Internal curing efficiency and key properties of UHPC influenced by dry or prewetted calcined bauxite aggregate with different particle size

Abstract

A strong and porous aggregate (calcined bauxite, CB) has been applied as novel internal curing (IC) agent in UHPC to counteract its high autogenous shrinkage. However, in view of intelligently UHPC mixture design to acquire satisfactory workability, mechanical properties, and low-shrinkage UHPC, the influences of CB particle size and IC water incorporation methods are not well explored yet, and thereby are investigated in this study. UHPCs with different particle sizes (1 ∼ 3 mm, 3 ∼ 5 mm, and 5 ∼ 8 mm) of CB and methods of prewetting CB or using dry CB with adding IC water during mixing were investigated, together with preparing a reference UHPC (no CB). Multiple microstructural characterization techniques were used to explore the underlying mechanisms. Results show that the UHPCs with any condition of CB aggregate all obtained promoted mechanical properties and significantly reduced autogenous shrinkage compared to the reference UHPC. The large CB led to the highest compressive strength increment while the small and medium CB resulted in similar increments; more shrinkage reduction (higher IC efficiency) was achieved with the medium or small CB (above 70% reduction) than with large CB (58% reduction). The prewetting practice resulted in slightly more strength increment than using dry CB with adding IC water during mixing, and these two practices reached similar IC efficiency. Microstructural analysis with TG test and BSE observation confirmed the promoted hydration in the UHPCs caused by the IC effect of CB aggregate; reduced total porosity was observed in these UHPCs with CB by MIP test.