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Pore structure of hydrated calcium silicates
Pore Structure of Hydrated Calcium Silicates I. Influence of Calcium Chloride on the Pore Structure of Hydrated Tricalcium Silicate J A N S K A L N Y , 1 I V A N O D L E R , AND J U L I U S H A G Y M A S S Y , JR. Institute of Colloid and Surface Science and Department of Chemistry, Clarkson College of Technology, Potsdam, New York 13676
Received August 13, 1970; accepted November 11, 1970 Wide pore and micropore analyses have been performed on tri-calcium silicate hydrated from 6 to 672 hours with and without calcium chloride as admixture. Samples were prepared by using a water to solid ratio of 0.33 on a weight basis. Two per cent of calcium chloride was added to some of the samples. Samples were hydrated at 25°C. Results obtained by water vapor and nitrogen adsorption were compared. It was found that changes in calcium silicate hydrate, due to hydration in the presence of calcium chloride, influence mainly the adsorption of nitrogen. In chloride-containing 28-day samples, nitrogen can penetrate 22% of the surface available to water vapor; in chloride-free samples, the surface available to nitrogen is only 5.5% of the water vapor surface. INTRODUCTION
the analysis of micropores t determines the entire width of the pore (8, 9). Calcium chloride is known to accelerate the hydration of calcium silicates, and its presence influences the physical characteristics of the products obtained. I n a recently presented paper, the authors showed t h a t the addition of calcium chloride to CasSiO5 influences not only the degree of hydration at a given point in time but, at the same time, the morphology of the main hydration product, a calcium silicate hydrate (10). Electron micrographs have shown t h a t the presence of calcium chloride changes the "spicule" morphology of the calcium silicate hydrate to one resembling thin crumpled sheets. Differences in the morphology of the other hydration product, calcium hydroxide, due to the presence of calcium chloride were found recently (11). The influence of these different morphological types on the physical properties of calcium silicate hydrate is not fully understood. However, the strength of cement pastes, when measured for the same degree of hydration, decreases considerably with the addition of calcium chloride (12).
Recently published papers have dealt with the pore size distributions of hydrated calcium silicates and portland cements using both nitrogen and water v a p o r as adsorbates (1-3). I n some other papers, results were given for tricalcium silicate, Ca3SiOs, which had been hydrated in the presence of calcium chloride (4-6), and for which nitrogen was used as the adsorbate. As far as the authors are aware, no detailed water v a p o r data are available in the literature on the pore structure of calcium silicates which had been hydrated to various degrees in the presence and in the absence of calcium chloride. H a g y m a s s y et al. and Mikhail el al. have recently published the t-curves necessary for the use of water v a p o r as the adsorbate, i.e., curves which give the statistical thickness of the film adsorbed on nonporous adsorbents as a function of relative pressure (2, 7). I n the analysis of wide pores, the thickness t represents a correction term; in 1 Present address: American Technical Center, Riverside, California 92502. Journal of Colloid and Interface Science, Vol. 35, No. 3, ~ a r c h 1971
434
P O R E S T R U C T U R E OF C A L C I U M S I L I C A T E S
0 to 1.0. A minimum of two weeks was allowed between the successive measurements to ensure equilibrium. The desiccator method was used (14). For the samples hydrated with and without calcium chloride for a period of 28 days, complete nitrogen adsorption-desorption isotherms were obtained. A commonly used BET apparatus was used for the measurements. The t-curve of Lippens et al. was used in the wide pore analysis (15), as the BET C-constants for both pastes were close to the value used by them. In both cases the "corrected modelless" and MP methods were used for pore structure analysis (8, 9). In the calculations of wide pores, an analytical method of calculation replaced the graphical integrations (16).
Because the calcium silicate hydrate is the main factor determining the physical properties of hydrated cement paste, the results indicate that calcium chloride has a negative influence on strength. Calcium silicate hydrate gel produced in the presence of 2 % of calcium chloride is considered by some authors to be outside the group of stable "tobermorite gels" (13). This paper presents the results of a study of the influence of calcium chloride upon the pore structure characteristics of calcium silicate hydrates as a function of hydration time and degree of hydration, using water vapor and nitrogen as adsorbates. EXPERIMENTAL
The pore structures of 10 partly hydrated tricalcium silicate pastes were analyzed for wide pores and micropores: four Ca3SiQ pastes hydrated for 1, 3, 10, and 28 days, and six Ca3SiO5 pastes with an addition of 2 % of calcium chloride hydrated for 6 and 12 hours, 1, 3, 10, and 28 days. All samples were mixed and hydrated at 25°C with a water to solid ratio of 0.33 in CO2-free atmosphere. More details are given elsewhere (10). The hydration was stopped by outgassing the samples to a constant weight at the vapor pressure of ice at -78°C, 5 X 10-4 mm Hg. Complete water vapor adsorption isotherms were obtained in the range p / p ~ =
RESULTS AND DISCUSSIONS From the BET region of the isotherms linear BET plots were obtained. The results are given in Table I, column 7. The molecular area of adsorbed water vapor was taken to be 11.4 A9. Degrees of hydration, determined from the ignition loss of D-dried samples (17), and final uptakes V, in milliliters per gram of D-dried paste, are given in columns 4 and 5, respectively. With the use of the available t-curves St values were obtained. Those giving the best agreement with SBET values are given in column 6. In all cases the o
TABLE
VAPOR SURFACE
WATER
435
I
AREAS
OF CAaSIO5
SAMPLES
Hydration No. 1
1 2 3 4 5 6 7 8 9 10
Sample 2
C~SiO5
Ca~SiOs -I2% CaCI2
BET H20a V~ (ml/gm)
St (me~gin)
Time
Degree
Paste
3
4
5
6
7
1 day 3 days 10 days 28 days 6 hr 12 hr 1 day 3 days 10 days 28 days
0.286 0.384 0.535 0.645 0.600 0.648 0.698 0.724 0.831 0.864
0.274 0.254 0.225 0.200 0.212 0.200 0.197 0.188 0.167 0.160
31 68 99 140 102 110 114 124 143 173
43 79 115 143 112 137 135 141 161 174
CaSo
C~Sh
TG
8
9
10
45 84 127 161 125 157 154 161 187 209
157 218 237 249 209 242 221 222 225 236
182 270 317 324 230 261 253 258 259 270
(m2/gm)
BETu~o surface areas are given in square meters per gram of D - d r i e d paste (Paste), total CasSiO5 in the paste (C~S0), h y d r a t e d p a r t of Ca~SiOs(C3SH), and calcium silicate h y d r a t e or t o b e r m o r i t e gel (TG). Journal of Colloid and Interface Science, Vol. 35, No. 3, March 1971
436
SKALNY, ODLER, AND HAGYMASSY TABLE II WATER VAPOR PORE STRUCTURE DATA. OF CA3SIO~ PASTES CALCULATED BY '~CORRECTED MODELLESS" AND " M P " METHODS
Wide pores No.
