Effect of bentonite on the sulfate durability of seawater coral-aggregate concrete

The depletion of natural construction resources and freshwater supplies has intensified the need for sustainable concretes utilizing sea-based materials. This study investigates the durability and microstructural performance of seawater-saturated sand concrete with bentonite clay under sulfate attack. Concrete mixtures were prepared by replacing natural coarse aggregates with 0–48% coral aggregate (CA) and partially substituting cement with 0–48% bentonite. In addition, sea sand and seawater were used as partial replacements for river sand and freshwater, respectively, while fly ash content was kept constant at 155 kg/m³. Compressive strength, porosity, and microstructural characteristics were evaluated after exposure to a 5% Na2SO4 solution for up to 180 days. Results show that compressive strength improved by approximately 10–15% at 12% CA replacement, while higher replacement levels (≥36%) caused a significant reduction of about 25–50% due to increased porosity and reduced matrix density.