# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
###### Abstract  
This article's aim is to analyze physical, mechanical, and fracture properties as well as the thermal investigation of geopolymer composites reinforced with flax, glass fiber, and also the hybrid combination of fibers. Two types of matrices were considered as composites matrices. The first composition was based on fly ash and river sand. The second matrix composition contained fly ash and glass spheres. The content of reinforcement was 1% by mass. Compressive strength and three-point bending fracture tests were performed. The values of fracture toughness and fracture energy were determined. The resonance method was used to verify the dynamic characteristics, such as the dynamic modulus of elasticity and the dynamic Poisson ratio. The results show that single-type fibers in composites based on fly ash and glass spheres did not affect compressive strength. However, introducing hybrid reinforcement increased compressive strength by about 10% compared to the reference specimens. Flax fibers and hybrid reinforcement ensured higher fracture toughness and energy. The results also revealed great potential for glass sphere application to geopolymer materials in terms of fracture mechanics and thermal properties. Despite the lower strength properties in relation to geopolymers based on sand aggregate, applying reinforced fibers into the composite with glass spheres enhanced the compressive strength compared to other materials. Materials modified with glass spheres have a thermal conductivity twice as low as that of materials containing river sand. geopolymer composites; cracking mechanics; natural fibers; glass fibers; glass spheres Article  
[PERSON], P., Z., P., K., B., K., D., L., M.  
different gradations aims to ensure mechanical properties, and volumetric stability, and reduce the price of the material. Usually, river sand is introduced as an aggregate, although research on the application of dune sands was also conducted. Still, this material with small gradations caused a decrease in compressive strength, mainly if this type of addition constituted one hundred percent of the aggregate portion [6]. Geopolymers contained in river sand are characterized by accelerated setting, higher mechanical properties, better compatibility and workability, lower water absorption, and similar shrinkage compared to only fly ash-based geopolymers. However, the introduction of sand into the geopolymer deteriorates properties such as leachability chlorides and the durability of the material [7]. Scientific research has shown that the greater the amount of the aggregate, the better the strength properties of geopolymers. However, the research presented in the literature indicates that the optimization of the volume of river sand or adaptation of other process waste can effectively positively affect the properties of geopolymer materials and thus reduce the consumption of natural mineral resources [8; 9; 10]. Glass spheres are one of the additives introduced to geopolymer for building materials to produce insulating materials. The introduction of glass spheres into geopolymers causes a decrease in the density of the material, and provides satisfactory strength properties and adequate thermal insulation, significantly reducing the thermal conductivity of materials. Both solid materials, in foamed form and thermal coatings modified with glass spheres, were investigated. Each time, these materials demonstrated that they developed thermal comfort and reduced energy consumption [11; 12; 13; 14; 15]. As a reinforcement of geopolymers, organic and inorganic fibers are often implemented. The application of inorganic fibers dates back to the last decades and aims to develop high-performance materials that meet structural requirements. The most commonly used inorganic fibers include carbon, aramid, polyethylene, glass, basalt, boron, boron carbide, boron nitride, zirconium oxide, silicon carbide, and silicon nitride.