Synergistic valorization of lepidolite slag and fly ash into self-foaming lightweight aggregates

A sustainable route is presented for converting lepidolite slag (LS) and fly ash (FA) into high-performance lightweight aggregates (LWAs) without external foaming agents. By exploiting interfacial chemistry between LS and FA, trace residual carbon in FA promotes sulfate decomposition under localized reducing conditions while alkali oxides from LS adjust melt viscosity and promote liquid-phase formation. Thermodynamic calculation and thermogravimetry/ derivative thermogravimetry (TG/DTG) analyses indicated that residual carbon and Fe redox cycling substantially lowered sulfate decomposition barriers, enabling gas generation within the optimal viscosity window for stable foaming. The optimal L40 formulation (LS: FA = 4: 5) sintered at 1130–1150°C yielded LWAs with bulk density of 0.26–0.49 g/cm³, compressive strength of 1.03–5.49 MPa, and water absorption below 1 %. Micro-CT analysis of typical LWAs revealed radially graded pore structures with dense surface layers and porous cores, simultaneously achieving high porosity and low water absorption. High-temperature sintering transformed unstable aluminosilicates into stable feldspar-based (47.3 %) and amorphous (48.7 %) matrices, achieving > 99.5 % immobilization of critical contaminants (e.g., Be). Economic analysis demonstrated production costs of only 69.36 CNY/m³, markedly below the market price of 150.00 CNY/m³. This work demonstrates a technically and economically viable waste-to-resource strategy that addresses dual industrial waste streams while providing fundamental insights into sulfate self-foaming mechanisms, contributing to the circular economy in the lithium extraction industry.