Upcycling spent coffee grounds into biochar-reinforced geopolymer mortars: sustainable structural materials with enhanced mechanical performance

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Spent coffee grounds constitute a significant portion of organic waste in landfills. Converting this waste into biochar via pyrolysis offers a sustainable approach to mitigate environmental impacts and enhance construction materials. Although biochar has been studied in cement-based systems, its use in geopolymer binders as a low-carbon alternative is still limited. This study developed an eco-friendly binder by combining geopolymer materials with biochar derived from spent coffee grounds. Raw spent coffee grounds and their biochar were used to partially replace ground granulated blast furnace slag at 1%, 3% and 5% by weight. Mortar samples were tested for workability, tensile, flexural and compressive strength. Results demonstrated that biochar outperformed raw spent coffee grounds. Compressive strength decreased with increasing replacement levels. However, at the optimal biochar dosage of 3%, tensile strength increased by up to 29.3%, and flexural strength by 8.9% compared to the reference mixture.