Sample
1
2
1 2 3 4 5 6 7 8 9 10
Ca~Si05
Ca3SiOs + 2% CaC12
Micro pores
V (ml/gm)
S (m2/gm)
V (ml/gm)
S (m~/gm)
V (ml/gm)
3
4
5
6
7
8
26.5 32.8 38.0 41.6 50.1 52.4 53.4 51.9 51.6 44.5
0.2628 0.2399 0.2005 0.1587 0.1887 0.1771 0.1778 0.1575 0.1295 0.1123
7.0 28.5 44.0 88.0 71.0 79.0 77.0 87.0 98.0 126.0
0.0027 0.0110 0.0173 0.0343 0.0276 0.0296 0.0283 0.0332 0.0376 0.0503
33.5 61.3 82.0 129.6 121.1 131.4 130.4 139.8 149.6 170.6
0.2655 0.2509 0.2178 0.1930 0.2163 0.2067 0.2061 0.1907 0.1671 0.1623
St values were lower than the SEET values. With increased degrees of hydration, the agreement between St and SBET values improved. Columns 8, 9, and 10 show the surface areas in square meters recalculated per gram of CasSi05, hydrated Ca3SiOs, and calcium silicate hydrate (or tobermorite gel), respectively. Because of higher degree of hydration at equal ages, higher BET~2o surface areas were obtained for samples contalning calcium chloride, when measured per unit of original Ca3SiQ present, but equal or slightly lower surfaces were obtained, when the calculations were based on the hydrated part of the Ca3SiQ. The low surface area of the chloride-free one-day sample is probably due to the very high lime to silica ratio of the calcium silicates produced in the early stages of hydration, as was shown elsewhere (a0). The wide pore analysis was performed using the adsorption branch of the isotherm above p / p ~ - - 0.45, which was the value at which the correction term (8) il ti
~
ASI
Total
S (ra~/gm)
(corr.)
1
became equal to or larger than the volume adsorbed V~. This indicates that, at lower relative pressures, in narrower pores, only adsorption takes place, and not capillary condensation. For the calculations of pore volumes and pore surfaces both parallelplate and cylindrical models were used. Because the cylindrical model gave results Journal of Colloid and Interface Science, Vol. 35, No. 3, lY[areh_~1971
in better agreement with both SBET and V~ values, only these results are presented in Table II, columns 3 and 4. Micropore analysis was performed up to the point on the V z - t plot at which the curves begin to deviate upwards, showing the beginning of capillary condensation in wide pores. Results obtained by analyzing the micropores are given in Table I1, columns 5 and 6. The total cumulative surfaces and volumes are given in columns 7 and 8. All given surfaces and volumes are expressed per gram of CasSiO5 paste. Cumulative surface results gave a very good agreement with SEETresults for samples hydrated in the presence of calcium chloride. The agreement for chloride-free samples was not as good. This may be due to a delay in the meniscus formation during adsorption-a phenomenon not infrequently encountered. The delay would lead to too low cumulative surface areas. A comparison of the hydraulic radii rh, defined as volume V of the pore system divided by its surface area S, is shown in Fig. 1. Both samples hydrated with and without the addition of calcium chloride have a micropore system with an average ~ of roughly 4 A. This average hydraulic radius remains constant as the hydration progresses. This suggests it may be a structural characteristic of one of the products of hydration, most probably of calcium silicate hydrate. If this suggestion is correct, then the volume of micropores should be propor-
437
P O R E S T R U C T U R E OF C A L C I U M S I L I C A T E S
tional to the degree of hydration. This appears to be valid for the chloride-containing samples, but it is not so for the chloride-free samples. One possible reason for this differenee may be the change with the degree of hydration of the CaO/SiO2 ratio of tobermorite gel in the chloride-free samples. It was found that in the chloride-free samples the lime-silica ratio decreases from a high starting value to a final value of about 1.5. On the other hand, the lime-silica ratio of samples containing calcium chloride did not change in the whole range of observation. This indicates that, in the chloride-containing samples, a lime-rich intermediate product was not formed at all, or that it was rapidly replaced by a product the composition of which did not vary in the later stages of hydration (10). '-
I
I
I
I
I
90--
7O
50 o,~ 50-
~
10-
-
-
MtCRO
.3
°1
I o
I"
.4 ,5 .6 .7 .8 DEGREEOFHYDRATION
FIG. 1. Hydraulic radius of h y d r a t e d Ca3SiO5 ea!eulated from the adsorption side of w a t e r vapor isotherms. E x p e r i m e n t a l points: © Ca3SiQ; O Ca3SiO5 -~ 2% CaCI~.
