EFFECT OF SILICA FUME ON CEMENT HYDRATION IN LOW POROSITY CEMENT PASTES

This paper presents some experimental test results on how silica fume affects the cement hydration in low porosity cement pastes. For cement pastes of low w/c ratios (less-than-or-equal-to 0.4) only a moderate increase in non-evaporable water content during a period of up to 550 days was observed. In the presence of silica fume, however, the non-evaporable water content decreased from 90 to 550 days, which is assumed to be due to a release of water during a polymerization of the silicates. Thus, the non-evaporable water content does not appear to be a valid measure for monitoring the hydration for cement paste containing pozzolanic materials such as silica fume, especially over a long period of time. The pozzolanic reaction started even before one day and continued up to 550 days. The reaction kinetics appears to be very much influenced by the level of w/c ratio. For decreasing w/c ratios from 0.4 to 0.3 and 0.2 a Ca(OH)2 contents of approximately 16, 12, and 8 % was observed, respectively. At 16 % replacement, approximately all of the Ca(OH)2 was consumed regardless of w/c ratio. Analysis of the pore solution composition showed that even for such a low w/c ratio as 0.2, a 16 % replacement of silica fume did not reduce the pH level to more than 12.7, which is slightly more than that of a saturated Ca(OH)2 solution.