Study the development of mould on prompt natural cementbased hemp concrete
The use of bio-based materials in construction is significantly improving. One of the bio-based materials is hemp concrete which is a composite obtained by mixing a binder and the nonfibrous part of hemp, called shiv. This specific hemp concrete developed by Vicat is made with Prompt Natural Cement (PNC). Hemp concrete is used for its insulating properties and its capacity to control humidity which is obtained due to the hydrophilicity of shiv. Therefore to reach the same level of binder hydration more water is needed in hemp concrete than in traditional concretes. This excess of water combined with insufficient drying of hemp concrete may quickly lead to the development of mould. In fact, the binder alkalinity is not sufficient to stop the microorganism development. This study presents results from two tests to quantify mould development in PNC-based hemp concrete. The first series of test was carried out respectively on the shiv only and on hemp concrete specimens which were a few hours and a few days old. During the first 24 hours the number of microorganisms identified on hemp concrete specimens decreases more significantly than on the raw shiv. So the hemp concrete formula prevents concrete from developing mould. A second part of the study was carried out on a hemp concrete of same formula than previously but coated with two different renders: one coating permeable to both liquid and vapour water while the second one is permeable to vapour water but not to liquid water. The coated hemp concretes had been exposed to external conditions for four seasons. After one year, some samples were removed in order to study the microorganism development. The tests show that the exterior coating must be well chosen. Using a waterproof coating which is permeable to moisture does not present development of mould.