Masonry facades

Sulphate attack is a chemical reaction between solutions of soluble salts such as potassium, sodium or magnesium and a component of lime or ordinary Portland cement. The reaction converts tricalcium aluminate into ettringite, which has a greater volume, resulting in the expansion and disintegration of mortars. Sulphate attack can also affect concrete floor slabs or foundations in certain circumstances.

Some bricks, for example Flettons, have a high sulphate content. If held in permanently wet conditions, the sulphates react with ordinary Portland cement used in mortar, causing the mortar to expand and disrupt the brickwork. The effects can be particularly damaging where brickwork has been rendered as the render restricts the evaporation of moisture.

Sulphate problems are likely to occur where:

• the brickwork can remain wet for long periods (for example, a rendered parapet wall);
• permeable Portland cement mortars (i.e. weak mortars) have been used;
• no sulphate resisting cement mixes have been used in the mortar;
• bricks with a high sulphate content (> 5%) are used;
• calcium aluminate content of the cement is above 8%; and
• bricks are not isolated from other sources of sulphates, such as groundwater.

Typical risk areas could be:

• exposed parapets and brick on edge copings;
• brick on edge cills;
• retaining walls subject to groundwater; and
• brickwork below DPC level.

Sulphates can be produced as a result of the oxidisation of pyrites and other sulphides.

The Mundic problem

Mundic is the old Cornish name for pyrites, a mineral found in abundance in some of the geological deposits in Cornwall and some parts of Devon. In the early 20th century, waste products from the areas mining industries (tin, copper, lead) were used to provide a cheap and local source of fine and coarse aggregate for the manufacture of locally-produced concrete blocks and in-situ concrete.

Some of this waste material contained unstable sulphides (pyrite being one of the main culprits), which deteriorated after the concrete became carbonated and also in damp conditions. There are 2 main mechanisms of deterioration, but both are based around the oxidisation of sulphide minerals in the aggregate resulting in the decomposition of the concrete and its expansion.

Parallel cracking along rendered block bed joints or map cracking of render finishes to in-situ concrete are a visual indicator of a problem. Rendered surfaces reduce the rate of carbonation and so it is possible for cavity walls to deteriorate from the inside out, meaning that by the time cracking and other visible manifestations of a problem have occurred the concrete is badly deteriorated.

Brick parapet wall suffering from sulphate attack; the mortar has expanded causing characteristic horizontal cracking. The render may have been applied to help repair the brickwork but has probably made matters worse. Reconstruction is the only practicable solution.