Chemical additives to control expansion of alkali-silica reaction gel: proposed mechanisms of control

Abstract

Calcium chloride, lithium chloride, and acetone have previously been shown to affect expansion caused by alkali-silica reaction (ASR), a deleterious reaction occurring between reactive siliceous minerals present in some aggregate and the strongly alkaline pore solution in concrete. Here, the effect of these chemical additives was examined by transmission soft X-ray microscopy and a quantitative elemental analysis, using ICP-OES. In examining the effect of calcium chloride on ASR gels, the formation of a calcium silicate product, believed to be non-expansive, was identified by X-ray microscopy. Additionally, the elemental analysis results suggest that the concentration of calcium ions in the pore solution, which is dependent upon the solubility of the chemical additive and the percent addition, relative to the concentration of silica species in solution is an important parameter for effective control of expansion associated with ASR. In examining the effect of lithium chloride, quantitative elemental analysis showed silica dissolution in solutions of 0.7 M NaOH + 0.1 M LiCl, but with silicon present in slightly lower concentrations than in 0.7 M NaOH solutions alone. However, X-ray microscopy showed less evidence of repolymerization of the dissolved silica into an expansive gel in the presence of lithium chloride as compared to the reaction of the ASR gel in alkaline (0.7 M NaOH) solutions without lithium. With acetone, the results, including X-ray images showing the formation of repolymerized gel in 0.7 M NaOH solution containing 10% v/v acetone, indicate that the use of acetone as a chemical additive may not be as effective as once believed in preventing expansion caused by ASR. It is proposed that any reduction in expansion by use of acetone is temporary and diminishes over time.