The increase in CO2 emissions from the cement industry over the years has had an important negative effect on the global environment. In this context, the use of supplementary cementitious materials for cement replacement has a decisive role in the cement industry since using waste from other industries has important environmental benefits, including a decrease in CO2 emissions from the decrease in cement use. Some research has shown that glass powder has adequate pozzolanic properties and that using it as a partial substitute for cement improves the bulk properties of concrete. However, it is important to evaluate the effect of the fineness of glass powder on its pozzolanic properties. The present investigation was conducted to understand the effect of glass powder particle size on the cementitious matrices, namely in their mechanical performance. An experimental campaign was performed to evaluate the pozzolanic activity index of the produced mixtures, made with 25% glass powder from grounded bottle waste. The results corroborate the tendency for an increase in the pozzolanic activity index with the decrease in glass powder particle size, establishing lower and upper thresholds for its use.