## 1 Introduction
### Types of Reinforcing Fibers in Concrete Composites--Analysis  
The brittleness of geopolymer materials is a commonly known problem. Portland cement-based materials (OPC) have a similar property. However, despite their brittleness, geopolymers are promising materials because of their compositional variability and their ability to be combined with other materials as a form of reinforcement. In his review, [PERSON] not only described the durability of OPCs but also focused on discussing the topic of geopolymer composites [33]. [PERSON]'s work mainly focused on the study of UHPC concretes and their mechanical properties [34]. Fiber-reinforced composite materials are most commonly used to provide increased static and fatigue strength, including specific strength. The positive effect of fibers on improving the strength properties and determining the onset of cracking can be confirmed, provided that the matrix material and reinforcement mixtures are properly selected. The correct choice of matrix materials increases the adhesion of reinforcing fibers to the matrix and changes the nature of crack propagation of the whole composite material, from fracture to more ductile. A matrix-fibers bond that is too strong can lead to fibers pulling out and under the action of force. A fibers-matrix interface that is too loose will not fulfill its reinforcing function [35; 36; 37]. The matrix only transfers the applied load to the fibers, which carry most of it. Fibers in composite materials are one of the most commonly used forms of reinforcement. They can have considerable lengths and diameters below 100 \(\upmu\)m. We distinguish between chopped fibers, long fibers, and short fibers, in the form of mats, fabrics, or tapes [38; 39]. Short fibers, due to their ease of dispersion, are an effective way to reinforce geopolymers, without the need for additional technological measures in the production process [40]. [PERSON] and [PERSON] proposed a classification of fibers, which are used as reinforcement in geopolymer materials. They distinguished five basic groups: carbon fibers, metal fibers, natural polymers, synthetic polymers, and other inorganic fibers [41]. If several types of reinforcing fibers are combined in a geopolymer, a synergic effect may be obtained, which may improve the mechanical, thermal, or functional properties of the whole geopolymer composite [42; 43; 44]. Cement composites using glass fibers as a reinforcing phase are frequently used in modern building and construction solutions. Thanks to their properties, glass fibers are used for structural materials with even very complex geometries. However, the problem is the cementitious environment, which leads to rapid corrosion of the popular types of S and E glass fibers, contributing to the disintegration of reinforced concrete structures [45; 46]. Many studies and scientific papers have been devoted to glass fibers as reinforcement in geopolymers [47; 48; 49; 50; 51; 52]. Another type of fiber used for reinforcement is carbon fiber. Such reinforcement is mainly used to improve the strength properties of geopolymers, mainly the bending strength. Thanks to the use of carbon fibers, the material does not crack quickly and brittle, and crack propagation is delayed. Carbon fibers have many advantages, including the following:  
* Low density;
* High tensile strength;
* High Young's modulus;
* Very good fatigue strength;
* Creep resistance;
* Good electrical conductivity;
* High abrasion resistance;
* Ability to absorb vibrations;
* High dimensional stability;
* High chemical resistance. The disadvantage of carbon fibers is the decrease of their properties in oxidizing environments at elevated temperatures (around 2000 \({}^{\circ}\)C). Alkaline environments protect the fibers against oxidation, making carbon fiber-reinforced geopolymers suitable for use at elevated temperatures [53; 54; 55].