The role of silica fume in activating composite cementitious materials blended with red mud and phosphogypsum

Using red mud (RM) and phosphogypsum (PG) as supplementary cementitious materials (SCMs) enabled large-scale industrial waste treatment and optimized resource utilization in cement production. Nonetheless, the mechanical strength of composite mortar and workability of composites manufactured by these materials were not optimal. This study introduced silica fume (SF) as a crucial activator to enhance hydration in RM-PG-OPC systems. Both the strength and workability of RM-PG-OPC cementitious system were improved by incorporating SF. Results indicated that the optimal component-mix ratio of RM, PG, and SF with replacing 30 % OPC, was 15:8:7. The compressive strengths of composite mortar were 27.30 MPa and 41.30 MPa, accounting for 103.40 % and 105.36 % of OPC (26.40Mpa and 39.20Mpa) at 3 and 28 days, respectively, with initial and final setting times of 290 min and 370 min. It was also revealed that the RM-PG-SF-OPC cementitious system exhibited the highest cumulative exothermic rate at 72 h, attributed to SF’s nucleation and pozzolanic effects. Based on the analysis of X-ray diffraction (XRD), thermogravimetric analyzer (TG), and scanning electron microscope-energy dispersive spectrometer (SEM-EDS), It was further demonstrated that SF involved in the RM-PG-SF-OPC cementitious system not only enhanced the leaching levels of Ca 2 + , Mg 2+ , and Al 3+ , but also facilitated reticular nucleation on SF particles, which promoted the formation of C-(A)-S-H, ettringite (AFt), and a denser gel structure. This research aims to optimize the RM-PG-OPC cementitious system by adding SF and provide guidance for large-scale treatment and efficient resource utilization of industrial solid wastes. • The compressive strength and workability were improved by adding the SF, RM and PG into cement. • The main impact of SF in the early hydration was to accelerate the rapid formation of AFt. • The function of SF in the late hydration was to continuously react with CH to form gels. • Due to the activation of SF, a mass of Ca 2+ , Al 3+ , Mg 2+ leached out from RM and PG in composite. • The role of SF was to promote the formation of C-(A)-S-H/M-S-H and densify these gels structure.