Application of biochar to mitigate the autogenous shrinkage and improve mechanical performance of engineered geopolymer composites prepared with different ground granulated blast-furnace slag contents

Biochar can be a promising additive for production of cementitious materials with enhanced mechanical performance and reduced carbon footprint. This paper investigated the autogenous shrinkage, compressive and flexural strengths, and tensile behaviour of biochar incorporated engineered geopolymer composites (BC-EGC) with different ground granulated blast-furnace slag (GGBFS) proportions. The results indicated that the addition of biochar mitigated the autogenous shrinkage of EGC by up to 13.4 %. The improvements in the mechanical properties due to biochar addition decreased with increasing GGBFS proportions. The compressive, flexural, tensile strengths, and strain capacity of EGC were enhanced by up to 26.7 %, 19.9 %, 18.0 % and 6.1 %, and 7.1 %, 5.2 %, 2.5 % and 1.0 % for GGBFS percentages from 20 % to 80 %, respectively. The strength and strain enhancements were mainly attributed to the internal curing and filling effects, and shrinkage mitigation due to biochar addition. The enhanced gel formation due to incorporating an appropriate quantity of biochar was verified by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). Furthermore, based on the correlation between the compressive, flexural, and tensile strengths and carbon footprints of BC-EGC, biochar (4 %) can be more effective for EGC with 40 % and 60 % GGBFS contents, regarding achieving higher strength output with unit carbon footprint.