The influence of KOH presence on the evolution of hydration and carbonation of high alumina cement mortars at two different curring temperatures (4 and 40 degrees C) has been studied. It has been confirmed that hydration reactions at both temperatures are accelerated with KOH presence and it has a great influence on hydrated and carbonated species. The massive deposition of carbonation products leads, in all cases to a decrease in mortar porosity which leads to an increase of mechanical strengths.