85 1. INTRODUCTION Each type of natural stone has an encoded speci fc disposition for certain behaviour under the impact of weathering agents. Te genesis, mineral composition, and structural-textural features of the rock are essential internal factors afecting the stability, durability, weathering resistance, and thus the rock material’s longevity. To increase the durability, the application of protec- tive coatings on natural stones exposed to external conditions in building facades, decoration etc. became the current stan- dard technique. Te coating serves as a suitable prevention from weathering. Practitioners in the restoration of historical sites, mainly statues and valued architectonical elements made of stone, are probably the most experienced in applying protec- tive mediums (Beck et al., 2003; Doehne & Price, 2010). Te domestic and international market ofers various impregnation coatings and promotes new products being under development. Te recently developed products against the negative impacts of external conditions are based on chemical nanotechnology, which means that the main particles are in nanometers. Teir ef- fect is tested and described mainly on high-quality natural stones used on historical buildings, e.g. marbles (Aldoasri et al., 2017; Ban et al., 2019). Te interactions between chemical products and the natural stone, as well as the impact of protective coatings on the stone material are continuously tested and analysed in chemical laboratories (Wheeler, 2005; Török et al., 2006; Pedna et al., 2016). Te producers evaluate the water resistance of the coating and the general impact of the formed protective layer that should repel water, make the material maintenance easier, and prolong its durability. Te available literature sources do not provide sufcient research data on the durability of protective coatings on natural stones or equivalent stone products. Ques- tions as how long does the protective efect last, or how ofen the coating has to be applied on the stone surface, are still unan- swered. Te problem of a di ferent impact when the coated stone is exposed to permanent water contact or just to sporadic rain, is also not resolved. Only the time, i.e. a long-lasting exposure, can truly verify the efect of the applied coating. However, some indication and material behaviour models can be determined by accelerated weathering tests in the laboratory. As good as no relevant data are available about protective coat- ings on highly porous stones, e. g. tufs. Teir particular structure, built by primary pores of the size of macro-pores (i.e. pores vis- ible by eye) and hollows between mineral grains, predetermine the rock to be an excellent material for decorative use. Typical properties of the tuf (i.e. light, highly abrasive, easy to carve and shape) make it a suitable stone for face ashlars, sculptural arts, or for production of small architectural elements. Tufs are widely used as building stone in Central Italy (Colella et al., 2017; Heap et al., 2018). But, this is a rock of lower strength that is prone to decay in a relatively short time. Te picturesque medieval stone towns and villages like famous Pitigliano (Fig. 1), atracting numbers of tourists and visitors, sufer under intense weather- ing. Both the preservation and remediation of these historical and cultural jewels require scientifc approaches. In 2008 an international symposium “Conservation and Sus- tainable Development of the Tuf Towns”, convened by the World Monument Funds, took place in Pitigliano, Civita di Bagnoregio and Orvieto (Barbacci & Peruzzeto, 2010). Te focus of the symposium was to address geological and cultural sustainability Efciency o f protective coating applied on a highly porous decorative tuf Tatiana Durmeková, Renáta Adamcová, Martin Maľa & Petra Grand Comenius University in Bratislava, Faculty of Natural Sciences, Department of Engineering Geology, Hydrogeology and Applied Geophysics, Bratislava, Slovak Republic, tatiana.durmekova@uniba.sk Abstract: The application of protective coatings can signifcantly infuence the essential physical properties of rocks used in building and decorative industries. This paper presents the results of laboratory tests demonstrating the positive efect of the protective coating Antipluviol S applied on a tuf from Tuscany in Italy, which is a weak volcanic rock of a very low strength and very high porosity. The efciency of the coating was measured by absorption tests, standard uniaxial com- pressive strength tests and cyclic frost resistance tests. After the coating application, the water absorption decreased from 39.1% to 31.3% by total rock immersion at atmospheric pressure conditions. The water absorption only by capillary suction showed much higher efciency of the coating. The change in the rock strength, resulting from the coating application, was less pronounced; the mean values rose from 5.8 to 6.8 MPa (17.2% increase). For all strength values are quite similar in both treated and untreated groups, as well as scattered within the groups, no relevance to the coating impact could be confrmed. But, even such a slight improvement of the monitored physical parameters has the efect of slowing down the rock’s weathering. This was proven by the results of the frost resistance test. Coated samples withstood all the standard 25 freeze-thaw cycles, whereas untreated samples began to disintegrate after the 17th test cycle. These fndings can help the maintenance of the famous historic architecture of picturesque tuf towns of Tuscany sufering from weathering, and contribute to the preservation of the cultural heritage. On the other hand, they showed how important such tests are when considering building stone import. Key words: Tuscany tuf, weathering, protective coating, water absorption, UCS, frost resistance Manuscript received 2021-04-14 Revised version accepted 2021-05-07 acta geologica slovaca, 13(1), 2021, 85–94