A Case Study: Calcium Silicate Bricks

 

Calcium silicate brick construction

Residential scheme built with calcium silicate bricks

Some time ago we were asked to investigate structural cracking in a large residential scheme in the West Midlands.

On visiting the scheme and noting cracks on the building I had a strong suspicion that the building had been constructed in Calcium silicate bricks but it should be noted that there is no definitive site test for identifying Calcium Silicate Bricks; positive identification can only be gained after laboratory analysis, specifically XRD (X-ray diffraction) where peaks in both quartzite and calcite will positively confirm calcium silicate construction. However, a basic understanding of these bricks and their properties can go some way to helping with correct site identification. Since we know there are a number of known problems associated with Calcium Silicate brick construction, it was of primary importance to identify the form of masonry construction.

Calcium silicate (sand lime and flint lime) bricks are manufactured by mixing lime, sand and/or crushed silicaceous or flint stone together, with enough water to allow the mixture to be moulded under high pressure. The bricks are then steam autoclaved so that the lime reacts with the silica to form hydrated calcium silicates. Pigments can be added during the mixing stage. In their natural state, calcium silicate bricks are white to a creamy off-white colour, but the addition of ochres (buff or cream colours), iron oxides (pink, red, brown or black) or chrome oxide (green) can enable a very wide variety of colours to be produced.

Close inspection of the bricks showed what appeared to be small particles of flint embedded up to 3mm in size.

This would be consistent with a calcium silicate brick as would the fact that a surface scrape of a brick proved them to be extremely soft. They also have no ‘fireskin’ as would be seen with a clay brick. They are often confused with concrete bricks but these are much harder and would not scrape quite so easily. Finally the factor which swung the balance of probability in favour of calcium silicate bricks was the colour differential below and just above DPC level. Calcium silicate bricks have a propensity for all colour variants to darken quite noticeably when wet. The wetter bricks below DPC level and just above dpc level (where the dpc has been bridged) is noticeably darker.

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