Industrial waste pollution in the cities and the areas beyond them is a burning issue today. The only solution is recycling the by-products of primary production. Thus, the magnesia caustics are widely used in construction. However, their application is limited due to their high hygroscopicity. Many scientists were involved into the study of this problem justifying their achieved positive result by either using different types of sealers or introduction of fine composition active mineral additives into the binder. Moreover, all these studies mainly focused on the improvement of the water resistance of magnesia composites, while hygroscopic regulation was not significant. So it is important to study the process of environmental moisture accumulation in the pores of the magnesia material. When establishing the causes of the high hygroscopicity of magnesium materials, it was found that its value depends on environmental conditions (temperature, pressure and relative humidity); the specific surface area of magnesia stone; density of the sealer for magnesia binder; the chemical composition of various modifying additives and their influence on the properties of magnesia stone and materials based on it; the composition of the hydrated phases of magnesia stone with additives and without them; the presence of pores, their size and number in the structure of the stone; the surface charge of a magnesia stone. Studies have shown that high-strength water resistant chlorine magnesia stone in combination with modifying additives allows obtaining a composite material with hygroscopicity less than 2 %.
