Study of the effects of fly ash on alkali-silica reaction in concrete

grantor: University of TorontoIt is generally accepted that low-calcium fly ashes (Class F) are more effective than higher-calcium fly ashes (Class C) in suppressing expansion due to alkali-silica reaction (ASR). This thesis presents the findings of five separate studies. The intent of these studies was to examine the relationship between fly ash composition and level of replacement and various fundamental aspects of ASR such as expansion, pore solution composition, hydrate composition, and the nature of reaction products. In addition, the role of calcium in reaction and expansion was investigated to provide insight into the mechanisms of expansion and the suppression of expansion. Ternary blends using combinations of fly ash and silica fume were studied as a viable solution to providing adequate protection from ASR. Finally, a microstructural examination by scanning electron microscopy was conducted to provide insight into the effect of fly ash on the distribution, composition, and morphology of reaction products in concrete specimens. It was determined that fly ash's CaO content and level of replacement influence its ability to mitigate expansion due to ASR. Generally, reductions in expansion and pore solution alkalinity were observed with decreasing lime contents and increasing replacement levels. Pore solution alkalinity was found to be a governing mechanism in ASR expansion, especially at 25% replacement. The influence of other factors, such as ash composition, was more marked at 50% replacement. Effective suppression of expansion was observed for all ashes, regardless of composition, at 70% replacement. Combinations of fly ash and silica fume to effectively suppress ASR expansion is viable. It was demonstrated using mortar bar tests and microscopy that the availability of Ca(OH) 2 is necessary to produce damaging reaction.M.A.Sc