Development of properties and microstructure of concrete with coral reef sand under sulphate attack and drying-wetting cycles

Coral reef sand is natural abundant solid resource on islands in the tropical region and is often used in the construction projects as fine aggregates where natural river sand is rare. The concrete structures on islands are usually subjected to drying-wetting cycles from sea-water due to the waves and tides, resulting in the deterioration of durability. In this study, Portland cement concrete is prepared with coral reef sand (CPC) and the effects of combined attack of drying-wetting cycles and sulphate are investigated and compared to concrete made with natural river sand (NPC). The corrosion products are ettringite and Friedel's salt for CPC and ettringite for NPC. More ettringite forms in the pores of CPC by immobilizing more sulphate from Na2SO4 solution, which causes strength loss of both CPC and NPC concrete. The salt concentrations have marginal effects on the mass change of CPC concrete. The loss of elastic modulus of CPC concrete is smaller than that of NPC concrete, due to the precipitation of ettringite in the pores of concrete, which compacts the structure of concrete.