EFFECTS OF POZZOLANIC REACTION ON THE EVOLUTION OF COARSE CAPILLARY PORE STRUCTURE AND PHASE CONSTITUTION IN CEMENT PASTES WITH MINERAL ADMIXTURES

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1 EFFECTS OF POZZOLANIC REACTION ON THE EVOLUTION OF COARSE CAPILLARY PORE STRUCTURE AND PHASE CONSTITUTION IN CEMENT PASTES WITH MINERAL ADMIXTURES S. Igarashi and A. Watanabe Department of Civil and Environmental Engineering, Kanazawa University, Kanazawa, Japan Abstract Capillary pore structures in cement pastes with mineral admixtures were revealed by a combination of SEM-BSE image analysis and calculations of volume changes during hydration of cement and pozzolanic reaction. Degrees of pozzolanic reaction were evaluated by the selective dissolution method. Characteristics of volume fractions of constituent phases were discussed from the viewpoint of how the pozzolanic reaction affected the capillary pore structure in the cement pastes. In cement pastes with silica fume and fly ash, the volume ratio of large capillary pores to the total porosity increased as the pozzolanic reaction proceeded. Less fine pores in the cement pastes with the admixtures resulted in the gap-graded pore structures. The gel/space ratio theory can be applied to the cement pastes with the admixtures. However, the ultimate strength predicted for the pastes containing the admixtures was lower than for the ordinary paste. This fact confirms that compressive strength is influenced by not only the total porosity but also features of the pore structure such as the relative volume ratio of coarse pores to the total porosity. 1. INTRODUCTION Various mineral admixtures such as fly ash and silica fume have been used for improving performances of concrete. For example, high strength and low permeability of concretes containing those admixtures are related to dense microstructure formed by the pozzolanic reaction. Therefore, for a better understanding of mechanism for high performances in the concretes, it is significant to elucidate characteristics in the microstructure linking them with kinetics of the pozzolanic reaction. SEM-BSE image analysis has been used to quantitatively evaluate the process of hydration of cement and characteristics of microstructure in cementitious materials. Unbiased evaluation for the amounts of constituent phases such as unhydrated cement and capillary pores is easily made in 2D cross sections of concretes, following the procedure based on a simple stereology principle. Reliable values for the degree of hydration of cement at given ages are also 1

2 provided by the SEM-BSE method. Taking this advantage, Scrivener et al [1] have used BSE image analysis to obtain control files on unhydrated cement in the Rietveld analysis. The present authors have shown that the degree of hydration determined by the image analysis method does not contradict the Powers model [2]. If a cement system contains a pozzolanic admixture, it is important to evaluate the degrees of pozzolanic reaction as well as the hydration of cement. However, it is difficult to directly quantify the pozzolanic reaction in the BSE images. The SEM-BSE method is based on the gray scale histogram. However, the threshold value to discriminate fly ash particles from other phases cannot be well determined in the gray scale. On the other hand, silica fume particles cannot be detected at a conventional magnification for BSE imaging. An alternative for evaluating the pozzolanic reaction in the BSE method may be quantifying the amount of calcium hydroxide in the images. Diamond [3] has shown that the amount of calcium hydroxide quantified by the BSE image analysis is almost comparable with that obtained by DTA. However, the range of gray scale for calcium hydroxide overlaps with those for other phases. Therefore, reliability of the image-based evaluation for calcium hydroxide is rather limited. In this study, the selective dissolution method is used to evaluate the degree of pozzolanic reaction [4,5]. The degree of the reaction determined by this method is used for calculating volume changes of phases involved in the pozzolanic reaction. Volume changes due to the hydration of cement are separately calculated based on a combination of the Powers model and the degree of hydration of cement, which is obtained from the BSE method. Combining these two mechanisms for volume changes, the whole volume fractions of constituent phases are calculated in fly ash- and silica fume-containing cement pastes. Effects of the mineral admixtures on characteristics of the constitution in the pastes are discussed with emphasis on features of capillary pore structure. Furthermore, development of compressive strength is also discussed in terms of the applicability of the gel/space ratio theory [6] to the cement pastes containing mineral admixtures. 2. Experimental 2.1 Materials and Mix Proportion of Cement Pastes The cement used was an ordinary Portland cement produced in Japan (Density = 3.15g/cm 3, Blaine fineness = 331cm 2 /g). A silica fume and a fly ash were used as mineral admixtures (Table 1). The replacement levels of silica fume and fly ash were 1% and 15%, respectively. A polycarboxylic acid type superplasticizer was used in cement pastes with silica fume. Water/binder ratio used was.4. Table 1 Physical properties of admixtures Density (g/cm 3 ) Specific surface area (m 2 /g) Ignition loss (%) SiO 2 (%) BSE image analysis and degree of hydration of cement Cylinders of 5mm in diameter and 1mm in length were produced. They were demolded at 24 hours after casting, and then cured in water at 2 C. At the prescribed ages, slices about 1mm thick were cut from cylinders for the BSE image analysis. They were dried by ethanol replacement and in the vacuum drying, and then impregnated with a low viscosity epoxy resin. 2

