Abstract This study has assessed the suitability of dehydroxylated (5 h at 750 °C) red, white and ball clays for the use as prime materials in the production of alkaline cements. The analytical methodology applied to quantify their potentially reactive phases included selective chemical attack, which was also used in conjunction with subsequent ICP analysis of the resulting leachate to determine their reactive SiO2/Al2O3 ratios. These results were compared with compressive strength values of the respective pastes activated with an 8-M NaOH solution and cured at 85 °C and 90% RH for 20 h. It was observed that when the reactive phase content was above 50%, the reactive SiO2/Al2O3 ratio in the starting materials had a larger impact than the amount of reactive phase on the developed strength of the cement material. In this context, fly ash was used as the reference material. Finally, to verify the accuracy of the results, a binder consisting of 70 wt.% fly ash and 30 wt.% dehydroxylated clay was activated with an 8-M NaOH solution. The reactivity of this cement was determined by chemical attack with 1:20 HCl (v/v) and the reaction products were characterised by powder X-ray diffraction and 29Si MAS NMR spectroscopy.