HYDRATION OF TYPE-K EXPANSIVE CEMENT PASTE AND THE EFFECT OF SILICA FUME .2. PORE SOLUTION ANALYSIS AND PROPOSED HYDRATION MECHANISM

This is the second part of a two-part paper on the hydration characteristics of Type K expansive cement pastes and the effect of silica fume addition. Data concerning expansion of pastes and quantitative analysis of the solid phases related to expansion were presented in Part I** Part II presents the portion of the study related to the pore solution analysis. The results presented in Parts I and II used to propose a hydration mechanism of expansive cement pastes and the effect of silica fume. In the expansive cement paste without silica fume, ettringite formation and the consequent expansion terminated due to the the depletion SO42- in the pore solution after 11 days of hydration In the presence of silica fume, the initial formation of ettringite was accelerated leading to an increased rate and magnitude of expansion. After two days of hydration, a reduction in the concentration of alkali cations in the pore solution was observed. This resulted in a decrease in the OH- concentration in the pore solution and subsequent decrease hi the rate of ettringite formation. The expansion stopped at about five days of hydration even though approximately 55% of the original C4A3S***BAR remained unhydrated and SO42-. Still remained in pore solution. It is speculated that the reduction of the pH of the pore solution after two days of hydration may provide an explanation for the termination of ettringite formation in the expansive cement paste containing silica fume.