INFLUENCE OF CEMENT COMPOSITION ON AUTOGENOUS DEFORMATION AND CHANGE OF THE RELATIVE HUMIDITY

Transcription

1 INFLUENCE OF CEMENT COMPOSITION ON AUTOGENOUS DEFORMATION AND CHANGE OF THE RELATIVE HUMIDITY Ole Mejlhede Jensen Department of Building Technology and Structural Engineering, Aalborg Univ., Denmark Abstract This paper deals with the influence of the cement composition on autogenous deformation and autogenous change of the relative humidity. The examination focuses on the clinker minerals C 3 S, C 2 S and C 3 A. The influence of the clinker minerals is examined experimentally by substituting cement with pure clinker minerals. The influence of clinker minerals on pore structure, chemical shrinkage and sensitivity to the relative humidity is summarized based on calculations and results from the literature. It is concluded that an increased C 3 A or gypsum content in the cement leads to a marked reduction in the autogenous deformation and autogenous RH-change of the cement paste. The content of C 3 S or C 2 S and the specific surface of the cement seem to have only little or no influence on these properties. 1. Introduction Even during sealed curing and at a constant temperature, a hardening cement paste will deform and the relative humidity within its pores will lower. This so-called autogenous deformation and autogenous relative humidity change (RH-change) may be so significant that the cement paste cracks if the deformation is restrained. Many mechanisms may lead to autogenous deformation and RH-change. Examples of these are expansion due to salt crystallization and lowering of the relative humidity due to dissolution of salts in the pore water. However, experiments indicate that the so-called self-desiccation and self-desiccation shrinkage caused by chemical shrinkage in the hardening cement paste are the main causes of autogenous RH-change and deformation (1). This paper forms part of an examination of the influence of different paramerters on autogenous deformation and autogenous relative humidity change. This paper focuses on the 143

2 influence of the cement composition. The influence of a number of other parameters e.g. w/c-ratio, silica fume addition, alkali content of the cement or the curing temperature has been reported previously (1-3). This knowledge is a basis for the understanding and possible control of autogenous deformation. 2. Experimental details 2.1 Materials Five different cements have been examined: 1) a reference cement consisting of pure white Portland cement, 2) the reference cement ground to a higher specific surface and 3,4,5) three synthetic cements. The synthetic cements were produced by blending 80% of the reference white Portland cement and 20% phase pure clinker minerals: C 3 S, C 2 S and C 3 A. Before blending with the white Portland cement the C 3 A was interground with 15% added gypsum in order to avoid flash set (4). This ratio of C 3 A/gypsum is typical for normal Portland cements. C 4 AF was not included in this study. The reason for this is that the typical composition of the ferrite phase in ordinary Portland cement clinkers differs markedly from that of C 4 AF. In addition, the ferrite phase is not well defined since it forms part of a solid solutions series and extensive substitution of especially Fe 3+ and Al 3+ may occur. Furthermore, the ferrite phase is, normally, a minor constituent of a Portland cement. Table 1 shows the calculated resulting cement compositions. The calculated compositions of the clinker mineral replaced cements are based on quantitative X-ray powder diffraction (QXRD) values for the reference cement. The values measured by QXRD are believed to be closer to the true composition of the cement than the Bogue calculated value. Table 1: Compositions of cements used (weight-%). According to QXRD the calcium sulfate in the white Portland cement exists as Bassanite (hemihydrate). The figure given has been converted to anhydrite. Reference cement 20% clinker mineral replacement by: Bogue QXRD C 3 S C 2 S C 3 A C 3 S 66 75± C 2 S 21 20± C 3 A ± C 4 AF CaSO ± The Blaine specific surface of the reference cement was 410±10 m 2 /kg. By ball-milling this cement for 1 hour the specific surface has been increased to 480 m 2 /kg. By weight the reference cement contains 1.96% free CaO and 0.17% Na 2 O eq. Pure clinker minerals, C 3 S (triclinic), ß-C 2 S (monoclinic) and C 3 A (cubic), were produced 144

