Study on mechanical properties and microstructures of UHPC containing calcined kaolin clay at elevated temperature

Calcined kaolin clay (CKC) as a kind of highly active admixture can be served as the reactive powder in UHPC preparation, which is beneficial to cost reduction and environmental protection. This study investigated the mechanical performances and microstructures development of UHPC blended with CKC at elevated temperature. The selected temperature is from 200–600℃ with a temperature gradient of 100℃. Mechanical properties of CKC contained UHPC at elevated temperature were evaluated with compressive and flexural strength tests. Hydration and microstructures of UHPC were determined with modern analysis and testing technologies including FTIR, XRD, TG/DTA, MIP and SEM measurements. Results indicates that the residual compressive strength of CKC contained UHPC increases first at 200℃ and then decreases from 300–600℃ while its flexural strength maintains a downward trend at elevated temperature. The residual compressive and flexural strengths of UHPC incorporating CKC are higher than that of control mixture at 600℃ due to the pozzolanic reaction of CKC consuming abundant portlandite. The amount evolution of portlandite in UHPC at elevated temperature demonstrates that new hydration reaction was generated in UHPC mixture in the former 300℃. The gel decomposition amount in CKC contained UHPC is higher than that of control mixture in the former 400℃, which correspondingly leads to a larger declining speed of compressive strength for UHPC incorporating CKC compared to blank mixture. The MIP and SEM results reveal that the pore structures of UHPC coarsen with growth of temperature and its porosity obviously increases after 400℃. The porosity increment of CKC contained UHPC is apparently less than that of control mixture from 400–600℃ since the widths and lengths of cracks generated in CKC contained UHPC are obviously less than that of blank mixture in the temperature range of 400–600℃. It is indicated that CKC can contribute to alleviate crack development of UHPC at high temperature.