Application of waste Eucalyptus-derived biochar for sustainable low-carbon and carbon-negative geopolymer: Drying shrinkage, mechanical performance, and acid resistance

Due to the highly porous, lightweight, and carbon-negative nature, biochar can be potentially used as an internal curing agent for sustainable geopolymer production. This paper studied the drying shrinkage, mechanical behaviour, and acid resistance of geopolymer pastes introduced with coarse (1.18 mm) and fine (0.425 mm) Eucalyptus-derived biochar and cured under 40 and 80 °C. The inclusion of coarse biochar declined the strength of the geopolymer despite the curing temperatures. The 28-day drying shrinkage was mitigated by 13.2 % to 15.9 % and 3.8 % to 5.4 % under 40 and 80 °C curing, respectively. The introduction of 2.5 % and 5 % fine biochar under 40 °C curing increased the compressive strength by 6 % and 12 %, respectively. The flexural strength was enhanced by 5 % with 2.5 % fine biochar. The enhancements in the mechanical performance were associated with the improved geopolymerisation induced by the internal curing effects of biochar, which can be verified by the microstructure analysis. After being exposed to 5 % sulfuric acid for 360 days, the samples with 2.5 % fine biochar demonstrated less mass loss, increased compressive strength, and improved strength retention ratio. The carbon emissions of the geopolymer were reduced by 27.9 % to 30.9 % with 2.5 % biochar. Increasing dosages of biochar to 10 % produced carbon-negative geopolymer (−34.5 to −10.9 kg CO₂/m³). The above results indicated that incorporating biochar in geopolymer could potentially be a viable solution to produce low-carbon geopolymer with improved shrinkage, mechanical, and durability properties.