Calcium Silicate Board (Asbestos)

Asbestos calcium silicate board is a rigid, lightweight insulation material composed of calcium silicate reinforced with asbestos fibres, typically amosite or chrysotile. The asbestos content usually ranges from 6 % to 20 %, although some formulations contain higher concentrations. The product was manufactured as flat boards, pre-formed pipe sections (half-shells), and block insulation for high-temperature industrial and commercial applications.

Calcium silicate insulation was widely used from the 1940s through the mid-1980s. As flat boards, it served as fire-rated wall linings, ceiling panels, and partition cladding. As pipe sections, it provided thermal insulation for hot-water and steam piping in industrial plants, power stations, and large commercial buildings. Block forms were used to insulate boilers, tanks, and process equipment.

The material is classified as mixed friability. Intact calcium silicate board is relatively stable and unlikely to release fibres during normal use. However, the material becomes significantly more hazardous when it is cut, drilled, sanded, broken, or when it deteriorates due to age, moisture, or mechanical damage. Crushed or powdered calcium silicate produces fine dust containing respirable asbestos fibres.

One notable concern with calcium silicate insulation is that it closely resembles modern, non-asbestos calcium silicate products. Visual identification alone is unreliable. Many contemporary insulation boards are also made from calcium silicate without asbestos, and the products are nearly indistinguishable in appearance. Positive identification requires laboratory analysis via polarised light microscopy (PLM).

Well-known asbestos-containing calcium silicate products include Johns-Manville Thermo-12 and Owens-Corning Kaylo, both of which were subjects of extensive litigation in the United States. In the UK, removal of calcium silicate pipe sections and boards may be classified as licensed or non-licensed work depending on the condition and scope. Good-condition calcium silicate insulation can often be managed in situ under a formal asbestos management plan.