Synergistic regulation of workability, mechanical properties, and thermal-shrinkage behavior of C60 self-compacting concrete with ultra-fine mineral admixtures

To mitigate the early-age cracking risk in C60 self-compacting concrete (SCC) induced by thermal stress and autogenous shrinkage, this study systematically investigates the effects of cementitious systems incorporating mono and blended ultra-fine admixtures (UMAs) derived from various industrial by-products, such as ultra-fine steel slag (SS), ultra-fine fly ash (FA), fly ash cenospheres (FAG), and ultra-fine phosphorus slag (PS), on the workability, mechanical properties, and thermo-shrinkage behavior of the concrete. The results demonstrate that the fresh properties are primarily governed by the particle morphology and ball-bearing effect of the UMAs. Notably, the SS and SS+FAG systems exhibited the lowest superplasticizer demand (1.69–1.72%), effectively reducing viscosity while enhancing passing ability. Although the dilution effect of UMAs universally retarded early-age strength development, their pozzolanic activity or potential hydration activity fostered sustained strength gain, with the 90d compressive strength reaching a peak of 93.7 MPa—a 14.1% improvement over the reference mix. Critically, the SS-based systems (SS and SS+FA), owing to their sluggish reactivity, delayed the hydration heat evolution and effectively mitigated shrinkage. The mono-SS system, in particular, reduced the 7d adiabatic temperature rise and autogenous shrinkage by 15.3% and 23.5%, respectively, compared to the reference. In contrast, while the PS system achieved the highest later-age strength, it also generated the most significant heat release and temperature rise. A comprehensive evaluation indicates that incorporating SS, either alone or in combination with FA, represents the most effective strategy for fabricating high-performance SCC with a desirable balance of excellent workability, high later-age strength, and low cracking potential.