The development of ettringite-based systems with low heat of hydration can be useful for industrial applications such as solidification of waste sludge with high water contents.Calcium aluminate cement (CAC) is often used as a precursor when mixed with calcium sulfates sources (anhydrite, hemihydrate or gypsum) to obtain ettringite.As calcium aluminate cements are known for providing large heat flow at early age due to their kinetics of hydration, additions like blast furnace slags can be combined to the CAC systems to lower temperature effects.However, low alkalinity of these systems can limit the reactivity of SCMs and different strategies are investigated to: i) activate slags using different nature and contents of calcium sulfates as well as low pH activators ii) study the corresponding hydration processes (hydrate features, kinetics) and final properties of the mixtures (mechanical strength).In particular, degree of reactions of slags and morphology of hydrates are investigated using SEM methods coupled with isothermal calorimetry methods and image analysis methods.