**3. Results and Discussion**

#### *3.1. Glass Powder Pozzolanic Activity*

The study of the pozzolanic activity index of glass powder based on the strength of cement mortars (physical method) indicates the moderate pozzolanic properties of glass. SAI gained 79% and 88% after 28 days and 90 days of curing, respectively. According to EN-450-1, the SAI should not be lower than 75% after 28 days and should not be lower than 85% after 90 days of hardening the mortar. The low pozzolanic activity of the glass powder was also confirmed by the chemical method. Based on the research, it was determined that the content of the reactive ingredients in the glass is about 9%. The content of reactive components in additives with pozzolanic properties, i.e., silica fume and fly ash, is 70–75% and 10–15%, respectively. According to literature reports, glass dust has good pozzolanic properties, and the reactivity increases as the fineness increases [12,16,31]. The lower-thanexpected pozzolanic activity of the analyzed glass powder is related to its insufficient fineness.

### *3.2. Cement Hydration*

The influence of glass powder on cement hydration was evaluated on the basis of the analysis of the hydration reaction rate and the microstructure of the hardening cement paste. Two samples were investigated, i.e., pure cement paste and paste with 10% glass powder addition. Cement replacement with glass powder leads to a decrease in the heat

release rate and the amount of heat released during hydration. This is of course due to the dilution of cement. At the same time, it was found that a 10% reduction in the cement content resulted in a reduction in the amount of heat released by only 6%. Thus, the relative reduction in heat released during hydration is smaller than the reduction in the cement content in the paste. This indicates a slight acceleration of hydration due to the addition of glass powder.

The influence of the glass powder additive on cement hydration was also examined on the basis of the microstructure analysis of the cement matrix in the early period of hydration. It was noticed that the C-S-H phase crystallizes on glass powder particles (Figure 2). Very fine particles of glass powder act as crystallization centers and provide an additional area where C-S-H nuclei can settle. Thus, a slight acceleration of hydration was observed.

**Figure 2.** SEM images show the crystallization of C-S-H phases on the glass powder surface after 2 h (**left**) and after 12 h (**right**) of hydration.

The interfacial region visible on the glass grain surface is dense, and it is similar to the matrix far from the surface. Calcium hydroxide crystals or increased local porosity were not observed in the area of the contact zone. In particular, the increased concentration of shrinkage cracks was not visible in this area. Heteronucleated small crystals of hydrated calcium silicates are visible on the surface of glass grains.
