Spent coffee grounds ash as a potential sustainable binder for mechanical and microstructural enhancement of alkali-activated mortars

Abstract

The global generation of spent coffee grounds (SCG) poses serious environmental challenges, including landfill occupation and greenhouse gas emissions. Although SCG and its derivatives have been investigated in construction materials, the application of spent coffee grounds ash (SCGA) as a partial replacement for ground granulated blast furnace slag (GGBFS) in alkali-activated systems remains insufficiently explored. This study investigates the feasibility of using SCG and SCGA as partial replacements for GGBFS in alkali-activated mortars and evaluates their effects on fresh properties and mechanical performance. Based on XRF analysis, SCGA was expected to contribute to the alkali-activation process due to its K₂O-rich compounds. SCG and SCGA were incorporated at replacement levels of 1, 5, 10, 15, 20, and 30%. The results showed that increasing the replacement ratio reduced workability, with a more pronounced reduction observed in SCG-based mortars. For SCG-containing mortars, compressive strength decreased at higher replacement levels, while tensile and flexural strengths reached their maximum values at 15% and 20% replacement levels, improving by approximately 19% and 7%, respectively, compared to the control after 56 days. The alkali-activated mortar containing 20% SCGA exhibited the best overall performance, with increases of 9%, 57%, and 45% in compressive, tensile, and flexural strengths after 56 days, respectively. XRD analysis revealed that K₂O-containing compounds in SCGA mainly existed as crystalline K₂CO₃ phases, which increased the system’s alkalinity and compensated for the reduction in aluminosilicate sources caused by GGBFS replacement. The findings demonstrate the potential of SCGA as a sustainable supplementary material for alkali-activated binder production and provide a practical approach for valorizing coffee waste in construction applications.