*3.3. Compressive Strength*

The compressive strength results of mortars at various curing ages with respect to the replacement of glass powder are shown in Figure 3. It is seen that, after 28 days of curing, the mortar compressive strength decreases with the increase in the glass powder content. The most visible mortar compressive strength deterioration, i.e., a 16% decrease, was observed in the case of mortar with a 20% replacement of cement with glass powder. This situation reverses at 90 and 180 days of age, when the partial cement replacement with glass powder does not lead to the compressive strength reduction. Even a slight increase is observed, i.e., 3% for mortar with a 5% glass additive and 4% for mortar with a 10% glass addition after 90 and 180 days of curing, respectively.

At an early age, glass powder acts only as an inert material causing the compressive strength reduction. The substitution of cement with glass powder leads to an increase in the water/cement ratio from a value of 0.5 to 0.63 for mortar with a 20% glass powder addition. Thus, due to the dilution of cement, a decrease in compressive strength is observed. The increase in the compressive strength at the later age of the curing of mortar with the glass powder additive may be attributed to the pozzolanic activity of glass powder. The pozzolanic reaction of reactive silica oxide in glass power and calcium hydroxide leads to C-S-H phases formation, which results in cement matrix microstructure densification and, consequently, a mortar compressive strength increase.

**Figure 3.** Compressive strength of mortars with glass powder addition.

### *3.4. Porosity*

The influence of glass powder on the microstructure was analyzed by mercury porosimetry. Cement was substituted with 10% and 20% glass powder by mass. The porosity and pore size distribution of mortars after 28 days of curing are shown in Figures 4 and 5. By increasing the amount of glass powder, the porosity increases from 0.58 cm3/g for the control mortar to 0.60 cm3/g and 0.66 cm3/g in the case of mortar with 10% and 20% glass powder addition, respectively (about 7% and 12%).

**Figure 4.** Total porosity of samples after 28 days of hydration.

**Figure 5.** Porosity of samples after 28 days of hydration.

An increase in the amount of the pore in the diameter range of 50–100 μm was observed. There is also a visible shift of the maximum of the pore size distribution towards the pores with larger diameters as the proportion of glass addition increases. This results in a compressive strength decrease. The unfavorable influence of glass powder on the porosity and pore size distribution is mainly due to the substitution of cement with coarser inert glass powder.