The continuity of the hydraulic radii versus degree of hydration plot suggests that, as far as water vapor adsorption is concerned, there is no significant difference between the chloride-free and chloride containing samples other than can be attributed to the degree of hydration. Both the total and wide pore hydraulic radii decrease with the degree of hydration. The fact that both chloride-free and chloridecontaining samples show similar total hydraulic radii in spite of very different fractions of micropores present, indicates some differences in pore structure. For example, Samples 4 and 6 have equal degrees of hydration. The total hydraulic radius for Sample 4 is 14.2 A_ and that for Sample 6 is 15.7 A. In Sample 4 about 30 %, in Sample 6 about 40 %, of the surface is located in wide pores. The two Ca3SiO5 samples hydrated in the absence and in presence of 2 % of calcium chloride for 28 days were analyzed by nitrogen adsorption-desorption. The results were different from those obtained with water vapor in that no micropores were found, as the vt - t plots did not show a downward deviation (15). Therefore, the "corrected modelless" method gave the complete analysis of the structure of the pores available to nitrogen. The results obtained are summarized in Table III. The sample hydrated in the presence of calcium chloride had almost five times as large a nitrogen surface area per unit of unhydrated Ca3SiOs, and almost four times as large a surface area per unit of the hydrated part of Ca3SiOa. This is unlike the water vapor results, which show a very similar surface area for both samples hydrated in the absence and in the presence of CaC12. Assuming that the free Ca(OH)2 has a negligible surface area, a 3.2-fold increase in nitrogen surface area of the tobermorite gel was found for the sample
TABLE III NITROGEN SURFACE AREAS OF CASSIO5 SAMPLES HYDRATED FOR 28 DAYS No.
4 10
Sample
Ca3SiO5 Ca3SiO5+ 2% CaC],
Degree of
hydration
0. 645 0.864
BETN'2 (m~/gm)
Vs (ml/gm)
St (m2/gm)
Paste
0.042 0.120
7.4 37.7
7.9 37.8
C~So 8.9 44.3
C~Sh
TG
13.8 51.3
20.9 66.9
Journal of Colloid and Interface Science, Vol. 35, No. 3, March 1971
438
SKALNY, ODLER, AND HAGYMASSY TABLE IV
Similar differences in surface areas of chloride-free and chloride-containing samples were obtained by Celani et al. (4), who reported for samples hydrated to the same degree without and with 2% of calcium Adsorption nesorptlon chloride a nitrogen surface area of 15.2 and No. Sample 78.9 m~/gm of paste, respectively. Collepardi V S V S (ml/gm) :r~2/gm) (ml/~m) (~nVgm) et al. (5, 6) published similar results. Distribution of the surface area in the 4 CasSiOs O.0414 6.33 0.0481 15.58 wide pores of the hydrated part of Ca3SiQ 10 CasSiO5-}- 0.1144 32.07 0.1281 51.45 samples hydrated for 28 days is presented in 2% Fig. 2; on the assumption that all the pores CaCI2 are associated with the products of hydration, the distributions are based on the D-dry hydrated in the presence of CaC12. This weights of hydrated material. Curve 1 was increase suggests that nitrogen molecules constructed from water vapor data and can penetrate the calcium silicate hydrate curves 2 and 3 from nitrogen data. All formed in the presence of calcium chloride curves were constructed from adsorption more easily, or that the structure is more results, with the use of a cylindrical pore model. The black points show the distribu"open." It was shown by Brunauer and coworkers tion of the surface area in samples hydrated (i) that for samples of Ca2SiO4 and cement in the presence of calcium chloride. Water pastes prepared with low water to solid vapor gives for CasSiO5 hydrated both in ratios, the adsorption branches of the the absence and in the presence of calcium nitrogen isotherms gave better results. The chloride almost the same distribution with a desorption branches gave cumulative vol- sharp peak at a hydraulic radius of about umes and surfaces which were much too 9 A. The calcium chloride-containing sample high. The same was found in the present analyzed by nitrogen gives a peak at about investigation. Results obtained by using the 11 A; the chloride-free samples gives a much cylindYical pore model are given in Table IV. smaller peak at about 13 ~. Thus, the pore The Ca3Si05 pastes prepared with the low distributions using nitrogen as adsorbate are water to solid ratio of 0.33 have shown a better agreement with the BET surface areas and with V~ values when analyzed from adsorption, but even the cylindrical pore 6 model gave surface area results which were somewhat low. This indicates a slight delay in meniscus formation. The hydraulic radii of the pore systems accessible to nitrogen for pastes hydrated without and with calcium chloride were 53.3 A and 31.8 A, respectively, using V~ and SBET values. The r~ values at equal degrees of hydration were not determined, but the much larger V~ in Table III, in spite of the larger degree of hydration, is sufficient ,o 20 3'o 4S 50 to show the more "open" structure of r~,£ Sample i0. The surface area values confirm FIG. 2. Surface area distribution of 28-day-old this since nitrogen can reach almost five hydrated CaaSiOa samples. Curve I: water vapor times as large a surface in samples contain- adsorption; curves 2 and 3: nitrogen adsorption. ing calcium chloride as in samples not con- Experimental points: 0 CasSiO5;• CasSiOa -~ 2% CeCIl. taining calcium chloride. PORE STRUCTVRE DACA SY NITROGEN ADSORPTION-I)EsORPTION OF CASSIO5 PASTES HYDRATED FOR 28 DAYS
Journal of Colloid and Interface Sc@nce, Vol. 35, No. 3, ~areh 1971
