Essay on surface science

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Applied Surface Science 275 (2013) 389–396

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A novel TiO2 –SiO2 nanocomposite converts a very friable stone into a self-cleaning building material
Luís Pinho, Farid Elhaddad, Dario S. Facio, Maria J. Mosquera ∗
Departamento de Química-Física, Facultad de Ciencias, Campus Universitario Río San Pedro, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain

a r t i c l e

i n f o

Article history:
Received 2 October 2012
Accepted 23 October 2012
Available online 5 November 2012
Non-ionic surfactant
TiO2 –SiO2 nanocomposite
Self-cleaning agent
Salt-resistant product

a b s t r a c t
A TiO2 –SiO2 nanocomposite material was formed inside the pore structure of a very friable carbonate stone by simple spraying of a sol containing silica oligomers, titania particles and a non-ionic surfactant
(n-octylamine). The resulting nanomaterial provides an effective adhesive and crack-free surface layer to the stone, and gives it self-cleaning properties. In addition, it efficiently penetrates into the pores of the stone, significantly improving its mechanical resistance, and is thus capable of converting an extremely friable stone into a building material with self-cleaning properties. Another important advantage of the nanocomposite is that it substantially improves protection against salt crystallization degradation mechanisms. In the trial described, the untreated stone is reduced to a completely powdered material after 3 cycles of NaSO4 crystallization degradation, whereas stone treated with this novel product remains practically unaltered after 30 cycles. For comparison purposes, two commercial products (with consolidant and photocatalytic properties) were also tested, and both alternative materials produced coatings that crack and provide less mechanical resistance to the stone than this product. These results also confirm the valuable role played by n-octylamine in reducing the capillary pressure responsible for consolidant cracking, and in promoting silica polymerization inside the pores of the non-reactive pure carbonate stone. © 2012 Elsevier B.V. All rights reserved.

1. Introduction
Natural stone of diverse types is employed as construction material around the world, for reasons of esthetic appeal and elegance but mainly for its durability. Demand for natural stone is, therefore, usually limited to the more durable varieties, such as granites, marbles and some sandstones. Another type of stone, pure carbonates, presents an exceptionally bright white color, which is much appreciated by consumers as a building material for floors, walls and external facades. However, this natural rock has low mechanical resistance and is easily stained, thus inhibiting it commercial application. Therefore, the development of a treatment product specifically intended to enhance the robustness and durability of carbonate stone, and with self-cleaning properties, should be of considerable interest for architecture and construction.
Since the early discovery of the self-cleaning properties of titanium dioxide [1], it has been considered to be the most efficient, stable and cheap photocatalytic material available [2,3]. In recent years, the application TiO2 to very widely different substrates, such as textiles [4–7], plastics [8–12] and glasses [13–16], has been

∗ Corresponding author. Tel.: +34 956016331; fax: +34 956016471.
E-mail address: (M.J. Mosquera).
0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. widely reported. However, its application to various types of stone has been much more limited [17–21]. It is commonly employed as an aqueous dispersion of titania particles [17,19,20]. The results obtained for these products on stone are not