Scenario-oriented functional mapping of ultrafine mineral admixtures in cementitious composites using a weighted activity–thermal–rheology index

The selection of ultrafine mineral admixtures is often based on isolated indicators such as strength, heat release or flowability, which cannot adequately reflect their scenario-dependent functions in cementitious composites. In this study, seven representative ultrafine mineral admixtures, including ultrafine metakaolin (UMK), ultrafine fly ash (UFA), fly ash microspheres (FAM), ultrafine limestone powder (ULP), ultrafine slag (UFS), ultrafine phosphorus slag (UPS) and ultrafine steel slag (USS), were systematically evaluated under a unified experimental basis. A weighted activity–thermal–rheology index (WATR) was proposed by integrating normalized 3d compressive strength, 7d cumulative heat release, and superplasticizer demand, evaluated under targeted experimental conditions (a water-to-binder ratio of 0.4, with 20% replacement for strength and thermal tests) and 30% replacement for the superplasticizer demand test). The results showed that UMK achieved the highest 3d compressive strength of 25.32 MPa and the highest E-WATR score of 0.612, indicating its suitability as an early-strength activating component. FAM showed excellent rheological compatibility and the highest average pore diameter reduction ratio, supporting its role as a rheology-regulating and pore-refining companion powder. USS exhibited the lowest 7d cumulative heat release of 273.0 J·g-1 and the lowest superplasticizer demand of 0.05%, leading to the highest T-WATR and R-WATR scores. Hydration and microstructural analyses revealed that UMK was dominated by pozzolanic CH consumption, UFS and UPS contributed mainly to later-age densification, while ULP behaved primarily as a dilution/nucleation filler. Based on the WATR evaluation and mechanistic evidence, three concrete design routes were proposed tailored to specific application scenarios: UMK + FAM for early-strength and high-durability concrete, standalone USS for low-temperature-rise mass concrete, and USS + FAM for high-fluidity mass concrete, respectively. This study provides a quantitative and scenario-oriented framework for functional selection and combination of ultrafine mineral admixtures. © 2026 Published by Elsevier Ltd.