Incorporación de aditivos fotocatalíticos en materiales conglomerantes para la descontaminación ambiental

Resumen

The main objective of this Doctoral Thesis has been to explore new pathways to improve the removal of gaseous pollutants by means of binding systems including photocatalytic additives. Firstly, TiO2 was incorporated in bulk into two types of calcium aluminate cements: one was a high alumina cement (white) and the other was a low alumina cement, rich in aluminoferrite (dark). NO abatement results under UV light proved that calcium aluminate cements were suitable binding systems to enhance the TiO2 photocatalytic effectiveness. Photocatalytic activity of mortars followed a dosage-response pattern. High alumina cement provided better results than the corresponding low alumina cement, due to the proved interaction between TiO2 and aluminoferrite phases of this latter material. This interaction reduced the TiO2 amount available for the photocatalytic process and produced iron titanate phases sensitive under visible radiation (confirmed by methyl orange dye degradation experiments). Furthermore, samples cured under more stringent conditions of temperature and relative humidity exhibited better NO removals than samples cured under more moderate conditions, due to the observed increase in porosity that allows NO molecules to reach the photocatalytic active sites. Secondly, three photocatalytic additives (TiO2 and two doped Fe-TiO2 and V-TiO2, aiming to widen the activity under visible light irradiation) were incorporated in bulk into four different type of building materials: Portland cement (PC), high alumina cement, low alumina cement and air lime. Photocatalytic activity studies showed best NO abatements under UV, solar and visible light for lime and for high alumina cement mortars due to the reaction between calcium carbonate and nitric acid (freshly generated during the photocatalytic reaction). Doped additives improved the NO abatements percentages under visible light, Fe-TiO2 always being more effective than V-TiO2. Selectivity values for NO degradation were very high showing a low release rate of harmful NO2. The presence of the photocatalysts enabled the hydrophilicity of the samples - as evaluated through the reduction of the water contact angle - to increase upon illumination, which can be associated with the self-cleaning ability of the treated mortars. The tendency of photocatalytic nano-particles to agglomerate was identified as a major drawback jeopardizing the efficiency of these photocatalytic systems. Therefore, the use of dispersing agents providing a better spreading of the nano-particles arises as a reasonable approach to improve the photocatalytic activity. Preliminary assessment of four different superplasticizers incorporated into air lime mortars modified with either nanosilica or metakaolin, as pozzolanic additives, was carried out. The molecular architecture of each polymer determined the adsorption mechanism onto C-S-H phases. Superplasticizers with lower anionic charge density and larger side chains exhibited the best results due to the steric repulsions between particles. Finally, in order to improve the photocatalytic performance, aqueous dispersions of photocatalytic additives and three polycarboxylate-based superplasticizers (52IPEG, 23APEG and 45PC6) as well as one naphthalene sulfonate formaldehyde polycondensate (PNS) were studied and applied as coatings onto both Portland cement and air lime mortars. NO abatements were improved for coatings containing polycarboxylate-based superplasticizers, which evidenced a clear relationship between NO degradation and the scattering of the photocatalytic nano-particles. The reduction of water contact angles for mortars subjected to the three sources of illumination (UV, solar and visible) also suggested an acceptable dirt-prevention ability of the mortars. Samples durability under accelerated weathering conditions preserved reasonable NO removal rates, 52IPEG, showing the largest steric repulsions, being the most efficient plasticizer.