Effects of Mineral Admixtures on the Alkali–Silica Reaction in Granite Manufactured Sand Mortar

The occurrence of the alkali–silica reaction (ASR) in granite manufactured sand concrete reduces its durability and service life. However, the combination and effect of different mineral admixtures on inhibiting ASR in granite-manufactured sand concrete and the in-depth understanding of the inhibition mechanism still need to be further explored. Therefore, this study accordingly analyzed the alkali activity of granite manufactured sand and the more active aggregates were selected to investigated the inhibitory effects of fly ash (FA), silica fume (SF), and slag (BS) mineral admixtures on the occurrence of ASR in mortar when used individually or together, as well as their optimal dosages; and XRD and SEM observations were analyzed to explore the mechanisms through which the mineral admixtures inhibited the ASR in the granite manufactured sand mortar specimen. The results indicate that: The existence of excessive ASR expansion in granite manufactured sand concrete, when the content of active sand was the most unfavorable, the ASR in the mortar specimen was the strongest and the corresponding expansion rate was the largest; otherwise, the intensity of the ASR in the mortar as weakened and the expansion rate was reduced; controlling the contents of alkali and active aggregates in the cementitious system to avoid the most detrimental proportions can alleviate the effects of ASR; the optimal contents of FA, SF, and BS individually were 30%, 20%, and 5%, respectively; and ASR was most effectively inhibited in the granite manufactured sand mortar when 5% SF was mixed with 5% FA or 5% BS; the inhibition of ASR in granite manufactured sand mortar is affected by the content of the active components as well as the particle fineness of the included admixture; the greater the content of active components, the smaller the average particle size and the better the inhibition effect. Finally, the considered admixtures were shown to inhibit ASR by reducing the total alkali content in the mortar and slowing the formation of ASR gel; generating C–S–H gel through the pozzolanic reaction with the calcium hydroxide cement hydration product, reducing the chances of contact between potassium and sodium ions and the active aggregate; and improving the microstructure of the interfacial transition zone and densifying the structure of the slurry, thereby impeding the diffusion of alkali to the interior.