Foam-based eco-lightweight controlled low-strength material for high-volume utilization of municipal solid waste incineration bottom ash

Abstract

The increasing scarcity of natural aggregates, coupled with the growing environmental burden associated with municipal solid waste incineration (MSWI) residues, poses a dual challenge for sustainable urban construction. Previous studies have primarily investigated MSWI bottom ash (IBA) or foam-modified mortars in isolation; however, the combined effects of IBA, fly ash, and pre-formed foam on controlled low-strength material (CLSM) have not yet been systematically evaluated. This study develops an integrated, waste-valorized CLSM system and systematically investigates the fresh-state, hardened, durability, and eco-mechanical performance of fifteen mixtures incorporating 0–40% IBA, 0–20% fly ash, and 0–20% pre-formed foam. Cylindrical specimens were tested in accordance with relevant ASTM standards to evaluate flowability, bleeding, setting time, water absorption, 3–28-day compressive strength, and durability under simulated acid rain and seawater immersion, supported by a normalized multi-criteria assessment (MCA) and eco-mechanical indicators (relative carbon emission [RCE] and relative energy consumption [REC]). The results indicate that increasing IBA content reduced flowability from 170 mm to 130 mm and increased water absorption by up to 49%, whereas foam incorporation significantly enhanced flowability to a maximum of 270 mm, albeit with an associated increase in water absorption to 5.44–5.91%. The 28-day compressive strength decreased from 3.213 MPa in the control mixture to 2.187 MPa at the highest combined waste and foam content; nevertheless, all mixtures exceeded the minimum 1 MPa requirement for CLSM applications. Durability losses ranged from 4.77% to 10.51% under acid rain exposure and from 8.34% to 16.61% under seawater immersion, with high-porosity mixtures being the most adversely affected. Despite these trade-offs, mixtures with moderate waste contents—particularly M5 (10% IBA, 10% fly ash, and 10% foam)—exhibited a well-balanced performance, achieving an MCA score of 0.66, a 28-day compressive strength exceeding 2.5 MPa, a flowability of approximately 210 mm, and 85–88% strength retention under aggressive environments. Overall, the findings indicate that the combined use of IBA, fly ash, and pre-formed foam can produce lightweight CLSM with a favorable balance between engineering performance and strength-normalized environmental efficiency, making it promising for non-structural applications such as coastal and municipal backfilling.