Microstructural and microanalytical studies of sulfate attack. II. Sulfate-resisting Portland cement: Ferrite composition and hydration chemistry

Abstract

A sulfate-resisting Portland cement and pastes made from it were studied by X-ray microanalysis of polished sections in the scanning electron microscope, X-ray diffraction and thermal analysis. The ferrite phase had the approximate composition Ca 2Al 0.8Mg 0.2Si 0.2O 5 with minor substitutions of Fe by Ti and Mn. The hydration products other than CSH and calcium hydroxide were small in amount and, for the most part, closely mixed with each other and with CSH. Ettringite was formed initially; some persisted for at least one year. Only a little monosulfate was formed. Mg appeared mainly to enter a hydrotalcite-type phase, and Fe a poorly crystalline hydrogarnet. Calculated X-ray powder diffraction patterns for the ferrite and hydrogarnet are given. Factors contributing to the superior resistance to sulfate attack include the low content of Al 2O 3 in the hydration products and the fact that much of this Al 2O 3 is present for a long time as ettringite. As a result, the potential for forming ettringite through reaction with sulfate from an external source is considerably less at all ages than is the case with an Ordinary Portland Cement.