Effect of curing temperature on the pore structure of tricalcium silicate pastes

Effect of Curing Temperature on the Pore Structure of Tricalcium Silicate Pastes A. B E N T U R Faculty o f Civil Engineering, Technion, Israel Institute of Technology, Haifa, Israel Received April 27, 1979; accepted A u g u s t 15, 1979 T h e pore structure of C~S pastes cured at 4, 25, and 65°C was investigated using m e r c u r y porosimetry and HzO and N2 adsorption. T h e 4 and 25°C pastes are not very different but the 65°C paste has a coarser pore structure. In this case there is an increase in macropore volume at the e x p e n s e of meso- and micropore volumesl T h e s e c h a n g e s are not a c c o m p a n i e d by variations in total porosity. Some possible relations b e t w e e n t h e s e c h a n g e s and the reduction in strength and drying shrinkage o f high-temperature cured pastes are discussed. I. I N T R O D U C T I O N

High-temperature curing is a common practice in concrete technology for obtaining high early strength. This type of curing induces significant changes in mechanical properties of mature pastes and concretes causing them to be weaker (1-3), and exhibit smaller time dependent deformations (creep and shrinkage) (4, 5). Several explanations have been suggested to account for these changes, some of them based on variations in the chemical structure of the hydration products (5) and some on changes in microstructure (1) and extent of hydration (1). However, few studies have attempted to investigate simultaneously changes in chemical composition, microstructure, and mechanical properties occurring in hightemperature cured pastes and concretes. Therefore, there are not sufficient data for assessing the validity of the different explanations. In an attempt to clarify some of these problems, an extensive investigation was initiated in which variations in mechanical, chemical, and microstructural characteristics induced by change in curing temperature were studied. Results of strength, creep, shrinkage, and chemical composition and some microstructural parameters were

reported in several publications (6-9). The work presented here is a continuation of this study, and it investigates the details of the changes in pore structure of C3S 1 pastes hydrated at 4, 25, and 65°C. Several studies of this kind have been reported in literature. Most of them dealt with determination of changes in pore structure using only one experimental technique: mercury porosimetry (10-12), HzO adsorption, and Nz adsorption (6, 13). Each of these test methods has some limitations and therefore conclusions reached on their basis are somewhat limited. Part of these limitations results from the fact that these techniques detect only part of the whole pore structure of cement paste. Mercury porosimetry is sensitive mainly to the structure of the larger pores of the system but not to the micropores which may be in the form of gel pores (14) or interlayer spaces (15). H20 and N2 adsorption methods are effective only in measuring the pore size distribution of pores smaller than several hundreds of angstroms in diameter. In addition, it is well known that, in the case of cement pastes, there is no agreement between the results ' C e m e n t chemistry notation is used: C = CaO; S = SiO2; H = HzO. 549

Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

0021-9797/80/040549-12502.00/0 Copyright © 1980by AcademicPress, Inc. All rights of reproduction in any form reserved.

550

A. B E N T U R

obtained by the various test methods (15, 16). These aspects of pore structure have been critically reviewed by Diamond (17). In view of these facts, there is no general agreement as to which testing technique gives a better estimate of the " t r u e " pore structure and, therefore, it is not possible to draw clear cut conclusions with regard to variations in the whole pore structure of cement paste using only one of these test methods. In order to overcome or bypass such difficulties, it was decided in this work to study the pore structure using all three techniques which are usually applied in cement research: mercury porosimetry and H20 and N2 adsorption. By doing so, two advantages may be gained: (a) These three techniques will provide information on the whole pore structure; (b) by comparing the results of the three techniques it would be possible to conclude whether they all indicate at least the same qualitative trends in changes in pore structure caused by an increase in curing temperature. In such a case, it would be possible to reach conclusions which are more reliable and less susceptible to doubts regarding the significance of each of the test methods. In an investigation of this kind, one is tempted to use one of the known cement paste models and analyze the results in terms such as gel pores according to the Powers model (14) or interlayer spaces according to the Feldman-Sereda model (15). However, in view of the uncertainties involved in these models, it was felt that, for the purpose of this work, it would be preferable to use the IUPAC classification (18) of pores into micro-, meso, and macropores, and present the analysis in terms of the effect of temperature on the volume and size distribution of each of these pore fractions. This kind of classification has some physical significance that may later provide a basis for discussing possible relations between changes in mechanical properties and pore structures. For example, shrinkage Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

