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Assessment of chemical compatibility and amelioration of stone consolidants with nanotechnology

Kapridaki Chrysi

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Summary

The necessity of developing novel materials for protection, consolidation and self-cleaning of building materials originates from the ever increasing atmospheric pollution, which induces considerable corrosion. To this end, and aiming at preserving the sustainability of monuments, as well as traditional and modern constructions, the present doctoral thesis developed novel nanotechnology materials, which show advanced properties and can be further produced on an industrial scale.Into this context, three novel consolidant and protective nanomaterials (STP-1, STP-2 and STP-4) were developed, which simultaneously combine self-cleaning, waterproofing and restoring the cohesion of the structure of building materials. The nanomaterials STP-1, STP-2 and STP-4 are environmentally friendly, their synthesis exhibits a small energy fingerprint and they are effectively applied on stone substrates of both monuments and modern buildings. The experimental study of this thesis consists of the following parts: (a) synthesis of transparent, hydrophobic and photocatalytically active nanocomposite materials SiO2-TiO2-PDMS with low energetic demand, (b) thorough physicochemical characterization, (c) application on real substrates and (d) evaluation of their effectiveness as protective, consolidating and self-cleaning materials.The synthesis of these nanomaterials was based on the sol-gel process after mixing titanium alkoxide (titanium tetra-isopropoxide, TTIP), silica alkoxide (tetraethyl orthosilicate, TEOS) and an organosilane (hydroxyl-terminated polydimethylsiloxane, PDMS). The synthesis was conducted in room temperature in the presence of oxalic acid (Ox), the concentration of which increases gradually in the three compositions, STP-1, STP-2 and STP-4. The numbers accompanying the compositions 1, 2 and 4 represent the low, intermediate and high concentration of Ox, respectively. Following their synthesis, the nanomaterials were thoroughly studied with respect to their physicochemical characteristics in order to test and establish their chemical compatibility with the building materials, as well as their photocatalytic efficiency.Examination of the nanostructures of the transparent novel materials confirmed the effectiveness of oxalic acid with respect to both creating homogeneous gels and forming photocatalitically active anatase TiO2 in room temperature. The size of the independent TiO2 nanoparticles has been estimated between 5 to 10 nm and 2 to 5 nm in the nanocomposite materials STP-2 and STP-4, respectively. Moreover, the lack of cracks and the cohesive structure of gels are due to the synergetic action of Ox and PDMS. The hydrophobic character of the nanocomposite materials is also noteworthy and is attributed to the presence of PDMS, which reverses the hydrophilic character of the SiO2-TiO2 system. Finally, the three nanomaterials have ranked according to their photocatalytic efficiency as following: STP-2 > STP-4 > STP-1. The increased efficiency of STP-2 is due to the high proportion of independent TiO2 nanoparticles within the silica matrix (5-10 nm). However, the contribution of the “hole-scavenger” oxalic acid to the increased photocatalytic efficiency of STP-2 and STP-4 seems to be very significant.After completing the physicochemical study and the evaluation of the photocatalytic action of the nanocomposite materials, the latter were applied to marbles and limestones. The assessment of the treatment revealed that the studied nanocomposite materials can successfully be used as strengthening, waterproofing and self-cleaning agents for marbles and limestones.

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