3 by heating mixtures of constituent oxides or carbonates. The ß-C 2 S was stabilized by addition of 0.4 mol-% H 3 BO 3. After formation, the clinker minerals were ball-milled to a specific surface area of approximately 370 m 2 /kg and the C 3 A interground with gypsum to 410 m 2 /kg. According to QXRD, Nuclear Magnetic Resonance (NMR) and free lime determination by ethylene glycol extraction, the phase purity of the clinker minerals was approximately 99%. Silica fume was added as a dry powder. The specific surface of the silica fume is 21 m 2 /g (BET-method). The chemical composition by weight-% is: SiO 2 : 89.9, Fe 2 O 3 : 0.59, Al 2 O 3 : 1.17, MgO: 1.67, SO 3 : 0.56, and Na 2 O eq.: The superplasticizer is a naphthalene based dry powder. The cement paste was added 1% superplasticizer on powder weight (cement + silica fume). 2.2 Mix The ingredients were mixed in a 5 liter epicyclic mixer. The water was added in two steps in order to assure dispersion of the silica fume. A w/c of 0.30 and 20% silica fume addition was used. The silica fume addition is given as percentage of the cement weight. The w/c is by weight, and does not include silica fume. The temperature of the ingredients was approximately 20 C. 2.3 Measuring technique and equipment Autogenous deformation was measured by a specially designed dilatometer, in which the cement paste is encapsulated in thin, corrugated PE moulds. This ensured insignificant restraint of the hardening cement paste and permitted measurements to commence shortly after casting, 30 min after water addition. During the tests the dilatometer bench with specimens was submerged into a thermostatically controlled glycol bath (30±0.1 C). Autogenous deformation was measured simultaneously on two identical samples. The measuring accuracy, which is limited by the diversity of the test specimens, was about 20 µstrain. A thorough description of the dilatometer and the sample preparation is published elsewhere (5). Autogenous RH-change was measured by a Rotronic Hygroscope DT (Rotronic ag, Basserdorf, Switzerland) equipped with WA-14TH and WA-40TH measuring cells, which was built into a thermostatically controlled box (30±0.1 C). Before and after every experiment the equipment was calibrated with saturated salt solutions in the range % RH. Autogenous RH-change was measured simultaneously on two identical samples. The measurements started about 12 hours after water addition. The measuring accuracy was below 1% RH. A thorough description of the Rotronic Hygroscope and the sample preparation is published elsewhere (2). 3. Results and discussion 145

4 The measurements of autogenous deformation and autogenous relative humidity change are shown in Figure 1 and 2. The labels refer to the cement types: Ref. is the paste based on the reference cement with a Blaine specific surface of approximately 410 m 2 /kg, and 480 m 2 /kg indicates the paste with the finer ground cement. C 3 S, C 2 S and C 3 A refer to the pastes where 20% of the reference cement has been replaced with clinker mineral. Figure 1: Autogenous deformation from setting time of five cement pastes at w/c=0.30 and 20% silica fume addition. The only difference between the reference paste and the other four pastes is the cement composition as indicated by the labels on the graphs: See text for explanations of labels. Temperature: 30 C. As seen, an increased specific surface of the cement accelerates the development of the autogenous deformation and autogenous RH-change of the cement paste. In addition, the total autogenous deformation and autogenous RH-change is increased. As supported by computer simulations (6) this is due to the increased reactivity and changes in the microstructure implied by the higher fineness of the cement. The specific surface of the cement, however, seems to be of secondary importance; the autogenous deformation and RH-change is only affected to a minor degree. Of more significant importance seems to be the clinker mineral composition of the cement. Figure 1 and 2 indicate that the clinker minerals of the cement - C 3 S, C 2 S and C 3 A have a distinctly different effect. Replacing 20% of the cement by pure C 3 S or C 2 S has no or very little influence whereas C 3 A significantly changes the course of deformation and RHchange. Both the autogenous deformation and the relative humidity change are reduced 146