PORE ST/%UCTURE OF CALCIUM SILICATES different, i t is interesting that the chloridecontaining samples analyzed by water vapor and nitrogen, and the chloride-free sample analyzed by water vapor, give identical surface area distributions for ~ores having hydraulic radii larger than 25 A. As stated earlier, electron micrographs have shown considerable differences in the morphology of the hydration products. Our work suggests that both the spicule and thin crumpled sheet types of calcium silicate hydrate are accessible to roughly equal amounts of water vapor. On the other hand, the surface area available to nitrogen in the chloride-free paste is only 25 % to 30 % of the surface area of the chloride-containing paste. Thus, apparently, nitrogen molecules penetrate into the spicules with greater difficulty than into the crumpled sheets. This, however, does not explain why, even in samples having crumpled foil morphology, the water vapor surface area is 4 to 5 times greater than the nitrogen surface. An explanation of similar observations on related systems has been put forward by Sereda et al. (18). In their work they ascribe this discrepancy to the fact that water can enter interlayer spaces of calcium silicate hydrates not accessible to nitrogen. It was shown in Table II that, with the exception of the two samples hydrated without calcium chloride for 1 and 3 days, all pastes had more than 50 % of their surface area in the mieropores. Brunauer et al. have shown that, in the absence of so-called ink-bottle pores, both nitrogen and water vapor can penetrate the entire pore system of silica gels, including mieropores (19, 20). In the case of calcium silicate pastes and portland cement pastes no micropores are accessible to nitrogen. This was attributed to the presence of ink-bottle pores, i.e., pores having narrow necks and wide bodies (1). The hydraulic radius of the pores inaccessible to nitrogen is given by: r~
=
(V~o
-
v,c~)/(s~x~o
-
S~),
and, using the values from Tables I and III, rh is 11.7 A for Sample 4, and 2.94 A for Sample 10. This shows that Sample 4 has far larger pores inaccessible to nitrogen than Sample 10. However, even Sample 10 must
439
have some ink-bottle pores. An r~ of 2.94 indicates that, for parallel-plate pores, the distance between the walls of the average o pore is 5.9 A, and for cylindrical pores the diameter is 11.8 A. The diameter of the nitrogen molecule is 3.5 A. CONCLUSIONS Water vapor and nitrogen adsorption measurements have shown that an addition of 2 % of calcium chloride not only changes the chemical composition, mechanical properties, and morphological characteristics of the hydration products of tricalcinm silicate but it also influences the pore volume and surface area distribution in the pore system. The great difference in the pore volume and pore surface available to water vapor and nitrogen is explained by the presence of ink-bottle pores, i.e., pores with constricted entrances and large bodies. The great difference in pore surface and pore volume available to nitrogen in pastes eontMning and not containing eMeium chloride suggests that the crumpled foil type of morphology is more open than the spicule type morphology. Either there are fewer ink-bottle pores in the former, or the entrances are less constrieted, or both. ACKNOWLEDGMENT The authors wish to express their great indebtedness to the National Science Foundation for the Grant GP-7746, which supported the research, and to Dr. Stephen Brunauer and Dr. G. J. C. Frohnsdorff for many helpful discussions. REFERENCES 1. BoDoR,
J., BRUNAUER,
E. E., SKAL~Y,
S.,
[{AGYMASSY,JR., J., AND YUDENFREUND, M.,
J. Colloid Interface Sci., accepted for publication. 2. MIKHAIL,R. SH., KABUL,A. M., ANDABO-ELENEIN, S. A., J. Appl. Chem. 19, 324 (1969). 3 I~AGYMASSY,JR. J., ODLE•, I., YUDENFlgEUND, M., SKALNY,J., A~D Bn~NAu~, S., J. Colloid Interface Sei., accepted for publication. 4. CELANI, A., COLLEPARDI, M., AND RIO, A., Ind. Ilal. Cemento 36, 669 (1966). 5. COLLEPARDI~ ~{[., ROSSI,
G.,
AND SPIGA,
M. C., Aead. NationaIe dei XL, Set I V
18 (1968). 6. COSL~I"AaDI,M., RossI, G. AND UsAt, G., Ind. Ital. Cemento 38, 657 (1968). Journal of Colloid an~ Interface Science,
Vol..35, No. 3, March 1971
440
S K A L N Y , ODLER, AND HAGYMASSY
7. HAGYMASS¥, JR., J., BRUNAUER, S., AND MIKHAIL, R. Sin, J. Colloid Interface Sei. 29, 485 (1969). 8. BRUNAV~a, S., MI~:E.~IL, R. Sin, AND BODOR, E. E., ,f. Colloid Sei. 24, 451 (1967). 9. MIKHAIL, R. SIt., BRUNAUER, S., AND BODOI~, E. E., J. Colloid Interface Sei. 26, 45 (1968). 10. ODLER, I., AN]:)SKALNY,J., 72nd Annual Meeting of the American Ceramic Society, Philadelphia, May 1970, to be published. 11. B~noEa, R. L., Unpublished results. 12. SKALNY,J., AND ODLER, I., Mag. Concrete Res. 19, 203 (1967). 13. KunczYK, H. G., AN1) SCHWI~TE, H. E., Tonind.-Ztg. Keram Rundschau 84, 585 (1960).
Journal of Colloidand InterfaceScier~ce,Vol. 35. No. 3, March 1971
14. K±NTRO, D. L., BRUNAUER, S., AND WEISE, C. H., Advan. Chem. Ser. 83, 199 (1961). 15. LIPPENS, B. C., LINSEN, B. G., AND D•BoER, J. It., J. Catal. 3, 32 (1964). 16. BODOR, ]~. ]~., ODLER, I., AND SKALNY, J., J. Colloid Interface Sci. 32, 367 (1970). 17. COeELAND, L. E., AND HAY~S, J. C., A S T M Bull. 194, 70 (1953). 18. FELDMAN, R. F., AN]) SEREDA, P. J., Mater. Construct. i , 509 (1968). 19. MIK~AIL, R.Stt., BRVNAV~R, S., AND BODOn, E. E., J. Colloid Interface Sci. 26, 54 (1968). 20. HAoY~assY, J~., J., AND BnUNAUER, S., J. Colloid Interface Sei., accepted for publication.
