Colonization by microorganisms is often observed on building materials and the aesthetic problems caused by this proliferation lead to high maintenance and repair costs. Curative and preventive methods have been developed. Nowadays, biocides are the most common treatment used against biocolonization. Their efficiency is, however, highly dependent on concentrations of active substances and therefore goes hand in hand with their environmental toxicity. It appears necessary to develop more environmentally friendly methods. Previous studies in our laboratory were dedicated to the study of the colonization of mortar surface by algae both in accelerated and in situ tests. The results showed that intrinsic characteristics of cement based mortars, such as porosity, roughness and surface pH, as well as chemical composition of the cement, affected algal fouling. A slowdown in microbial growth on calcium aluminate cement (CAC) based mortars was notably observed. Taking these results into account, the aim of the present study is the development of preventive treatments based on CAC without adverse effects on the environment. We are working on a CAC based coating applied in a thin layer of 1mm on the surface of a cementitious substrate. The objective is to maintain the slowing effect of CAC on microbial growth while reducing costs by decreasing the amount of CAC used, since it is only applied at the mortars surface. In order to remain free of the variables linked to their intrinsic properties, a single formulation of substrate (mortar) based on Portland cement (CEMI, 52,5N, Holcim) was used as reference. The bioreceptivity of the mortars with the CAC based coating is characterized by image analysis during accelerated tests and compared with reference mortars. The experiment consists in periodically sprinkling an algal suspension on top of mortar samples inclined at 45° in a glass chamber. The green algae used in this study, which have been selected thanks to their representativeness, are Klebsormidium flaccidum. The addition of nanoparticles in the coating will be considered. The aim would be to insert antimicrobial substances (such as TiO₂, silver or copper nanoparticles) on the surface of mortars while maintaining the CAC efficiency. One of the main issues of this process would be to protect the nanoparticles from leaching.