This paper in concerned with the role of calcium content in fly ash used to stabilise soft soils through alkaline activation with sodium-based alkaline activators. A 10 molal sodium hydroxide solution was used to activate fly ash at solution: ash ratios of 2: 1 and 4: 1, and at a solution: (soil + ash) ratio of 0.40. Fly ash type C, with higher calcium content (FAC), and type F, with lower calcium content (FAF) were used separately, and soil-cement (SC) and soil-lime (SL) mixtures were also tested. Comparing the strength of the FAC and FAF mixtures, the results showed that short-term compressive strength was higher for FAC. However, after longer curing FAF mixtures achieved significantly higher strength levels than SC or SL mixtures. The same relative behaviour was reported when FAF strength results were compared with SC or SL mixtures, i.e., the former achieved lower strength than the latter in the short term, but for longer curing periods FAF strength was significantly higher. XRD (X-ray diffraction), SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy) analysis showed material modifications that can be related to the strength results. The main conclusion is that low calcium fly ash is a better source for long term soft soil stabilisation with alkaline activation than high calcium fly ash.