Collaborative recycling of red mud and FGD-gypsum into multi-shell cold bonded lightweight aggregates: Synergistic effect, structure design and application in sustainable concrete

Abstract

In this study, novel multi-shell CLAs were prepared by synergistically using red mud (RM) and flue gas desulfurization gypsum (FGD). The performance optimization, mineral compositions, microstructures, and pore size distributions of the CLAs were investigated in detail. The results indicated that the synergistic using of RM and FGD significantly improved the strength of the CLAs, and that the multi-shell structure design by encapsulating, incorporating, and coating EPS, EPP, and hydrophobic shell effectively reduced the density and water absorption of the CLAs. Based on the synergistic effect between RM and FGD, the ettringite, C(N)-A-S-H and U-phase were the main minerals of the CLAs, which jointly contributed to the strength development of the CLAs. The multi-shell structure including lightweight core, intermediate shell and the hydrophobic outer shell further reduced the density and water absorption of the CLAs. Hence, viable CLAs with loose bulk density of 940 kg/m3, compressive strength of 7.3 MPa and 1 h water absorption of 6.91 % were obtained. The environmental impact and durability of the CLAs were evaluated acceptable. Furthermore, the prepared CLAs were employed as aggregates for lightweight aggregate concrete (LWC) preparation. The workability and mechanical strength of the LWC were investigated, and the LWC with density of 1880–2100 kg/m3, compressive strength of 39.8–54.5 MPa, and splitting tensile strength of 2.44–2.95 MPa was obtained. Finally, the carbon emissions and cost of the LWC were calculated as 288.5 kgCO2/m3 and 335.31Yuan/m3 (¥), respectively, which were 36.2 % and 23.1 % lower than those of traditional concrete. Overall, this research may provide a green approach for converting RM and FGD into value-added CLAs, with a potential application in sustainable concrete.