Comprehensive evaluation of fresh, mechanical, durability, and thermal properties of one-part alkali-activated slag composites with hybrid fibers and activated carbon

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 (PPF), 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 % PPF) 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 % PPF yielded the lowest (26.17 MPa). PPF-rich systems delivered the best flexural performance (up to 4.34 MPa at 0 % AC + 2 % PPF) 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 GF-rich blends. After 300 °C exposure, several mixtures gained strength (+14.35 % for 0 % AC + 2 % PPF), but at 600 °C losses reached −45.36 % (15 % AC + 2 % PPF) 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 85 % GBFS + 15 % AC + 1 % GF + 1 % PPF). Optimal performance was achieved with ≤ 7.5 % AC and GF-dominant fiber blends, balancing sustainability with strength, toughness, and durability.