3.2 Leaching influence on capillary porosity

The volume fractions of the main phases and the capillary porosity given by the model for the six cement pastes are presented in Table 6. The results are in good agreement with what is already known concerning the increase of capillary porosity with the water-to-cement ratio and the reduction in CH content with silica fume addition (Huang & Feldman 1985, Regourd 1987). 

Previously, (Bentz et al. 1992) have shown the important effect of CH dissolution on the porous network percolation of tricalcium silicate cement pastes. We studied the influence of the total dissolution of CH and also of AFm and AFt on the increase in the capillary porosity for the six cement pastes. The model values of capillary porosity of the unleached, and the CH, AFm and Aft-leached cement pastes are given in Table 7. The results show an important capillary porosity increase after total CH dissolution for the different pastes. To evaluate the effect of the CH, AFm and AFt dissolution, we calculated the capillary porosity ratios after and before leaching. As shown in Fig. 4, the total CH dissolution has a very important effect on the increase in the capillary porosity and particularly for pastes with lower water-to-cement ratios. The total dissolution of AFm and AFt also contributes to the increase in capillary porosity, but in a much smaller way, due to the fact that their volume fractions in the unleached cement paste are much lower, i.e. 0.16 % AFm and 2.4 % AFt compared to 16.4 % CH in the CEMI at w/c = 0.5. For the different systems, with and without silica fume, the capillary porosity due to hydrate dissolution increases with water-to-cement ratio.

Figure 4. Leaching effect of CH, AFm and AFt on the increase of capillary porosity.

In the study of transfer phenomena, the connected capillary porosity is an important and critical parameter to be taken into account. It is known that the entire capillary porosity system is not totally connected. The connectivity depends on the capillary porosity value. For high values, beyond 0.4, the capillary porosity is almost fully connected. Thus, the dissolution of hydrates can strongly increase the connected capillary porosity fraction (fraction of total sample volume). It is important to remember that this study can predict properties of fully leached areas in the cement paste, but it is not made to simulate the progression of the leaching front. The connected capillary porosity fraction was computed for the six cement pastes before and after leaching of hydrates, using CEMHYD3D. The model values are given in Fig. 5. The results indicate that CH dissolution significantly increases the connected porosity fraction, connecting previously isolated pores. This effect is highlighted in the cement pastes with w/c = 0.25. The AFm and AFt dissolution contributed slightly to the connectivity of the porous network because their original volume fraction in unleached cement pastes was lower.

Figure 5: Leaching effect of CH, AFm and AFt on the increase of the connected capillary porosity fraction (expressed as a fraction of total sample volume).