Physicochemical investigation of Portland cement pastes prepared and cured with seawater

The direct use of both seawater and sea sand in concrete production has been becoming attractive for some marine and coastal engineering where the availabilities of freshwater and river sand are limited. To further expand the use of seawater (e.g., using seawater as both mixing and curing water), this work provided fundamental research regarding the effects of using seawater as the mixing and curing water on the physicochemical properties of Portland cement pastes. The sub-samples at different depths of the seawater mixing and curing samples were extracted and separately analyzed. The chemical changes were quantitatively investigated, and the relation between the physical behaviors and the chemical changes was studied. The results showed that in the outer region of the samples, the ettringite content was significantly increased, but the content of Friedel’s salt was slightly reduced. Moreover, a large amount of calcium hydroxide was dissolved, but correspondingly, magnesium hydroxide (MH) crystals with various particle sizes were formed. Also, the sodium ions in the seawater were incorporated into the structure of calcium silicate hydrate gel, resulting in the formation of silica dimers with a shorter silica chain and the increase of nanopore volume (increasing by 22% in the inner region and 36% in the outer region). In addition, seawater increased the ion transport rate, but the blocking effect of the MH crystals on the samples largely decreased the rate. The changes in the crystalline and amorphous hydration products potentially influenced the strength development.