Photocatalytically active coatings for cement and air lime mortars: enhancement of the activity by incorporation of superplasticizers
Coatings made with water dispersions of different nano-particles of photocatalytic additives (titania and titania doped with iron and vanadium) were prepared with diverse superplasticizers, SPs, to optimize the atmospheric NO removal efficiency when applied onto cement- and air-lime mortars. The use of different polycarboxylate-based superplasticizers (52IPEG, 23APEG and 45PC6) prevented nano-particles from agglomeration. The steric hindrance, provided by a large density and length of side chains, was ascertained as the most effective repulsion mechanism and 52IPEG was the most efficient SP. In PC- and air-lime mortars, the coatings with polycarboxylate-based SPs improved the NO removal rates as compared with the SP-free coating: an average increase of NO degradation by 15% under UV and by 76% under solar light was found. This finding was related to the drop in the agglomeration of the photocatalysts, with more exposed active sites and a decrease of the electron-hole recombination rates. Capillary water absorption and water vapour permeability values showed that the coatings did not alter the performance of the mortars. SEM examination showed that the use of SPs enhanced the distribution of the photocatalysts yielding thinner coating layers and boosting the percolation of the active material within the mortars. Accelerated weathering showed a moderate reduction of NO removal efficiency. Coating with 52IPEG was the most efficient in preserving the activity. Measurements of Ti content showed a low washout of the TiO2 nano-particles, supporting the long-run activity of these coatings.