The experimental study aims to prevent the alkali-silica reaction (ASR) of mortars made with reactive aggregates, where the volume rates of cement ranging from 10% to 50% are substituted by mineral admixtures such as Granulated Slag, Natural Pozzolan and Silica Fume. Their effectiveness in countering the ASR adverse effects; such as weight and dimensional variations, the width and density of cracks and mechanical performance losses was studied. The results obtained show that the rate and the nature of the mineral admixtures influence their effectiveness, for Granulated Slag and Natural Pozzolan the ideal rate is 30%, for Silica Fume the 10% rate seems effective in countering ASR adverse effects. This effectiveness is ensured by the pozzolanic reaction favored by the high temperature of the test and the alkalinity of the system.