This contribution highlights the influence of pore solution composition on the activation and hydraulic reactivity of ternesite.The hydration of pastes with different contents of ternesite and belite, content with and without presence of [Al(OH)4]-as activator was investigated.Neat ternesite systems showed poor reactivity in water even after 7 days and beyond, whereas [Al(OH)4]-activation leads to high reactivity at early ages, but only moderate reaction rate at later ages once the [Al(OH)4]-is depleted from the solution.When combining ternesite and belite, [Al(OH)4]-activation gives a much higher reaction rate and extent of ternesite in comparison to belite.This is partly explained by the release of additional SO4 from ternesite leading to the formation of AFm and AFt as main hydration product.Ternesite has proven to be more reactive than systems containing belite and anhydrite, with the same stoichiometric ratios as for ternesite (i.e.2Ca2SiO4·CaSO4), in the presence of [Al(OH)4]-.