A sustainable replacement for TiO₂ in photocatalyst construction materials: hydroxyapatite-based photocatalytic additives, made from the valorisation of food wastes of marine origin

The use of waste materials and by-products in building materials is of increasing importance to improve sustainability in construction, as is the incorporation of photocatalytic materials to both combat atmospheric pollution and protect the structures and façades. This work reports the innovative use of photocatalytic hydroxyapatite (HAp) based powders, derived from Atlantic codfish bone wastes, as an additive to natural hydraulic lime mortars. HAp is the main component of bone, and hence is non-toxic and biocompatible. This is the first time that such a calcium phosphate-based photocatalyst, or indeed any fish/marine derived wastes, have been added to building materials. A key factor is that this HAp-based photocatalyst contains only 1 wt% TiO₂, the material usually used as a photocatalyst in construction materials. As we only add 1–5 wt% of our total HAp-based material to the mortar, this means our photocatalytic mortars only contain 0.01–0.05 wt% titania (100–500 ppm), two orders of magnitude less than the quantities of 2–10 wt% TiO₂ which are usually needed. Our photocatalyst is made from a sustainable waste stream by simple solution and thermal processing, and thus with a much smaller impact on the environment. Specimens were made by either traditional intermixing techniques, or by a post-curing coating procedure. All showed gas-phase photocatalytic activity for abatement of NO_x pollutants under solar light. With intermixing, NO_x abatement of 6.3–8.3% was observed. However, for coated mortars, superior NO_x conversion rates were achieved of 7.1% and 23.8%, with 1 and 5 wt% additions, respectively. These results show the potential of this naturally-derived photocatalyst for applications in the construction industry, leading to lower atmospheric pollution and the creation of more durable/lower maintenance building façades, and environmentally sustainable materials for the preservation of cultural heritage.