Class A1 magnesia boards and refractory systems

Reactive magnesium oxide reacts with magnesium chloride solution to form magnesium oxychloride cement — the "Sorel" binder behind modern magnesia boards. The panels that come out of that reaction are non-combustible, dimensionally stable and light enough to cut installation time substantially.

01What makes a magnesia board different

A magnesia board is a composite: MgO and MgCl₂ form the binder, perlite or sawdust lowers the density, and glass-fibre mesh on both faces carries the mechanical strength. Compared with gypsum board or cement board, the result is:

• Class A1 non-combustible, with 1–3 hour fire ratings in tested wall and shaft assemblies.

• Moisture- and mould-resistant — it does not soften or grow mould in humid interiors and semi-exposed areas.

• Light and workable — it cuts, nails and screws with ordinary tools, and takes tile, paint and laminate directly.

• High strength-to-weight, reducing structural load in high-rise and prefabricated systems.

02Two things that decide board quality

The activity of the MgO. The reaction rate between MgO and MgCl₂ solution controls setting time, early strength and exothermic behaviour. Activity that is too high causes flash set and cracking; too low, and the board never develops full strength. Consistent activity batch after batch is what keeps a production line stable.

Resistance to efflorescence. Excess unreacted chloride migrates to the surface and appears as white "sweating" in humid weather. Modifiers and a correctly balanced MgO : MgCl₂ ratio are what prevent it — which is why board makers specify MgO by activity, not just by assay.

03Beyond boards

The same MgO goes into fire door cores, exterior cladding substrates, shaft-wall and partition systems, and — at the refractory end — into basic refractory bricks and castables for kilns and furnaces, where its high melting point and slag resistance matter more than binder chemistry.

04Recommended grades