characteristics are more likely to be associated with the structure of mesopores [where capillary processes can take place (19)] and micropores [where processes ~ c h as changes in interlayer spaces (20) or variations in disjoining pressure (19) occur]. Intuitively one would expect that strength is more likely to be sensitive to the volume and structure of the macropores. II. E X P E R I M E N T A L

The C3S used in the present work was prepared at the Portland Cement Association and was ground to 3800 cm2/g Blaine fineness. Pastes of 0.4 water solids ratio were mixed in vacuum and cast as 12.5-mm cubes. They were continuously cured in lime water at specified temperatures (4, 25, and 65°C). At predetermined ages, hydration was stopped by drying of the cubes at 105°C. The weight loss on drying of saturated surface dry samples was used as a means for determining total porosity, assuming water density of 1.0 g/cm~. Fragments of these cubes were tested by mercury porosimeter using a 60,000 psi Aminco instrument. 2 Cumulative pore size distribution curves were calculated, using the Washburn equation assuming cylindrical pores, contact angle of 117°C, and surface energy of 484 dyne cm-'. These data for oven-dried portland cement pastes were reported by Winslow and Diamond (21) who also presented a comprehensive discussion on the application of mercury porosimetry in cement research. Part of the dried cubes were ground into chunks smaller than #50 sieve and were tested for N2 adsorption by the dynamic method with helium as carrier gas, using the Quantasorb of Quantachrome Corp. instrument. Similar chunks were analyzed for H20 adsorption by the gravimetric method using evacuated desiccators held at constant temperature, with dilute sulfuric acid solu2 T h e tests were carried out at the laboratories of the School of Civil Engineering, P u r d u e University.

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FIG. 1. Cumulativepore size distribution curves of low and high degree of hydration C~S pastes cured at 4, 25, and 65°C, determinedby mercury porosimeterand H=O and N2 adsorption. tions used to control relative humidity according to the data of Bray (22). Pore size distribution curves were calculated from the adsorption isotherms using the procedure described by Linsen and Van den Heuvel (23) with the assumption of cylindrical pores. The t curves published by Lippens et al. (24) were used for Nz adsorption analysis and those of Hagymassy et al. (25) for HzO adsorption. The theory, limitations, and possible significance of these three test methods for determining the pore structure of cement pastes were critically reviewed by Diamond (17). Specimens similar to the ones studied in this work were tested also for strength, shrinkage, and chemical composition. These results are given in other publications (6-9). III. RESULTS Typical cumulative pore size distribution curves are shown in Fig. 1 for low and high degree of hydration C3S pastes cured at 4°C. The trends in this figure are also typical for the 25 and 65°C pastes. In most cases, the pore volume detected by mercury

porosimeter is the largest while the N2 adsorption gives the lowest results. The differences between the mercury-detected pore volume and that detected by Nz and H20 adsorption is large at low degrees of hydration (Fig. la) but decreases with increased hydration. It is apparent from these curves that the larger difference at the earlier stages of hydration is mainly the result of the pores larger than 300 A that can be detected easily only by mercury porosimetry. At higher degrees of hydration when the volume of these pores decreases, the mercury porosimetry curves tend to approach those of the adsorption tests. These trends can be readily explained on the basis of the filling of the larger pores of the paste [termed by Powers (14) capillary pores] with hydration products. It was found that, at high degrees of hydration, the mercury and H20 adsorption curves tend to intersect at the 4 and 25°C pastes but they are still far apart in the 65°C paste. This difference is mainly the result of the existence of a considerable volume of macropores in the 65°C paste and their absence in the 4 and 25°C pastes, as can be Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

