The effects of silica fume and fly ash on reducing the risk of concrete damage due to alkali-silica reaction are presented in this thesis. One silica fume and twenty fly ashes of a wide range of chemical composition were investigated. The effect of the selected materials on expansion when used with high-alkali Portland cement and a siliceous limestone reactive aggregate (Spratt) was evaluated using the concrete prism and the accelerated mortar bar tests. Portland cement pastes and lime/pozzolan blends were prepared using similar combinations of materials used in the expansion tests. These samples were used to study the effects of fly ash and silica fume on the pore solution alkalinity and composition of hydration products. The study showed that the efficiency of fly ash in limiting the risk of deleterious expansion is linked to its efficiency in lowering the pore solution alkalinity. High-calcium and high-alkali fly ashes are generally less effective in this respect compared to low-calcium ash. Silica fume was found to be effective in reducing the pore solution alkalinity at early ages; however, the alkalinity was found to increase at ages later than 28 days. Combinations of moderate levels of silica fume (5%) and high-calcium fly ash (20-30%) were found to be effective in limiting the expansion even after three years. This was attributable to the high capacity of such combinations to achieve and maintain low pore solution alkalinity. The silica fume reduces the pore solution alkalinity at early ages and the fly ash maintained the low alkalinity at later ages. The study showed that, for low and moderate-alkali fly ash, the 14-day expansion in the accelerated mortar bar test could be used to predict the 2-year expansion of concrete prisms. The results from the lime/pozzolan blends confirmed that the hydration products of pozzolan play the dominant role in reducing the pore solution alkalinity of cementitious systems containing pozzolans. The two-year expansion of concrete prisms containing fly ash could be correlated reliably to the alkalis available from the ash as determined by leaching mature paste samples in leaching solution of alkali ion concentration of 0.25 moles per litre.