Hydration Process and Pore Structure of Portland Cement Paste Blended with Blastfurnace Slag

Transcription

1 Hydration Process and Pore Structure of Portland Cement Paste Blended with Blastfurnace Slag J. Zhou 1, G. Ye 1, 2* and K. van Breugel 1 1) Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands 2) Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Technologiepark-Zwijnaarde 904 B-9052, Ghent (Zwijnaarde), Belgium Abstract: In this study, the hydration process and pore structure of portland cement (PC) paste blended with 10-90% blastfurnace slag (BFS) with the water/solid (w/s) ratios of 0.4 and 0.5 were investigated. Experimental results indicate that BFS accelerates the hydration of PC and leads to the occurrence of an additional peak. In PC paste blended with BFS, the presence of capillary pores and gel pores was detected clearly. The addition of BFS results in a finer pore structure with higher porosity. Keywords: Portland cement, Blastfurnace slag, Hydration process, Pore structure. 1. Introduction The use of BFS in PC is widespread in the Netherlands, because of its good durability, economical advantage and environmental benefit. BFS with about 95% glass, which consists of monosilicates like those in PC clinkers [1, 2], is latent hydraulic. From literature, it has been shown that BFS reacts with water for a short time and then stops, when it is mixed with water. The reason for this phenomenon is the formation of a water impermeable layer on the surface of BFS particles [3]. The latent hydraulicity of BFS need be activated by activators, such as the solutions of alkalis or sulphates [4]. The hydration of PC produces calcium lime (liberated by hydration of the clinker silicates) and sulphate (from the gypsum). Thus, the combination of BFS and PC forms a group of cements with good reactivity and performance. Many studies concerning the hydration process of PC blended with BFS have been carried out in past decades [5, 6, 7, 8]. Both constituents can react with water and their hydration processes affect each other. Escalante- Garcia and Sharp [5] have reported that BFS accelerates the hydration of C 3 S, C 3 A and C 4 AF to different extents. According to Uchikawa [6], the reaction of BFS can be activated by CH, when ph is up to It is responsible for the occurrence of an additional peak in the curves of rate of heat evolution of PC blended with BFS [7, 8] Corresponding author. Tel: ; Fax: ; Yeguang@tudelft.nl The pore structure is the key factor, which determines the ingress of harmful substances and the durability of concrete structures. Smolczyk [9] reported that PC pastes blended with BFS contain more gel pores and fewer capillary pores than pure PC pastes. According to Pigeon and Regourd [10], the average pore size becomes small after 28 days curing, when the BFS content increases from 28.3% to 66.0%. Up to now, the effect of BFS content on the hydration process and the development of pore structure of PC paste has not been studied extensively. In this study, the heat evolution was measured by isothermal conduction calorimeter to monitor the hydration process. The pore structure was investigated by mercury intrusion porosimetry (MIP). 2. Materials and Methods 2.1 Materials In this study, CEM I 32.5R and BFS were mixed in the laboratory. The chemical compositions of CEM I 32.5R and BFS are given in Table 1. The mineral composition of CEM I 32.5R, calculated with modified Bogue equation [11], is 63.6% C 3 S, 9.7% C 2 S, 7.3% C 3 A and 9.7% C 4 AF. Figure 1 shows the particle size distribution of CEM I 32.5R and BFS measured by gravity sedimentation. The fineness of BFS is higher than that of CEM I 32.5R. The average particle diameters of CEM I 32.5R and BFS are µm and µm, respectively. Two w/s ratios of 0.4 and 0.5 were chosen. The BFS content varied from 10% to 90% by weight. 1