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observed in Fig. 2. The 65°C curve has a threshold diameter of about 1300 ,~ diameter. The 4 and 25°C pastes have, on the other hand, a smaller threshold diameter of about 300 A and a sharp increase in the curve starting at about 250 A diameter. These trends reflect a marked change in the pore structure of the 65°C paste. They will be discussed in detail later on. Theoretically, all three test methods should be effective in measuring pores in the range of approximately 30 to 300 A (mesopores). In order to compare the results in this range, the H20 and N~ curves were shifted to coincide with the mercury curve at the point of 300/~ diameter (Fig. 1). Comparison of these shifted curves indicates that, even in this range, there is no agreement. Usually, the H20 adsorption detects more pore volume in this range than the other two methods which tend to be similar although not identical. Evaluation of the significance of the similarities and differences observed in these curves is beyond the range of this work. It will be discussed in more detail in a future publication that will also include data from other systems. Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

IV. DISCUSSION

In this discussion a detailed analysis of the pore size distribution curves will be presented, and on its basis some conclusions will be drawn about the effect of curing temperature on the pore structure of C3S pastes. Some relations between the effect of curing temperature on pore structure and mechanical properties (compressive strength and drying shrinkage) will also be presented. As discussed in the Introduction, the pore structure will be divided into three parts following closely the division defined by IUPAC (18): macropores (d > 300 •), mesopores (30/~ < d < 300 ~), and micropores (d < 30 ~). Macropores will be analyzed on the basis of mercury porosimeter results and mesopores on the basis of mercury porosimeter and N2 and H20 adsorption. No direct measurements of the micropores were carried out in this work. However, the pore volume of the micropores was calculated indirectly from the difference between the total pore volume obtained by weight loss of saturated sur-

553

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face dry samples dried to 105°C and the volume of pores with diameter larger than 30 A which was derived from mercury porosimetry results. It should be mentioned that a detailed analysis of the effect of curing temperature on the micropore structure of CaS pastes was carried out by Skalny and Odler (13) using the "MP" analysis of H20 adsorption isotherms.

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ever, also independent of curing temperature. Macroporosity tends also, as expected, to decrease with increased hydration, but in this case curing temperature has a marked effect: Throughout the whole hydration period, the 65°C paste has much greater macropore volume, compared to the 4 and 25°C pastes. The latter two are similar. The trends in the mesopore volume are just the opposite. In all cases it increases with hydration but the 65°C paste has consistently smaller mesopore volume compared to the 4 and 25°C pastes which are similar. In Fig. 4, cross-plots showing the effect of curing temperature on total, macro-, meso-, and microporosity of well-hydrated C3S pastes are presented. They indicate that increase in curing temperature from 25 to 65°C is associated with decrease in microand mesoporosity and increase in macroporosity. Variations in these pore volumes Journal of Colloid and Interface Science,

Vol.74, No. 2, April1980

554

A. BENTUR

are relatively small when the temperature is raised from 4 to 25°C. Total porosity is practically independent of curing temperature. This latter observation suggests that curing temperature does not greatly affect the volume of hydrated solids which grow into the spaces between the CzS grains that were initially filled with water. However, the variations in macro-, meso-, and microporosity indicate that curing temperature has a marked effect on the arrangement of these growing hydration products. It should be emphasized that the changes in pore structure are not the only ones taking place and that increase in curing temperature also causes variations in the structure of the C - S - H particles themselves, increasing their degree of silicate polymerization (5, 6) and decreasing their specific surface area (6, 13). It is reasonable to assume that the decrease in macroporosity with increased hydration is the result of growth of reaction products. The fact that this reduction is smaller in the 65°C paste (leading to much larger macroporosity at high degrees of hydration) indicates that, at this temperature, the hydration products are more closely packed and are therefore less efficient in filling the macropore volume. This is consistent with the lower meso- and microporosity of the 65°C paste. In such an interpretation it is implied that the micro- and mesopores are pores enclosed between hydrated particles. The concept of enclosed micropores between hydration products is not inconsistent with the various models proposed for cement paste. Most of Powers' gel pores (14) and Feldman-Sereda's interlayer spaces (15) may be classified in the micropore fraction. However, it is not obvious whether all the mesopores can be considered as pores enclosed between hydrated particles. The fact that increased hydration is associated with increase in mesopore volume may support this concept. But it should not be ruled out that part of this increase may be the result of converJournal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

