Investigation on combined use of biochar and ground granulated blast furnace slag as a supplementary admixture in concrete

Concrete is the most used material globally, with cement production causing 8% of emissions. Waste-based supplementary cementitious materials (SCMs) offer a partial cement replacement to address climate goals. The present study explores using Ground Granulated Blast Furnace Slag (GGBS) and biochar as SCMs to elevate concrete’s sustainability while maintaining structural performance. GGBS, sourced from steel production, was used at 20% and 40%, while biochar, derived from wood waste through pyrolysis, was incorporated at 3%, 4%, and 5% by weight of cement. The effects of these replacements were evaluated through compressive strength tests at 7 and 28 days, as well as microstructural analyses employing scanning electron microscopy (SEM) and energy-dispersive X-ray analysis. Findings revealed that GGBS enhanced workability due to its finer particles and pozzolanic activity while aiding long-term strength development. However, including biochar, particularly at higher percentages, led to a reduction in compressive strength, attributed to its porous structure and high carbon content, which weakened the interfacial transition zones (ITZ) and increased voids in the matrix. SEM analysis confirmed the highly porous nature of biochar, which interfered with the formation of calcium silicate hydrate (C-S–H), while EDAX showed a significant presence of carbon in biochar and GGBS, further explaining the dilution of strength. At 7 days, the compressive strength of concrete decreased by 29.4% (22.84 MPa) for 20% GGBS with 3% biochar, and up to 52.1% (15.49 MPa) for 40% GGBS with 5% biochar, compared to the control mix (32.34 MPa). At 28 days, the reduction ranged from 26.7% (25.02 MPa) for 20% GGBS with 4% biochar to 54.6% (15.49 MPa) for 40% GGBS with 5% biochar, relative to the control mix (34.14 MPa). Despite the reduction in early strength, the GGBS and biochar blends offer promise for applications focused on long-term durability and sustainability. This research highlights the need for careful optimization of mix proportions to find a middle ground between environmental benefits and mechanical performance.