The incorporation of waste glass into concrete significantly decreases cement consumption, thereby reducing CO2 emissions, and fostering a circular economy. Due to its high amorphous silica content, glass exhibits pozzolanic properties that enhance the long-term strength, durability, and resistance to chemical attack of concrete. However, depending on its production, glass can contain high alkali levels, which may increase the risk of alkali-silica reaction in concrete. Utilizing waste glass with fineness less than 45μm can mitigate this issue, as the material becomes more reactive and exhibits enhanced pozzolanic characteristics. In this study, waste glass from a window glass manufacturer in Greece, was evaluated as a potential pozzolanic material. Following an analysis of the raw material's properties, including chemical composition, density, and mineralogical characteristics, the waste glass was ground to a fineness of under 45 μm and employed as a supplementary binder for cement paste production. Various proportions of waste glass (0%, 10%, 20%, 30%, and 50%) were combined with Portland cement to create composites, which were prepared with standard consistency. The influence of the addition of the waste glass on properties such as heat of hydration and setting time of the pastes were measured immediately after mixing. Shrinkage was monitored over a period of 40 days. Additionally, the pozzolanic behavior of the waste glass—cement pastes was analyzed using XRD and TG at various ages. The flexural and compressive strength of the samples were tested at 7, 28, and 90 days, showing an initial decrease of the strength at 7 days, up to 60%. However, overtime, the mixtures containing glass showed a higher increase in strength compared to the reference mix (higher than 50%). The performance evaluation of the waste glass confirmed its pozzolanic properties, indicating its potential as a supplementary cementitious material.