sion of macropores into mesopores due to reduction of their size by the growing hydration products. Regardless of the exact nature of the micro- and mespores, Figs. 3 and 4 clearly indicate a shift in pore structure in the 65°C paste which, at high degrees of hydration, contains more macroporosity at the expense of micro- and mesoporosity. This shift in pore structure is exhibited in marked changes in pore size distribution curves, as was already shown in Fig. 2. In this figure it can be clearly seen that the 65°C paste has much more porosity in the macropore range and much less in the mesopore range. The slope of the 65°C curve in this latter range is much less than that of the 4 and 25°C curves. The 4 and 25°C curves are similar although by no means identical. Such similarity was also noted in Figs. 3 and 4. Comparisons of the cumulative pore size distribution curves in the mesopore range of well-hydrated pastes using the results of all three test methods are presented in Fig. 5. The mercury porosimetry and N~ adsorption curves show much smaller pore volume for the 65°C paste. In both cases there are only small differences between the 4 and 25°C paste, but they are not identical. The H20 adsorption also indicates smaller mesoporosity for the 65°C paste but, in this case, the differences are not as large as those shown by the N2 and mercury measurements. It is obvious from Fig. 5 that the shape of the curves obtained by the various test methods differs considerably. However, in view of the limitations and uncertainties of the significance of each of these measurements, it was felt that a more detailed analysis of the differences reflected in the shape of these curves would not be justified. Therefore, the conclusions reached at this stage are more qualitative in nature. The most important observation based on Fig. 5 is that the 65°C paste has smaller mesoporosity than the 4 and 25°C pastes which tend to be similar but not identical to each other. All three testing techniques indicate the same trends.

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2. The Structure of the Meso and Macro Pores It was argued that a detailed analysis of the p o r e size distribution curves would be too p r e s u m p t u o u s . H o w e v e r , at the same time, it does not seem fight to ignore completely the information contained in the shape of these curves. It was, therefore, decided to refer to average p a r a m e t e r s , such as median diameters and hydraulic radii which give some general information on pore size distribution.

The effects of curing t e m p e r a t u r e and degree of hydration on macro- and mesopore median diameters obtained by m e r c u r y p o r o s i m e t r y are shown in Fig. 6. The decrease in m a c r o p o r e median diameter with increased hydration p r o b a b l y reflects the growth of hydration products into the larger pores of the system which were initially filled with water. The 4, 25, and 65°C tend to cross over, indicating that variations in curing t e m p e r a t u r e do not cause, in m o s t cases, v e r y large changes in the median diameter of the m a c r o p o r e s . A similar effect of cross Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

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over is observed in the mesopore median diameter curves in Fig. 6. In contrast to the macropore curves, increased hydration is not associated with very large changes in the median diameter of the mesopores, suggesting that the shape of the mesopores, as reflected by their median diameter, is not very sensitive to changes in degree of hydration or curing temperature. However, when the median diameters of the mesopores obtained by H20 and N2 adsorption are considered (Fig. 7), the conclusions are not always the same. In these cases too, increased hydration is not associated with drastic changes in median diameter. But they indicate variations with curing temperature. In both cases the 65°C has lower median diameter. The differences are very small in the H20 adsorption, but are relatively large in the N2 adsorption curves. It is interesting to note that the same trends are Journal of Colloid and Interface Science,

