Investigation of alkali-silica reaction by transmission soft x-ray microscopy

The soft x-ray transmission microscope XM-1 was used to examine the alkali-silica reaction, a deleterious reaction that produces expansion in some concrete structures. Reactions of silica gel in alkaline solutions (pH⩾12.4) in the presence of pore solution cations (Na+, Ca++) and a chemical additive (CaCl2) were observed, and the morphology of the reaction products were examined to investigate the mechanisms of expansion and expansion control. From the investigation of the effect of pore solution cations, it was found that reactive silica combined with alkalis present in the pore solution to produce a reaction gel capable of swelling, while the reaction of silica in the presence of calcium ions alone resulted in the formation of a nonswelling product—calcium silicate hydrate. Reaction of the silica gel in the presence of CaCl2 also resulted in the formation of calcium silicate hydrate.