The process and mechanism of alkali — silica reaction using fused silica as the reactive aggregate Available to Purchase

As part of a detailed study to investigate the processes and mechanism of alkali—silica reaction (ASR), OPC mortars with 50% of the aggregate replaced by highly reactive fused silica (FS) were immersed in 1M KOH and saturated NaCl solutions at 38°C for 150 days. The expansion of the mortars was measured every 5 days for the first 30 days and every 15 days thereafter, and compared with the expansion of a series of OPC control mortars without FS. Maximum expansions (up to 2·2%) were observed in the FS mortars immersed in NaCl. Polished mortar specimens were studied using back-scattered electron imaging techniques and electron microprobe analysis to determine the extent of reaction of the FS and to obtain quantitative chemical analyses of the ASR gels. ASRfrequently appeared to initiate inside the reactive silica grains rather than at their peripheries, possibly reflecting the higher surface energies of microcracks and other internal discontinuities. The amount of visible reaction of the FS appeared greater in the KOH mortars, although they expanded less than the NaCl mortars. Element mapping of partially reacted FS grains demonstrated that alkali cations (Na⁺, K⁺) are clearly capable of diffusing into the silica structure, whereas the larger hydrated Ca²⁺ ions are not. The ASR gels generated in the NaCl mortars had low CaO contents < 1 wt%) compared with those produced in the KOH solution (∼14·5 wt%). This appears to provide conclusive evidence that high calcium contents inhibit the expansion of ASR gels. The implications of these experimental results for elucidation of the mechanism of ASR are discussed. The importance of absorption of alkali cations (Na⁺, K⁺) onto the surface of the reactive aggregate during the early stages of reaction and the availability of Ca²⁺ ions in the cement pore fluids in controlling the expansion behaviour of the resultant ASR gels are stressed.