Microstructural Studies of Alkali-Silica Reaction in Fly Ash Concrete Immersed in Alkaline Solutions

Abstract

This article presents expansion and microstructural data for a series of concrete mixes containing reactive flint aggregate, with a range of fly ash levels, exposed to various alkaline salt solutions. This study was undertaken to determine whether fly ash has any influence on alkali-aggregate reaction beyond changes in pore solution chemistry; in these tests the external source of alkalis should neutralize pore solution effects. Fly ash was found to be effective in reducing expansion even after extended periods (44 months) of exposure in 1N NaOH at 80°C, notwithstanding the presence of abundant reactive silica and an inexhaustible supply of alkali hydroxides. Higher levels of ash (40%) prevent damaging expansion and cracking in this environment despite considerable evidence of reaction. In some cases, flint grains had been completely removed by dissolution. The addition of Ca(OH)₂ at the mixing stage was found to increase the expansion of all the concretes; the effect on concrete with 40% ash was most marked, the expansion increasing by nearly 20 times. The most noticeable difference between deteriorated control specimens (no ash) and concrete with 40% ash was the formation of a calcium-alkali-silica rim on certain flint grains in concrete without ash. Such particles were invariably sites of expansive reaction with cracks emanating from them. The absence of such a feature in concrete with 40% ash is probably linked to the reduction in Ca(OH)₂ at the cement-aggregate interface. It is possible that the formation of this reaction rim produces expansive forces itself or acts as a semi-permeable membrane preventing diffusion of alkali silicate solution from the reaction site, thereby leading to osmotic pressure generation. Regardless of the actual mechanism, the presence of Ca(OH)₂ appears to be critical for the development of expansion due to alkali-silica reaction. It was observed that the alkalis of the reaction product were distributed in bands. In the Portland cement concrete specimens, the distribution of the gel consisted of a high calcium reaction rim at the aggregate-cement interface with a sodium-rich silica gel adjacent to it, followed by a potassium-rich silica gel. The potassium-rich silica gel appears to have a crystalline, needle-like structure, whereas the sodium-rich silica gel is amorphous. In fly ash concrete specimens in which the formation of calcium-rich reaction rim was prevented, it was observed that the sodium-rich gel had diffused into the surrounding cement matrix, and the potassium-rich gel had remained within the original aggregate boundary.