## 1 Introduction  
This paper discusses the problem of leaching of hazardous substances from mineral-wood-based geopolymer and cement mortars. The partial substitution of wool waste with metakaolin and CEM II cement is investigated as a method to prevent leaching. The earliest information regarding geopolymers goes back to ancient times, when it had various applications. Some sources state that the ancient Egyptians used similar solutions in the construction of the pyramids [1]. The concept of geopolymers was introduced into the literature by Professor [PERSON] in 1978 [2]. What exactly is a geopolymer? Geopolymers are synthetic, nonorganic, i.e., not containing carbon atoms in their structures, polymers of aluminosilicates. The basic reaction behind the formation of this material is the synthesis of silicon and aluminum. In other words, geopolymers are synthetic aluminosilicate polymers characterized by an amorphous internal structure. The definition geopolymer can be presented by analysing its two parts: \"geo\" and \"polymer\", the former indicating that t is a material with a structure that imitates raw materials found in nature, hence, the 'geo' in the name [3]. Furthermore, like a polymer, it is formed of multiples of repetitive minor particles, hence, 'polymer'. The base of a geopolymer composition is a powdered material whose chemical composition is dominated by aluminosilicates. The other major component of a geopolymer is an activator--a chemical mixture whose task is to create an alkaline environment in which the binding reaction will take place [3]. Geopolymer is an innovative environmentally friendly building material, commonly referred to as 'green concrete'. According to the sources, the production of geopolymer uses up to three times less energy compared with classic Portland cement concrete. The same is true for CO2 emissions. Geopolymers are regarded as the ecological successor to Portland cement as classical concrete production generates several times more harmful greenhouse gases. The production of one tonne of Portland cement emits approximately 1 tonne of CO2 into the atmosphere. The synthesis of geopolymer generates 4-8 times less CO2 [4; 5]. Mineral wool is a widely used construction material, performing the function of cold, fire, and noise insulation. Waste from this building material is generated during both the demolition and production process [6]. Mineral wool waste poses a significant environmental challenge. In the age of thermomodernisation of buildings, massive amounts of this waste are being generated, reaching an estimated annual of 2.5 million tonnes in the European Union. Due to the difficulty of recycling, most of this ends up in landfills. The management of mineral wool waste is a difficult issue. Due to the specific characteristics of the waste, it is stored in industrial landfills and, thus, contributes to environmental degradation. Nevertheless, due to its low bulk density, high volume, poor compressibility, and the consequential high elasticity, mineral wool causes problems, even in landfilling. Its characteristics adversely affect the landfill stability [7]. The synthesis of geopolymer may offer a method to counteract this occurrence. Geopolymers can be obtained from various raw materials, including fly ashes [8; 9], bottom ashes [10], glass powder [11], natural rocks [12], and mineral wool [13] and metakaolin [14], which are the subjects of research in this article. A problem that the production of wool-based geopolymers can generate is the leaching of environmentally hazardous substances. This issue was covered in a previous article of the team [15], where acceptable concentrations of sodium and strong alkalinity were found to be exceeded. An additional challenge is the reduced leachability of phenol and formaldehyde. This paper addresses the issue found in that article regarding the exceedance of permissible leaching standards for geopolymer mortars. The replacement of mineral wool with metakaolin aims to reduce the leaching of exceeded parameters.