Received August 13, 1970; accepted November 11, 1970 Wide pore and micropore analyses have been performed on tri-calcium silicate hydrated from 6 to 672 hours with and without calcium chloride as admixture. Samples were prepared by using a water to solid ratio of 0.33 on a weight basis. Two per cent of calcium chloride was added to some of the samples. Samples were hydrated at 25°C. Results obtained by water vapor and nitrogen adsorption were compared. It was found that changes in calcium silicate hydrate, due to hydration in the presence of calcium chloride, influence mainly the adsorption of nitrogen. In chloride-containing 28-day samples, nitrogen can penetrate 22% of the surface available to water vapor; in chloride-free samples, the surface available to nitrogen is only 5.5% of the water vapor surface. INTRODUCTION
the analysis of micropores t determines the entire width of the pore (8, 9). Calcium chloride is known to accelerate the hydration of calcium silicates, and its presence influences the physical characteristics of the products obtained. I n a recently presented paper, the authors showed t h a t the addition of calcium chloride to CasSiO5 influences not only the degree of hydration at a given point in time but, at the same time, the morphology of the main hydration product, a calcium silicate hydrate (10). Electron micrographs have shown t h a t the presence of calcium chloride changes the "spicule" morphology of the calcium silicate hydrate to one resembling thin crumpled sheets. Differences in the morphology of the other hydration product, calcium hydroxide, due to the presence of calcium chloride were found recently (11). The influence of these different morphological types on the physical properties of calcium silicate hydrate is not fully understood. However, the strength of cement pastes, when measured for the same degree of hydration, decreases considerably with the addition of calcium chloride (12).
Recently published papers have dealt with the pore size distributions of hydrated calcium silicates and portland cements using both nitrogen and water v a p o r as adsorbates (1-3). I n some other papers, results were given for tricalcium silicate, Ca3SiOs, which had been hydrated in the presence of calcium chloride (4-6), and for which nitrogen was used as the adsorbate. As far as the authors are aware, no detailed water v a p o r data are available in the literature on the pore structure of calcium silicates which had been hydrated to various degrees in the presence and in the absence of calcium chloride. H a g y m a s s y et al. and Mikhail el al. have recently published the t-curves necessary for the use of water v a p o r as the adsorbate, i.e., curves which give the statistical thickness of the film adsorbed on nonporous adsorbents as a function of relative pressure (2, 7). I n the analysis of wide pores, the thickness t represents a correction term; in 1 Present address: American Technical Center, Riverside, California 92502. Journal of Colloid and Interface Science, Vol. 35, No. 3, ~ a r c h 1971
434
P O R E S T R U C T U R E OF C A L C I U M S I L I C A T E S
0 to 1.0. A minimum of two weeks was allowed between the successive measurements to ensure equilibrium. The desiccator method was used (14). For the samples hydrated with and without calcium chloride for a period of 28 days, complete nitrogen adsorption-desorption isotherms were obtained. A commonly used BET apparatus was used for the measurements. The t-curve of Lippens et al. was used in the wide pore analysis (15), as the BET C-constants for both pastes were close to the value used by them. In both cases the "corrected modelless" and MP methods were used for pore structure analysis (8, 9). In the calculations of wide pores, an analytical method of calculation replaced the graphical integrations (16).
Because the calcium silicate hydrate is the main factor determining the physical properties of hydrated cement paste, the results indicate that calcium chloride has a negative influence on strength. Calcium silicate hydrate gel produced in the presence of 2 % of calcium chloride is considered by some authors to be outside the group of stable "tobermorite gels" (13). This paper presents the results of a study of the influence of calcium chloride upon the pore structure characteristics of calcium silicate hydrates as a function of hydration time and degree of hydration, using water vapor and nitrogen as adsorbates. EXPERIMENTAL
The pore structures of 10 partly hydrated tricalcium silicate pastes were analyzed for wide pores and micropores: four Ca3SiQ pastes hydrated for 1, 3, 10, and 28 days, and six Ca3SiO5 pastes with an addition of 2 % of calcium chloride hydrated for 6 and 12 hours, 1, 3, 10, and 28 days. All samples were mixed and hydrated at 25°C with a water to solid ratio of 0.33 in CO2-free atmosphere. More details are given elsewhere (10). The hydration was stopped by outgassing the samples to a constant weight at the vapor pressure of ice at -78°C, 5 X 10-4 mm Hg. Complete water vapor adsorption isotherms were obtained in the range p / p ~ =
RESULTS AND DISCUSSIONS From the BET region of the isotherms linear BET plots were obtained. The results are given in Table I, column 7. The molecular area of adsorbed water vapor was taken to be 11.4 A9. Degrees of hydration, determined from the ignition loss of D-dried samples (17), and final uptakes V, in milliliters per gram of D-dried paste, are given in columns 4 and 5, respectively. With the use of the available t-curves St values were obtained. Those giving the best agreement with SBET values are given in column 6. In all cases the o
TABLE
VAPOR SURFACE
WATER
435
I
AREAS
OF CAaSIO5
SAMPLES
Hydration No. 1
1 2 3 4 5 6 7 8 9 10
Sample 2
C~SiO5
Ca~SiOs -I2% CaCI2
BET H20a V~ (ml/gm)
St (me~gin)
Time
Degree
Paste
3
4
5
6
7
1 day 3 days 10 days 28 days 6 hr 12 hr 1 day 3 days 10 days 28 days
0.286 0.384 0.535 0.645 0.600 0.648 0.698 0.724 0.831 0.864
0.274 0.254 0.225 0.200 0.212 0.200 0.197 0.188 0.167 0.160
31 68 99 140 102 110 114 124 143 173
43 79 115 143 112 137 135 141 161 174
CaSo
C~Sh
TG
8
9
10
45 84 127 161 125 157 154 161 187 209
157 218 237 249 209 242 221 222 225 236
182 270 317 324 230 261 253 258 259 270
(m2/gm)
BETu~o surface areas are given in square meters per gram of D - d r i e d paste (Paste), total CasSiO5 in the paste (C~S0), h y d r a t e d p a r t of Ca~SiOs(C3SH), and calcium silicate h y d r a t e or t o b e r m o r i t e gel (TG). Journal of Colloid and Interface Science, Vol. 35, No. 3, March 1971
436
SKALNY, ODLER, AND HAGYMASSY TABLE II WATER VAPOR PORE STRUCTURE DATA. OF CA3SIO~ PASTES CALCULATED BY '~CORRECTED MODELLESS" AND " M P " METHODS
Wide pores No.
