| Study of expansion associated with the alkali-silica reaction in concrete win enhance understanding of the damage mechanisms and will provide more dependable means for predicting performance and for limiting damage. However, in the past, studies have been limited by the lack of appropriate characterization techniques. The reaction which occurs between reactive silica or silicates present in some aggregates and alkalis in the pore solution produces an alkali-silicate gel. Damage to concrete is caused by expansion believed to be generated by swelling of the gel. Removal of water from the gel, as required by most existing high-resolution techniques, introduces artifacts in the specimens.; A microscopy technique, never previously applied to concrete technology, was used to study the alkali-silica reaction with the objective of improving understanding of the mechanisms of the alkali-silicate gel expansion. Transmission soft x-ray microscopy is a high-resolution technique where reactions occurring in wet samples can be imaged over time at standard pressure. Transmission soft x-ray microscopy was used to study the effects of pore solution cations (Na+, Ca++) and some chemical additives which have been shown to affect expansion (CaCl2, LiCl, acetone) on the morphology of an alkali-silicate gel obtained from FURNAS Dam, Brazil.; 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 non-swelling product, i.e. calcium silicate hydrate. In the investigation of the effect of some chemical additives, a structure resembling calcium silicate hydrate formed in a solution containing sodium hydroxide and calcium chloride, suggesting that the relative concentration of calcium ions to silica species in solution may influence expansion. In the presence of sodium hydroxide and lithium chloride, the amount of silica repolymerization was reduced as compared to when only alkalis were present. However, in the presence of sodium hydroxide and acetone, much repolymerization was evident in the x-ray images, suggesting that the use of acetone does not effectively control expansion. |
