<p>Fly ash–cement mixture containing a very high amount (70–80%) of fly ash needs activation to accelerate setting and hardening, and to increase early compressive strength. In the previous parts of this series of publications, the mechanisms of chemical activation using Na₂SO₄+Ca(OH)₂, and mechanical activation (grinding of fly ash and cement) were discussed. In this work, combined chemical and mechanical activation was proposed and the aim of the study was to investigate the processes occurring in fly ash–cement pastes activated in this way. This combined activation method consisted in adding Na₂SO₄ and Ca(OH)₂ in solid form to the fly ash and cement mixture, and then grinding all the components concomitantly in a planetary ball mill before water adding. The following methods were used to investigate the hydration/activation processes: calorimetry, thermal analysis, infrared spectroscopy, X-ray diffraction and SEM microscopy coupled with EDS. Early and later hydration periods of the activated blends were considered. It was shown that greater comminution of the mixture caused by longer grinding time results in an increase in the reactivity of the system, and the formation of solid hydration products is accelerated. Precipitation of ettringite is inhibited in the first hours of hydration and favoured in later periods. Mechanism of hydration/activation was discussed in detail.</p>