5 considerably when 20% of the cement is replaced by C 3 A and a small amount of gypsum. Figure 2: Autogenous RH-change of five cement pastes at w/c=0.30 and 20% silica fume addition. The only difference between the reference paste and the other four pastes is the cement composition as indicated by the labels on the graphs: See text for explanations of labels. Temperature: 30 C. The total shrinkage of the C 3 A replaced cement seems to be reduced due to two different mechanisms, see Figure 1. A part of the shrinkage reduction is due to an induced expansion observed right after set, and a further shrinkage reduction relates to the long term shrinkage development. The former may be due to expansion from ettringite formation counteracting the shrinkage. More ettringite is produced in this paste since it contains an increased amount of C 3 A and gypsum. As seen from the curves in Figure 2 the C 3 A replaced cement has a reduced autogenous RHchange. Most likely, this is connected with the reduction of the long term shrinkage development. Self-desiccation shrinkage is an important type of autogenous deformation. Self-desiccation shrinkage is caused by the chemical shrinkage which is linked to the hydration process. Chemical shrinkage leads to a successive reduction in the meniscus radii. This reduces the relative humidity in the cement paste (self-desiccation), increases the tensile stress in the pore water, increases the compressive stress in the solid phase, and consequently leads to an outer shrinkage (self-desiccation shrinkage) (1,7,8). 147

6 The observed influence of C 3 A on autogenous deformation and RH-change may be examined in this light. However, several factors play a role for the tendency of a cement for self-desiccation: - the chemical shrinkage accompanying the cement hydration, - the sensitivity to the relative humidity of the cement hydration, - the pore structure of the cement paste. These factors are treated separately in the following paragraphs. 3.1 Chemical shrinkage of clinker mineral hydration After setting, the chemical shrinkage is a measure of the amount of pore volume which has been emptied of liquid water due to hydration reactions. This is a consequence of the hydration products taking up less space than the reacting materials. During sealed hydration this leads to formation of inner gas-filled pores and a successive reduction in the meniscus radii. Chemical shrinkage, therefore, is the direct cause for self-desiccation. If the pore structure is sufficiently fine an increased chemical shrinkage will lead to an increased selfdesiccation. The amount of empty pores formed can be calculated from the equation of reaction, the molar masses and the densities of the components. As an example the chemical shrinkage produced from the reaction C 3 A + 6 H C 3 AH 6 is calculated as 17.6 cm 3 /100 g C 3 A. Mchedlov-Petrossyan (9) has presented a number of idealized reaction equations for the hydration of clinker minerals. A calculation of the chemical shrinkage has been carried out from these equations. The results are given in Table 2 together with data of densities and molar masses. The calculated values for the chemical shrinkage given in Table 2 are only indicative. In the calculation only the chemical shrinkage due to chemical reactions is taken into consideration. By adsorption of water to surfaces of the cement gel produced, a compaction of the water may also occur and contribute to the chemical shrinkage. Furthermore, the hydration of cement clinker minerals generally cannot be described by simple reaction equations which Table 2 is based on. However, the calculated values for chemical shrinkage correspond well with experimentally obtained values by Powers (13); After 28 days hydration Powers measured the chemical shrinkage produced by pure C 3 S to app. 5 cm 3 /100 g, for C 2 S app. 1 cm 3 /100 g and for C 3 A to app. 16 cm 3 /100 g when it contained 10% gypsum. For comparison the chemical shrinkage of Portland cement normally is around 5 cm 3 /100 g cement. The picture thus seems clear: During hydration C 3 A produces much more chemical shrinkage than C 3 S, which again produces more chemical shrinkage than C 2 S. That means, an increased C 3 A content should result in a larger total chemical shrinkage. 148

7 Table 2: Densities, ρ, and molar masses, M, of substances (10-12) and calculated chemical shrinkage, V, from the hydration of the indicated clinker minerals. Substance ρ (g/cm3) M (g/mol) V (cm3/100 g clinker) C 3 S C 2 S C 3 A C2SH C3S2H C4S3H C6S6H C5S6H C5S6H C5S6H C2S3H CS2H C4AH C4AH C3AH C2AH CAH A0.5H C3S C2S (ß) C3A CH H Sensitivity to the 1.00 relative humidity A cement whose hydration is unaffected by a drop in the relative humidity may selfdesiccate more vigorously than a cement whose hydration is sensitive to the relative humidity. If, for example, a cement is unable to hydrate below 80% relative humidity, selfdesiccation will be restricted by this value. The ability of pure clinker minerals to hydrate under reduced relative humidities has been examined in a previous project (14). In a series of experiments pure clinker minerals were exposed to different relative humidities for different times, and the degree of hydration was subsequently Degree measured. of hydration Figure 3 (%) shows some of the results C3A 20 C3S C2S Relative Humidity (%)

