Hybrid fly ash cement is a binder with a composition between that of pozzolanic fly ash cement and alkali-activated fly ash cement. Its production requires less cement clinker than ordinary Portland cement, facilitating a much-needed reduction in the carbon dioxide footprint related to the production of high-clinker-containing cement. Research on activation methods is required to overcome the low early age strength and slow strength development in hybrid fly ash cements. In this study the activation of a South African siliceous fly ash (70%) for use along with Portland cement (30%) in a hybrid alkaline binder was investigated. Both chemical (the addition of sodium sulfate) and mechanical (milling) activation of fly ash was studied. The aim of this work was to develop a better understanding of the characteristics of compressive strength development of hybrid fly ash cement mortars over an extended curing period of up to a year. The results indicated that the combination of mechanical and chemical activation (sodium sulfate) can be considered a viable solution to improving early strength and slow strength development, so that hybrid cements are able to comply with the strength requirements of the BS EN 197 cement specification.