The aim of this research is to find alternative plasters to cement plasters for waterproofing roofs. The earth is mixed with sand and stabilised with cement and a paste made of lime, tannin, alum (Ammonium sulphate) and water. Tannin is extracted by soaking into water broken seeds of an Indian tree, named “kaddukai” in Tamil Nadu. Its botanical name is Terminelia Chebula. The lime paste is prepared by mixing powdered alum with lime and tannin juice and extra water.
Three coats of plaster are done with different proportions of these components. The last coat, which is the most waterproof, is done with a 5 mm thick plaster composed of soil, sand and lime paste. No cement is added to the latter. Note that cement is giving strength to the plaster and also helping the waterproofing, but the effectiveness of this waterproofing is given by the combination of clay in the soil, lime, alum and tannin.
The results obtained with this waterproofing are excellent at the beginning: see table hereafter for the training centre of the Auroville Earth institute.
An exceptional rainfall in February 2000 showed the effectiveness of this stabilised earth waterproofing. 402 mm of rainfall occurred in 5 days, and there was neither any leakage nor any dampness inside the vaults. The following comparative survey was measured with an electronic hygrometer:
 Training centre of the Earth Institute: Entirely waterproof with stabilised earth |
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Preparing the lime-alum-tannin paste
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Preparing a stabilised earth plaster for waterproofing: Mix of soil, sand, cement and the lime-alum-tannin plaster
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Waterproofing a vault with stabilised earth plaster
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