Intrinsic strength and microstructure of hydrated C3S

CEMENTand CONCRETERESEARCH. Vol. 6, pp. 583-590, 1976. PergamonPress, Inc. Printed in the United States.

INTRINSIC STRENGTHAND MICROSTRUCTURE OF HYDRATEDC3S A. Bentur(x) Faculty of Civil Engineering, Technion Israel Institute of Technology Haifa, Israel (Refereed) (Received March l , 1976; in final form May 3, 1976)

ABSTRACT In previous work i t was shown that addition of gypsum to hydrating C3S changes the intrinsic strength of the CSH gel. In the present work an attempt was made to find whether this change could be correlated with variations in pore structure, measured by nitrogen adsorption. The results indicated that at the stage where intrinsic strength is independent of gypsum content (about 40% hydration) the analysis of the adsorption and desorption curves could not reveal any changes in the microstructure. At the stage where the intrinsic strength decreases with increase in gypsum content (about 60% hydration) the pore analysis indicated changes in microstructure. The pure C~S could be described better by a cylindrical pore model while the highest gypsum content paste was better accounted for by a parallel plate model. In frUheren Arbeiten wurde gezeigt, dass die Beigabe von Gips zu hydratisierendem C3S die innere Starke von CSH Gel ver~ndert. In dieser Untersuchung wurde versucht herauszufinden ob die Ver~nderung der St~rke mit den Ver~nderungen der Porenstruktur, mit Nitrogen Adsorption gemessen, in Verbindung gebracht werden kann. Die Ergebnisse zeigen, dass die Analyse der Adsorption-und Desorptions Kurven keine Ver~nderungen der Mikrostruktur zeigten wenn die inhere St~rke nicht vom Gipsinhalt (ungefahr 40% Hydration) beeinflusst ist. Wenn die innere St~rke durch die Vergr6sserung des Gipsinhaltes (ungefahr 60% Hydration) abnahm, zeigte die Mikrostruktur Ver~nderungen. Das reine C3S wird besser mit einem zylindrischen Poren~dell dargestellt besch~ieben wahrend der pasten mit dem hBchsten Gipsinhalt besser mit einem parallel plate n~del veranschaulicht beschrieben wird (X)Present address: Departn~nt of Civil Engineering, University of Illinois at Urbana, I l l i n o i s 61801. 583

584

Vol. 6, No. 4 A. Bentur

Introduction In previous work(l ) i t has been shown that addition of gypsum to hydrating C~S paste accelerates the rate of hydration and changes the i n t r i n sic strengtB of the CSH gel formed. The variations in intrinsic strength are accompanied by changes in the C/S ratio of this gel. In Figure l , part of the results are presented, showing the dependence of strength and C/S mole ratio on the gypsum content of 40% and 60% hydrated pastes. Since in this figure a comparison is made between strength of pastes hydrated to the same extent, the variations in strength reflect changes in intrinsic strength. At 40% hydration, the C/S ratio and strength are practically independent of gypsum content. At 60% hydration there is a trend for increase in C/S ratio and decrease in strength with increase in gypsum content.

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FIG. l The effect of gypsum on strength and C/S mole ratio of 40% and 60% hydrated C3S pastes. I t was suggested{l~"" that the variations in intrinsic strength can be the result of changes in the C/S mole ratio of the paste. Another possible explanation may be based on variations in pore structure which could result from the interaction of gypsum with the CSH gel. The present work was designed to investigate this possibility. Two methods of pore analysis were to be employed for this purpose, nitrogen a~sorption for the determination of the low range of pore sizes (up to ca. 300 A) and mercury intrusion porosimetry for the bigger pores. In the present paper the results of nitrogen adsorption measurements are presented. The choice of nitrogen adsorption to measure pore size distribution lead inevitably to problems of interpretation, since the pore volume determined by adsorption methods depends on the vapor used. The pore structure of hydra-. ted calcium silicates has been studied extensively by many investigators(2)'{5) and i t is well established that there is a big difference between surface area and pore volume measured by nitrogen and water adsorption. Feldman and Sereda(5) have provided a model of hydrated cement, based on the premise that the nitrogen analysis will yield the actual pore structure, since interlayer water ~ t o r t s water adsorption results. According to Brunauer and co-workers~Qj however, nitrogen penetrates to part of the pores

