Effect of metakaolin addition and seawater mixing on the properties and hydration of concrete

•Metakaolin improved compressive strength by refining pore structure without altering porosity.•Seawater increased compressive strength at early age without altering pore structure.•Chloride in seawater was immobilized by hydrocalumite with metakaolin addition.

The effect of metakaolin (MK) and seawater mixing on the properties and hydration products of concrete was studied by analytical techniques. MK improved compressive strength by refining pore structure without altering porosity. Seawater increased compressive strength at early age without altering pore structure. The combination of MK and seawater improved both early and later age strength. Hydration products in pastes containing MK were Portlandite, ettringite, hemicarboaluminate, monocarboaluminate and C-S-H gels. In pastes with seawater, instead of hemicarboaluminate and monocarboaluminate, hydrocalumite was identified. MK promoted hydrocalumite formation. MK decreased CH content with increase of MK content and age. Seawater increased CH content at early age with MK content. The combination increased CH content at early age and decreased later. At 28 days, most chloride in seawater was immobilized by hydrocalumite. The combination improved properties of concrete at both early and later age.