# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
###### Abstract
The addition of inorganic fibers significantly improves the physical, mechanical, and thermal properties of composites [16]. Glass fiber is one of the most critical reinforcements of different composites. The benefits of the usage of glass fibers are, i.a., improvement of tensile strength and low production costs. The structure of the glass fibers is amorphous, and Young's modulus of the glass fiber is the same as that of bulk glass. They increase the stiffness and reduce the deformability of the composites. They are also characterized by good thermal insulation properties. The main disadvantages of glass fibers are relatively reduced long-term strength, and poor resistance to moisture and alkaline media. Glass fibers introduced into geopolymers cause an increase in flexural and compressive strength and an enhancement in Young's modulus of composite. However, these properties depend on the amount and type of applied fibers. An increase in the aging resistance of geopolymer materials containing glass fibers was also observed concerning materials without reinforcement, which indicates an improvement in long-term strength [17; 18; 19; 20; 21]. Natural fibers are being increasingly applied as the advantages of natural fibers are their biodegradability, renewability, relatively high strength properties, and no negative environmental impact. Since geopolymer materials are produced at a relatively low temperature, not exceeding 80 \({}^{\circ}\)C, there is a growing interest in using natural fibers to reinforce geopolymer composites. The most commonly introduced natural fibers include coconut, cotton, flax, hemp, jute, kenaf, sisal, abaca, banana, and palm. Natural fibers have significant advantages over synthetic fibers but are not free from disadvantages. Due to the structure of natural fibers (lignin and hemicellulose and the resulting hydroxyl groups), these materials are affected by moisture. They also have low dimensional stability and low fire resistance [22]. Flax fibers are natural fibers that belong to one of the strongest fibers related to their low density. The strength of flax fibers can reach over 2 GPa, with deformation up to about 4%. Still, these properties depend on the plant variety, length, and diameter of the fibers, the processing parameters, the applied finishing, and any defects on the surface [23]. Studies on composites reinforced by flax fibers have presented significant improvement in strengthening geopolymers during flexural loading by about 20-30%. However, properties that strongly depend on used fibers may also reduce the compressive and tensile strength. The decrease in mechanical properties is connected with the increase in porosity of the geopolymer (which acts as a stress concentrator). However, scientific research has proven the advantage of using flax fibers as a reinforcing material for geopolymers and eco-friendly additives obtained from renewable sources. The study pointed out that the porosity can be minimized by appropriate surface treatment, increasing workability, or being considered a naturally occurring structure with better thermal and acoustic properties [24; 25; 26; 27; 28]. Intensive research on geopolymer composites with organic and inorganic reinforcements has revealed the advantages and disadvantages of geopolymer materials. The disadvantages of geopolymers include a brittle structure and lower tensile strength, so the introduction of reinforcement is a promising solution. The fibers and particles distributed in the geopolymer matrix are designed to couple cracks and reduce the tendency of cracks to propagate. Extensive research demonstrates the need to address the identified issues. Geopolymer composites with hybrid fiber-fiber reinforcement are intensively produced and examined. Examples of this type of reinforcement can be the combination of steel fiber with polypropylene fiber, basalt fiber with polypropylene, steel fiber with Kevlar fiber, and many others.