Behaviour of a protective coating for bio-based materials used for energy rehabilitation regarding salt attack

Abstract

This study focuses on the behaviour of three lime-based coatings against salts attack intended to protect straw biobased insulation composites as part of an energy retrofit of building. Salts are among the main causes of mortar degradation. Their sources are various between interior ones as mortar constituents and mixing water, and others external as groundwater in contact with gypseous soils, rainwater polluted by sulfur dioxide or di-icing salts that infiltrate the coating by capillary actions or by spaying. In this paper ageing tests are carried out by total immersion in salt solution to expose the samples to salts attack by using sodium sulfate and sodium chloride solutions. The studied mortars are composed by lime, calcareous sand and two additives used also in the biobased insulation materials. The reference mortar without additives is used to highlight the effect of the two additives (air entrainer and casein protein). The third mortar is used with the addition of a small percentage of fine straw about 7.5 %.  This percentage allows the optimized mortar to meet all criteria for an insulation coating. The results show that the coating with straw and additives are more resistant than those based on the reference mixture without additives, even when ageing tests are carried out with the most aggressive sodium sulfate solution. This resistance is linked to the induced porosity of the additives which minimizes the crystallization pressure.