In-depth analysis of the synergistic effects of red mud, circulating fluidized bed fly ash and desulfurization gypsum in the preparation of foam lightweight soil: Reaction mechanism, performance optimization and carbon emission analysis

Foam lightweight soil (FLS) is a kind of porous lightweight engineering material with low density, high fluidity, and controllable strength. It is widely used in civil engineering in the fields of filling, roadbed reinforcement, and slope support. This study developed a novel FLS through synergistic utilization of red mud (RM), circulating fluidized bed fly ash (CFA), and desulfurized gypsum (DG). The response surface method (RSM) was used to design 16 sets of experiments and to predict the mechanical properties of the FLS. Moreover, the reaction mechanisms, compound synergistic effects and carbon emissions of the solid wastes were analyzed in depth. The results revealed that when the RM dosage was less than 50 %, the unconfined compressive strength (UCS) of FLS tended to increase and then decreased with increasing CFA dosage and decreasing DG dosage. The prediction result of the RSM indicated that the 7 d and 28 d UCS of FLS can be more than 1.8 MPa and 2.1 MPa, respectively, at solid waste doses greater than 50 %. Microanalysis revealed that the UCS of FLS was influenced by both the hydration products and the pore structure. Too much RM incorporation leads to more macropores and connecting pores in FLS, which is not conducive to strength. In addition, the carbon emission and social cost of carbon of the FLS were approximately 45.476 kgCO₂/m³ and 2.85$, respectively. A comparison of results from other studies revealed that each cubic meter of FLS reduces ${\text{E}}_{{\text{CO}}_2}$ and SCC by approximately 75.2∼86.7 % while ensuring that the performance meets the required engineering requirements. This paper provides a theory for the preparation of solid waste-based FLS in roadbed filler and slope support.