This study investigates the sulfate resistance of binary, ternary and quaternary mixes of Portland clinker,calcareous fly ash, slag and limestone.Neat and limestone cements expanded to damage within less than 80 days of exposure.A 45 wt.% clinker replacement by calcareous fly ash reduced the expansion significantly.However, the sulfate resistance varied between the two studied ashes.We observed that the expansion of mortars largely correlated to the amount of monosulfate in paste prior to exposure.Although the two ashes had similar alumina content, and thus similar potential to form monosulfate, their pastes contained very different amounts of this hydration product.The reason for that seems to be the fly ash reaction kinetics and the speed of supply of alumina to the system.