Microstructural evolution and compressive strength enhancement of mortar containing spent coffee ground biochar as a sustainable filler

This research investigates the influence of spent coffee ground biochar (SCGBC) on the mechanical and microstructural properties of cement mortar as part of sustainable construction practices. Mortar specimens were prepared with various SCGBC contents (0, 1, 2, 3, and 5 wt% of cement) and tested for compressive strength (7, 14, and 28 days). Phase composition and hydration characteristics were examined using XRD, SEM-EDS. The 1 wt% SCGBC sample revealed the highest compressive strength (15.03 tons at 28 days), approximately 6% higher than the control. The enhancement is attributed to the micro-filler and nucleation effects of biochar, which promote denser C-S-H gel formation and stronger interfacial transition zones (ITZ). XRD patterns confirmed typical hydrated phases—portlandite, calcite, quartz, and gypsum—with decreasing portlandite intensity as SCGBC increased, indicating mild pozzolanic reactions. SEM-EDS analysis revealed reduced porosity and higher Ca/Si ratios at 1 wt%, while excessive biochar (>3 wt%) caused pore coalescence and particle agglomeration, resulting in strength loss. These findings demonstrate that coffee-waste- derived biochar can be effectively utilized as a green additive for improving mechanical performance and reducing the environmental impact of cement-based materials.