. 8: SEM observations of CAC hydrated at 70°C


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However the overall porosity continues to decrease from 0.5 to 3 hours, indicating that continuing hydration of CA dominates the microstructural development. Isothermal curing at 70°C leads to a high early degree of reaction, by XRD it appears that the CA is almost totally consumed (Fig. 6) after 3 hours. Already at 0.5 hours the total bound water, more than 20% and increases only slightly as the hydrates change (Fig. 4). The C2AS peak by XRD shows a drop similar to that seen at 20°C in the first hour (again probably dilution), but there seems to be a slow continuing reaction of this phase over the first day. Temperature history from 20 to 70°C In this experiment, the paste is initially cured at 20°C for 1 hour to keep a similar induction period to that at 20°C. After one hour, the temperature is increased at a rate of 10°C/h up to 70°C. Figures 9, 10 and 11 show respectively XRD, TGA and SEM results of CAC paste progressively heated from 20 to 70°C. The sequence of hydration can be deduced from XRD and TGA results. Below 50°C, hydration is dominated by the formation of C2AH8 according to XRD while TGA shows the same two peaks (100°C and 160°C), already commented on for the 70°C cure.

Fig. 9: XRD results of plain CAC cured with increasing temperature ramp.

Fig. 10: TGA results of plain CAC cured with increasing temperature ramp.

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2.3h (40°C)

On the left, bright C2AH8 intermixed with dark AH3. The matrix is filling.

4h (60°C)

Growth of C2AH8 and AH3. A dense matrix is already formed.

8h (70°C)

The C2AH8 conversion is initiating and revealed by C3AH6 clusters on the edge of C2AH8 plates. A progressive density of the matrix is shown on the right image.

24h (70°C)

C3AH6 is present as well in CAC grain than intermixed with AH3. The porosity development looks minimized.

Fig. 11: SEM observations of CAC hydrated with increasing temperature ramp.


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Once the paste temperature reaches 60°C, the intensity of C2AH8 decreases progressively and C3AH6 and AH3 develop. After 6 hours (70°C), no more metastable hydrates remain in the system and the microstructure does not develop further up to 24 hours. XRD also shows that the CA is completely consumed after 6 hours

POROSITY QUANTIFICATION In the case of the 20°C cure, the porosity decreases within the first 24 hours due to the massive precipitation of hydrates. In the cases where the paste undergoes curing at 70°C the formation of the stable hydrates results in more macroporosity. Micrographs of the fully converted pastes (24hrs) immediately and progressively heated at 70°C are compared in Figure 12.

Fig. 12: SEM micrographs of CAC immediately (left) and progressively cured at 70°C, at 24h.

TGA indicates a similar nature and amount of hydrates (Fig. 13); although there are some subtle differences – for the paste progressively heated there is still a significant weight loss from 100 to 200 °C and the peak for C3AH6 is smaller. In the micrographs there is clearly less porosity for the progressively heated sample compared to that immediately cured at 70°C.

Fig. 13: TGA results of CAC paste immediately and progressively cured at 70°C, at 24h.

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Image analysis results (average and standard error of measurement) are given in Table 2. The density of the phases was used to convert volume to weight %. Table 2. SEM-IA quantification of samples cured immediately and progressively at 70°C, after 24h. T0

Water

T24h Immediate 70°C cure

Progressive 70°C cure

Density

% vol

% wt

% vol

% wt

% vol

% wt

1.00

55.3

28.6

-

-

-

-

CA

2.95

29.8

45.4

0.6 ± 0.04

0.9 ± 0.07

0.5± 0.06

0.7 ± 0.1

C2AS

3.04

9.8

15.4

4.0 ± 0.2

5.9 ± 0.5

4.3± 0.2

6.0 ± 0.3 1.8 ± 0.03

CT

4.04

1.7

3.6

1.6 ± 0.04

3.1 ± 0.1

1.0± 0.01

C2AH8

1.95

-

-

8.3 ± 0.2

7.9 ± 0.2

9.7± 0.2

8.6± 0.2

C3AH6

2.52

-

-

34.8 ± 1.1

42.6 ± 1.6

38.2± 1.1

43.9 ± 1.5

AH3

2.40

-

-

34.0 ± 0.6

39.7 ± 0.8

35.5± 0.8

38.9 ± 1.0

Porosity

-

-

-

16.7 ± 1.1

-

10.8 ± 0.2

Total

-

100

100

100

100

100

100

Bound water

-

-

-

-

29.1 ± 0.17

-

29.4± 0.16

Image analysis indicates similar amounts of the solid phases, C3AH6 and AH3. The total bound water which would be contained in these phases would be 28%, which is somewhat higher than the 22% measured by TGA, but a reasonable agreement considering the previous freeze drying of the samples, the fact that TGA only goes up to 500°C and the error in the image analysis. The image analysis confirms that the coarse porosity (which can be resolved in the SEM) is significantly higher in samples cured immediately at 70°C versus those progressively heated. The solid density calculated from the image analysis is very close to the values measured. MIP (Fig. 14) also shows a higher total porosity for the immediately cured sample (26.2%) compared to the progressively cured sample (22.7%), but the breakthrough pore size is similar and in both samples much larger than the sample cured isothermally at 20°C. Due to the limited resolution of the SEM Image analysis the porosities by MIP are significantly higher.

