## 1 Introduction  
### Geopolymer Concrete and Fiber-Reinforced Composites  
Composite materials have been used for centuries in many industries. Mankind is constantly striving to improve and obtain materials with better properties while generating the lowest possible production costs. Already before our era, the Israelites, to improve the strength and durability of bricks, added straw and horsehair to mud, which acted as reinforcement [1, 2]. Since then, composites have developed on a much larger scale. Nowadays, composites reinforced with fibers have become a fundamental group of materials used in the construction industry, in air transport, or on the ground [3, 4, 5, 6]. The addition of suitable fibers to reinforce the material can not only improve the mechanical properties but also change the fracture character of the material from brittle to more flexible [7, 8, 9]. Unfortunately, composite materials, despite their many advantages, have a drawback that concerns their recycling and the carbon footprint of a given product. More and more emphasis is being put on the above aspects, which makes it necessary to look for more environmentally friendly alternatives, e.g., in the form of reinforced composites of natural origin [10; 11]. Concrete is one of the most widely used composite materials in the world. Steel bars and fibers are mainly used to reinforce concrete structures, but other reinforcement methods, such as fibers or particles made from organic or inorganic materials, are increasingly used. Currently, the industry with the highest environmental burden and most polluting emissions is construction. The building sector is responsible for a huge energy consumption that is estimated to be around 40% worldwide, of which residential buildings consume 27% of energy and contribute 17% of CO2 emissions to the environment. This is due to rapid population growth and changing living standards. This high demand for energy translates into significant pollution and environmental degradation. To avoid the negative effects of excessive energy consumption, sustainable building materials are increasingly being developed. In recent years, a promising alternative to Portland cement is composites based on so-called geopolymers. These are much less harmful to the environment and generate less carbon dioxide. In addition, significantly lower energy consumption during the production process is noticeable. These values are two to three times lower than the energy consumption for Portland cement [12; 13; 14; 15; 16]. Geopolymers are a class of amorphous, inorganic, aluminosilicate polymers whose properties depend on the type of base material used and the type and amount of activator used to produce them. In addition, it is possible to control the external factors of the polycondensation process (temperature and heating time), which also affects the subsequent properties of the obtained material [12; 17]. These materials are also suitable for the immobilization of hazardous waste (radioactive waste, asbestos) or toxic waste (mercury, lead, arsenic) [14; 17; 18]. This group of binders is characterized, among others, by high compressive strength [19; 20], resistance to acids, chlorides, and sulfates [21; 22; 23], thermal resistance (up to about 800 \({}^{\circ}\)C) [24; 25], or good frost resistance [26; 27]. Furthermore, no corrosion of steel reinforcement occurs in geopolymers, and the material shows good adhesion properties with steel [12; 17]. The current metallurgical and mining industries produce huge amounts of post-production waste materials, the storage of which in the environment may pose an ecological problem, leading to the contamination of soils and surface waters. The use of waste streams from mining and processing industries for the production of geopolymer materials is not only a positive environmental aspect but also contributes to the development of a circular economy. In addition, the use of post-production industrial waste reduces its quantity in landfills and contributes to their revalorization [10; 28; 29; 30].