MICROSTRUCTURAL AND MICROANALYTICAL STUDIES OF SULFATE ATTACK .2. SULFATE-RESISTING PORTLAND-CEMENT FERRITE COMPOSITION AND HYDRATION CHEMISTRY

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 Ca2Al0.8Fe0.8Mg0.2Si0.2O5 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 Al2O3 in the hydration products and the fact that much of this Al2O3 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.