Occurrence of large silica fume-derived paticles in hydrated cement paste

Abstract

In an examination of a 1-year old hydrated silica fume bearing cement paste a number of large (35–80 μm) rounded siliceous particles were found that had apparently been derived from the coarse fraction of the silica fume. Calcium had diffused inward from the periphery of the grains, and in most cases had reached the centet. The Ca:Si mole ratio near the outer zone was almost the same as that in the surrounding matrix CSH gel, but decreased continuously with distance toward the center. Potassium and sodium were also found within the particles, but were distributed inversely to the calcium, i.e. increasing toward the center. The reaction product generated here appears to be CSH and not potentially expansive alkali silica reaction product gel, although such gel might be produced with high alkali cements.