Synergistic effect of silica fume and ultra-fine glass microsphere on macro properties and hydration of coarse-aggregate UHPC

This study investigated the synergistic effect of silica fume (SF) and ultrafine glass microspheres (UGM) in coarseaggregate ultra-high performance concrete (CA-UHPC). Comprehensive assessments were conducted on macro properties, encompassing fluidity, mechanical properties, dry shrinkage, and chloride penetration resistance, while the hydration process was analyzed through the hydration heat, hydration products, and microstructure. The findings reveal that the fluidity of CA-UHPC exhibits a diminishing trend with escalating SF content but significantly improves with the incorporation of UGM. CA-UHPC containing 8 % SF attains the highest 28-day compressive strength of 166.9 MPa, while further increasing in SF content leads to reduced compressive strength. The introduction of SF and UGM effectively enhances the flexural strength by fostering stronger bonding between fibers and the matrix. Both SF and UGM increase dry shrinkage but enhance chloride penetration resistance. Notably, the combined use of SF and UGM significantly reduces the DRCM by 66.7 %. SF reduces the heat release during the acceleration stage and consumes more Ca(OH)2 than generated by Portland cement hydration, while UGM exhibits higher reactivity than FA and contributes more to the binder hydration. The combination of SF and UGM further improves the hydration degree and reduces porosity, resulting in a finer pore structure. This study underscores the advantages of incorporating SF and UGM in CA-UHPC, providing valuable insights for future research and applications in the field of UHPC.