Building surfaces degrade rapidly upon exposure to moisture, water, UV radiation, temperature variations, and other external factors. Since moisture is a critical cause of surface damage, hydrophobic surface treatment for both existing and new building materials is crucial to prevent the deterioration process, dirt deposition or aggressive agents activity.
Surface protection against moisture has been used for thousands of years1. It began with waxes and oils in Mayan culture and since then, various types of coatings and impregnates have been developed.
For example, when a concrete surface is treated with a water repelling agent, the properties of the surface typically turn from hydrophilic to hydrophobic. Hence, water droplets are prohibited from entering the concrete, while water vapor is still allowed to pass through.
Surface protection can be classified in different ways, according to the main generic components, function, and cure requirements, or by specific properties; i.e., the thickness of the surface film and penetration depth.
According to Levi et al. there are three main classes of protective treatments2:
- Hydrophobic impregnation ("pore-lining treatment") - produces a water-repellent surface; based on silanes and siloxanes
- Impregnation ("pore blocking treatment") - produces a discontinuous thin film; based on polymers, such as acrylics and epoxies with low viscosity
- Coatings - produce a continuous layer on the surface; based on epoxy, polyurethane, vinyl, and acryl compounds.
In contrast to other surface treatments, hydrophobic impregnation does not change the original/aesthetic appearance of the surface. After hydrophobic impregnation, the surface pores and capillaries are penetrated, so that they are internally lined but not filled.
Hydrophobic impregnation treatments alter the surface tension of mineral substrates, such as concrete or brick, and thus a water-repellent surface is created to keep water and aggressive water-soluble salts out. It is a transparent, non-film forming protection system, that can effectively increase the durability of building material. However, it is irreversible.
Impregnation consists of an upper layer of ‘coating’ (fulfilling a hydrophobic function caused by significantly reduced surface tension and molecular attraction) and a lower layer, which ensures the entire ‘coating’ adheres to the treated substrate.
The application of silica-based hydrophobic materials began in Germany in the 1950s. Since then, a lot of effort has been put into product development, and many commercial materials are currently available on the market. New products for hydrophobic impregnation are mostly based on concentrated silanes and siloxanes compounds. They are also solvent-free and available on the market as liquids, creams or gels.
So far, no clear criterion can be used to assess the effectiveness of hydrophobic treatment applied to construction products. Despite the extensive research work on the effectiveness of hydrophobic treatments in improving building material durability, little is known about surface treatment influence on aesthetic performance.
A current issue is the proper selection of a hydrophobic product, as no universal product can be used for the hydrophobic treatment of all mineral substrates. Construction materials made from limestone, sandstone, concrete, and ceramics, require the selection of the optimal type of hydrophobic agent, which entails research and objective evaluation.
Long-lasting protection and aesthetics are two critical requirements of today's building materials. Producers must continually optimize their formulas and more importantly, methods for evaluation, to achieve the highest levels of performance and understand the mechanism of protective treatment.
- P. J. Koblischek, Protection of Surfaces of Natural Stone and Concrete through Polymers, Surface Engineering, 62-71, 1990.
- M. Levi, C. Ferro, D. Regazzoli, G. Doteli, A. Lo Presti, Comparative evaluation method of polymer surface treatments applied on high-performance concrete, Journal of Materials Science 37, 4881 – 4888, 2002.