Vol. 74, No. 2, April

1980

exhibited when hydraulic radii (defined as volume of mesopore divided by their calculated cumulative surface area) instead of median diameters are considered. Typical results of hydraulic radii, showing the effect of temperature in well hydrated pastes, are presented in Table I. In this case too, the 65°C paste exhibits lower values for N2 adsorption, but similar values to the 4 and 25°C pastes in the case of H20 adsorption and mercury porosimeter. At this stage it is felt that explanation of the different effects of curing temperature observed by the various testing techniques would be premature, since the question of the significance of each of these methods is not resolved. The threshold diameters and median diameters of the whole pore structure detected by mercury show sensitivity to degree of hydration and curing temperature (Fig. 8). Increased hydration is associated with reduction in both diameters. This kind of reduction was reported in other studies (21) and it reflects the filling of the pores with hydration products. The decrease in these two diameters is smaller in the 65°C paste, resulting in larger diameter values at high degrees of hydration. The median and threshold diameters of the 4 and 25°C pastes are not much different throughout all the hydration period. These observations indicate that increase in curing temperature is associated with formation of a "coarser" pore structure consisting of larger average pores. The increase in these diameters at 65°C curing is due, at least in part, to the increase in macropore volume on the expense of meso- and micropore volumes that was observed in the previous section. Some of the shifts ,in pore size distribution observed for the 65°C CzS paste are in agreement with other works. Skalny and Odler (13) studied the structure of wide pore (with hydraulic radious larger than 9:A) of C~S pastes using H20 adsorption. These wide pores are equivalent to the me:sopores plus micr0pores as defined in t h e p r e s e n t work. They found that the volume and

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hydraulic radii of these pores in 28-day hydrated pastes tend to increase with curing temperature. Their conclusion that increase in hydration temperature is associated with formation of "coarser" structure is in agreement with the trends observed in the present work. It was shown here that this "coarsening" takes place mainly in the macropores and not in the mesopores. Such distinction could not be made in Skalny and Odler's work due to the limitation of H20 adsorption technique which is ineffective in measuring size distribution of pores larger than several hundreds of angstroms in diameter. The effect of thermal curing on the pore structure of Portland cement pastes was studied by Auskern and Horn (10) and Sellevold (11) using mercury porosimeter [97°C curing (11) and high-pressure steam curing (10)]. In both studies it was observed that increase in curing temperature is associated with increase in pore volume accessible to mercury, in agreement with the results of the present work (Fig. 2). However, only Auskern and Horn (10) reported increase in threshold diameter for the thermally

cured paste (similar to the increase observed in the present work). In contrast to these studies, Diamond (12) did not find any significant changes in the pore structure detected by mercury porosimeter of 28-dayold cement pastes cured at 6 and 40°C. Most of these studies indicate coarsening of the pore structure in high-temperature curing. The differences in the details between the various works may be the result of differences in curing conditions and chemical composition of the hydrating calcium silicates.

TABLE I Effect of Curing Temperature on Hydraulic Radii of Well-Hydrated C3S Pastes Hydraulic radious (A) Curing temperature (°C)

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Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

558

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The marked change in the pore structure of C3S paste when curing temperature is ~oooo (A~m) raised to 65°C may serve as an explanation \ to changes in mechanical properties ob50o0 \ "\. served by several investigators for hightemperature cured pastes, such as the re~0oo duction in strength and drying shrinkage \ \ " "\.~s'c of well-hydrated cement and C8S pastes ,Ioo¢ (1-6). One of the explanations put forward to account for strength reduction was that increase in curing temperature tends to desoc press hydration at later stages, thus leading LU '2OC Fto lower strength (1). However, in another £ publication (6) it was shown that the 65°C ¢ I I I I J [ paste has lower strength than the 4 and 25°C M~IRM pastes (which are similar) even when the ~ DIRME.TE~ 5000 comparison is made on the basis of equal degrees of hydration. It should be pointed '2000 out that all these pastes have similar total porosity (Figs. 3 and 4) and, therefore, ~ooo ~. '\,~°c "~"~" changes in this parameter cannot account for strength variations. Helmuth and Verbeck (1) hypothesized also that the strength reduction in high-temperature cured pastes may have something to do with internal changes in pore structure. Feldman and I I I I I 4 ~0 GO 70 8O ~0 Beaudoin (28) suggested that difference in DEGNEE OF HY~RTUoN ~,°/b strength between cementitious systems havFIG. 8. Effect of degree of hydration and curing ing equal total porosity may be the result of temperature on threshold diameter and median diam- differences in morphological characteristics eter of the pore structure of C3S pastes detected by which involve crystal bonding and differmercury porosimeter. ence in the density of the products. The microstructural changes observed in the 3. Some Relations with Mechanical present work, which show an increase in Properties macropore volume in the 65°C paste at the There have always been attempts to cor- expense of meso- and micropore volumes, relate various mechanical properties with may support such an explanation. One would parameters related to pore structure, expect that increase in the content of larger especially total porosity. The object of this pores in the paste, as in the case at 65°C chapter is to present a brief discussion that hydration, would be detrimental to strength. indicates that a careful study of the pore The 4 and 25°C pastes which have similar structure may be useful in explaining some macroporosity have similar strength. aspects of mechanical properties which are Changes in pore structure may also have not always well understood. It should be an influence on the reduction in drying emphasized that there is no intention here shrinkage of the 65°C cured C3S paste (8, 9). of presenting a comprehensive treatment of There is reason to believe that shrinkage the relations between mechanical proper- is the result of processes taking place within ties and pore structure. the finer pores of the system: Drying of •

Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

559

SILICATE PASTE PORE STRUCTURE

water from the mesopores causing capillary effects (19), and from the micropores inducing shrinkage strains which may be the result of mechanisms such as changes in disjoining pressure (19) and interlayer space (20). On this basis it would be expected that the volume and size distribution of the microand mesopores would have an effect on shrinkage. In Table II the volume content of these pores and drying shrinkage strains of well-hydrated C3S pastes cured at 4, 25, and 65°C are presented. It can be seen from this table that an increase in curing temperature from 25 to 65°C is associated with approximately a 50% reduction in micro- plus mesoporosity and in drying shrinkage strains. However, this is not the trend in the 4°C paste. It has a little more micro- plus mesoporosity compared to the 25°C paste but it shrinks almost four times as much. Therefore, the notion of the relation between the pore structure and shrinkage may explain the variations between the 25 and 65°C paste, but not between the 4 and 25°C. Probably other factors, such as size distribution of the microand mesopores and silicate structure (5), must be taken into account, in addition to variations in pore volume. V. CONCLUSIONS

(1) The 4 and 25°C pastes have similar pore structure, although not identical. (2) Increase in curing temperature to 65°C results in coarsening of the pore structure which is exhibited by an increase in macropore volume at the expense of mesopore and micropore volumes. These changes are not accompanied by changes in total porosity, suggesting that the shifts in pore structure are the result of rearrangement of the growing hydration products rather than change in their volume. (3) The shift in pore structure in the 65°C paste is also indicated by larger threshold and median diameters of the porosity detected by mercury porosimeter.

TABLE II Shrinkage and Volume of Micropores plus Mesopores of Well-Hydrated C3S Pastes Cured at 4, 25, and 65°C Curing temperature (°C)

Degree of hydration (%)

Drying shrinkage (%)

Micro- plus mesopososi(y (cm*/g)

4 25 65

86 88 87

0.720 0.197 0.110

0.204 0.185 0.085

(4) The median diameters of the macropores are not sensitive to curing temperature but they decrease with increased hydration. (5) The median diameters of the mesopores obtained by mercury porosimeter and HzO adsorption are not very sensitive to curing temperature and degree of hydration. The N2 adsorption measurement indicates, on the other hand, reduction in this diameter at 65°C curing. (6) The larger macroporosity of the 65°C paste may account for its lower strength. (7) The 65°C paste exhibits approximately 50% reduction in drying shrinkage and in mesopore plus micropore volumes. These two effects may be related. On the other hand, the 4°C paste has only slightly more porosity in this range compared to the 25°C paste, but it shrinks almost four times as much. This indicates that other factors besides the content of micro- plus mesopores may exert significant influence on shrinkage. ACKNOWLEDGMENTS Part of the experimental work was carried out at the School of Civil Engineering, Purdue University and at the Department of Civil Engineering, University of Illinois at Urbana-Champaign, while the author was visiting there. The hospitality, assistance, and useful discussions with Professors S. Diamond, R. L. Berger, and J. F. Young of these universities are gratefully acknowledged. The author is a "Bat Sheva Fellow" at the Technion, Israel Institute for Technology. The support of Bat Sheva de Rotschield Foundation for the Advancement of Science and Technology is appreciated. Journal of Colloid and Interface Science, Vol. 74, No. 2, April 1980

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