## 3 Results
### Microstructure of Concrete  
Figures 12 and 13 show exemplary images of the microstructure of the analyzed reference concrete without the addition of glass flour and polypropylene fibers. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Sample**} & \multicolumn{3}{c}{**Average Compressive**} & \multirow{3}{*}{**Average Weight (g)**} & \multirow{3}{*}{**Average Weight (g)**} & \multirow{3}{*}{**Average Weight (g)**} \\  & & **(MPa)** & & & & **Weight** \\ \cline{2-6}  & \multirow{2}{*}{**Reference**} & **After 150** & & & **Before** & **After 150** & **Loss** \\  & **Sample** & **Freezing** & & **(\%)** & & **Freezing** & **(\%)** \\ \hline BZ & 43.4 & 41.2 & \(-\)5.1 & 2362 & 2353 & 0.381 \\ \hline M1 & 52.6 & 46.4 & \(-\)11.8 & 2303 & 2293 & 0.434 \\ \hline M2 & 47.3 & 37.7 & \(-\)20.3 & 2280 & 2265 & 0.658 \\ \hline M3 & 44.6 & 34.2 & \(-\)23.3 & 2274 & 2254 & 0.880 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of the average decrease in compressive strength of concrete samples subjected to freezing and the average loss in weight of samples made on the basis of glass flour and polypropylene fibers. Figure 12: Microstructure of reference concrete (grains of clinker relics and fine quartz aggregate are marked). The microstructure of the analyzed reference concrete was compact and tight, without excessive air voids or regular, spherical air pores. Numerous relics of clinker were present. No significant content of other secondary cement components was found, which confirms the use of CEM I cement for the preparation of concrete samples. The C-S-H phase was properly formed with a compact and tight structure. In most cases, the contact zone between the aggregate grains and the cement grout was tight and properly formed. Areas of discontinuous contact zone between aggregate grains and grout were few, which indicates proper mixing and compaction of the concrete mix and proper care of the internal areas of hardened concrete (Figure 13). Cracks that occurred in the cement matrix were caused by the sample preparation due to the drying of C-S-H gel. The aim of SEM microstructural analysis was to discover the potential causes of the observed increase in compressive strength between the reference sample and the M1 sample and the causes of the observed decrease in compressive strength in samples M1, M2, and M3. Additionally, the microstructure observations might explain the slight increase in freeze-thaw resistance of concretes containing polymer fibers compared to the reference one. Figures 14-16 present examples of the observed microstructure of concretes M1, M2 and M3. In the cement matrix of M1-M3 samples, grains of clinker relicts were observed, along with glass flour and polymer fibers with a diameter of about 30 \(\upmu\)m. Also, some grains of GGBS (ground granulated blast furnace slag) were observed. Some of the GBBS might be present in the CEM I cement as a minor constituent, according to the EN 197-1 standard. Comparing the microstructure of M1-M2-M3, it was observed that by increasing the content of glass flour in the cement matrix instead of a part of cement, the cement matrix became more porous and cracked. Especially when comparing the M2 and M3 samples. In the M3 sample, there were observed areas with high porosity, probably caused by the difficulties with the compaction of the samples during molding. In the M3 sample, the C-S-H gel was more porous than in other samples, which might be caused by the lower content of clinker in the cement matrix. The C-S-H phase was sealed and well developed in M1 and M2 samples, especially in areas located near the clinker relicts. Some regular spherical air voids were observed in the M1 sample. Figure 13: Reference concrete grout microstructure and C-S-H phase mapping around clinker relic grain. Figure 14: Microstructure of M1 concrete sample. Figure 15: Microstructure of M2 concrete sample.