# Hybrid Geopolymer Composites Based on Fly Ash Reinforced with Glass and Flax Fibers
## 2 Materials and Methods  
### Materials  
Geopolymers were made using an alkaline promoter based on sodium, fly ash (bulk density--1.1 g/cm\({}^{3}\)) (Elektrocieplownia Skawina, Skawina, Poland), and river sand (bulk density--1.52 g/cm\({}^{3}\)) or glass spheres (BIOSPACE, diameter around 0.1 mm). Glass fibers (Krosglass S.A., Poland, length \(\sim\)3 mm, diameter \(\sim\)10 \(\upmu\)m, density--2.7 g/cm\({}^{3}\), [PERSON]'s modulus \(\sim\)70 GPa) and flax fibers (Hempiflax Group Sp. Z o.o., Poland, length \(\sim\)5-10 mm, diameter 40 to 80 \(\upmu\)m, density--1.5 g/cm\({}^{3}\), [PERSON]'s modulus \(\sim\)60 GPa) were used as reinforcement. The short fibers were selected because they have advantages in processing and allow for the use of traditional methods, the same properties as for pure matrix materials. In this study, we also selected the shortest available fibers, taking into account the laboratory scale of our experiment. In the case of natural fibers, it is not possible to obtain the fibers with strict dimensions, because they are obtained in the process of plant growth. For the study, we select the shortest fibers available on the market.  
For the activation process, a 10-molar (10 M) solution of sodium hydroxide NaOH in combination with a solution of sodium silicate (water glass with a modulus of 2.5 molar and a density of about 1.45 g/cm\({}^{3}\)) in a ratio of 1:2 was used. This solution was prepared by adding an aqueous solution of sodium silicate and water to the solid sodium hydroxide. The solution was allowed to equilibrate to ambient temperature before use. The first stage of manufacture was mixing dry ingredients: fly ash, aggregate, and reinforcing fibers, then adding an alkaline solution and continuing mixing until a homogeneous mass was achieved. The prepared mass placed in molds was subjected to vibrations to remove air bubbles on a vibrating table. The next step was to put them in a laboratory dryer (SLW 750 STD, POL-EKO-APARATURA, Wodzislaw Slaski, Poland) for 24 h at a temperature of 75 \({}^{\circ}\)C and atmospheric pressure. After that, specimens were stored under laboratory conditions until the tests were performed.  
The histograms of the grain distribution and the curve of the cumulative grain distribution of materials are presented in Figure 1. Table 1 shows the particle size distribution of applied raw materials, and Table 2 describes the composition of the produced geopolymer composites.  
The particle size of the base materials significantly affects the physical and mechanical properties of geopolymer materials. The smaller the particle sizes of ashes intended for geopolymer synthesis, the greater their reactivity during the geopolymerization process. Smaller particle sizes have increased active surfaces, which promotes a faster dissolution reaction and the formation of a geopolymer gel in condensation polymerization [11].  
Table 3 shows the results of the X-ray analysis of the used fly ash. Based on diffraction tests, quartz and mullite and a small amount of anhydrite and hematite characterized the fly ash.  
The phases of the materials present in the fly ash belong to the crystalline phases. It is worth noting that some studies of geopolymer materials indicate that even a small amount of quartz may positively affect mechanical properties. In contrast, the presence of other minerals may have opposite effects. An example is illite, which causes changes in geopolymer morphology and leaching behavior [36; 37; 38]. The investigated fly ash has a large number of quartz and mullite which allows us to use it in the geopolymerization process.