Produced by the high temperature calcination of magnesium carbonate or magnesium hydroxide, magnesium oxide can also be obtained by treating magnesium chloride solutions such as seawater and calcium carbonate (lime). Varying the temperature during production varies the reactivity, with high temperature calcined magnesia being less reactive (heavy), and lower temperature calcined magnesia being more reactive and more caustic (light). Magnesium oxide is a refractory material, making it useful for its high thermal and low electrical conductivity. It is the principal material used in fireproofing building construction materials, and magnesium oxide wall boards are resistant to fire, termites, moisture, mold, mildew, and weight/pressure. Heavy magnesium oxide is also used in manufacturing certain varieties of concrete, and in treating wastewater and drinking water that has been contaminated with heavy metals. Due to its refractory nature, it is also one of the primary components of heating element tube construction for home, commercial, and industrial heating units.