Valorization of spent coffee grounds ash as a novel bio-derived alkaline activator for ground granulated blast-furnace slag mortars

Due to the increasing demand for alternative and waste-derived construction materials, this study investigated spent coffee grounds ash (SCGA) as a bio-derived alkaline activator for GGBFS-based alkali-activated mortars. SCGA was used at levels corresponding to 16%, 31%, and 51% of the equivalent K₂O content, and its performance was compared with that of KOH-activated mortars. The reference mixture without an alkaline activator exhibited very low compressive strengths of 1.30 and 1.57 MPa at 28 and 56 days, respectively, highlighting the importance of an alkaline environment for GGBFS activation. Among the SCGA-containing mixtures, AAM-SCGA-16% exhibited the best performance, with compressive strengths of 31.08 and 33.04 MPa at 28 and 56 days, respectively, while AAM-SCGA-31% and AAM-SCGA-51% achieved 15.80 and 17.43 MPa, and 12.22 and 13.35 MPa, respectively. The tensile and flexural strengths of AAM-SCGA-16% were 2.13 and 6.20 MPa, respectively. Increasing the SCGA content to 31% and 51% reduced the mechanical properties, with tensile strengths of 2.08 and 1.63 MPa and flexural strengths of 4.92 and 4.74 MPa, respectively. Although KOH-activated mortars generally exhibited higher mechanical strengths, SCGA demonstrated a viable waste-derived alternative for GGBFS activation. XRD and FTIR results indicated that the alkaline activity of SCGA was mainly associated with K₂CO₃, providing a slower and more controlled activation process compared with KOH.