The effect of coconut shell ash on the mechanicaldurability and microstructural properties of hybridfiber-reinforced alkali-activated composites

The incorporation of activated carbon (AC) into construction materials offers a sustainable pathway for valorizing agricultural waste, enhancing certain performance aspects, and reducing the environmental footprint of cementitious systems. In this study, the effects of coconut-shell-derived AC and hybrid fiber reinforcement on the mechanical and durability properties of one-part alkali-activated slag composites (AACs) were evaluated. Sixteen mixtures were produced by replacing ground granulated blast-furnace slag (GBFS) with AC at 0%, 7.5%, and 15% by mass, and adding 2 vol% total fibers as glass fiber (GF), polypropylene fiber (PPL), or hybrids. Increasing AC content consistently reduced flowability (up to −11.8% at 15% AC) and oven-dry density (from 2226 kg/m3 in reference to 1985 kg/m3 at 15% AC + 2% PPL) while increasing porosity (from 3.66% to 13.09%) and water absorption (from 2.59% to 6.77%). The reference mixture reached 28-day compressive and flexural strengths of 47.73 MPa and 2.49 MPa, respectively. The highest compressive strength (48.61 MPa) was obtained with 0% AC + 2% GF, while 15% AC + 2% PPL yielded the lowest (26.17 MPa). PPL-rich systems delivered the best flexural performance (up to 4.34 MPa at 0% AC + 2% PPL) and minimized drying shrinkage (601 με vs. 1065 με in unreinforced control). Sorptivity increased markedly with AC (1.19 → 5.84 kg/m2), especially in GFrich blends. After 300 °C exposure, several mixtures gained strength (+14.35% for 2% PPL + 0% AC), but at 600 °C losses reached −45.36% (2% PPL + 15% AC) and at 900 °C all mixes lost > 76% of initial strength. After 90-day immersion, 10% H2SO4 caused up to 53.6% strength loss, whereas Na2SO4 attack resulted in minimal losses (as low as 3.44% in 15% AC + 1% GF +85% GBFS + 1% PPL). Optimal performance was achieved with ≤ 7.5% AC and GFdominant fiber blends, balancing sustainability with strength, toughness, and durability.