Use of fly ash, blast furnace slag and chemical gypsum for the synthesis of calcium sulfoaluminate-based cements

This paper presents a study on early strength cements based on calcium sulfoaluminate, C4A3$. These cements can be produced at temperatures ~300°C lower than normal portland cement and they also can be synthesized using industrial process wastes and by-products such as fly ash, blast furnace slag, chemical gypsum and other waste materials containing reactive sulfate and alumina. Cements designed to contain C4A3$, Beta.C2S and C$ or C4A3$, calcium sulfosilicate, C5S2$, and C$ have been synthesized using (a) pure analytical reagent (AR) calcium carbonate or commercial limestone as the source of CaO; (b) fly ash, blast furnace slag, bauxite, clay or aluminaas the source of Al2O3 and SiO2; and (c) natural gypsu, phosphogypsum or desulphogypsum as the source of sulfate. Ettringite, C6A$3H32, generated by the hydration of C4A3$ and C$ is responsible for the high early strength of these cements. The hydration of the silicate phase and the presence of C5S2$ contribute to the ultimate strength. These ettringite-containing cements do noy expand, and in fact they have dimensional stabilities similar to portland cement. In these type of cements durability problems may arise from the poor resistance of ettringite to carbonation. Due to the higher resistance to carbonation of another calcium sulfoaluminate hydrate, monosulfate (C4A$H12), the investigation has been extended to a composition which included brownmillerite, C4AF, whose presence promotes the conversion of ettringite to monosulfate during hydration.