3 After the resin hardened at room temperature, the slices were finely polished with SiC papers. The polished surfaces were finished with diamond slurry for a short time. Samples were examined using an SEM equipped with a quadrupole backscatter detector. The BSE images were acquired at a magnification of 5. Ten fields in each specimen were randomly chosen and analyzed. Each BSE image (e.g. Fig.1) consists of pixels. The size of one pixel is about.22.22μm. A dynamic thresholding method was used to make binary segmentation based on the gray level histogram. Pixels for unhydrated cement particles and for pores were tallied so as to obtain area fractions of the two phases. The volume fractions of cement gel (i.e. CSH and calcium hydroxide crystals) were calculated by combination of the Powers model with the image-based degree of hydration. The degree of hydration of cement (α) was calculated by Equation (1). UH α = 1 i UH (1) where UH i : area fraction of unhydrated cement particles at the age of t i, UH : initial area fraction of unhydrated cement particles (i.e. t i =) Representative BSE micrographs of silica fume- and fly ash-containing cement pastes are given in Fig.1. As found in Fig.1(b), brightness of unhydrated cement particles is sufficiently different from that of fly ash. It is easy to determine the threshold value between unhydrated cement and fly ash particles in the gray level. Therefore, the same procedure as for the paste without admixtures could be used for determining the degree of hydration of cement in the cement pastes containing the admixtures. (a) (b) Fig.1 BSE images at 1 day: (a) Silica fume-containing (b) Fly ash-containing 2.3 Compressive strength tests Cylinder specimens of 5mm in diameter and 1mm in height were prepared according to JIS R 521 and JSCE-F56. The specimens were cured under the same condition as those used in the SEM examinations. Compressive strength tests were conducted at the age of 1,7,28 and 91days. 3

4 2.4 Loss on ignition of cement pastes and the degree of reaction for fly ash and silica fume Small pieces of cement paste were taken from the middle of specimens after the compressive strength test. They were dried by ethanol replacement. Then, they were dried in the oven at 15 C for 24h. The dried samples were ignited at 15 C. The loss on ignition of cement paste was calculated in accordance with JIS R 522. The dried samples were also treated with HCl and Na 2 CO 3 solution. The degree of reaction of pozzolanic materials was determined by the insoluble residue and the loss on ignition [4,5]. 2.5 Calculation of phase constituents The volume of hydration products of cement was calculated by applying the Powers model to the results of image analysis [2]. In the calculation, the volume of cement gel produced by the hydration of 1cm 3 dry cement was assumed to be 2.1cm 3. The non-evaporable water content in the reacted cement is assumed to be about 23% by mass. Chemical shrinkage was also assumed to be.254 of the volume of non-evaporable water. The porosity of cement gel used in the calculation was 28%. The gel pores were assumed to be saturated with gel water. The volume of cement gel was estimated using the degree of hydration determined by Equation (1). The volume of capillary pores was obtained by subtracting the volume of residual unhydrated cement and the calculated volume of cement gel from the initial volume of the mixture. Thus, differences in the volume fraction between the calculated capillary pore volumes and the coarse pore ones obtained by the image analysis represent the volume fractions of fine pores of which diameters are less than the resolution of the image analysis (.2μm in this study). As for the calculation for cement pastes containing silica fume and fly ash, it was assumed that the pozzolanic reaction was expressed as the following equation [7], and that physical properties of CSH formed by the pozzolanic reaction were assumed to be the same as those produced by the hydration of cement [8]. S CH H C SH (2) Volume changes of solid phases involved in the pozzolanic reaction were also calculated based on the equation (2) and the reaction degree of pozzolanic material was determined by the insoluble residue method. Combining the volume changes due to the cement hydration and the pozzolanic reaction, the whole volume fractions of constituent phases were obtained for fly ash- and silica fume- containing cement pastes. 2.6 Characterization of coarse capillary pore structure In order to extract features of pore size distributions from a binary image, the equivalent diameter of a pore was used as a geometric measure [9]. Each pore with irregular shape was labeled by the rule of 8-neighbor connectivity. The labeled pores whose areas were tallied by pixels, were converted to the equivalent circles with the same area as the original pores. Then, all the circles were scaled by their diameters. The cumulative pore volume vs. the equivalent diameter curves were plotted by sorting and cumulating areas of those scaled circles. 4

