Concrete exposed to seawater is susceptible to its corrosive effects. The effects are more pronounced above the tidal zone than where the concrete is permanently submerged. In the submerged zone, magnesium and hydrogen carbonate ions precipitate a layer of brucite (magnesium hydroxide: Mg(OH)), about 30 micrometers thick, on which a slower deposition of calcium carbonate as aragonite occurs. These mineral layers somewhat protect the concrete from other processes, which includes attack by magnesium, chloride and sulfate ions and carbonatation. Above the water surface, mechanical damage may occur by erosion by waves themselves or sand and gravel they carry, and by crystallization of salts from water soaking into the concrete pores and then drying up. Pozzolanic cements and cements using more than  of blast furnace slags as cementitious material are more resistant to seawater than pure Portland cement. Seawater attack presents aspects of both chloride and sulfate attacks.