Vol. 6, No. 4

585 C3S HYDRATION, STRENGTH, PORESTRUCTURE

only, and therefore, the meaning of pore structure determined by nitrogen adsorption is unclear. They proposed that the inability of nitrogen to penetrate into all the pores is mainly ~ result of their morphology which is of ink bottle shape. Other studies( ? ) have linked changes in nitrogen adsorption results to morphological characteristics of CSH, whereas water adsorption is essentially constant. Thus, even in view of Powers-Brunauer model i t could be expected that nitrogen adsorption would be sensitive to microstructural changes, although the character of this change could not be determined from the nitrogen analysis. The choice of nitrogen adsorption is based on its sensitivity to microstructural changes, even though doubt exists as to the validity of nitrogen measurement. The results based on this technique are however to be interpreted on a comparative, rather than absolute basis, and therefore do not require selection of any "true" surface area or pore volume model. Experimental The experimental program consisted of analyzing the pore structure of O, 4 and 9% gypsum pastes hydrated to about 40% and O, 2, 4 and 9% gypsum pastes hydrated to about 60%. In the former case the intrinsic strengths are similar (Figure l) while in the latter this strength tends to increase with decrease in gypsum content. I t could be expected that pore structure analysis of pastes of similar and of different intrinsic strength might serve as a basis for evaluating the importance of pore structure in controlling strength. Mixtures of C3S and gypsum were paste hydrated at 0.43 water cement ratio, at 20° C. At varlous ages samples were tested for strength and chemical constitution by X-ray diffraction The me)b9ds employed for paste preparation, testing and drying are described ~IsewheretIy. The adsorption and desorption of nitrogen was carried out by the dynamic method using the Quantasorb of Quantachrome Corporation. Adsorptio)^, and desorption curves were obtained by a procedure described by Karp et al.£ vJ With this instrument i t is impossible to determine the amount of gas adsorbed at saturation. The highest measurement made was at 0.97 relative pressure. Degassing was carried out by purging the powder sample with a pure stream of nitrogen at lO0° C. General

Results and Discussion

Two methods of pore structure analysis were employed for the interpretation of the adsorption and desorption curves. The f i r s t one was based on the calculation of pore size distribution and cumulative surface area assuming that the adsorption and desorption are the result of two mechanisms: physical adsorption on the pore walls and capillary conden)@~on in the inner capillary volume. The t curve published by Lippesn et al. t~uj was used for the f i r s t mechanism. The calculation of the capillary condensation is based on the Kelvin equation. For that purpose, an assumption has to be made regarding the shape of pores. Comparison between calculated cumulative surface area assuming different pore structures and the BET surface area can serve as a means for determining which pore model better f i t s the internal structure. A detailed description of this method was given by Linsen and van dan Heuvel. ( l l ) I t is shown there that for a parallel plate structure the calculated cumulative surface area from the adsorption curve will be smaller than the true surface area, while that calculated from the desorption curve will be

586

Vol. 6, No. 4 A. Bentur

equal to i t . In the case of cylindrical pores the situation is reversed: The adsorption calculated surface area is equal to the true surface area, while the desorption calculated area is larger. The second me~R~ employed is the t method which was f i r s t suggested by Lippens and ~¢.Boer.£'~/ A summary of this method was given by Linsen and van den Heuvel.£l l ) The adsorption results can be drawn in the form of v-t curve, (v-volume adsorbed at a certain relative pressure; t-thickness of adsorbed layer at the same relative pressure). At small values of t , the v-t curve is a straight line, indicating that only the physical adsorption mechanism is active. The surface area can be calculated from the slope of this line. At higher values of t deviation from this line may take place; upward deviation indicates the existence of pores where capillary condensation can occur, such as in cylindrical pores. Downwarddeviation may be the result of a structure which consists of small or parallel plate shaped pores. In the present discussion the results will be analyzed in terms of two models: parallel plate structure and cylindrical pores. An attempt w i l l be made to find which of them accounts better for the adsorption-desorption curves. Cumulative Surface Area Analysis In Table I, the experimental and calculated surface areas (m2 per gm. of original C3S) are presented. The gypsum content of the pastes is given in column 2 and the degree of hydration of the C3S in column 3. Specimens l , 2 and 3 hydrated to a similar extent (about 40% hydration). All of them exhibited similar intrinsic strength (Figure l ) . Samples 4 to 7 hydrated to about 60% In this case the pure C3S has the highest intrinsic strength and the 9% gypsum paste has the lowest. The BET surface areas, SBET, and those obtained from the v-t curves, St , are presented in columns 4 and 5, respectively. The deviations between these two values is about I0% for samples I to 4 and about 2% for samples S to 7. The calculated cumulative surface areas based on the adsorption curves are given in column 6 (for cylindrical model) and column 7 (for TABLE I Calculated and experimental surface areas

