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flowers of concrete with a dash of lime

Architect's memo 58: May 1999

It is the nature of Portland cement to release free lime during the hydration (or hardening) process. Carried on the flux of water this lime reaches the surface of the concrete, plaster or (ex the mortar) brick, as a white deposit.

Caught in its first fresh bloom, it is easily removed with more water. Maturity however, sees it combining with atmospheric carbon dioxide to form much more intractable efflorescence.

Although the release of free lime cannot be prevented, its transport can be inhibited by ensuring that the amount of water conduit is minimised. High water/cement ratios invariably lead to high levels of efflorescence and the use of water-reducing additives and mortar plasticisers is recommended.

Throughout the life of the materials, water transport can result in efflorescence, whether the water comes from leaks or high moisture differentials either side of the cementitious fabric.

Other elements of concrete design can impact on the potential for efflorescence. Pozzolanic additives have a double benefit and they allow for a reduction in Portland cement (this reducing the source of free lime) by reacting with the free lime, converting it to a useful, cementitious material.

Careful placement and compaction of concrete can lead to denser surfaces that restrict the passage of water and hence free lime. Conversely, porous concrete affords easy passage.

Where efflorescence is exposed to the weather it will weather off, although this process may take up to two years. It can be removed by various methods but none is simple.

Abrasion will work, but the level of abrasion may need to be high depending on the degree of carbonation and the profile of the surface. Successful methods range from stiff bristle brushes and lots of elbow grease, to abrasive blasting. The latter method not only has health hazards during the operation, but also will change the surface texture.

Efflorescence is soluble in acids and treatment with dilute hydrochloric acid is efficacious. In order to avoid saturating the surface with soluble calcium salts and starting a similar process over again, the surface should first be soaked with water and the acid treatment applied when the glisten goes off the surface. A final rinse is necessary.

Most troublesome, especially over brick, is when efflorescence occurs subsequent to the application of a clear protective coating. The coating does not allow liquid water through it and as the water escapes through the film by vapourisation, it leaves behind the disfiguring efflorescence permanently under the film. In this case, total removal of the film will be necessary before the efflorescence can be dealt with.

Perhaps the most elegant method of preventing the blooming of efflorescence is the early treatment of the surface with a penetrative oligomeric polysiloxane. Liquid water cannot wet these invisible, non film-forming materials and must vapourise before it can carry on its journey. This causes the lime to be deposited at the boundary of the penetration, down below the surface. Resene Aquapel is such a product and a noted de-flowerer.

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