Sample
1
2
1 2 3 4 5 6 7 8 9 10
Ca~Si05
Ca3SiOs + 2% CaC12
Micro pores
V (ml/gm)
S (m2/gm)
V (ml/gm)
S (m~/gm)
V (ml/gm)
3
4
5
6
7
8
26.5 32.8 38.0 41.6 50.1 52.4 53.4 51.9 51.6 44.5
0.2628 0.2399 0.2005 0.1587 0.1887 0.1771 0.1778 0.1575 0.1295 0.1123
7.0 28.5 44.0 88.0 71.0 79.0 77.0 87.0 98.0 126.0
0.0027 0.0110 0.0173 0.0343 0.0276 0.0296 0.0283 0.0332 0.0376 0.0503
33.5 61.3 82.0 129.6 121.1 131.4 130.4 139.8 149.6 170.6
0.2655 0.2509 0.2178 0.1930 0.2163 0.2067 0.2061 0.1907 0.1671 0.1623
St values were lower than the SEET values. With increased degrees of hydration, the agreement between St and SBET values improved. Columns 8, 9, and 10 show the surface areas in square meters recalculated per gram of CasSi05, hydrated Ca3SiOs, and calcium silicate hydrate (or tobermorite gel), respectively. Because of higher degree of hydration at equal ages, higher BET~2o surface areas were obtained for samples contalning calcium chloride, when measured per unit of original Ca3SiQ present, but equal or slightly lower surfaces were obtained, when the calculations were based on the hydrated part of the Ca3SiQ. The low surface area of the chloride-free one-day sample is probably due to the very high lime to silica ratio of the calcium silicates produced in the early stages of hydration, as was shown elsewhere (a0). The wide pore analysis was performed using the adsorption branch of the isotherm above p / p ~ - - 0.45, which was the value at which the correction term (8) il ti
~
ASI
Total
S (ra~/gm)
(corr.)
1
became equal to or larger than the volume adsorbed V~. This indicates that, at lower relative pressures, in narrower pores, only adsorption takes place, and not capillary condensation. For the calculations of pore volumes and pore surfaces both parallelplate and cylindrical models were used. Because the cylindrical model gave results Journal of Colloid and Interface Science, Vol. 35, No. 3, lY[areh_~1971
in better agreement with both SBET and V~ values, only these results are presented in Table II, columns 3 and 4. Micropore analysis was performed up to the point on the V z - t plot at which the curves begin to deviate upwards, showing the beginning of capillary condensation in wide pores. Results obtained by analyzing the micropores are given in Table I1, columns 5 and 6. The total cumulative surfaces and volumes are given in columns 7 and 8. All given surfaces and volumes are expressed per gram of CasSiO5 paste. Cumulative surface results gave a very good agreement with SEETresults for samples hydrated in the presence of calcium chloride. The agreement for chloride-free samples was not as good. This may be due to a delay in the meniscus formation during adsorption-a phenomenon not infrequently encountered. The delay would lead to too low cumulative surface areas. A comparison of the hydraulic radii rh, defined as volume V of the pore system divided by its surface area S, is shown in Fig. 1. Both samples hydrated with and without the addition of calcium chloride have a micropore system with an average ~ of roughly 4 A. This average hydraulic radius remains constant as the hydration progresses. This suggests it may be a structural characteristic of one of the products of hydration, most probably of calcium silicate hydrate. If this suggestion is correct, then the volume of micropores should be propor-
437
P O R E S T R U C T U R E OF C A L C I U M S I L I C A T E S
tional to the degree of hydration. This appears to be valid for the chloride-containing samples, but it is not so for the chloride-free samples. One possible reason for this differenee may be the change with the degree of hydration of the CaO/SiO2 ratio of tobermorite gel in the chloride-free samples. It was found that in the chloride-free samples the lime-silica ratio decreases from a high starting value to a final value of about 1.5. On the other hand, the lime-silica ratio of samples containing calcium chloride did not change in the whole range of observation. This indicates that, in the chloride-containing samples, a lime-rich intermediate product was not formed at all, or that it was rapidly replaced by a product the composition of which did not vary in the later stages of hydration (10). '-
I
I
I
I
I
90--
7O
50 o,~ 50-
~
10-
-
-
MtCRO
.3
°1
I o
I"
.4 ,5 .6 .7 .8 DEGREEOFHYDRATION
FIG. 1. Hydraulic radius of h y d r a t e d Ca3SiO5 ea!eulated from the adsorption side of w a t e r vapor isotherms. E x p e r i m e n t a l points: © Ca3SiQ; O Ca3SiO5 -~ 2% CaCI~.
