Photocatalytic performance of calcium aluminate cements modified with TiO2

Abstract

The photocatalytic activity of powdered TiO2 (75% anatase, 25% rutile) was tested after the incorporation of the additive in mortars of calcium aluminate cement. Two different CACs were used according to their chemical and mineralogical composition: one of the CAC was a high alumina cement (H-CAC, 70.9% of Al2O3) while the other one was a low alumina cement (L-CAC, 42.0% of Al2O3) with a large amount of ferrite. One batch of the prepared mortars was tested after being subjected to 28 days under a normal curing regime (20ºC and 95% RH). A second batch was tested after a curing regime of 60ºC and 100% RH (for 24 hours) and then 20ºC and 95% RH. This second curing regime intended to induce the conversion reaction, which is responsible for the formation of stable calcium aluminate hydrates at the expense of metastable hydrates. Different properties such as workability, setting time and compressive strength were determined in the tested mortars in order to study the influence of the TiO2 addition. The photocatalytic effect was assessed by means of the NOx abatement under UV irradiation. The TiO2-H-CAC mortars were found excellent systems to decrease NO2 concentrations in connection with the more abundant presence of aluminates. The better NO2 retention improved the NOx removal. An chemical interaction between ferrite and TiO2 was suggested to take place in the L-CAC mortars as evidenced by XRD, SEM, EDAX and zeta potential analyses. This interaction originated two new iron titanate phases, namely pseudobrookite and, to a lesser extent, ilmenite. The reduced band-gap energy of these compounds compared with that of TiO2 improved the photocatalytic efficiency of L-CAC samples in the visible spectrum, as proved by Methyl Orange degradation tests, conferring a self-cleaning ability to the TiO2-bearing iron-reach L-CAC under visible illumination.