2 Table 1 Chemical composition of CEM I 32.5 R and BFS (The chemical composition of PC was obtained from the producer and that of BFS was measured by energy dispersive X-ray analysis). Oxide CEM I 32.5 R (%) BFS (%) CaO SiO Al 2 O Fe 2 O MgO K 2 O Na 2 O SO Cl the information of the pores with the diameter larger than µm can be collected. 3. Results and Discussion 3.1 Isothermal Conduction Calorimeter Figures 2-5 show the rate of heat evolution and cumulative heat evolution of PC blended with BFS at 20 C with the w/s ratios of 0.4 and 0.5 in the first 50 hours. There are three peaks in the curves of the rate of the heat evolution of pure PC. The peak I was described to be attributed to the hydration of C 3 A, the hydration of free lime and the wetting of PC [14]. According to Taylor [1] and Odler [15], the peaks II and III are associated with the hydration of C 3 S and C 3 A, respectively. In this study, the peak III is higher than the peak II, which is probably because of the high content of C 3 A in this PC. Figure 1 Particle size distribution of CEM I 32.5 R and BFS measured by gravity sedimentation. 2.2 Isothermal Conduction Calorimeter Isothermal conduction calorimeter tests were carried out by TAM Air 314 at 20. Every measurement was started after about 10 minutes after water mixing and lasted 168 hours. The first so-called wetting peak in the curves of the rate of heat evolution cannot be recorded. This peak lasts several minutes, and its heat evolution amounts to a few percent of total heat evolution of hydration. Thus, the influence of the first peak can be neglected. 2.3 MIP PC and BFS were mixed with water by the HOBART mixer. Subsequently, samples were rotated at a speed of 5 r/min in a room with the temperature of 20 C for 24 hours. After sealed curing at 20 C, the sample was split into small pieces around 1 g at 1, 3, 7, and 28 days. Then, freeze-drying was applied, because of the low damage of the pore structure of cement pastes [12]. The MIP measurement was carried out on the freezedried samples at 16 C. The maximum mercury intrusion pressure of 210 MPa was used. The relation between intrusion pressure P and the pore radius r was described by the Washburn equation based on a model of cylindrical pores [13]. The contact angle of 141 and the surface tension of 485 mn/m were assumed. Therefore, 2 Figure 2 Rate of the heat evolution, referred to the mass of total powder, of PC blended with BFS with the w/s ratio of 0.4 at 20 C. Figure 3 Total heat evolution, referred to the mass of total powder, of PC blended with BFS with the w/s ratio of 0.4 at 20 C.

3 In order to check whether BFS has reacted, the total heat evolution, referred to the mass of PC, is plotted in Figure 6. This value increases very fast with the increase in the BFS content. The total heat evolution is up to an apparent value of 900 J/g at 50 hours, when the BFS content is 90%. It exceeds the actual potential heat evolution of PC, which is 445 J/g calculated by Bogue equation [1]. This cannot be explained by the acceleration of PC due to BFS. The hydration of BFS should contribute a part of heat evolution. Figure 4 Rate of the heat evolution, referred to the mass of total powder, of PC blended with BFS with the w/s ratio of 0.5 at 20 C. Figure 5 Total heat evolution, referred to the mass of total powder, of PC blended with BFS with the w/s ratio of 0.5 at 20 C. In the curves of the rate of the heat evolution of PC blended with BFS, peaks I, II and III are also observed. For samples with 70% or 90% BFS, the peaks II and III cannot be distinguished. This is because of the low content of PC. Both the peaks II and III are shifted to earlier times as the BFS content increases. This indicates that BFS accelerates the hydration of C 3 S and C 3 A related to these two peaks. The same observation has been reported by Escalante-Garcia and Sharp [5]. They attributed the acceleration to the effect of dilution, which means that, because of the slow reaction of BFS, there is more water available for PC particles to react. In this study, for the samples with the w/s ratio of 0.4, as the BFS content increases from 0% to 90%, the water/cement ratio varies from 0.4 to 4. Ye [12] has done an experiment to investigate the effect of water/cement ratio on the hydration process of pure PC. According to him, the higher water/cement ratio results in the retardation of the peaks II and III. Only the dilution effect cannot explain this acceleration clearly. 