Mechanism and optimisation of sintered ultra-lightweight aggregates with sand washing slurry and red mud

Sand washing slurry (SWS), red mud (RM) and sawdust are major solid wastes produced by the construction, mining and wood processing industries, respectively. The accumulation of these wastes poses significant environmental hazards and is a substantial waste of resources. In this study, a novel rapid sintering method was used to prepare ultra-lightweight ceramsite aggregates (ULC) using SWS, RM and sawdust as raw materials, achieving a utilisation rate of the SWS exceeding 60%. ULC aggregates with a loose bulk density of 264.3 kg/m 3 were produced. The influence of material formulation on the performance of the ULC aggregates was studied. The phase changes and expansion mechanism of the ULC aggregates were examined from microscopic and chemical perspectives. The synergistic effect of RM and sawdust significantly enhanced the foaming effect of the ceramsite (i.e. porosity), reducing its loose bulk density while slowing the rate of strength reduction. Additionally, increasing the contents of RM and sawdust promoted ceramsite expansion. When the mass ratio of SWS, RM and sawdust was 29:12:1, with a hematite (Fe 2 O 3 ) content of 13% and sawdust content of 3%, preheating at 500°C for 5 min followed by sintering at 1150°C for 5 min produced ULC with a compressive strength of 1.7 MPa. This study provides a promising method for producing ULC aggregates from industrial solid wastes.