Insight into the synergic effects of circulating fluidized bed fly ash, red mud and blast furnace slag in preparation of ultrahigh-performance concrete: Reaction mechanism and performance optimization

Abstract

Ultrahigh-performance concrete (UHPC) is a kind of special concrete with excellent mechanical properties and good flexibility that has attracted much attention. Partial replacement of cement with supplementary cementitious materials (SCMs) can significantly reduce costs and ensure UHPC performance. The composite preparation of cementitious materials using the physicochemical properties of different solid wastes has been proved to be effective. On this basis, the highly alkaline solid waste red mud (RM), highly pozzolanic activity solid waste containing sulfate circulating fluidized bed fly ash (CFA) and high pozzolanic activity solid waste blast furnace slag (BFS) were utilized to prepare low cement clinker UHPC. 20 groups of experiments were designed by response surface methodology (RSM). Then, the optimization and prediction of the performance of low cement UHPC was explored using simplex lattice method. In addition, the hydration degree and pore structure were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), mercury intrusion porosimetry (MIP) and back scattered electron (BSE). In the case of 60% cement blending, the best compressive strength (128.15 MPa) occurred in 9% CFA, 28% RM and 3% BFS. The results demonstrated that the increasing strength existed with the addition of RM and a high content of CFA had a negative impact on performance development. In the cases of low cement systems (<45%), however, the compressive strength increased and then decreased as CFA content increased from 0 to 40% or RM content increased from 0 to 70%, which meant that a compound synergistic existed among the three solid wastes. Microstructure analysis demonstrates that the hydration degree and pore structure of UHPC were effectively improved due to synergies among CFA, RM and BFS. These results implied that the compound synergy is more significant in low clinker systems and beneficial to the development of strength. This paper provides a reference for low cement clinker and the low-cost preparation of UHPC.