Abstract: A method of producing a supplementary cementitious material, includes providing at least one waste material selected from quarry sludge, aggregate washing sludge and road cleaning sludge, removing excess water from said waste material so as to provide a dry waste material, and either: mixing the dry waste material with a source of calcium sulphate to obtain a raw material mixture, and calcining the raw material mixture at a temperature of 700-900° C. to obtain the supplementary cementitious material, or: calcining the dry waste material at a temperature of 700-900° C. to obtain a calcined waste material, and mixing the calcined waste material with a calcined source of calcium sulphate to obtain the supplementary cementitious material.