5 3. RESULTS AND DISCUSSION 3.1 Degrees of hydration and of pozzolanic reaction Fig.2 shows the degrees of hydration of cement in cement pastes with and without mineral admixtures. The hydration degrees in the pastes with the admixtures are greater than those in the ordinary cement paste even at early ages of 1 and 7days. It seems that the hydration of cement was accelerated by the presence of fine particles of the admixtures [1]. However, the Degree of Hydration Age (Days) Degree of Pozzolanic Reaction Age (Days) Fig.2 Degrees of hydration of cement in cement pastes with and without admixtures Fig.3 Degrees of pozzolanic reaction subsequent changes in the degree of hydration were not appreciable in the silica fume-containing paste. On the other hand, the fly ash-containing paste exhibited continuous increase in the degree at long ages. It is found from Fig.2 that the hydration of cement in the silica fume paste almost ceased after 28days. Fig.3 shows the degrees of pozzolanic reaction in fly ash- and silica fume-containing pastes. About 1% of silica fume had reacted after the age of 1day. The degree of reaction of silica fume increased steeply as to attain about 8% at 91days. In contrast, the reactivity of fly ash used in this study was quite low. Little reaction had occurred during the initial 7days. The degrees of the reaction of fly ash at 28 and 91days were at most 4 and 18%, respectively. This suggests that most of the fly ash was left in the fly ash pastes. However, it should be noted that as expected, reaction of fly ash continued longer than silica fume. Fig.4 shows the calculated volume fractions of constituent phases in cement pastes. Volume fractions of silica fume appreciably decreased after 7days, as mentioned above. Correspondingly, the amounts of cement gel produced in the silica fume pastes were greater than those in the ordinary cement pastes. On the other hand, the amounts of coarse capillary pores in the silica fume pastes were almost the same as those in the ordinary cement pastes. As a result, the total porosity in the silica fume pastes was smaller than that in the ordinary pastes. 5

6 It should be noted that differences in capillary pore structure between the cement pastes were featured by the amount of fine pores at long ages. If it is assumed that pore structure in Volum e Fraction Unhydrated C em ent Adm ixture H ydration Products Fine Pores Coarse Pores Age (D ays) (a) (b)silica Fum e (c) Fig.4 Volume fractions of constituent phases in cement pastes with a mineral admixture cement paste is like hierarchical network, less fine pores suggest that there are few fine paths which interconnect the coarse pores in the silica fume- containing pastes. In other words, the capillary pores in the silica fume pastes are less interconnected to each other so that a discontinuous (gap-graded) pore structure was formed at long ages. On the contrary, in the ordinary cement pastes, coarse pores seem to be more interconnected since the paste has more fine pores as connecting paths. Based on the pore structure revealed by the MIP method, many workers have suggested that more refined pore structure has been developed by the pozzolanic reaction in silica fume pastes. However, it is found from Fig.4 that such an evolution of refined pore structure does not mean the reduction in coarse pores. The amount of coarse pores in the cement pastes with silica fume is almost the same as in the ordinary cement pastes. Namely, coarse pores are still left even in matured silica fume pastes as well as in the ordinary cement pastes. In fly ash-containing pastes, the amount of fly ash decreased a little because of its low reactivity. However, the volume fraction of cement in fly ash pastes continuously decreased even after 28days. Compared to the ordinary pastes, more coarse pores are present in the fly ash pastes at long ages. However, fine pores in fly ash pastes did not decrease appreciably as in the silica fume pastes. As a result, more continuous pore structure in which coarse pores are connected to fine pores, is formed in the fly ash pastes. Taking into account the fact that the reduction in fine pores was consistent with the degree of the pozzolanic reaction in the silica fume pastes, less interconnected pore structure in the silica fume paste resulted in little change in the degree of hydration of cement after 28days since water cannot easily reach unhydrated cement particles. On the other hand, more interconnected pore structure has developed in the fly ash pastes. As a result, the hydration of residual cement could continue longer at long ages since water could be supplied through the continuous pore network. 6