The continuity of the hydraulic radii versus degree of hydration plot suggests that, as far as water vapor adsorption is concerned, there is no significant difference between the chloride-free and chloride containing samples other than can be attributed to the degree of hydration. Both the total and wide pore hydraulic radii decrease with the degree of hydration. The fact that both chloride-free and chloridecontaining samples show similar total hydraulic radii in spite of very different fractions of micropores present, indicates some differences in pore structure. For example, Samples 4 and 6 have equal degrees of hydration. The total hydraulic radius for Sample 4 is 14.2 A_ and that for Sample 6 is 15.7 A. In Sample 4 about 30 %, in Sample 6 about 40 %, of the surface is located in wide pores. The two Ca3SiO5 samples hydrated in the absence and in presence of 2 % of calcium chloride for 28 days were analyzed by nitrogen adsorption-desorption. The results were different from those obtained with water vapor in that no micropores were found, as the vt - t plots did not show a downward deviation (15). Therefore, the "corrected modelless" method gave the complete analysis of the structure of the pores available to nitrogen. The results obtained are summarized in Table III. The sample hydrated in the presence of calcium chloride had almost five times as large a nitrogen surface area per unit of unhydrated Ca3SiOs, and almost four times as large a surface area per unit of the hydrated part of Ca3SiOa. This is unlike the water vapor results, which show a very similar surface area for both samples hydrated in the absence and in the presence of CaC12. Assuming that the free Ca(OH)2 has a negligible surface area, a 3.2-fold increase in nitrogen surface area of the tobermorite gel was found for the sample
TABLE III NITROGEN SURFACE AREAS OF CASSIO5 SAMPLES HYDRATED FOR 28 DAYS No.
4 10
Sample
Ca3SiO5 Ca3SiO5+ 2% CaC],
Degree of
hydration
0. 645 0.864
BETN'2 (m~/gm)
Vs (ml/gm)
St (m2/gm)
Paste
0.042 0.120
7.4 37.7
7.9 37.8
C~So 8.9 44.3
C~Sh
TG
13.8 51.3
20.9 66.9
Journal of Colloid and Interface Science, Vol. 35, No. 3, March 1971
438
SKALNY, ODLER, AND HAGYMASSY TABLE IV
Similar differences in surface areas of chloride-free and chloride-containing samples were obtained by Celani et al. (4), who reported for samples hydrated to the same degree without and with 2% of calcium Adsorption nesorptlon chloride a nitrogen surface area of 15.2 and No. Sample 78.9 m~/gm of paste, respectively. Collepardi V S V S (ml/gm) :r~2/gm) (ml/~m) (~nVgm) et al. (5, 6) published similar results. Distribution of the surface area in the 4 CasSiOs O.0414 6.33 0.0481 15.58 wide pores of the hydrated part of Ca3SiQ 10 CasSiO5-}- 0.1144 32.07 0.1281 51.45 samples hydrated for 28 days is presented in 2% Fig. 2; on the assumption that all the pores CaCI2 are associated with the products of hydration, the distributions are based on the D-dry hydrated in the presence of CaC12. This weights of hydrated material. Curve 1 was increase suggests that nitrogen molecules constructed from water vapor data and can penetrate the calcium silicate hydrate curves 2 and 3 from nitrogen data. All formed in the presence of calcium chloride curves were constructed from adsorption more easily, or that the structure is more results, with the use of a cylindrical pore model. The black points show the distribu"open." It was shown by Brunauer and coworkers tion of the surface area in samples hydrated (i) that for samples of Ca2SiO4 and cement in the presence of calcium chloride. Water pastes prepared with low water to solid vapor gives for CasSiO5 hydrated both in ratios, the adsorption branches of the the absence and in the presence of calcium nitrogen isotherms gave better results. The chloride almost the same distribution with a desorption branches gave cumulative vol- sharp peak at a hydraulic radius of about umes and surfaces which were much too 9 A. The calcium chloride-containing sample high. The same was found in the present analyzed by nitrogen gives a peak at about investigation. Results obtained by using the 11 A; the chloride-free samples gives a much cylindYical pore model are given in Table IV. smaller peak at about 13 ~. Thus, the pore The Ca3Si05 pastes prepared with the low distributions using nitrogen as adsorbate are water to solid ratio of 0.33 have shown a better agreement with the BET surface areas and with V~ values when analyzed from adsorption, but even the cylindrical pore 6 model gave surface area results which were somewhat low. This indicates a slight delay in meniscus formation. The hydraulic radii of the pore systems accessible to nitrogen for pastes hydrated without and with calcium chloride were 53.3 A and 31.8 A, respectively, using V~ and SBET values. The r~ values at equal degrees of hydration were not determined, but the much larger V~ in Table III, in spite of the larger degree of hydration, is sufficient ,o 20 3'o 4S 50 to show the more "open" structure of r~,£ Sample i0. The surface area values confirm FIG. 2. Surface area distribution of 28-day-old this since nitrogen can reach almost five hydrated CaaSiOa samples. Curve I: water vapor times as large a surface in samples contain- adsorption; curves 2 and 3: nitrogen adsorption. ing calcium chloride as in samples not con- Experimental points: 0 CasSiO5;• CasSiOa -~ 2% CeCIl. taining calcium chloride. PORE STRUCTVRE DACA SY NITROGEN ADSORPTION-I)EsORPTION OF CASSIO5 PASTES HYDRATED FOR 28 DAYS
Journal of Colloid and Interface Sc@nce, Vol. 35, No. 3, ~areh 1971
