Influence Mechanism of Curing Regimes on Interfacial Transition Zone of Lightweight Ultra-High Performance Concrete

This study aims to clarify the effects of curing regimes and lightweight aggregate (LWA) on the morphology, width and mechanical properties of the interfacial transition zone (ITZ) of ultra-high performance concrete (UHPC), and provide reference for the selection of lightweight ultra-high performance concrete (L-UHPC) curing regimes and the pre-wetting degree LWA. The results show that, under the three curing regimes (standard curing, steam curing and autoclaved curing), LWA is tightly bound to the matrix without obvious boundaries. ITZ width increases with the water absorption of LWA and decreases with increasing curing temperature. The ITZ microhardness is the highest when water absorption is 3%, and the microhardness value is more stable with the distance from LWA. Steam and autoclaved curing increase ITZ microhardness compared to standard curing. As LWA pre-wetting and curing temperatures increase, the degree of hydration at the ITZ increases, generating high-density CSH (HD CSH) and ultra-high-density CSH (UHD CSH), and reducing unhydrated particles in ITZ. ITZ micro-mechanical properties are optimized due to hydration products being denser.