Effects of biochar on strength and CO2 mineralisation behaviour of MgO enhanced CS-GGBS geopolymer

The escalating accumulation of industrial solid wastes and carbon-intensive cement production has intensified climate change and extreme weather events, driving the demand for sustainable construction materials that synergise waste utilisation with carbon sequestration. This study systematically investigates the effects of magnesium oxide (MgO) content, durian shell biochar (DSB) dosage, and its modification on the compressive strength, carbon dioxide (CO2) capture and mineralisation performance, phase evolution, and microstructure of copper slag-granulated ground blast furnace slag (CS-GGBS) based geopolymer composites. The results reveal that 5% magnesium oxide incorporation in CS-GGBS geopolymers facilitates magnesium silicate formation, while optimal biochar addition enhances both geopolymerisation and carbon dioxide capture. Among them, the alkaline-modified biochar with larger pores (45.9678 nm) facilitates polymer gel encapsulation of unreacted particles, improving structural integrity. Co-grinding magnesium oxide with biochar enhances early strength and preparation-stage carbonation. Furthermore, carbon dioxide mineralisation predominantly occurs within the matrix during preparation and curing, while surface deposition dominates during carbon dioxide exposure, filling microstructural defects that become interwoven with the geopolymer network. The CS-GGBS composite with 5% magnesium oxide and 1.5% alkaline-modified biochar (24-h treatment) determined as optimal mixture, exhibiting 40.2 MPa compressive strength, 0.88981 mg/g carbon dioxide capture, and 4.25 Wt.% of mineralisation potential at least.