Effect of Ultra-Fine Mineral Admixture on the Performance of Ultra-High-Performance Concrete

The ultra-high performance of ultra-high-performance concrete (UHPC) has led to a significant increase in cement mixing, resulting in an increase in CO₂ emissions. Using mineral admixtures to replace cement partially is an effective way to reduce the cement amount. In this study, to investigate the effect of ultrafine fly ash (UFA) and ultrafine mineral powder (UMP) on the performance of UHPC, a series of experiments involving single admixture use of UFA, UMP, and their compound mixing were conducted. Results show that the cement dosage in UHPC can be reduced to 800 kg/m³ by mixing UFA and UMP. Compared with single mixing, compound mixing can effectively improve the fluidity and mechanical properties, and the optimal improvement effect is achieved when the dosage of UFA and UMP is equal. From the viewpoint of microscopic pore structure, the compound addition of ultrafine mineral admixtures can significantly improve the pore size distribution of UHPC by reducing the number of large pores and fissures and increasing the number of micropores and mesopores. From the perspective of microscopic hydration, ultrafine mineral admixtures can promote the cement hydration reaction, generating more C-S-H gels, enhancing the compactness of UHPC, and improving its mechanical properties and durability.