8 Figure 3: Degrees of hydration of pure clinker minerals C 3 S, C 2 S and C 3 A, after 3 months water vapour exposure at different relative humidities at 20 C (14). The measurements show that the clinker minerals C 3 S, C 2 S and C 3 A have fundamentally different sensitivities to relative humidity. C 2 S hydration is hampered to a greater extent by decreased relative humidity than is C 3 S hydration, which again is more sensitive than C 3 A hydration. This observation agrees with thermodynamic calculations (15). From the experiments the limiting relative humidity for hydration of the C 3 S, C 2 S and C 3 A has been found to be approximately 85%, 90% and 60% respectively based one year water vapour exposure at 20 C. From these results it is expected that an increased C 3 A content should improve the ability to hydrate at reduced relative humidities. 3.3 Pore structure A fine pore structure is potentially much more vulnerable to self-desiccation than a coarse structure. As an extreme example of a coarse pore structure, the relative humidity inside a bucket of water will stay at 100% even when all but the last drop of water is removed. On the contrary if, for example, half the evaporable water of a well hydrated cement paste is removed the relative humidity may drop below 50%. When silica fume is added to cement paste a significant stronger self-desiccation is observed (1,2). This is not surprising because: 1) the pozzolanic reaction of silica fume has a high chemical shrinkage compared to Portland cement, and 2) the pozzolanic reaction of the silica fume is relatively insensitive to a drop in the relative humidity, 3) silica fume refines the pore structure of the cement paste (16). In the case of C 3 A the picture is more complex. The foregoing paragraphs have shown that C 3 A significantly increases the chemical shrinkage as well as promote hydration at lower relative humidities of the cement paste. Viewed separately, this should increase the selfdesiccation and self-desiccation shrinkage. However, the opposite effect is seen from Figures 1 and 2. This may be because the pore structure has an overriding effect so that the net result is a reduced self-desiccation and self-desiccation shrinkage. Experimentally, it has been shown (17) that the pore structure formed during C 3 A hydration is much coarser than the pore structure formed by C 3 S and C 2 S hydration. The above discussion has only focused on how the C 3 A hydration itself contributes to autogenous deformation and RH-change. However, the observed reduction of autogenous deformation and RH-change by C 3 A may, potentially, be caused indirectly. Previous experiments have shown that the bulk of the observed autogenous deformation and RHchange is caused by the silica fume and only a minor part is related to the cement (1,2). Possibly, the silica fume reaction is influenced by the C 3 A or gypsum. This could for example be due to that other pozzolanic reaction products are formed as a consequence of the presence of an increased amount of C 3 A or gypsum. These reactions may be more 150

9 sensitive to the relative humidity, produce less chemical shrinkage or not lead to the same refinement of the pore structure, c.f. the discussion above. A reduced reactivity of the silica fume due to a lower content of calcium hydroxide could be a contributing factor; calcium hydroxide is mainly formed during the hydration of the silicate clinker minerals. Further research is needed to find the exact cause of the marked influence of C 3 A or gypsum on autogenous deformation and RH-change. A number of experiments could clarify the above suggestions including: Autogenous deformation and RH-change of cement pastes where the C 3 A and the gypsum content are varied separately. Autogenous deformation and RH-change of cement pastes with added C 3 A and gypsum with varying specific surfaces. Influence of C 3 A and gypsum addition on the pozzolanic reaction of silica fume - followed for example by NMR. Influence of C 3 A and gypsum on pore structure, for example measured by mercury intrusion porosimetry (mainly the pore size range nm is interesting). Tazawa and Miyazawa (18) have examined the autogenous shrinkage in pastes of 7 different commercial cements. Based on these experiments they conclude that an increased C 3 A content leads to an increased autogenous shrinkage. These results are, apparently, in conflict with the results presented in this paper. One reason for this could be that Tazawa and Miyazawa have compared the cements based only on their Bogue calculated clinker mineral composition. Several other parameters influence the autogenous deformation of a cement paste, e.g. the content of gypsum, free lime and alkalis. Tazawa and Miyazawa have not taken this into account. Another reservation concern the experiments performed in this paper. They are based on pure clinker minerals and should, therefore, be interpretated with caution. The clinker minerals in a normal Portland cement contain some percent foreign oxides incorporated in their structure (19). Such impure clinker minerals may have somewhat different hydration properties compared to the "pure" clinker minerals used in this project. Even the type and amount of stabilizer used for forming ß-C 2 S is reported to affect the rate of the hydration of the ß-C 2 S (20). 4. Conclusion Measurements indicate that an increased C 3 A or gypsum content in the cement leads to a marked reduction in the autogenous deformation and autogenous RH-change of the cement paste. The content of C 3 S or C 2 S and the specific surface of the cement seem to have only little or no influence on these properties. Potentially, the crack tendency during hardening of 151

