Wikipedia
Sorel cement (also known as magnesia cement) is a non-hydraulic cement first produced by Frenchman Stanislas Sorel in 1867.
The cement is a mixture of magnesium oxide (burnt magnesia) with magnesium chloride with the approximate chemical formula MgCl(OH)( HO), corresponding to a weight ratio of 2.5–3.5 parts MgO to one part MgCl. A variant uses zinc oxide with zinc chloride instead of the magnesium compounds.
In use it is usually combined with filler materials such as sand or crushed stone. It is used for grindstones, tiles, artificial stone, and even artificial ivory (e.g. for billiard balls). It can withstand 10,000 – 12,000 psi (69 - 83 MPa) of compressive force whereas standard Portland cement can typically only withstand 7,000 - 8,000 psi.
Its chief drawback is its poor water resistance, making it unsuitable for construction applications. In addition, it is relatively expensive compared to standard concrete and gypsum.