Combined regulation of silica fume reactivity and coarse aggregate skeleton toward high-strength and low-shrinkage ultra-high performance concrete

To address the issues of large total shrinkage and high cost in ultra-high performance concrete (UHPC), this study developed high-strength and low-shrinkage UHPC by adjusting the reactivity of silica fume and introducing coarse aggregate. The hydration process, phase composition, microstructure, compressive strength, and volume stability of the UHPC system were systematically characterized. The results indicate that the pozzolanic activity of silica fume follows the order of SF1 > SF3 > SF2. Highly reactive silica fume (SF1) can promote the consumption of portlandite (CH) and the formation of amorphous gels. The 28 d non-crystalline fraction of the U-SF1 group reaches 45.82%, while its CH content decreases to 2.05%. In contrast, the U-SF2 sample incorporating low-reactivity silica fume still retains relatively high contents of clinker minerals and CH, accompanied by a slower hydration process and consequently increased porosity. SF3 possesses moderate pozzolanic reactivity and can effectively balance the strength and shrinkage performance of UHPC. The U-SF3 group without coarse aggregate exhibits a 28 d compressive strength of 191.16 MPa and a 60 d total shrinkage of 1090.24 μm/m. In comparison, U-A500 incorporating 500 kg/m3 coarse aggregate delivers a lower 60 d total shrinkage of 820.38 μm/m, with a satisfactory 28 d compressive strength of 178.60 MPa. CT reconstruction results also reveal that U-A500 has a porosity of 3.27% with a uniform and acceptable spatial distribution of steel fibers.