Mechanical and microstructural regulation of biochar-geopolymer composites from industrial slag wastes

Eco-friendly geopolymers were prepared by alkali activation of ground-granulated blast-furnace slag (GGBFS) using calcium carbide slag (CS), with rice-husk biochar (BC, 0–30 wt%) incorporated as a functional additive. The optimal formulation CS/GGBFS = 15:85 with 1 wt% BC achieved a 28-day compressive strength of 27.3 MPa and a low porosity of 30.14%. Low BC dosages (≤5 wt%) markedly enhanced early-age and ultimate strength, whereas excessive BC (≥10 wt%) disrupted matrix continuity, increased porosity, and reduced strength. Microstructural analyses confirmed that low BC contents promoted calcium–aluminosilicate–hydrate gel formation through internal curing and nucleation effects, while high BC contents enlarged pores and induced stress concentrations. The strength–porosity relationship followed the Balshin model, indicating a critical porosity of ∼36% to maintain compressive strength ≥20 MPa. Overall, incorporating 1 wt% BC in a CS15:GGBFS85 system maximizes mechanical performance while enabling the sustainable co-utilization of industrial and agricultural wastes. • 1% biochar boosts strength (9-22.8%) with porosity reduced to 30.14%. • Optimal CS/GGBFS ratio (15:85) maximizes C-A-S-H gel and ettringite formation. • Biochar >5% raises porosity (30.14-41.91%) and degrades strength. • Biochar enables internal curing and nucleation for denser matrices. • Solid waste-biochar synergy supports carbon neutrality via resource co-utilization.