Magnesium oxide, or Magnesia, is a white hygroscopic solid mineral which occurs naturally as the mineral periclase, and is a source of magnesium. In aqueous media, the mineral combines quickly with water to form magnesium hydroxide. The term “Milk of Magnesia” was introduced to describe a white, aqueous and mildly alkaline suspension of magnesium hydroxide. Milk of magnesia is used to alleviate indigestion, constipation and heartburn. Magnesium oxide has therefore come to be known as an antacid and mild laxative, while having a number of non-medicinal uses as well.
The mineral is also prized as a refractory material–a solid material that is physically and chemically stable at high temperatures. The material is cast as crucibles because of two key attributes: high thermal conductivity and low electrical conductivity. Worldwide, the largest consumer of Magnesia is the refractory industry, which consumed about 56 % of the production in the United States in 2004, the remaining 44 % being applied in agricultural, chemical, construction, environmental, and other industrial applications. Magnesium oxide is also a principal ingredient in the fireproofing of construction materials. Magnesium oxide wallboards are strong and have valuable characteristics such as fire, termite and moisture resistance, as well as resistance to mold and mildew.