Magnesite (Magnesium Oxychloride) Flooring
Magnesite flooring is not commonly used at the current time but was very popular for domestic applications in the period 1920 to 1940, and for Local Authority housing between 1945 and 1960. It is usually a reddish pink colour although some floors were coloured using pigments. It was commonly applied on top of concrete ground floor slabs. Magnesite flooring was made from a mixture of calcined magnesite and magnesium chloride solution with various fillers (e.g. wood flour, sawdust, asbestos). It was typically laid between 10 and 25mm thick, but two coat applications could be up to 50mm thick. Magnesite floorings are very vulnerable to dampness and if there is any doubt about moisture protection, it should be replaced. Chlorides may migrate from the magnesite into the concrete below and corrode any reinforcement or gas and water pipes. Most magnesite floors are now coming to the end of their life, only if they can be guaranteed to remain unaffected by moisture can they be used as a screed to receive other floorings, otherwise the flooring should be replaced.
Identification: It is easy to confuse magnesite with a pigmented sand/cement screed; however the following features can aid identification. Magnesite flooring is electrically conductive and so an electrical resistance moisture meter will give a full scale damp reading even when the material is bone dry. Also, a small lump when held in gas flame will glow brightly, like a gas mantle at the edges.
Further reading: PYE, PW and HARRISON, HW. Floors and Flooring, BRE, 2003.