10 high performance concretes may be reduced by increasing the C 3 A or gypsum content in the cement. Acknowledgements Assistance from Dr. Jim Fletcher and Daniel Brew, both of the University of Aberdeen in the QXRD analysis and production of clinker minerals, is gratefully acknowledged. 5. References 1. Jensen, O.M. and Hansen, P.F., Autogenous deformation and change of the relative humidity in silica fume modified cement paste, ACI Mat. J. 93 (6) (1996) Jensen, O.M. and Hansen, P.F., Autogenous relative humidity change in silica fumemodified cement paste, Adv. Cem. Res. 7 (25) (1995) Jensen, O.M. and Hansen, P.F., Influence of temperature on autogenous deformation and relative humidity change in hardening cement paste, Cem. Concr. Res. 29 (4) (1999) Soroka, I., Portland cement paste and concrete, p. 10 (Macmillan, London, 1979). 5. Jensen, O.M. and Hansen, P.F., A dilatometer for measuring autogenous deformation in hardening Portland cement paste, Mater. Struct. 28 (1995) Bentz, D.P., Garboczi, E.J., Haecker, C.J., and Jensen, O.M., Effects of cement particle size distribution on performance properties of Portland cement-based materials, Cem. Concr. Res. 29 (1999) Hua, C., Acker, P. and Ehrlacher, A., Analyses and models of the autogenous shrinkage of hardening cement paste: I. Modelling at macroscopic scale, Cem. Concr. Res. 25 (7) (1995) Buil, M., Contribution a l etude du retrait de la pâte de ciment durcissante, Rapport de recherche des Laboratoire des Ponts et Chaussées (LPC), 92 (1979). 9. Babushkin, V.I., Matveyev, G.M. and Mchedlov-Petrossyan, O.P., Thermodynamics of Silicates (Springer, Berlin, 1985). 10. Lea, F.M., The chemistry of cement and concrete, (Edward Arnold, London, 1970). 11. Roberts, W.L., Rapp, G.R., Weber, J., Enclyclopedia of minerals, (Van Nostrad Reinhold, New York, 1974). 12. Taylor, H.F.W., Cement Chemistry, (Academic, London, 1992). 13. Powers, T.C., Absorption of water by Portland cement paste during the hardening process, Ind. Engng. Chem. 27 (7) (1935) Jensen, O.M., Hansen, P.F., Lachowski, E.E. and Glasser, F.P., Clinker mineral hydration at reduced relative humidities, Cem. Concr. Res. 29 (1999) Jensen, O.M., Thermodynamic limitation of self-desiccation", Cem. Concr. Res. 25 (1) (1995) Sellevold, E.J., Condensed silica fume in concrete, FIP State of art report (Thomas Telford, London, 1988). 17. Eckart, A., Ludwig, H.-M. and Stark, J., Hydration of the four main Portland cement clinker phases, ZKG Int., 48 (8) (1995)

11 18. Tazawa, E., and Miyazawa, S., Influence of constituents and composition on autogenous shrinkage of cementitious materials, Mag. Concr. Res., 49 (178) (1997) Taylor, H.F.W., Modification of the Bogue calculation, Adv. Cem. Res. 2 (6) (1989) Nurse, R.W., The Dicalcium Silicate Phase, Proc. Symp. Chem. Cem. London, 1952,