High performance and low carbon flowable solidified soil based on construction muck and alkali-activated solid wastes

Flowable solidified soil (FSS) provides an effective route for the resource utilization of construction muck (CM) toward engineered backfill. This study develops a low-carbon, high-performance FSS based on composite binders composed of cement and alkali-activated solid wastes. Lithium slag (LS) dosage, cement dosage, and activator dosage were selected as design variables. Response surface methodology (RSM) was used to model flowability and compressive strength at 28 days, while life-cycle assessment (LCA) quantified the global warming potential (GWP) and cumulative energy demand (CED). Based on these models, NSGA-III coupled with entropy-weight TOPSIS was used to identify the best compromise mixture under coupled performance and carbon-emission objectives. The results showed that FSS flowability and strength can be regulated through mix design, while GWP is mainly governed by cement and activator dosages. Microstructural results showed that C-S-H gels, partly incorporating Al and Na, together with AFt, promoted interparticle bridging and pore filling, thereby densifying the stabilized matrix and contributing to strength development. Multi-objective optimization identified the recommended mixture as 4% LS, 2% cement, 14% GGBFS, and 12.3% activator. Validation tests gave a flowability of 279 mm and a 28 d compressive strength of 6.05 MPa, with prediction errors below 5%, supporting the applicability of the optimization framework within the tested variable ranges.