Thermal Shock-Resistant Cement

Abstract

We studied the effectiveness of sodium silicate-activated Class F fly ash in improving the thermal shock resistance of Secar #80 refractory cement. We performed a multiple heating -water cooling quenching cycle test to evaluate cement thermal shock resistance. In one cycle, the 200°C-autoclaved cement was heated at 500°C for 24 hours, and then the heated cement was rapidly immersed in water at 25°C. This cycle was repeated five times. The phase composition of the autoclaved #80/Class F fly ash blend cements comprised four crystalline hydration products, boehmite, katoite, hydrogrossular, and hydroxysodalite, responsible for strengthening cement. After a 5-cycle heat-water quenching test, three crystalline phase transformations in this autoclaved cement were observed: boehmite ? ?-Al2O3, katoite ? calcite, and hydroxysodalite ? carbonated sodalite. Among these phase transformations, the hydroxysodalite ? carbonated sodalite not only played a pivotal role in densifying cementitious structure and sustaining the original compressive strength developed after autoclaving, but also offered an improved resistance of #80 cement to thermal shock. In contrast, the autoclaved Class G well cement with and without Class F fly ash and silica flour failed this cycle test, generating multiple cracks in the cement. The major reason for such impairment was the hydration of lime derived from the dehydorxylation of portlandite formed in autoclaved cement, causing the cement’s volume expansion.