Development of Multifunctional Coatings for Protecting Stones and Lime Mortars of the Architectural Heritage

Abstract

Unique multifunctional coatings, comprising a 3D superhydrophobic agent and two nanostructured photocatalysts (solar-light sensitive 50/50 and 10/90 TiO 2 -ZnO nanoheterostructures), compatible with the inorganic substrates of the Built Heritage, have been designed.The synthesized nanoparticles showed an enhanced photocatalytic activity (tested by NO degradation) as compared with the raw TiO 2 and ZnO materials.Dispersing agents were used to optimize the coatings, avoiding agglomeration of the photocatalytic nanoparticles and increasing the stability of the suspensions. Four distinct dispersions were optimized and applied as coatings onto stony materials used in the Built Heritage, such as sandstone, lime mortar, granite and limestone. Their effectiveness was assessed by assessing hydrophobicity of the surfaces (static water contact angle), photocatalytic activity and self-cleaning as well as water vapour permeability of the treated specimens. These transparent coatings demonstrated high compatibility with the construction materials of the Architectural Heritage and showed a synergistic effect rendering a minimized water absorption, self-cleaning ability evidenced by the reduced adsorption of soiling deposits and a reasonable degradation of any trace that might be adsorbed, as well as a protecting hydrophobic environment for the photocatalyst.

Self-cleaning and de-pollution efficacies of photocatalytic architectural membranes

“…Those aged samples remained within ±6°, on average, with respect to the contact angle of unexposed (pre-bleached) specimens. This result agrees with another report in the literature showing small changes in surface hydrophilicity of photocatalytic building coatings subjected to accelerated climatic aging [72]. By contrast, field aging of TiO2-modified polymeric coatings showed a significant decrease in contact angle values after one year of field exposure in an urban setting [23].…”

Evolution of C–S–H in lime mortars with nanoparticles: Nanostructural analysis of afwillite growth mechanisms by HRTEM

“…26 The particle size affects their properties, 27,28 so incorporating nanomaterials facilitates the formation of C-S-H gel. 29 Previous studies report the effectiveness of nano-SiO 2 in the formation of C-S-H. [30][31][32][33][34] Likewise, there is greater effectiveness in samples with low crystallinity, that is, being the amorphous variety of SiO 2 achieves a faster growth of the C-S-H. 35 Besides, the incorporation of nano-Ca(OH) 2 has proven to be highly effective in lime mortars, 24,[36][37][38] being very useful in restoring historical mortars of great values within the cultural heritage. [39][40][41][42] Afwillite (Ca 3 Si 2 O 7 ⋅3H 2 O), amongst the C-S-Hs, plays a significant role in the hydration process of cement pastes and concrete, 43,44 prompting microstructure modifications.…”

“…Likewise, there is greater effectiveness in samples with low crystallinity, that is, being the amorphous variety of SiO 2 achieves a faster growth of the C–S–H 35 . Besides, the incorporation of nano‐Ca(OH) 2 has proven to be highly effective in lime mortars, 24,36–38 being very useful in restoring historical mortars of great values within the cultural heritage 39–42 …”

Aqueous miscible organic solvent treated NiTi layered double hydroxide De-NOx photocatalysts

“…In contrast to the traditional catalytic reduction technologies which need high temperatures to obtain a correct De-NOx efficacy, photocatalysis works under mild conditions (room temperature and ambient pressure) by using abundant and natural resources in the air: oxygen, water and sunlight irradiation [10]. This alternative strategy has already been applied in field trials using materials available in the commercial sector, demonstrating a potential real life application [11][12][13]. Commercial materials typically contain TiO2 as a photocatalyst additive since it possesses chemical stability and a high band gap value (3.0 -3.2 eV).…”