INTRODUCTION Alternative materials to reduce the carbon footprint of the cement and concrete industries have been widely studied in recent years [1-15]. Among the alternatives@ the inclusion of high volume fly ash concretes@ or 100% slag or fly ash concretes are some of the most viable options [3-4]. This work is focused on a hybrid system with 50% normal portland cement@ 50% fly ash and sodium sulfate as an activator. Although in most research studies this system is seen to be technically viable@ there are still questions about what really affects its performance@ from the material technology to the concrete mixture proportion. Different fly ashes and activators including sodium sulfate@ quicklime and lime are evaluated. Fly ash fineness and composition are related to mortar microstructure using X-ray fluorescence@ X-ray diffraction and scanning electron microscopy analyses. The final objective is to identify the parameters influencing material performance and finding the most viable system.