The effect of (and the potential of recycled) superabsorbent polymers on the water retention capability and bio-receptivity of cementitious materials

•The bio-receptivity of mortars increased due to the use of superabsorbent polymers.•Recycled superabsorbent polymers (SAPs) improve sustainability and circularity.•The overall SAP macroporosity is a determining factor for bio-colonization.•The water-retaining capacity of the SAPs is beneficial for green wall development.•The bio-receptivity was confirmed by colorimetric analysis of the fouled regions. The evolution of rural to urban areas has a negative impact on the availability of green zones. The lack of space gives green concrete walls the opportunity to introduce vegetated areas in highly urbanized cities. However, cementitious materials are not the best substrate for plants. Still, diverse microbial communities are suitable for colonizing cementitious materials, allowing later introduction of other plants. One of the limiting factors for microbial growth under environmental conditions is the availability of water for the microorganisms. In this study, the bio-receptivity of cementitious materials under laboratory conditions was investigated, studying relevant parameters such as the porosity needed for the colonisation. Superabsorbent polymers (SAPs) were added in order to improve the water retention capacity and the colonisation of cementitious materials. These polymers, crosslinked networks able to absorb fluids up to hundreds of times their own weight, promote bio-receptivity and clear algal growth was observed. Recycled SAPs, obtained from a cleaning and crushing action of hygienic products, may show an added benefit in terms of sustainability and a circular economy. The best results were obtained with a SAP leading to a well-distributed overall macroporosity near the surface. [Display omitted]