Floor screeds and finishes

Although no longer popular in the UK, magnesite floor finishes and screeds are sometimes found in industrial buildings, workshops, etc. in Europe. It is made of caustic magnesia, additions (fillers), an aqueous salt solution of bivalent metals - generally magnesium chloride - and additives (like colouring materials). Magnesium chloride is an acid, hygroscopic and corrosive salt. The material was popular in housing (particularly local authority housing) between 1945 and 1960 where it was sometimes mixed with red and yellow pigments to give a mottled appearance. (Domestic floors: repairing or replacing floors and flooring – magnesite, tiles, slabs and screeds, BRE Good Building Guide 28 part 4, 1997) In some cases, sawdust, wood flour or even asbestos were used as a filler. The thickness of magnesite ranges from 10-25mm (single coat work) and 50mm in 2-coat work.

Magnesite screeds are never used externally. They offer a dust free, high abrasion and impact resistant surface of low heat conductivity and good resistance to mineral oils, solvents and fuels.

After hardening, the pores in the magnesite screed are filled with magnesium chloride solution. However, above a relative humidity of 32%, magnesium chloride remains damp and can cause corrosion in ferrous building materials that may be in contact with it. The hygroscopic nature of magnesite makes it unsuitable for use in damp conditions. If the material is covered with an impervious floor covering, trapped moisture can cause the magnesite to degenerate. Sometimes, swelling and expansion of the finish can occur.

Occasionally, timber floors in light industrial buildings were improved by laying magnesite screeds reinforced with wire mesh.

To prevent the migration of chlorides from magnesite into a reinforced concrete sub-base, it is important to ensure that a suitable impervious barrier is provided between the two materials. Failure to do this can cause corrosion problems, possibly evidenced by disruption in the flooring caused by expansion of a buried component or reinforcement bar.

In the article Corrosion Damages Caused By Cast Magnesite Floor Screed (Otto-Graf-Journal, Vol. 14, 2003) Beul and Menzel describe the effects of corrosion in a modern industrial floor slab finished with magnesite but without a separating layer. The building had been constructed in 1997. By 2000 it had begun to show evidence of small bulges in the floor finish close to columns. The bulges were found to coincide with the location of steel studs within the concrete base, the studs having relatively low cover of less than 20mm. The uniformity of the apparent damage on all the areas covered with magnesite screed and evidence from chloride profiles in different locations established a correlation between progressive chloride contamination, depassivation of the buried studs and subsequent corrosion. In areas where the studs were located at greater depth, corrosion had not occurred, although repeated tests in 2003 demonstrated that chloride penetration had increased.