7 3.2 Coarse capillary pore size distribution Fig.5 shows coarse pore size distribution curves for the cement pastes with and without admixtures. Differences between the curves were clearly seen at 1 and 7 days. The differences are characterized by the amounts of large pores of which diameter are greater than 2-3 μm. Less large pores are present in silica fume pastes than in ordinary cement pastes until 7 days. Namely, dense microstructure had already developed in silica fume pastes even at early ages. However, pore size distributions in silica fume paste after 28days were almost the same as those in the ordinary pastes. Fly ash pastes always had more coarse pores than ordinary cement pastes in the 2D distribution. However, a remarkable decrease in the porosity was Cumulative Pore Volume (cm 3 /cm 3 ) Cumulative Pore Volume (cm 3 /cm 3 ) (a) 1 day Pore Diameter (μ m) (c) 28 days Pore Diameter (μ m) (b) 7 days Pore Diameter (μ m) S ilica F um e.2 (d) 91 days Pore Diameter (μ m) Fig. 5 Coarse capillary pore size distribution in cement pastes found in the fly ash paste between 28 and 91 days. There were little differences in the distributions between the cement pastes at 91 days. The pore size distribution shown in Fig.5 is obtained by the 2D circles equivalent to the areas of original pores with irregular shape. Thus, the meaning of pore size is obviously different from that estimated by the MIP method. However, taking into account of 2-point correlation function in terms of second-order stereology [11], pore size shown in Fig.5 may provide a rough measure which reflects continuity of coarse pores in a random pore structure. 7

8 Large areas of pores are also derived from long continuous pores. The more long pores, the more lineal paths between arbitrary two points are present in a paste. Namely, a large equivalent pore in the distribution could be interpreted as presence of a long continuous path in a certain direction. Therefore, the characteristic diameter at which porosity starts to steeply increase with decreasing pore diameter in the distribution may be a significant property in pore structure. Fig.6 shows comparison of characteristic pore diameters among the cement pastes. They decreased with time. The silica Characteristic Pore Diameter (μm) Age (Days) Fig.6 Characteristic diameter of coarse capillary pores in cement pastes fume pastes had smaller characteristic diameters while the fly ash paste had greater ones than the ordinary cement pastes. This implies that continuity of coarse pores in fly ash pastes is greater than the ordinary cement pastes. Such a continuity of pore structure would be significant in determining durability of concrete. 3.3 Development of compressive strength and applicability of the gel/space ratio theory Fig.7 shows the evolution of compressive strength in cement pastes. The strength of silica fume pastes is always greater than that of ordinary cement pastes. Fly ash pastes exhibited smaller strength than others. However, the strength of the fly ash pastes still increased monotonically even after 28days while the strength of other pastes increased little at long ages. Fig.8 shows the relationship between compressive strength and coarse capillary pore porosity in the cement pastes with and without mineral admixtures. There is a good correlation between them both. Namely, coarse capillary porosity is a dominant parameter to control the compressive strength. Sensitivity of strength to the coarse capillary porosity is Compressive Strength (N/mm 2 ) Age (Days) Fig.7 Compressive strength of cement pastes Compressive Strength (N/mm 2 ) Fit () Fit () Fit () Coarse Capillary Porosity (cm 3 /cm 3 ) Fig.8 Compressive strength vs. coarse capillary porosity in cement pastes 8

9 changed by the incorporation of mineral admixtures. Fig.9 shows the relationship between the compressive strength and the total porosity calculated in the cement pastes. The regression lines for the pastes with the admixtures are different from that for the ordinary cement pastes. If the ultimate strength of cement pastes with the admixtures is extrapolated from the regression line, the predicted strength for them is smaller than that for ordinary cement pastes. Compressive Strength (N/mm 2 ) Fit () Fit () Fit () Total Porosity (cm 3 /cm 3 ) Compressive Strength (N/mm 2 ) Fit () Fit () Fit () Gel/Space Ratio Fig.9 Relationship between compressive strength and the total porosity in cement pastes Fig.1 Compressive strength vs. gel/space ratio for cement pastes Fig.1 shows the relationship between compressive strength and the gel/space ratios. It is clearly seen that the gel/space ratio theory by Powers and Brownyard [6] can be applied to the cement pastes with the mineral admixtures. Namely, the development of compressive strength is expressed as a function of the gel/space ratio. However, the regression lines also predict lower strength for the admixture - containing cement pastes than for ordinary cement pastes. Namely, it is implied from Figs.9 and 1 that fully matured pastes with those mineral admixtures would exhibit smaller strength than the pastes without the admixtures. This is different from a general agreement that the strength of cementitious materials with mineral admixtures can be greater than that for the ones without admixtures. It is likely that greater strength for concretes with the admixtures at long ages results from more homogeneous and dense microstructure formed by the pozzolanic reaction. Such a discrepancy in the strength development at long ages may be explained from the characteristics of capillary pore structure and its spatial distribution. In other words, the ratio of coarse pores to the total porosity can be a significant parameter to control the strength evolution in cement pastes. As found in Fig.4, coarse capillary porosity in silica fume containing pastes is almost comparable with that in ordinary pastes at long ages. However, fine capillary porosity in the silica fume pastes is considerably smaller than the control pastes. Namely, dense structure due to the pozzolanic reaction is formed while isolated coarse pores are still left unoccupied in the silica fume pastes. Of those coarse pores, Hadley grains could be included. At matured ages, coarse pores are more representative for the whole pore structure in silica fume pastes. Strength of cement pastes is usually more sensitive to greater flaws or pores. As long as those great flaws are dispersed uniformly, the strength of paste can 9

