## 1 Introduction  
Increasing concentrations of global greenhouse gas (GHG) emissions (especially CO\({}_{2}\)), worldwide energy use, and amount of waste constitute currently the largest environmental problems. A large volume of CO\({}_{2}\) emissions can be attributed to building industry, whether related to construction materials production or energy needed for heating and cooling of insufficiently insulated buildings [1; 2]. For example production of Portland cement (PC) contributes to circa 5-7% of the global CO\({}_{2}\) emissions [3; 4]. At present, there are efforts to find an alternative to PC that would have a low carbon footprint. One of the candidates may be cements based on the magnesium oxide (MgO). Magnesium oxychloride cement (also known as Sorel cement) is formed by the reaction of a light-burnt MgO powder with a solution of magnesium chloride (MgCl\({}_{2}\)) [5], where the light-burnt (also called caustic c) MgO is produced by calcination of magnesite (MgCO\({}_{3}\)) at a temperature of circa 750 \({}^{\circ}\)C [6]. This lower temperature, compared to \(\sim\)1450 \({}^{\circ}\)C used in the production of Portland clinker, allows the use of alternative fuels [3]. MOC cement-based composites are able to absorb high amount of CO\({}_{2}\) from the atmosphere during their service life to form carbonates and hydroxycarbonates, which leads to their denser microstructure and to higher strength of resulting material [4; 7]. The production of MgO from magnesite releases more CO\({}_{2}\) per ton than in the production of Portland cement (1.7 t of CO\({}_{2}\)/t of MgO vs. 1 t CO\({}_{2}\)/t of PC) [3]. However, taking into account the carbonation of the MOC cement composites, when MOC is able to sequestrate more than half of the CO\({}_{2}\) produced during MgCO\({}_{3}\) calcination (the resulting CO\({}_{2}\) emissions decreased therefore to around 0.5-0.6 t of CO\({}_{2}\)/t), and given the total environmental impact during the life cycle of MgO ascertained by LCA, the MOC cement can be considered as environmentally friendly material [3]. Among superior properties of MOC cement composites are: early compressive strength, excellent fire resistance, low alkalinity (pH of 8-10), short setting time, and anti-abrasion performance [8; 9]. MOC cements are also known by their ability to incorporate a higher amount of various fillers compared to PC, namely granite waste [10], fly ash [10; 11], cenospheres derived from the fly ash [12], biomass ash [13], wood [6], recycled tire rubber [14], and waste plastics [15]. Currently, MOC cement is used for the production of industrial floors, for ornamental applications, stucco, grinding wheels, and for different types of panels used for fire protection, as decoration or for sound and thermal insulation [7; 16]. As an air-dried cement material, MOC cement composites lose their compressive strength after immersion in water because of the decomposition of hydration products. This limits their wider use in construction sector [9; 17]. In literature, there are studies that report on improving the poor water resistance of MOC cement. Authors investigated the use of additives as soluble phosphates [18], fly ash [11; 17; 19; 20], rice husk ash with addition of phosphoric acid, calcium lignocellulosic, and acrylic emulsion [21], the phosphoric acid [22; 23], silica fudehume [19], and glass powder [21]. The evaluation of the MOC cement composites water-resistance is to date done only by the compressive strength measurement before and after the immersion in water. However, in the building practice it is also necessary to know other important and for specific applications even crucial parameters of construction materials, such as the water- and water vapor transport properties, heat transport, and storage properties, etc. As a result of plastics versatility, durability, light-weight, and other intrinsic properties, the plastics demand is still increasing worldwide. The world plastic production was 359 million tons in 2018 and the European production constituted of approx.