Strength development, hydration reaction and pore structure of autoclaved slag cement with added silica fume

Under continuous hydrothermal treatment the strength of portland cement paste decreases with curing time and the pore structure coarsens. It was found in this study that the compressive strength of slag cement paste containing 67.5 wt.% ggbfs also decreases with time after 24 hour hydrothermal processing, but with a small addition of silica fume to the slag cement, the cement strength increases and the pore structure densifies when processed under comparable conditions. Based on observations XRD and SEM, these changes are attributed to: (a), changes in the hydration reactions and products by highly reactive silica fume, such that amorphous products dominate and the strength reducing phase α-C 2SH does not form; (b), slower hydration of slag, partially caused by the decreased pH of the pore solution, favors the formation of a dense pore structure; and (c), the space filling properties of the micro particles of silica fume.