A magnesite floor — also called a magnesium oxychloride floor, Sorel cement floor or, in German speaking practice, Steinholz — is a seamless floor finish made from burnt magnesium oxide gauged with a magnesium chloride solution and mixed with wood flour, cork, sawdust or mineral fillers. The mix sets into a hard, warm, slightly resilient surface. In Europe it was laid mainly between about 1900 and 1970, in schools, offices, hospitals, houses and on ships’ decks.
Why it was popular
- Seamless and dust free — no joints, so easy to keep clean
- Warm and resilient underfoot, unlike bare concrete
- Thin section — a few centimetres is enough
- Colourable — pigmented in a wide range of shades, often red brown or green, sometimes with inlaid patterns
Where it goes wrong
The problem is the chloride. As long as the floor stays dry, nothing happens. Introduce moisture — a leak, mopping water, rising damp, an unsealed substrate — and chlorides are released that attack steel.
- Corrosion of reinforcement. Where the floor sits directly on a concrete slab or on steel beams, the reinforcement or the steelwork can corrode. Rusting steel expands and spalls the concrete; in old floors on steel joists this is a genuine structural risk
- Swelling and breakdown. The floor itself loses cohesion under sustained moisture, going soft and crumbly
- Attack on services. Copper and steel pipes buried in the floor can be corroded
How to deal with it
- Identify it. A seamless, pigmented, slightly resilient floor from that period is suspect, and it often smells of wood when abraded. Only laboratory analysis of a sample gives certainty
- Keep it dry. A sound, dry magnesite floor with no moisture exposure can simply stay in place
- Have the structure assessed where the floor sits on steel beams, where there are signs of leaks, or where rust staining shows on the ceiling below
- Remove it with dust control and extraction, and dispose of the debris according to the waste contractor’s requirements
- Never simply cover it over with a levelling compound or a resin floor — you trap the moisture and the process continues underneath
Related terms
- Screed
- Floor finish
- Resin floor
- Corrosion
- Chloride class
- Concrete decay
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