**1. Introduction**

Concrete is the most-often-used construction material, and in the largest quantities, all over the world. The production of concrete significantly impacts the environment, as it consumes major amounts of fossil fuels and raw materials and is energy-intensive. Cement production has the largest carbon-footprint since it leads to the release of carbon dioxide and enormous energy consumption. The production of one ton of cement leads to emissions of about 0.8 tons of CO2 into the atmosphere. Therefore, alternative materials that can substitute cement as a binder in concrete production are currently being sought. Researchers are mostly focused on the use supplementary cementitious materials (SCM) as a partial or complete replacement of cement in mortar and concrete production, i.e., fly ash, slag and silica fume. The use of SCMs contributes to improving the material properties and durability and also brings economic and ecological benefits. However, the decarbonization policy, the transformation towards alternative renewable energy resources and the departure from coal combustion significantly reduce the availability of fly ash [1]. The worldwide slag supply is also quite limited compared to the demand for concrete production [2]. Therefore, scientific research is being conducted worldwide on the possibility of using other materials that can be used as an SCM in the production of concrete as a substitute for cement.

One of the potential alternative materials is recycled powder glass. Glass is a very common material used for the production of everyday appliances such as glasses, dishes and windowpanes, as well as for the production of glass packaging: bottles and jars. Due to the limited-service life, these products must be recycled or landfilled. It has been

estimated that the annual worldwide volume of waste glass stored in landfills is about 200 million tons [3]. According to the European Container Glass Federation, the average collection for the recycling rate for glass packaging reached the rate of 78% in 2019 [4]. The unrecycled glass is stored in landfills, causing environmental problems. In recent decades, there has been increasing interest in the use of waste glass as a substitute for cement [5–7]. The utilization of ground glass in construction production allows for effective waste management and contributes to the reduction in waste glass volumes in landfills, which is currently a serious problem [8,9]. Most of all, it reduces the consumption of natural resources and greenhouse gas emissions. Previous research related to the assessment of the use of powder glass in cementitious composites revealed that an up to 10–30% replacement of cement with glass powder led to no significant deterioration or even an improvement in the properties and durability of mortar and concrete [10–15]. The effect of glass powder on the properties of cement composites depends mainly on the fineness of the glass and is more pronounced in the later curing time [3,8,11,16,17]. Researchers observed a decrease in the mortar and concrete compressive strength in the early curing time with an increase in the substitution level of cement with glass powder. However, in the later curing time, a comparable compressive strength to that of the control cement composites or even a slight increase in strength were reported [12,18–20]. Compressive strength improvement is associated mostly with the filler effect of glass powder, which leads to the formation of a denser and less permeable cement matrix.

The effect of glass powder on the properties of cementitious composites is related not only to the filler effect but also to the pozzolanic activity [12,13,19,21–26]. The amorphous structure of glass enables its dissolution in a highly alkaline pore solution and acts as a pozzolana in the cement matrix [27–30]. The pozzolanic reactivity of glass powder increases with the decreasing particle diameter [12,16,31]. The pozzolanic reactivity of glass powder contributes to the formation of C-S-H phases, which, in turn, leads to the densification of the cement microstructure. Thus, the compressive strength increases as the glass powder amount increases.

This paper presents research results on the use of glass powder as a partial cement replacement with a substitution level of 0–20%. The pozzolanic activity and influence of glass powder on cement hydration were analyzed. The porosity, the microstructure of the interfacial transition zone and the mechanical properties of mortar were also investigated. Due to the relatively low pozzolanic activity of the glass powder, its effect on cement hydration is small. The replacement of cement with ground glass up to 10% does not lead to the deterioration of mortar properties. In the later curing time, a slight increase in the mortar compressive strength with the glass powder addition was observed. The influence of glass powder on cement hydration and mortar properties is related mainly to the filler effect and the heteronucleation of the C-S-H phase on the glass powder surface.
