Utilization of industrial by-products as binders and fine aggregates for one-part lightweight controlled low-strength materials: Turning waste to value approach

This study investigated the fresh and hardened properties of controlled low-strength material (CLSM) using ground coal bottom ash (GBA) mixed with ground calcium carbide residue (GCR) as the primary binder. Sieved coal bottom ash (SBA) fully replaced the river sand by volume. The effects of sodium metasilicate pentahydrate (SM) at 2 %, 4 %, 6 %, and 8 % by weight of binder as a solid activator for one-part CLSM were also examined. Fresh and hardened properties, including flowability, hardening time, bleeding, ball drop, unconfined compressive strength (UCS), and elastic modulus, were evaluated. The microstructures of the CLSM pastes were analyzed using X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). All CLSM mixtures exhibited high flowability (>200 mm). The proportions of GBA and GCR significantly influenced the fresh and hardened properties. Replacing river sand with SBA resulted in lightweight CLSM; however, complete replacement increased hardening time, bleeding, and ball drop value and reduced UCS. An optimal SM dosage of approximately 4 % resulted in a 28-day UCS of 2.54 MPa, bleeding of 0.6 %, hardening time of 33 h, and a ball drop value of 68.5 mm, meeting typical CLSM requirements. XRD and SEM-EDS results revealed that the GCR acted as a crucial source of calcium oxide. Their effective reaction to strength development in C-S-H and N-A-S-H gels, particularly when activated by 4 % SM, positively contributed to the observed compressive strength and denser matrix of the CLSM. Incorporating GBA, GCR, and SBA offers a sustainable, low-carbon approach for CLSM production, promoting waste valorization and conserving natural resources. • GBA and GCR were used as hybrid precursors to fully replace cement in CLSM. • SBA completely replaced river sand to produce lightweight CLSM with reduced density. • SM enabled one-part alkali activation for CLSM mixtures. • An optimal 4 % SM dosage enhanced strength and reduced bleeding and hardening time. • CLSMs made with 100 % waste had low CO₂ emissions (22–59 kg/m³).