A ²⁹Si MAS NMR study of the pozzolanic activity of condensed silica fume and the hydration of di- and tricalcium silicates

The pozzolanic activity of condensed silica fume replacing 15% of di- and tricalcium silicates at a w/(c + s) ratio of O·70 has been studied by solid-state ²⁹Si magic angle spinning nuclear magnetic resonance. The hydration characteristics of the two cement minerals have been studied by the same technique in mixtures with and without silica fume. The water in the mixtures was pre-mixed with alkalis to partly simulate the initial solution formed when water is mixed with cement, since alkalis are believed to strongly affect the conversion rate of silica fume. The combination of NMR data with information from thermogravimetry (TG) has been utilized to balance the total chemical reaction for each mixture, and thereby the empirical formula of the CSH gel as well. The consumption rate of silica fume is found to be faster than the hydration of the minerals after three to seven days. Furthermore, the average chain length of the linear polysilicate anions in the CSH gel is longer in mixes with silica fume than without. The latter is accompanied with a lower CIS ratio in the gel. The C/S ratio of the gelfrom the pure cement minerals tends to increase with age. Silica fume accelerates the hydration of the cement minerals at early age, but retards the hydration at a later stage under the current conditions.