Wool of promise for composite bricks

Clay Technology magazine
10 Feb 2011
Making composite brick

Would you live in a house made out of clay, sheep’s wool and seaweed? Researchers from the universities of Strathclyde and Seville are hoping the unfired composite brick will catch on, with the revived interest in natural and locally-sourced building materials.

To form the bricks, the team has blended different proportions of alginate, Scottish sourced wool and clay soil, supplied by Scottish brick manufacturers such as Errol, Ibstock and Raeburn. Tests have been performed to assess the mechanical strength of three differing compositions.

In mechanical tests, the group found the addition of alginate and wool could result in a composite with compression strength that ranges from 2.0 to 3.75MPa, depending on the soil specimens used.

Alginate is extracted from the cell walls of brown algae and acts as a natural polymer. It is already used to make dental impressions and for thickening soups, so it is an economical and easily available choice to stabilise the bricks during drying.

Carmen Galán Marín at the University of Seville, Spain, explains, ‘In our case, the alginate helps to hold the mix of soil when drying. It also improves the workability of the mixture.’

While the alginate stabilises the material, the wool fibre provides reinforcement. According to the team, the inclusion of wool in the brick sample increased compression strength by 37%.

To produce good resistance, raw, unprocessed wool, cut to 10mm-long fibres, is added to the soil and alginate.

Like reinforced concrete, the fibre-reinforced bricks are said to withstand more strain due to the redistribution of forces inside the bricks. Moreover, they do not crumble suddenly like unreinforced bricks, but develop small cracks and deform slowly before giving way.

The composite is said only to perform well where no artificial additives are introduced to the Scottish Blackface sheep fibre. Another aim for the work is to create a material suitable for adverse climate conditions

‘Researchers have been testing piles of wool fibre that had been on damp roads in Scotland,’ says Galán Marín. ‘These piles of fibre had been there for a long time, over 50 years. If the wool is in a wet environment it has a very long life. The Scottish Blackface sheep has a very interesting fibre. It is long and can be kept in a wet climate well,’ says Galán Marín

Furthermore, ‘The materials used, being natural ones, mean that you can re-use the materials, or just throw them into the earth’.

In the future, the group plans to establish the optimum length and orientation of the fibres, as well as conducting more research on the thermal properties of the bricks.