Fig. 14: MIP results (cumulative pore volume) of CAC pastes cured at 20°C, 20 to 70°C and immediately at 70°C cure, at 24h.


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DISCUSSION The experimental methodology presented here aims at studying the microstructure development of CAC pastes cured in different thermal regimes. In the system cured at 20°C, the total bound water (Fig. 4) evolves monotonically for the first 6 hours of hydration becoming constant thereafter. This trend is consistent with XRD results and corroborates the rapid hydration of CAC. It is not clear what causes the hydration to stop at the end of the calorimetry peak. The limitation could be the amount of water - the measured bound water to 500°C is 22% compared to 28% water present at the start, which in may be a reasonable agreement considering the measurement protocol as discussed above. It is interesting to note that the maximum bound water values are very similar in all the curing regimes. However space filling could also be the factor limiting the degree of reaction. SEM shows a rapid densification of the matrix within the first four hours of hydration and slow down in hydration appears to correspond to filling of the matrix with hydrates – i.e. lack of space for further precipitation of hydrates. TGA results remain difficult to interpret and very sensitive to the crystallization of the hydrates. In 20°C cure a broad peak does not distinguish CAH10 overlapped with C2AH8 peaks. On the contrary, a higher temperature of cure improves the crystallization of C2AH8 whose the dehydration peak becomes well distinct from CAH10. In the systems where AH3 and C3AH6 hydration dominates, dehydration peaks are very close (respectively at 300 and 350°C). The degree of crystallization may be improved in the immediate 70°C cure compared to the progressive heating, leading to a better discrimination of their respective peaks. Mass loss from TGA is useful to measure the total bound water in every system and evaluate the degree of hydration. Whatever the cure the total bound water reaches 22% after in day of hydration, with different kinetics of reaction. At ambient and increasing temperatures, the total bound water evolves over the first few hours while, at 70°C, the degree of hydration is very high from the first 30 min. Whatever the cure condition the total bound water obtained at 24 hours can be compared with the initial water content theoretical (28%), assuming that freeze drying method can partially affect the hydrates structure. Moreover SEM image analysis applied on samples cured at 70°C gives pertinent results at 24 hours with a total bound water of 28%. The hysteresis curves from MIP show the effect of the nature and the assemblage of hydrates on the volume and the connectivity of pores. At 20°C cure, the critical pressure, required to initiate the mercury percolation, corresponds to a diameter of 0.05 micron compared to 1 micron in the cures at 70°C. The quantification of total porosity in systems cured at 70°C shows interesting differences. Image analysis measures pores volume (16.7% ± 1.13 and 10.8% ± 0.20 respectively for immediate and progressive 70°C) with an accurate distribution of pore within the matrix. The porosity measured with MIP confirms this difference between the samples, although the values measured are much higher due to the limitations on resolution in the SEM. The differences in porosity correspond to higher strengths observed for samples cured progressively to 70°C [8] as opposed to those cured immediately at 70°C

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CONCLUSION The microstructure of calcium aluminate cement has been studied from the first minutes until 24 hours of hydration. Cement pastes cured in three different isothermal temperature histories develop various kinetics of reaction with different products of hydration. At 20°C and in presence of Li2SO4, the hydration of CAC leads to a massive precipitation of CAH10. The development of metastable hydrates is stopped after few hours because of the lack of water or available space. The dense microstructure development is then completed giving very low porosity. At high temperature, C3AH6 and AH3 precipitate with different kinetics according to the rate of temperature increase. In the case of immediate cure at 70°C, metastable hydrates (mainly C2AH8) precipitate instantaneously and convert rapidly (3 hours) into C3AH6 and AH3. If the temperature increases progressively, the preliminary precipitation of metastable hydrates tends to fill the matrix and their total conversion occurs later on. The progressive assemblage of the hydrates limits the porosity development and its impact on mechanical strength.

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

K. L. Scrivener, A. Capmas. Calcium Aluminate Cements, Chapter 13. Lea’s Chemistry of Cement and Concrete. New York, 1998. S. Richard. Structure et proriétés élastiques des phases cimentières à base de mono-aluminate de calcium. PhD Thesis. Paris, University Paris VI, 1996. P. J. French, R. G. J. Montgomery, T. D. Robson. High concrete strength within the hour. Concrete, Vol 3, August 1971, pp 253–8. P. F. G. Banfill. Superplasticisers for Ciment Fondu: Part 2 Effects of temperature on the hydration reactions. Advances in Cement Research, Vol 28, 1995, pp 151–7. H. G. Midgley. The use of thermal analysis methods in assessing the quality of high alumina cement concrete. Journal of Thermal Analysis, Vol 13, 1978, pp 515–24. V. S. Ramachandran. Handbook of Thermal Analysis of Construction Materials. William Andrew Inc, New York, 2002. D. Damidot, A. Rettel, A..Capmas. Action of admixtures on Fondu Cement: Part 1 lithium and sodium salts compared. Advances in Cement Research , Vol 8, 1996, pp 111–9. J. Ideker and K. Folliard, private communication.