Phase and porosity changes in a calcium aluminate cement and alumina system under hydrothermal conditions

When refractory castables are dried, hydrothermal conditions may result inside the bodies if the H2O cannot escape from the material. Under such high-pressure conditions, problems such as explosive spalling can arise. As different curing temperatures during the hydration of calcium aluminate cement (CAC)-bound castables lead to the formation of different hydrate phases, different microstructures can develop in the hardened material. This study presents the changes in porosity and in the mineralogical composition of a refractory castable model system under hydrothermal conditions depending on the curing temperature (5, 23 and 40 degrees C). Quantitative X-ray diffraction (QXRD) measurements show that different hydrate phases are formed during curing, while C(3)AH(6) and boehmite are formed in the same quantities after hydrothermal treatment in an autoclave at similar to 11 bar/180 degrees C. Although the mineralogical composition after autoclaving is not different, the three samples differ in their microstructure. Mercury intrusion porosimetry measurements reveal that although the total porosity after autoclaving is the same, the 40 degrees C samples have a higher proportion of large pores. SEM images also show that the appearance of C(3)AH(6) in the 40 degrees C autoclaved samples varies, which originates from the starting phase composition and microstructure after curing.