3 Figure 6 Total heat evolution, referred to the mass of PC, of PC blended with BFS with the w/s ratio of 0.4 at 20 C. An additional peak S is observed in the curves of PC blended with BFS. This peak is more obvious as the BFS content increases (Figure 2, 4). The peak S always follows the peak II, which is associated with the formation of CH. This suggests that the reaction of BFS need to be activated by the CH. The BFS content is the main factor influencing the peak S. The value of peak S first increased, as the BFS content increased up to 50%, and then decreased. This peak shifted to earlier times as the BFS content increases except the mixtures with 90% BFS. This may be explained by the reaction of BFS and CH. When the content of BFS is relatively small and CH is abundant, the availability of BFS accelerates this reaction; when the content of BFS is high and CH is not enough, this reaction is limited by the amount of CH. 3.2 MIP Definition of Terms There are several terms used to describe a pore structure: capillary pore, gel pore, total porosity, effective porosity, inkbottle porosity and critical pore diameter [12]. The capillary pore is the empty or waterfilled space between hydration products or hydration products and unhydrated cement particles. The diameter of capillary pore varies from 10 nm to 10 µm. The gel pores are formed within the hydration products and have the size of 0.5 nm to 10 nm in diameter. The total porosity is the sum of capillary porosity and gel

4 porosity. In an MIP test, the effective porosity is defined as the value of mercury removed during extrusion procedure. It is a crucial factor, which determines the water permeability. The inkbottle pore is the large pore into which mercury intrudes through a narrow neck (Figure 7). The inkbottle porosity can be calculated as the total porosity minus the effective porosity (Figure 8). The critical pore diameter relates to peaks in differential curve (Figure 9). Normally there are two distinct peaks in the differential curve: the first one corresponds to a diameter about 1 µm and the second one corresponds to a smaller diameter around 50 nm. According to Katz et al. [16], the pores with diameter larger than the critical pore diameter create a connected path through the sample. the volume of solid phase, including gel pores, increases, the pore space is filled, and some pores are even blocked by the formed hydration product at later age. Some pores become inkbottle pores or isolated pores (Figure 7). Consequently, the total volume of pores and the ratio of the effective pores gradually decrease. The change of pores into the isolated pores is another reason for the reduction of total porosity, because MIP cannot detect the isolated pore. Figure 8 Definition of the total porosity, the inkbottle porosity, the effective porosity. Figure 7 Change of the capillary pore into the inkbottle pore or the isolated pore Total Porosity and Effective Porosity Table 2 gives the total porosity, the effective porosity and the ratio of the latter to the former in PC paste blended with BFS with the w/s ratios of 0.4 and 0.5 at 1, 3, 7 and 28 days. With the same w/s ratio and BFS content, the total porosity and the effective porosity decrease as the curing age increases. The ratio of effective porosity to total porosity also decreases. After PC and BFS are mixed with water, PC reacts with water to produce the hydration product, the volume of which is approximately 1.6 times of the volume of PC [17]. As Figure 9 Pore size distribution: the differential curve from MIP test Table 2 Total porosity, effective porosity and the ratio of the latter to the former in PC paste blended with BFS with the w/s ratios of 0.4 and 0.5 at 1, 3, 7 and 28 days BFS Total porosity (%) Effective porosity (%) Effective/total (%) w/s content Curing age (days) Curing age (days) Curing age (days) (%)

