## 4 Discussion  
Analysis of the results of the compressive strength of concrete in different ripening periods indicates that the strength of concrete increases over time, reaching its highest values for longer periods. This phenomenon is typical for the cement hydration process, in which chemical reactions take place that cause the concrete to harden and develop its  
Figure 16: Microstructure of M3 concrete sample.  
Figure 17: Microstructure of M2 sample and transition zone between glass flour grain and cement matrix.  
strength. The obtained results also indicate that the content of glass flour and a constant number of polypropylene fibers have an impact on the development of strength over time. As the content of the additives used increases, differences in strength can be observed at different time stages. However, there is a limit to the optimal amount of flour to which the compressive strength increases. This suggests that the presence of additives may affect the rate and nature of the cement hydration process because of the interaction between flour, fiber, and cement, which consequently affects the development of concrete strength over time. The research confirms the results of other authors. [PERSON] and [PERSON] [45] showed that cement partially replaced by glass flour has a beneficial effect on the mechanical properties of the hardened composite. Studies have shown that the maximum increase in compressive strength was achieved for samples with 10% flour, and above this value, there is already a noticeable decrease in this property [45]. Polypropylene fibers have an antispasmodic effect, protect the composite against cracks and scratches [46], and improve the frost resistance of concrete. Research with the addition of polypropylene fibers is described in [PERSON] and [PERSON] [47], which focuses mainly on showing their impact on frost resistance. Reference concrete was compared with concrete containing class Ia (single) and class Ib (fibrilated) microfibers. The authors observed that with an increase in the percentage content of polypropylene fibers, the consistency of the concrete mix decreases, the density of the concrete mix decreases with the increase in the percentage content of single fibers, the addition of single microfibers causes a decrease in the compressive strength of the composite after 28 days, and frost resistance is significantly improved compared to reference concrete [48].  
The analyzed microstructure of M1-M3 concrete samples containing additions of polymer fibers and glass flour has shown that the addition of larger amounts of glass flour as a substitute for cement might cause problems with the compaction of the concrete mix, as observed in the M3 sample. This is probably due to the decrease in the workability of concrete mix caused by the increase in water demand from glass flour.  
Observed in the cement matrix of the M1 sample, several spherical air voids might be the cause of the observed slight increase in freeze/thaw resistance of this concrete. Although the amount of air voids was too low to make concrete fully resistant to this type of aggression, in the microstructures of other samples (M2 and M3), this type of air void was not observed. In the microstructure of the M3 sample, there were observed areas with high porosity and cracks, which might decrease freeze/thaw resistance even more.  
Examining the microstructure of the transition zone between grains of glass flour and the cement matrix, it was discovered that silicon ions from glass migrate into the cement matrix and calcium ions in the opposite direction. It proves that grains of glass flour react with the cement matrix. It might be the cause of the observed slight increase in compressive strength of the M1 sample compared to the reference sample (without glass flour). Although the reactivity of glass flour grains might be the cause of the corrosion of cement composites by the potential alkali aggregate reaction, which needs further investigation.