Biochar-Enhanced Geopolymers from Construction and Demolition Waste: Toward Carbon-Negative Construction Materials

Abstract The construction sector is a major contributor to resource consumption, waste generation, and global impacts, underscoring the need for circular strategies while reducing environmental burdens. In this study, biochar-based geopolymers incorporating construction and demolition waste are investigated as a pathway that closes material loops while enabling carbon mitigation. An experimental design was first applied to evaluate the influence of biochar incorporation (0–7.50 wt %) on material performance. A compromise formulation containing 5.00 wt % biochar was identified, providing a favorable balance between strength and matrix integrity, with a compressive strength of 24.9 ± 0.9 MPa after 28 curing days, alongside reduced porosity and water absorption relative to the control. The selected formulation was subsequently assessed using prospective life cycle assessment to quantify environmental impacts under current (2020) and future net-zero (2050NZ) energy scenarios. Under present-day conditions, the geopolymer exhibits a carbon footprint of ∼116 kg CO2-eq t–1, corresponding to a ∼86% reduction compared with Portland cement, driven by clinker avoidance and permanent carbon sequestration associated with biochar (∼146 kg CO2-eq t–1). Under the 2050NZ scenario, the geopolymer becomes net-negative (–107 kg CO2-eq t–1), whereas cement remains carbon-positive due to residual process emissions. The multi-indicator assessment reveals trade-offs in water consumption and land occupation associated with biomass sourcing and alkaline activators, although most energy- and emission-related impacts improve under future decarbonized systems. Overall, the integrated experimental-prospective framework demonstrates that biochar-based geopolymers represent a promising class of circular, low-carbon, and potentially carbon-negative construction materials required for deep decarbonization of the built environment.