PORE ST/%UCTURE OF CALCIUM SILICATES different, i t is interesting that the chloridecontaining samples analyzed by water vapor and nitrogen, and the chloride-free sample analyzed by water vapor, give identical surface area distributions for ~ores having hydraulic radii larger than 25 A. As stated earlier, electron micrographs have shown considerable differences in the morphology of the hydration products. Our work suggests that both the spicule and thin crumpled sheet types of calcium silicate hydrate are accessible to roughly equal amounts of water vapor. On the other hand, the surface area available to nitrogen in the chloride-free paste is only 25 % to 30 % of the surface area of the chloride-containing paste. Thus, apparently, nitrogen molecules penetrate into the spicules with greater difficulty than into the crumpled sheets. This, however, does not explain why, even in samples having crumpled foil morphology, the water vapor surface area is 4 to 5 times greater than the nitrogen surface. An explanation of similar observations on related systems has been put forward by Sereda et al. (18). In their work they ascribe this discrepancy to the fact that water can enter interlayer spaces of calcium silicate hydrates not accessible to nitrogen. It was shown in Table II that, with the exception of the two samples hydrated without calcium chloride for 1 and 3 days, all pastes had more than 50 % of their surface area in the mieropores. Brunauer et al. have shown that, in the absence of so-called ink-bottle pores, both nitrogen and water vapor can penetrate the entire pore system of silica gels, including mieropores (19, 20). In the case of calcium silicate pastes and portland cement pastes no micropores are accessible to nitrogen. This was attributed to the presence of ink-bottle pores, i.e., pores having narrow necks and wide bodies (1). The hydraulic radius of the pores inaccessible to nitrogen is given by: r~
=
(V~o
-
v,c~)/(s~x~o
-
S~),
and, using the values from Tables I and III, rh is 11.7 A for Sample 4, and 2.94 A for Sample 10. This shows that Sample 4 has far larger pores inaccessible to nitrogen than Sample 10. However, even Sample 10 must
439
have some ink-bottle pores. An r~ of 2.94 indicates that, for parallel-plate pores, the distance between the walls of the average o pore is 5.9 A, and for cylindrical pores the diameter is 11.8 A. The diameter of the nitrogen molecule is 3.5 A. CONCLUSIONS Water vapor and nitrogen adsorption measurements have shown that an addition of 2 % of calcium chloride not only changes the chemical composition, mechanical properties, and morphological characteristics of the hydration products of tricalcinm silicate but it also influences the pore volume and surface area distribution in the pore system. The great difference in the pore volume and pore surface available to water vapor and nitrogen is explained by the presence of ink-bottle pores, i.e., pores with constricted entrances and large bodies. The great difference in pore surface and pore volume available to nitrogen in pastes eontMning and not containing eMeium chloride suggests that the crumpled foil type of morphology is more open than the spicule type morphology. Either there are fewer ink-bottle pores in the former, or the entrances are less constrieted, or both. ACKNOWLEDGMENT The authors wish to express their great indebtedness to the National Science Foundation for the Grant GP-7746, which supported the research, and to Dr. Stephen Brunauer and Dr. G. J. C. Frohnsdorff for many helpful discussions. REFERENCES 1. BoDoR,
J., BRUNAUER,
E. E., SKAL~Y,
S.,
[{AGYMASSY,JR., J., AND YUDENFREUND, M.,
J. Colloid Interface Sci., accepted for publication. 2. MIKHAIL,R. SH., KABUL,A. M., ANDABO-ELENEIN, S. A., J. Appl. Chem. 19, 324 (1969). 3 I~AGYMASSY,JR. J., ODLE•, I., YUDENFlgEUND, M., SKALNY,J., A~D Bn~NAu~, S., J. Colloid Interface Sei., accepted for publication. 4. CELANI, A., COLLEPARDI, M., AND RIO, A., Ind. Ilal. Cemento 36, 669 (1966). 5. COLLEPARDI~ ~{[., ROSSI,
G.,
AND SPIGA,
M. C., Aead. NationaIe dei XL, Set I V
18 (1968). 6. COSL~I"AaDI,M., RossI, G. AND UsAt, G., Ind. Ital. Cemento 38, 657 (1968). Journal of Colloid an~ Interface Science,
Vol..35, No. 3, March 1971
440
S K A L N Y , ODLER, AND HAGYMASSY
7. HAGYMASS¥, JR., J., BRUNAUER, S., AND MIKHAIL, R. Sin, J. Colloid Interface Sei. 29, 485 (1969). 8. BRUNAV~a, S., MI~:E.~IL, R. Sin, AND BODOR, E. E., ,f. Colloid Sei. 24, 451 (1967). 9. MIKHAIL, R. SIt., BRUNAUER, S., AND BODOI~, E. E., J. Colloid Interface Sei. 26, 45 (1968). 10. ODLER, I., AN]:)SKALNY,J., 72nd Annual Meeting of the American Ceramic Society, Philadelphia, May 1970, to be published. 11. B~noEa, R. L., Unpublished results. 12. SKALNY,J., AND ODLER, I., Mag. Concrete Res. 19, 203 (1967). 13. KunczYK, H. G., AN1) SCHWI~TE, H. E., Tonind.-Ztg. Keram Rundschau 84, 585 (1960).
Journal of Colloidand InterfaceScier~ce,Vol. 35. No. 3, March 1971
14. K±NTRO, D. L., BRUNAUER, S., AND WEISE, C. H., Advan. Chem. Ser. 83, 199 (1961). 15. LIPPENS, B. C., LINSEN, B. G., AND D•BoER, J. It., J. Catal. 3, 32 (1964). 16. BODOR, ]~. ]~., ODLER, I., AND SKALNY, J., J. Colloid Interface Sci. 32, 367 (1970). 17. COeELAND, L. E., AND HAY~S, J. C., A S T M Bull. 194, 70 (1953). 18. FELDMAN, R. F., AN]) SEREDA, P. J., Mater. Construct. i , 509 (1968). 19. MIK~AIL, R.Stt., BRVNAV~R, S., AND BODOn, E. E., J. Colloid Interface Sci. 26, 54 (1968). 20. HAoY~assY, J~., J., AND BnUNAUER, S., J. Colloid Interface Sei., accepted for publication.