Mineralogy of aluminous cements and their reactivity
This project was initiated to determine the effects of cooling conditions on the mineralogy of aluminous cements fired in oxidizing and reducing atmospheres. Using a vertical tube furnace, compositions covering the range of aluminous cements were melted at a predetermined temperature and at various oxygen partial pressures. They were subsequently cooled at different rates. Product identification was undertaken by X-ray diffraction, optical and scanning electron microscopy, and analytical electron microscopy. The clinker microstructures were found to differ radically with cooling rate, and the nature of the phases crystallizing proved to be very sensitive to the Fe2+/Fe3+ ratio of the parent melt. The order of crystallization and the phase composition of the individual phases were also affected by thermal history. Phases grown with fast undercooling exhibited departures from solid solution limits established under equilibrium. Furthermore, the coexistence of different phase assemblages within a same clinker was pointed out. With respect to the phases determined, more attention was paid to CA and pleochroite, for which latter high resolution transmission electron microscopy studies were carried out. Mixed pleochroitegehlenite intergrowths were characterized. Another aim of the project was to investigate the consequences of the mineralogy of the clinker on reactivity and strength of the cement. To this end, microcalorimetry experiments were performed to study the reactivity of cement up to 24 hours, and the compressive strength development of pastes was measured for ages up to 28 days. The mineralogical transformations taking place during the hydration of cement were explored, using the X-ray diffraction technique. As a result, different trends could be outlined. Reactivity and strength were found to be affected by cooling rate and FeO content of the cement, the respective influence of which differed with the overall chemical composition of the clinker.