<p>The growing demand for sustainable low-carbon binders has driven interest in alkali-activated materials. This study investigates areca nut husk biochar (ANB) as a carbonaceous micro-filler in alkali-activated slag (AAS) mortar under ambient curing conditions. ANB was incorporated at 1%–5% (mass fraction) of binder to systematically evaluate its influence on workability, mechanical properties (compressive strength, flexural strength, and ultrasonic pulse velocity), and durability characteristics (electrical resistivity, water absorption, porosity, high-temperature resistance, and sulfuric acid resistance). Results demonstrated that the addition of ANB reduced workability due to increased water absorption and high cation exchange capacity. The optimum dosage of 3% ANB achieved the 28-day compressive and flexural strengths of 81.85&#xa0;MPa and 9.72&#xa0;MPa, representing an increase of 14.15% and 5.19% over the control mix, respectively. The water absorption and porosity were reduced by 24.40% and 30.54%, respectively, while electrical resistivity decreased by 29.96%, indicating a refined pore structure as well as enhanced compactness and conductivity. The AAS mortar retained 34.83% of its strength at 500&#xa0;°C, indicating good thermal stability. Under H₂SO₄ exposure, the 3% ANB mix exhibited 0.35% mass loss and 15.31% strength loss, compared to 2.08% and 24.01% for the control mix, demonstrating superior acid resistance. Microstructural analyses confirmed the formation of C–S–H and C–(A)–S–H hydrated gels through the nucleation and filling effect of biochar, leading to matrix densification. Overall, ANB serves as a multifunctional and sustainable filler that optimizes the performance of AAS mortar and promotes the valorization of agro-waste towards next-generation carbon-efficient sustainable construction materials.</p> Graphical abstract <p></p>