Nowadays composite materials are used in a wide range of industries, including wind energy, aeronautics and transport. With the massive use of composite materials with thermosetting resins and glass fibers, a recycling problem has emerged, since their complex and cross-linked structure does not allow conventional recycling methods. Currently, the main options for their end-of-life are landfilling or incineration with energy recovery. In this study, we evaluate the valorization of glass fiber-thermoset composite waste in concrete mixtures. We study the behavior of a composite waste in an alkaline environment. Thermo-gravimetric analysis and Total Organic Carbon measurements show that the resin mass fraction in the composite waste undergoes degradation, releasing organic products in the solution. Isothermal calorimetry shows that these organic compounds delay the setting time of cement pastes. Ultimately, we find that the composite powder exhibits a pozzolanic activity comparable to other products like fly ash and glass powder.