From porous coral to dense coral concrete: Ultra high performance geopolymer based concrete via coral sand gradation optimization and simplex centroid design

Coral aggregate concrete used in island and marine infrastructures often exhibits low strength, high porosity, and rapid deterioration. This study develops an ultra-high-performance geopolymer coral concrete (UHSGC) through coral aggregate refinement and statistically guided mixture optimization. Mechanical milling improved coral particle sphericity and reduced surface roughness, enabling denser aggregate packing. Twelve tailored gradations were evaluated, and the Zone II distribution provided the lowest void ratio and superior early mechanical performance. A two-stage design strategy was adopted: an orthogonal design first identified suitable fineness modulus, steel fiber dosage, and sand-to-binder ratio, followed by a simplex centroid mixture design to refine paste, fine coral sand, and coarse coral sand proportions. The optimized UHSGC, comprising 40 % paste, 34 % fine coral sand, 26 % coarse coral sand, and 2 % steel fibers, achieved 158.0 MPa compressive strength and 3.88 % porosity at 28 days. Durability tests demonstrated high stability, with 140.3 MPa residual strength after 180 days of sulfate-chloride immersion and chloride diffusion coefficients in the “moderate-to-high resistance” range. A simplified environmental assessment indicated reduced embodied carbon and cost compared with Portland cement-based coral concrete. The results confirm a viable and sustainable pathway for producing dense, durable, and high-strength coral-based geopolymer concrete for marine applications.