Producing Self-cleaning, Transparent and Hydrophobic SiO<inf>2</inf>-crystalline TiO<inf>2</inf> Nanocomposites at Ambient Conditions for Stone Protection and Consolidation

Three innovative strengthening, protective and self-cleaning agents for marble and calcareous stones were synthesized by using a simple sol–gel route at ambient conditions. TiO2 nanoparticles and hydroxyl-terminated polydimethylsiloxane (PDMS) were incorporated in a tetraethoxysilane (TEOS) matrix in the presence of oxalic acid (Ox). Both the Ox concentration and the addition sequence of the reagent agents were assessed through the evaluation of the microstructure and of the physicochemical and self-cleaning properties of all the synthesized nanocomposites. The presence of Ox contributed to the production of both homogeneous materials and photoactive independent domains of anatase TiO2 crystals at ambient conditions. In addition, the role played by Ox as a hole-scavenger increased the photocatalytic activity of the nanocomposites. The synthesized crack-free, homogeneous, transparent and photoactive nanocomposites provide self-cleaning, water repellency and consolidation properties to building substrates, while respecting their aesthetic qualities

Producing photoactive, transparent and hydrophobic SiO2-crystalline TiO2 nanocomposites at ambient conditions with application as self-cleaning coatings

Nowadays, the enhancement of atmospheric pollution is dramatically increasing the presence of soiling on buildings in every city of the world. Thus, the development of photocatalysts as self-cleaning coatings is a promising challenge. The first object of this work was to develop a simple synthesis route for obtaining SiO2-crystalline TiO2 nanocomposites at ambient temperature. Thus, it meets the requirements to produce photoactive coatings on buildings and, additionally, it can be used in other applications requiring low temperature. The second objective was to gain insights into the structure of these materials and to establish the relationship between their structure and their performance as photocatalysts. The synthesis process involves mixing titanium and silicon alkoxides in the presence of oxalic acid. An organic silica oligomer is also added to reduce surface energy and consequently, to give hydrophobic properties to the material. These products were applied as a coating on marble specimens in order to investigate their effectiveness. The results obtained highlight that oxalic acid acts a chelating agent of the titanium precursor, giving rise to a homogeneous TiO2–SiO2 material. In addition and most importantly, oxalic acid produces photoactive anatase crystals at ambient temperature. From our investigation on the structure of these materials, we conclude that the formation of independent domains of TiO2 inside of SiO2 matrix, and the anatase crystal size are key factors for improving the photoactivity of the coatings. We also conclude that the oxalic acid concentration plays a crucial role in the formation of this optimum structure

Modified Tetraethoxysilane with Nanocalcium Oxalate in One-Pot Synthesis for Protection of Building Materials

A nontoxic strengthening and protective agent for porous calcareous stones and cement mortars has been synthesized in a one-pot synthesis via the sol–gel method, incorporating nanoparticles of synthesized amorphous calcium oxalate monohydrate (ACO) in tetraethoxysilane (TEOS). Calcium hydroxide and oxalic acid added into TEOS produce ACO, which is then incorporated into the silica matrix, while oxalic acid also acts as catalyst for TEOS hydrolysis. The crack-free nanocomposite derived possesses a uniform microstructure with average pore diameter of 2.73 nm and particles of approximately 7–15 nm in size. The ACO incorporated into the silica matrix gives good interfacial compatibility between the nanocomposite and building materials, such as calcareous stones and cement mortars, and improves their mechanical properties. The hybrid nanocomposite can penetrate inside the lithic substrate and acts as a strengthening agent with protective effect against environmental loading. This one-pot synthesis allows an easy and cost-effective industrial scaling up

The protection of marble surfaces: the challenge to develop suitable nanostructured treatments

Marbles have been extensively used in historical architecture owing to their good mineralogical and microstructural properties, durability, and aesthetic quality. Nevertheless, the protection of historical marbles in outdoor conditions is a difficult task, mainly because of their low open porosity. An overview of nanostructured protective treatments based on the use of SiO2, TiO2, ZnO, and Ag nanoparticles to confer superhydrophobic, self-cleaning, and antifouling properties to the surface is proposed. Particular attention is devoted to the development of photocatalytic nano-TiO2-based treatments. In this regard, advantages, drawbacks, and critical issues in the use of nanocomposites are covered. Recent advances using modified innovative TiO2 nanoparticles, in dispersion and as nanocomposites, are reported. Nanocomposites based on suitable TiO2 nanoparticles seem very promising, and a comparison of the results obtained in controlled lab conditions and on real deteriorated surfaces is also presented