5 Figure 10 shows the effect of the BFS content on the total porosity and the effective porosity of the samples with the w/s ratio of 0.4 at 3 and 28 days. The increasing BFS content leads to the increase in the total porosity. This can be explained by the low reactivity of BFS. The addition of BFS results in the decrease in the overall degree of the hydration of the mixture of PC and BFS (Figure 2-5), and a corresponding reduction of the amount of hydration products. Figure 10 Total porosity and effective porosity of the samples with the w/s ratio of 0.4 at 3 and 28 days In view of the effective porosity, the minimum value is found for 70% BFS content. It means that the addition of BFS decreases the connectivity of pores. Figures 11 and 12 [18] show the BSE image of the PC paste blended with 10% and 50% BFS with the w/s ratio of 0.5 at 3 days, respectively. BFS particles with relatively small size can fill the gaps between large PC particles. Compared with the sample with low BFS content, there are much more inkbottle pores and the pore size is smaller in the sample with the high BFS content. Figure 12 BSE image of PC paste blended with 50% BFS with the w/s ratio of 0.5 at 3 day Pore Size Distribution There are two pore systems in the pure PC paste: capillary pore and gel pore [12]. For PC paste with BFS, the differential curve measured from intrusion procedure shows two principal peaks, corresponding to the capillary pore and the gel pore (Figure 9), respectively. This indicates that the blended cement paste also consists of these two pore systems. Figure 13 shows the pore size distribution of the samples with the w/s ratio of 0.4 and 70% BFS content at 1, 3, 7 and 28 days. As curing time elapses, the size and volume of capillary pores decrease in the first 3 days. After 7 days, the first peak corresponding to capillary pores even disappears. On the other hand, the volume of gel pores increase in the first week, and then their amount and size decrease. This phenomenon is also observed on other samples. Figure 11 BSE image of PC paste blended with 10% BFS with the w/s ratio of 0.5 at 3 days 5 Figure 13 Differential curves measured from intrusion procedure of the PC paste blended with 70% BFS with the w/s ratio of 0.4 at 1, 3, 7 and 28 days At the beginning, the paste comprises the unhydrated PC and BFS, and water-filled space, i.e. capillary pores. As the hydration proceeds, the capillary pores are generally filled with hydration products. The formed C- S-H can be distinguished in two parts [19], viz. (1) outer C-S-H, with larger gel pores, in the originally water-

6 filled spaces, (2) inner C-S-H, with very fine gel pores, within the boundaries of the original unhydrated grains. The former is rapidly formed during the first few days. It is followed by the formation of the latter. Thus, the formation of hydration products decreases the size and of capillary pores and increases the amount of gel pores. Meanwhile, the average gel pore diameter gradually decreases in the hardening cement paste. Figures 14 and 15 show the effect of the BFS content on the pore size distribution of samples with the w/s ratio of 0.4 at 1 and 28 days. The critical pore diameter increases with the increase in the BFS content at 1 day. After 28 days curing, the pore structure of PC paste blended with 70% BFS is the finest. It means that the reaction of BFS has a positive effect on the development of pore structure. At early age, the reaction of BFS is not activated and has no contribution to the filling of the capillary pores. After the ph of pore solution is up to 12.6, BFS starts to react with CH to produce the hydration product. At later age, there is only one peak in the curves, which relates to gel pores within C-S-H. The increasing BFS content up to 70% results in a denser C-S-H. Figure 15 Differential curves measured from intrusion procedure of the PC paste blended with BFS with the w/s ratio of 0.4 at 28 days The diameter of effective pore measured from extrusion procedure is from µm to 10 µm. Experiment results show that samples have one critical pore diameter and it varies from 0.05 µm for sample with 70% BFS and 0.4 w/s ratio at 28 days to 8.88 µm for sample with 90% BFS and 0.5 w/s ratio at 1 day. This critical pore diameter is larger than that measured from the intrusion procedure (Figure 16). It indicates that the effective pore system is mainly composed of large capillary pores. Figure 16 also shows that the curve measured from extrusion procedure is higher than the curve measured from the intrusion procedure at the diameter from 1 µm to 10µm. This is probably because of the damage of the pore structure at high intrusion pressure. Figure 14 Differential curves measured from intrusion procedure of the PC paste blended with BFS with the w/s ratio of 0.4 at 1 day. Figure 16 Differential curves measured from both intrusion and extrusion procedure of the PC paste blended with 70% BFS at 1 day Conclusions The effect of BFS content on the hydration process and pore structure of PC paste has been experimentally studied. BFS accelerates the hydration of PC. The reaction of BFS activated by CH results in the

7 occurrence of an additional peak S. This peak becomes more obvious as the BFS content increases. In the PC paste blended with BFS, the addition of BFS increases the total porosity. The presence of capillary pores and gel pores was detected clearly. As curing age increases, the amount and size of capillary pores decreases. On the other hand, the amount of gel pores increases in the first 7 days, and then their size becomes smaller. When the BFS content is not higher than 70%, the higher BFS content results in a finer pore structure and a denser C-S-H at later age. For the BFS content is higher than 70%, i.e. 70% to 90%, we found that the microstructure becomes more porous with the increase in the BFS content. References [1] H. F. W. Taylor, Cement chemistry, (Academic Press, London, 1990). [2] J. Bijen, Benefits of slag and fly ash, Construction and Building Materials 10 (5) (1996) [3] M. Regourd, J. H. Thomassin, P. Baillif, and J. C. Touray, Blast-furnace slag hydration. Surface analysis, Cem. Concr. Res. 13 (1983) [4] M. Regourd, Cements made from blastfurnace slag, in Lea s chemistry of cement and concrete, 4 th Edn, edited by P. C. Hewlett, (Arnold, London, 1998). [5] J. I. Escalante-Garcia and J. H. Sharp, Effect of temperature on the hydration of the main clinker phases in Portland cements: Part II, Blended cements, Cem. Concr. Res. 28 (9) (1998) [6] H. Uchikawa, Effect of blending components on hydration and structure formation, in Proceedings of 8 th ICCC, Rio de Janeiro-Brasil, 1986 (Secretaria de CIQC, Rio de Janeiro, 1986) Vol. 1, [7] X. Wu, D. M. Roy, and C. A. Langton, Early stage hydration of slag-cement, Cem. Concr. Res. 13 (1983) [8] J. I. Escalante-Garcia and J. H. Sharp, The effect of temperature on the early hydration of Portland cement and blended cements, Adv. Cem. Res. 12 (3) (2000) [9] H.G. Smolczyk, Slag structure and identification of slag, in 7 th ICCC, vol. 1, Paris, 1980, pp [10] M. Pigeon and M. Regourd, Freezing and thawing durability of three cements with various granulated blast furnace slag contents, Proc. CANMET/ACI Conference. ACI Publication SP-79, vol. 2, Montebello, Canada, 1983, pp [11] H. W. F. Taylor, Modification of the Bogue calculation, Adv. Cem. Res. 2 (6) (1989) [12] G. Ye, The microstructure and permeability of cementitious materials, PhD thesis, (Delft university press, Delft, 2003). [13] E.W. Washburn, in Proc. The National Academy of Sciences, PNASA, [14] D. J. Lee, Heat of hydration measurements on cemented radioactive wastes Part I: cement-water pastes, (United kingdom atomic authority, Winfrith, 1983). [15] I. Odler, Hydration, setting and Hardening of Portland cement, in Lea s Chemistry of cement and concrete, 4 th Edn, edited by P. C. Hewlett, (Arnold, London, 1998). [16] A. J. Katz and A. H. Thompson, Quantitative prediction of permeability in porous rock, Physical Review B 34 (1986) [17] T.C. Powers, L.E. Copeland, J.C. Hayes and H.M. Mann, Permeability of Portland cement paste, ACI Journal Proceedings 51, pp , [18] J. Zhou, G. Ye and K. van Breugel, Hydration of portland cement blended with blastfurnace slag at early stage, Proc. of RILEM Symposium September 11-13, Quebec Canada, in press. [19] I. G. Richardson and J. G. Cabrera, The nature of C-S-H in model slag-cements, Cem. Concr. Comp. 22 (2000)