Portland Cement Mortar Incorporating Coffee Husk Ash as a Supplementary Cementitious Material: Preliminary Insights on Mechanical and Environmental Performance

The use of Coffee Husk Ash (CHA) as a supplementary cementitious material can reduce Portland cement consumption, but its higher water demand and processing requirements may compromise mechanical and environmental performance. In this context, this study evaluated seven mortar formulations with 0%, 3%, 5%, and 10% cement replacement by mass under constant and variable water-to-binder (w/b) ratios. Fluidity, seven-day saturated surface-dry bulk density, and compressive strength at 7, 14, and 28 days were assessed. Furthermore, an attributional cradle-to-gate life-cycle assessment quantified ten impact categories per cubic meter of produced mortar using TRACI 2.1. Increasing CHA content reduced fluidity, density, and strength. At 10% replacement, maintaining w/b = 0.70 yielded a 28-day strength of 20.53 MPa, whereas increasing w/b to 0.81 to recover fluidity yielded 17.57 MPa, compared with 22.32 MPa for the control. Eight environmental impacts decreased progressively, with global-warming reductions reaching 6.70%. Conversely, ecotoxicity and fossil-fuel depletion increased with increasing CHA dosage, plausibly reflecting the energy-related environmental burdens of ash processing. Environmental differences between paired constant- and variable-w/b mixtures did not exceed 0.18 percentage points. MC3, containing 3% CHA at w/b = 0.70, provided the preferred preliminary compromise, retaining 97.54% of control strength and achieving 112% fluidity. This formulation reduced eight environmental indicators by 0.62–2.00%. Overall, these production-based comparisons support limited CHA incorporation with controlled water content, while durability, service life, and application-specific performance require further validation.