Characterization of black crusts developed on historic stones with diverse mineralogy under different air quality environments.

Black crusts (BCs) are one of the most critical alteration forms found on stones belonging to architectural heritage. Since they could be considered as passive samplers of atmospheric pollution, it would be plausible to establish relations between the air contamination and the BCs. With this aim, we have characterized BCs collected on historic buildings from two Spanish cities (Granada and Vigo) with different polluted atmospheres, as well as formed on stone substrates of varied mineralogy and texture. Likewise, in order to assess the impact of the atmospheric pollutants on the growth of BCs, quartz fiber filters were used as surrogate substrates and placed nearby the studied buildings to collect and analyze the aerosol particulate matter (PM). To this end, an array of complementary analytical techniques was used to evaluate the mineralogy, chemical composition, and texture of the BCs and to establish the correlation with the ions, OC (organic carbon), and EC (elemental carbon) detected in the PM on the quartz fiber filters. As result, BCs developed on carbonate substrates from Granada show more complex structure than those from Vigo, which are thinner because of frequent rain episodes. In both cities, NaCl, Pb-Cl, and Ca-Cl-rich particles, Ca-phosphate particles and clusters of Ba-sulfate-rich particles were detected. However, metal-rich rounded particles were more abundant in Granada's BCs, including soot particles. BCs from Granada were richer in carbonaceous components (OC and EC) than the Vigo's BCs. Although in the filters PM did not show EC-mainly due to traffic-, in the BCs from both locations OC and EC were detected. Therefore, this different composition was related to the mineralogy of the stones and the higher pollution of Granada in contrast to the industrial and sea-exposed city of Vigo.

Evaluation of side effects of mechanical cleaning with an anionic detergent on granite cladding tiles.

One of the aims of Goal 11 of the UN's 2030 Agenda for Sustainable Development is to make human settlements sustainable. Some effort should therefore focus on developing maintenance strategies for buildings. Polished granite cladding tiles are very popular for use as flooring material in private and public buildings. These tiles are often treated by mechanical cleaning with detergents and scrubbers, and anionic detergents without chelating agents are usually used to prevent damaging the material. In this study, the anionic detergent Teepol® was applied to granite tiles in order to determine the effect of the treatment on the surface properties of this stone, i.e. appearance, colour, gloss, roughness and hydrophobicity. The results obtained were explained by scanning electron microscopy observations, which enabled characterisation of the microtexture of the surfaces, chemical contamination and any damage to the different granite-forming minerals. The results were compared with those obtained for tiles cleaned by the same mechanical procedure but with tap water in place of detergent and for tiles treated by immersion in water without any mechanical action. In summary, the properties evaluated were affected by the extraction of the typical oxyhydroxylated iron forms that fill cracks in the granite and by the formation of new cracks due to the mechanical action. Moreover, colour changes and the chemical contamination (C-rich deposits with Na, S, Cl, Ca and Mg) hindered the effectiveness of the cleaning procedure.

Moving toward Smart Cities: Evaluation of the Self-Cleaning Properties of Si-Based Consolidants Containing Nanocrystalline TiO2 Activated by either UV-A or UV-B Radiation.

This study evaluated the self-cleaning ability and durability of Si-based consolidants (an ethyl silicate consolidant and a consolidant based on nanosized silica) spiked with nanocrystalline TiO2 activated by either UV-A radiation (spectral region between 340 and 400 nm, and main peak at 365 nm) or UV-B radiation (spectral region between 270 and 420 nm, and main peak at 310 nm). Granite samples were coated with consolidant, to which nanocrystalline TiO2 was added at different concentrations (0.5, 1, and 3%, by wt.). Diesel soot was then applied to the coated surfaces, and the samples were exposed to UV-A or UV-B radiation for 1650 h. The surface color changes, relative to the color of untreated granite, were determined every 330 h by color spectrophotometry. Slight color changes indicated a recovery of the reference color due to the degradation of the soot. The final surfaces of both the untreated and treated surfaces were compared by stereomicroscopy and scanning electron microscopy. The main findings were that: (1) In general, the consolidant containing nanosized silica induced the most intense photocatalytic activity. In the more compact xerogel coating formed by the nanosized silica, more TiO2 nanoparticles were available to interact with the radiation. (2) For all consolidant mixtures, soot degradation remained constant or decreased over time, except with ethyl silicate with 0.5 wt % TiO2 (no self-cleaning capacity). (3) Soot degradation increased with the concentration of TiO2. (4) The UV-B radiation was the most effective in terms of soot degradation, except for the surface coated with the ethyl silicate and 3% wt. TiO2.