Performance of Cement Mortars with Very-Low Dosages of Biochar as a Fine-Aggregate Replacement: Effects of Biochar Feedstock and Content

Biochar incorporation in cementitious materials has been widely studied as a cement or binder replacement, but its use specifically as a very-low-dosage fine-aggregate substitute, expressed directly as a fraction of aggregate mass rather than binder mass, remains largely unexplored. This study addresses that gap by systematically comparing three lignocellulosic biochars, corn-stalk (CB), hemp (HB), and beech-wood (BB), incorporated at 0.5% and 1.0% of fine-aggregate mass in a Portland-composite cement mortar (CEM II/B-M (S-L) 42.5R), evaluating fresh-state, hardened-state, transport, and microstructural (SEM–EDS) properties up to 90 days. Results show that even these very low additions produce measurable, feedstock-dependent effects: CB caused the greatest reduction in fresh flow and density, while BB had the smallest effect on workability but produced the largest increase in entrapped air and capillary absorption (+20.5% at 1%, 28 days), along with numerical reductions in compressive (−9.5%), flexural (−12.2%), and splitting tensile strength (−8.2%) at 90 days; of these, only the compressive-strength reduction was confirmed as statistically significant. Feedstock, rather than dosage alone, governed the magnitude of response, and even the most affected mixture retained ~90% of reference compressive strength. SEM–EDS confirmed biochar–matrix integration and linked microstructural heterogeneity to the observed property trends. These findings establish that sub-1% fine-aggregate-level biochar additions are technically viable, with feedstock selection as the critical design variable.