Mechanism of Hydration of Condensed Silica Fume in Calcium Hydroxide Solutions


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Title: Mechanism of Hydration of Condensed Silica Fume in Calcium Hydroxide Solutions

Author(s): Michael W. Grutzeck, Scott Atkinson, and Della M. Roy

Publication: Special Publication

Volume: 79


Appears on pages(s): 643-664

Keywords: blended cements; calcium hydroxides; hydration; pozzolans; silica.

Date: 5/1/1983

Condensed silica fume was mixed with nominally saturated Ca(OH)2 solutions at 21° and 38°C. The water to solid ratio (w/s) of the suspensions was approximately 1500. Hydration was monitored as a function of time. Samples were taken at 0.08, 0.25, 1, 3, 8, 24 and 55h. After centrifuging, the liquid and solid portions of the samples were analyzed using conventional wet chemical and SEM techniques, respect ive ly. Upon termination of the experiments, the remaining solid residues were also characterized using x-ray diffraction methods and IR spectroscopy. Results obtained suggest that the condensed silica fume undergoes rapid dissolution in the Ca(OH)2 solution . It is proposed that the silica in solution reaches supersaturation with respect to a new phase in a matter of minutes. This phase is seen to precipitate as a silica-rich layer on the surfaces of the condensed silica fume. Once formed, this layer tends to alter, in part dissolving and in part becoming a substrate for conventional C-S-H formation. Some of the observed characteristics of condensed silica fume blended cements are discussed in light of the above model of reaction.