This study aims to elucidate the alteration mechanism of mortar surface reacted with seawater and the relationship between characteristics of secondary products on the surface and salt concentration of seawater. The experiment was carried out in closed system using two kinds of mortars which were made from high-early-strength portland cement (HES) or alumina cement (AL) and silica sand, and two kind of artificial seawater with quarter or twice the salt concentrations. Tablet type specimen (3.5-cm diameter, 1.0-cm height) was treated with seawater of 200.0 mL at room temperature during 91 days. Results of the experiment showed that secondary products were formed on the surface of altered tablet. These were characterized by SEM-EDS and XRD as brucite, Mg (OH)2, and aragonite, CaCO3, under low salt concentration of seawater. Production of brucite was remarkable at HES, while that of aragonite was remarkable at AL. Under high salt concentration of seawater the production of aragonite was promoted. Besides, analysis of cross section of altered tablet illustrated that the thin layer of brucite was formed at first, and then columnar or needle-shaped aragonite was formed on the layer.
