Ceramics-Silikáty 62 (4), 311-324 (2018) www.ceramics-silikaty.cz doi: 10.13168/cs.2018.0027 Ceramics – Silikáty 62 (4) 311-324 (2018) 311 THE EFFECT OF THE SODIUM SULPHATE SOLUTION EXPOSURE ON PROPERTIES AND MECHANICAL RESISTANCE OF DIFFERENT KINDS OF RENDERS MILENA PAVLÍKOVÁ*, JAROSLAV POKORNÝ*, # ONDŘEJ JANKOVSKÝ**, MARTINA ZÁLESKÁ*, MARTIN VAVRO***, KAMIL SOUČEK***, ZBYŠEK PAVLÍK* * Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 166 29 Prague 6, Czech Republic ** Department of Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic *** Department of Geomechanics and Mining Research, Institute of Geonic of the AS CR, v.v.i., Studentská 1768, 708 00 Ostrava – Poruba, Czech Republic # E-mail: Ondrej.Jankovsky@vscht.cz Submitted July 14, 2018; accepted August 24, 2018 Keywords: Sulphate attack, Renders, Mechanical resistance, X-Ray CT, Chemically induced damage The efect of 5 % water solution of the sodium sulphate and the reference environment of distilled water on changes in mechanical resistance of the commercial dry render mixes was researched in the paper. The prism-shaped specimens cured 28 days in humid and stable conditions were immersed in water or sodium sulphate water solution. At chosen time, the particular specimens were collected, dried to constant mass, and subjected to tests. The maximum time of specimen exposure to moisture or salt action was 168 days. Within the performed experiments, length changes, mass gain, mechanical and basic properties were measured in order to reveal the efect of sulphate corrosion on researched materials. Additionally, pore size distribution measurement and X-Ray CT analysis for the most damaged render were done, in order to characterize the disruptive impact of sulphate solution on porous microstructure of lime-metakaolin-based render. The obtained data revealed the high capacity of porous space of the tested renders for salt storage. For shorter times of exposure to salt action, most of the studied renders exhibited improvement in mechanical resistance. On the other hand, after the full flling of the porous space, the evoked crystallization pressures led to the decrease in mechanical resistance and materials damage. However, one type of studied renders originally designed for restoration of salt laden masonry maintained its excellent mechanical properties even after 168 days of sulphate exposure. INTRODUCTION Renders represent group of construction materials used for surface fnishing of buildings or their particular structures. In terms of their degradation, several decay mechanisms can be distinguished, whereas water has the major infuence. Corrosive media for renders mainly includes pressure water, chloride ions, sulphate ions, carbon dioxide and other harmful acids, alkali, or salt solutions [1]. In case of pure water presence, most of the disruptive efects would not arise. On the other hand, water often becomes a transport medium for diferent types of salts that signifcantly contribute to materials deterioration [2-5]. Soluble salts may derive from acid rains or marine environment, but they are more often associated with the rising damp [6-9], and in respect to the climatic and environmental conditions of building structure negatively afect materials durability due to their physical and chemical action. Soluble salts are considered as principal agent of decay of porous building materials and a source of great frustration to those involved in the conservation of historic and older buildings. The behaviour of salts may seem unpredictable since they can remain dormant for long periods and then suddenly become active causing damage and disfguring historic fabric. Repeated wetting and evaporation cycles due to daily/seasonal microclimatic changes lead to the cyclic precipitation of salts and to the progressive decay of building materials, and thus whole structures [10]. Sulphate corrosion of porous building materials is caused by sulphate ions that are present for example in subsoil with high level of underground water [11], [12], in salts used for winter maintenance , and in sea water and thus in marine environments [13], where also chloride and magnesium ions are contained. The mechanism of sea water attack on construction materials is very diferent than what is happening in the case of pure sulphate attack [14]. The sulphate induced degradation mechanism of cement based composites depends on materials external conditions as temperature, humidity, moisture content and concentration of sulphate ions. Moisture afects especially the rate of chemical reactions, and implicates