Substituting Slag with Calcined Clay to Mitigate Conversion in Calcium Aluminate Cement

Blast furnace slag is the most widely researched supplementary cementitious material (SCM) used to both reduce the carbon footprint of calcium aluminate cement (CAC) and inhibit conversion. Due to problems with the availability and cost of slag, there is a need to investigate other possible SCMs and their application potential in CAC. In this study, calcined clay with medium kaolin content (40% kaolinite) available in the region was used. The conversion process was accelerated by curing at an elevated temperature (38°C). The progress of conversion was evaluated by the development of compressive strength up to 56 days. At the same time, changes in the reaction products were analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and the pore structure by mercury intrusion porosimetry (MIP). For the CAC samples cured at 38°C, TGA and XRD confirmed the conversion of metastable hydrates to stable hydrogarnet, resulting in the formation of new capillary pores, a significant increase in critical pore diameter, and a 60% loss of compressive strength. When slag or calcined clay was added to CAC, no decrease in compressive strength was observed after curing at elevated temperatures. When calcined clay was added to CAC, strätlingite hydrate was formed, which led to compaction of the structure and inhibition of conversion. This study has shown that the use of calcined clay instead of slag in CAC can effectively inhibit conversion, resulting in stable compressive strength regardless of the curing temperature.