MGO% SiO2% CaO% LOI%
<95   <1   <1.5  <3   
<94   <2    <2   <4
<92   <3    <2   <5
<90   <4    <2   <6
<87   <5    <3   <7
<85   <6    <3   <8
<80   <8    <3   <10

Magnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO. It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to protect it from moisture. Magnesium hydroxide forms in the presence of water (MgO + H2O -Mg(OH)2), but it can be reversed by heating it to separate moisture.

Magnesium oxide was historically known as magnesia alba (literally, the white mineral from Magnesia), to differentiate it from magnesia negra, a black mineral containing what is now known as manganese.

Applications
A refractory material is one that is physically and chemically stable at high temperatures. "By far the largest consumer of magnesia worldwide is the refractory industry, which consumed about 56% of the magnesia in the United States in 2004, the remaining 44% being used in agricultural, chemical, construction, environmental, and other industrial applications.

Cement
MgO is one of the raw materials for making Portland cement in dry process plants. If too much MgO is added, the cement may become expansive. Production of MgO-based cement using serpentinite and waste CO2 (as opposed to conventional CaO-based cement using fossil fuels) may reduce anthropogenic emissions of CO2.

Desiccant
MgO is an efficient moisture absorbent used by many libraries for preserving books.

Medical
In medicine, magnesium oxide is used for relief of heartburn and sore stomach, as an antacid, magnesium supplement, and as a short-term laxative. It is also used to improve symptoms of indigestion. Side effects of magnesium oxide may include nausea and cramping. In quantities sufficient to obtain a laxative effect, side effects of long-term use include enteroliths resulting in bowel obstruction.

Other

MgO is used as an insulator in industrial cables, as a basic refractory material for crucibles and as a principal fireproofing ingredient in construction materials. As a construction material, magnesium oxide wallboards have several attractive characteristics: fire resistance, moisture resistance, mold and mildew resistance, and strength.
It is used as a reference white color in colorimetry, owing to its good diffusing and reflectivity properties. It may be smoked onto the surface of an opaque material to form an integrating sphere.
It is used extensively in electrical heating as a component of "CalRod"-styled heating elements. There are several mesh sizes available and most commonly used ones are 40 and 80 mesh per the American Foundry Society. The extensive use is due to its high dielectric strength and average thermal conductivity. MgO is usually crushed and compacted with minimal airgaps or voids. The electrical heating industry also experimented with aluminium oxide, but it is not used anymore.
Pressed MgO is used as an optical material. It is transparent from 0.3 to 7 μm. The refractive index is 1.72 at 1 μm and the Abbe number is 53.58. It is sometimes known by the Eastman Kodak trademarked name Irtran-5, although this designation is long since obsolete. Crystalline pure MgO is available commercially and has small use in infrared optics.
It is packed around transuranic waste at the Waste Isolation Pilot Plant, to control the solubility of radionuclides.
An aerosolized solution of MgO is used in library science and collections management for the deacidification of at-risk paper items. In this process, the alkalinity of MgO (and similar compounds) neutralizes the relatively high acidity characteristic of low-quality paper, thus slowing the rate of deterioration.
It is also used as a protective coating in plasma displays.

Precautions
Magnesium oxide is easily made by burning magnesium ribbon which oxidizes in a bright white light, resulting in a powder. However, the bright flame is very hard to extinguish and it emits a harmful intensity of UV light. Inhalation of magnesium oxide fumes can cause metal fume fever.