Mechanical and durability characterization of lightweight geopolymer concrete incorporating coconut shell aggregates and steel fibers

Abstract Rapid urbanization and industrialization have intensified environmental challenges, driving the need for sustainable construction materials that balance structural performance with eco-friendliness. This work develops lightweight ambient-cured geopolymer composites by partially replacing expanded polystyrene (EPS) aggregates with 10% and 20% coconut shell aggregate (CSA) by volume, while incorporating steel fibers in plain, twisted, and mat forms to enhance mechanical performance. Physical characteristics including density (up to 1404.23 kg m −13 ), workability, and ultrasonic pulse velocity were assessed alongside mechanical properties such as compressive strength, tensile strength, flexural strength, and impact resistance. Compressive strength increased from 5.01 MPa to 18.92 MPa with 20% CSA and reached 21.38 MPa when reinforced with twisted fibers. Tensile and flexural strengths peaked at 3.90 MPa and 5.65 MPa, respectively, for the 20% CSA mix with twisted fibers, attributed to superior crack bridging and stress transfer capabilities. Impact resistance was notably enhanced in fiber-reinforced composites. Sorptivity results indicated that plain and twisted fibers significantly reduced water absorption rates, with values as low as 0.14 mm/min 0.5 , highlighting improved durability, whereas mat fibers exhibited less reduction. Empirical models for compressive and tensile strengths based on mix composition are proposed. The transition from brittle to ductile failure modes underscores the synergistic effects of CSA and steel fibers in lightweight geopolymer composites, supporting their potential for durable and sustainable structural applications.