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Sulphate Attack in UK Homes

After the second world war there was a shortage of building materials. In areas of mining and industry, red shale (colliery spoil) and blast furnace slag were often used as sub-base materials beneath concrete floors. But red shale can be a big problem for buildings as it may contain significant concentrations of water-soluble sulphate. The presence of it under a concrete floor can cause what is known as the ettringite form of sulphate attack.

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Understanding Sulphate Attack

Lack of Membrane
 
This form of sulphate attack usually occurs in older properties where the ground-bearing Portland cement concrete floor slab has been cast directly onto the sub-base with no damp proof membrane in between. In such circumstances, water-soluble sulphates present in the red shale (or other sub-base types like industrial slags and ashes) can dissolve in any moisture present within the sub-base. The resulting solution, if in direct contact with the underside of the concrete floor slab, has the potential to react with aluminates within the cement matrix to form a sulpho-aluminate complex (ettringite).
 
Upward “heave”, lateral movement, cracking and delamination
 
The ettringite reaction product occupies a greater volume than the original reactants, and the corresponding expansive effects can give rise to significant disruption of the concrete floor fabric, including upward “heave”, lateral movement, cracking and delamination. This can also affect the structural integrity of walls and foundations, for example.
 
Mortgage lender surveys
 
Unfortunately, red shale and other industrial by-products were not identified as potential problem materials until the 1970s and no records were kept of how many houses it was used in. When people buy property and have a survey carried out by a mortgage lender, they sometimes pick up cracking/heaving in a ground floor concrete slab and there is always a chance this could be sulphate attack.
 
Red shale within a property can result in considerable costs to a homeowner to rectify. This is not usually covered by insurance.
 
Remedial measures
 
Kiwa’s Sulphate Testing Service can help. Kiwa CMT can sample and analyse both the concrete and underlying fill material. This will determine whether “sulphate attack” has occurred or, alternatively, whether the sub-base has levels of water-soluble sulphate that still have the potential to lead to sulphate attack of the concrete, should conditions change regarding presence/availability of moisture for example. This will also determine whether remedial measures are required in order to obtain a mortgage on the property.