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Vol. 6, No. 4

587 C3S HYDRATION, STRENGTH, PORESTRUCTURE

parallel plate model). In columns 8 and 9 the desorption calculated cumulative surface areas are presented: Column 8 for cylindrical model and column 9 for parallel plate model. I t can be noted that for the 40% hydrated samples (No. l to 3) both models give similar results which are in about I0% range from the BET surface areas (comparison of columns 4, 6 and 9). Thus, both models account equally well for the pore structure. I f at this stage of hydration there are any variations in the pore structure, they cannot be detected by this method of analysis. In the 60% hydrated samples (No. 4 to 7) the surface areas calculated on the basis of the cylindrical model are in good agreement with the BET areas of the O, 2 and 4% gypsum pastes: The adsorption calculated surface areas (column 6) are within 3% from the BET areas (column 4), while the values of desorption calculated areas (column 8) are higher. In the case of 9% gypsum paste the area calculated for cylindrical model deviates about 25% from the BET area. The parallel plate model yields better agreement in this case (column 9) but even here the difference is s t i l l great (ca. 17%). v-t Curves v-t curves of 40% and 60% hydrated pastes are presented in Figures 2 and 3, respectively. From Figure 2 i t can be observed that there is similari t y in the curves of the 40% hydrated pastes. Generally, all of them follow closely to the straight line typical to the low range t-values, indicating that the active mechanism is physical adsorption. This behavior might be the result of parallel plate structure with large spacing between plates, or some other kind of large pores where capillary condensation occurs only at high values of relative pressure. The similarity in this case is not inconsistent with the results of the cumulative surface area analysis. A

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588

Vol. 6, No. 4 A. Bentur

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FIG. 3 v - t curves of 60% hydrated pastes Comparison of the v-t curves of the 60% hydrated pastes (Figure 3) shows that the pore structure is being changed due to the presence of gypsum. The pure C3S curve deviates upward, at about t = 7 X while that of the 9% gypsum deviates downward at about t = 6 A. The former behavior which is a r e s u l t o f capillary condensation might indicate the existance of cylindrical pores while the l a t t e r behavior may be due to parallel plate structure. These conclusions are generally in agreement with the cumulative surface area analysis. Cylindrical pore structure f i t t e d the pure C~S while parallel plate model gave better agreement in the case of 9% gypsum paste. The v-t curves of the 2 and 4% gypsum pastes (Figure 3) indicate that their pore structure is intermediate between the two extremes. At f i r s t , there is upward deviation similar to that of pure C3S but later on a downward deviation becomes dominant, as in the case in 9% gypsum paste. Such an intermediate characteristic was not exhibited in the cumulative surface area analysis: The 2 and 4% gypsum pastes are well accounted for by a cylindrical pure model which is also typical to pure C3S paste. Specific Surface Area In Figure 4, the BET specific surface areas (m2 per gm. hydrated C~S) of the 40% and 60% hydrated pastes are plotted against the gypsum content. A~ 40% hydration this area is independent of the gypsum content. This is in agreement with the conclusion derived from the former analysis that showed no indication of variation in pore structure at this stage.