10 increase to a certain degree. However, presence of coarse pores strongly affects the evolution of strength in fully matured paste with little porosity. On the other hand, the volume ratio of coarse pores to the total porosity was low in the pastes without admixtures since there still exist lots of fine pores at long ages. As a result, strength of silica fume pastes would be predicted smaller even if the same gel/space ratios as in the pastes without admixtures are attained in the admixture-containing pastes at long ages. Comparison of strength evolution between pastes with and without admixtures confirms that strength depends on not only the total porosity but also characteristics of pore structure. Presence of coarse pores left in mature pastes seriously affects the development of strength at long ages. 4. CONCLUSIONS Pore structure and phase constituent in cement pastes with and without mineral admixtures were revealed by a combination of the SEM-BSE image analysis and the selective dissolution method. The major results obtained in this study are summarized as follows: The degree of hydration of cement was higher in the pastes with the mineral admixtures than in ordinary cement pastes. The addition of mineral admixtures accelerated hydration of cement at early ages. Pore size distribution in the silica fume pastes was more discontinuous. The amount of fine capillary pores was small at long ages. Dense microstructure in which residual large capillary pores might be directly surrounded by the cement gel matrix with less fine capillary pores, was formed at long ages. Pore networks in the fly ash pastes were more interconnected. A little reaction product by the pozzolanic reactivity of fly ash did not effectively fill in the networks. Good correlations between the compressive strength and the coarse capillary porosity were found in the cement pastes with and without admixtures. However, the sensitivity of the strength to the coarse capillary porosity depends on types of admixture. The gel/space ratio theory can be applied to the cement pastes with the admixtures. However, the predicted ultimate strength for the pastes with the admixtures was lower than for the ordinary cement pastes at a greater gel/space ratio. This is explained from the difference of pore structure, where the ratio of coarse capillary pore volume to the total porosity is increased in the cement pastes with admixtures. REFERENCES [1] Scrivener, K.L., et al, 'Quantitative study of Portland cement hydration by X-ray diffraction / Rietveld analysis and independent methods', Cement and Concrete Research, 34(9) (24) [2] Igarashi, S., Kawamura, M., Watanabe, A., 'Analysis of cement pastes and mortars by a combination of backscatter-based SEM image analysis and calculations based on the Powers model', Cement and Concrete Composites, 26(8) (24) [3] Diamond, S., 'Calcium hydroxide in cement paste and concrete-a microstructural appraisal', in 'Materials Science of Concrete, Special Volume: Calcium Hydroxide in Concrete (eds. J. Skalny, J. Gebauer, I. Odler)', (The American Ceramic Society, Westerville, 21) [4] Lam, L., Wong, Y.L. and Poon, C.S., 'Degree of hydration and gel/space ratio of high-volume fly ash/cement systems', Cement and Concrete Research, 3(5) (2)

11 [5] Tang, F.J. and Gartner, E.M., 'Influence of sulphate source on Portland cement hydration', Advances in Cement Research, 1(2) (1988) [6] Powers, T.C. and Brownyard, T.L., 'Studies of the physical properties of hardened cement pastes (Nine parts)', Journal of American Concrete Institute, (43) ( ). [7] Young, J.F. and Hansen, W., 'Volume relationships for CSH formation based on hydration stoichiometry', in MRS Symp. Proc., 85 (1987) [8] Neville, A.M., 'Properties of Concrete', 4th Edition, Longman, Harlow (1998). [9] Diamond, S. and Leeman, M.E., 'Pore size distributions in hardened cement paste by SEM image analysis', in MRS Symp. Proc., 37 (1995) [1] Gutteridge, W.A. and Dalziel, J.A., Filler cement: the effect of secondary component on the hydration of Portland cement, Cement and Concrete Research, 2(6) (199) [11] Howard, C.V. and Reed, M.G., 'Unbiased Stereology', 2nd Edition (BIOS Scientific Publishers, Abingdon, 25). 11