# Factors Affecting the Efficiency of Hydrophobic Coatings--Experience from Application on Sandstone  
[PERSON]  
1 Cemenius University in Bratislava, Faculty of Natural Sciences, Department of Engineering Geology, Hydrogeology and Applied Geophysics, Ilkovicova 6, 842 15 Bratislava, Slovakia; [EMAIL_ADDRESS] (L.D.); [EMAIL_ADDRESS] (T.D.)  
[PERSON]  
1 Cemenius University in Bratislava, Faculty of Natural Sciences, Department of Engineering Geology, Hydrogeology and Applied Geophysics, Ilkovicova 6, 842 15 Bratislava, Slovakia; [EMAIL_ADDRESS] (L.D.); [EMAIL_ADDRESS] (T.D.)  
[PERSON]  
1 Cemenius University in Bratislava, Faculty of Natural Sciences, Department of Engineering Geology, Hydrogeology and Applied Geophysics, Ilkovicova 6, 842 15 Bratislava, Slovakia; [EMAIL_ADDRESS] (L.D.); [EMAIL_ADDRESS] (T.D.)  
Footnote 27: email: {tatiana.  
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27: email: {[EMAIL_ADDRESS]}  
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the most important factor; (b) efficiency and durability of the coatings depend on the stone type; (c) every impregnation should be tested prior to its technical application; (d) even after such a long time, coatings can still be efficient, especially if polysiloxane-based ones were used [10]. Similarly, supporting findings were published [11] on hydrophobic effects lasting more than 20 years if correctly applied.  
Impregnation can also contribute to the strengthening of softer materials, even if this is not its main function. Additionally, a higher frost resistance of impregnated highly-porous tuffs was presented when compared to untreated ones in the freeze-thaw tests [12].  
There are many different hydrophobic products available on the market today that have been researched and can be applied to natural stones [6; 13; 14; 15]. Silicon-based systems are the most popular water repellents in use. However, in individual experimental studies, there are significant differences in the test conditions: different protective products, types of treated building material (mortar, brick, stone, etc.), methods of coating application, external or laboratory research conditions, drying/curing time of the coating after its application, and the like. All this makes a formulation of general conclusions about the use of protective coatings on stones difficult.  
Many authors agree that a sufficient depth of penetration into the treated material is the key requirement for achieving the desired durability of a coating [13; 16; 17]. The depth of penetration depends on several parameters, such as porosity of the treated material, water content or degree of saturation, contact time of the substrate with a coating, and the related consumption of a coating [18]. It turns out that the time of coating hardening, or so-called curing time, also plays an important role in the durability of the coating. curing time is the time period of undisturbed action of the coating after its application to the material. This is the aspect which the present paper is focused on.  
Partial results of laboratory testing of two siloxane-based hydrophobic coatings are presented in this paper. The treated rock is well-known as Horice sandstone, which has been widely used in the construction industry as building, decorative, or sculptural stone for a long time. In the presented research, attention is paid to the effect of the curing time of used coatings. Two-day and two-week hardening of the coatings are compared under the same conditions of coating application. Moreover, the effect of the number of applied coating layers was also assessed.  
The efficiency of the coatings applied on the sandstone under the specified conditions was evaluated by checking the depth of penetration into the treated sandstone, as well as by water absorption tests. Mainly tests of the absorption by capillarity were carried out, approved as a suitable method in previous research [19; 20].