Vol, 6, No, 4

589 C3S HYDRATION, STRENGTH, PORESTRUCTURE

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The 60% hydrated pastes exhibit a trend for increase in specific surface area with increase in gypsum content. Such variations which might reflect changes in pore structure are not inconsistent with the conclusions derived from the two former methods of analysis which.indicated that such changes occur. Collepardi and Marchese(7)and Skalny et al.(8) found that a similar trend for increase in the nitrogen measured sure'ace'-area 9~urs when C3S is hydrated in the presence of CaCl . Collepardi and Marchese~I ; showed that this increase is associatedwithmorp~olog~ca~ ~ e ) ~ t ~ e P ~ S ~ ~l~st~ a z" the form of crumpled f i i Sc ~t~ by a rolled sheet structure. They suggested that intrusion of nitrogen into the space in the rolled sheets is more d i f f i c u l t and this might explain the lower specific surface area. ) i ~ l a r explanation might be applied to the present results: Copelandet al.tm~; reported that the morphology of PUre C~S gel can be described by f i b r e s structure of rolled sheets, while in the presence of gypsum small platelets are predominant. The fact that parallel plate model was found to f i t better the pore structure of 9% gypsum paste may be the result of the existance of such platelets. The higher specific area of the 9% gypsum paste may thus be explained on the basis of this morph919gical change, using the same argument suggested by Collepardi and Marcheset~) for the case of CaCl2. Conclusions The various methods of interpretation of the nitrogen adsorptiondesorption curves show that in the presence of gypsum there is a trend for change in the pore structure of 60% hydrated pastes from one that can be described by a model of cylindrical pores to a structure which can be better accounted for by a parallel plate model. Yet, the different methods of interpretation do not lead to the same conclusions regarding the course of this change with the increase in gypsum content. This is probably due to the fact that two very simplified models were employed for the characterization of the complex pore structure of the CSH gel. An interesting feature is the fact that in the stage where intrinsic compressive strength is independent of gypsum content (40% hydration) the nitrogen pore analysis could not detect any differences in structure while at the stage where increase in 9ypsum content was followed by decrease in intrinsic strength (60% hydration), the pore analysis showed that changes in the internal morphology occur. These general parallel trends indicate that pore shape may be a factor influencing intrinsic strength. However, since the various methods of pore analysis do not agree on the course of change of pore structure at 60% hydration, i t is in doubt whether these parallel trends can serve as a basis for evaluating the importance of pore structure in controlling intrinsic strength.

590

Vol. 6, No. 4 A. Bentur

The present pore analysis is limited to the smaller size pores which are detected by nitrogen adsorption. In view of the doubt as to the validity of this technique i t should be emphasized that the results should be interpreted on a comparative basis. The possible effect of larger pores which can be measured by mercury intrusion porosimeter(14) is a subject for further work. Acknowledgment The author wishes to thank Bat Sheva de Rotshield Foundation for the Advancement of Science in Israel for supporting this work. Part of the experi mental work was carried out at the laboratories of the Israel Ceramic and Silicate Institute. The hospitality of the Institute is gratefully acknow, ledged. References I.

A. Bentur, "The Effect of Gypsum on the Hydration and Strength of C3S Pastes," to be published in J. Amer. Ceram. Soc.

2.

S. Brunauer, D. L. Kantro and C. H. Weise, Canadian J. of Chem, 3._77,714,

(1959). 3.

R. Sh. Mikhail, L. E. Copeland and S. Brunauer, Canadian J. Chem., 42,

426, (1964). .

J. Hagymassy, I. Odler, M. Yudenfreund, J. Skalny and S. Brunauer, J. Coll Inter. Sci.o 38, 20 (1972).

5.

R. F. Feldman and P. J. Sereda, Materiaux et Constructions, ~, 509, (1968)

6.

S. Brunauer, I. Odler and M. Yudenfreund, Highway Research Board Record, No. 38, 89, (1970).

7.

M. Collepardi, and B. Marchese, Cem. Concr. Res., 2_, 57, (1972).

8.

J. Skalny, I. Odler and J. Hagymassy, J. Colloid and Interface Sci., 35, 434, (1971).

9.

S. Karp, S. Lowell and A. Mustacciuo, J. Anal. Chem., 44, 2395, (1972).

lO.

B. C. Lippens, B. C. Linsen and J. H. deBoer, J. Cat., 3_, 32, (Ig69).

II.

B. G. Linsen, and van dan Heuvel, "Pore Structures," from The Solid Gas Interface, E. Alison Flood (ed.), Marcel Dekker, Inc., V. I f , Chap. 35, 1025-I053 (1967).

12.

B. C. Lippens, and J. H. deBoer, J. Catal., 4--, 319, (1965).

13.

L. E. Copeland, E. Bodor, T. N. Chang and C. H. Weise, J. Portland Cem. Ass. Res. Develop. Lab., 9_, I , 61, (1967).

14.

S. Diamond, Cem. Concr. Res., l_, 531, (1971).