In order to evaluate the risk of the source and quantity of cement SO3 on the expansion properties of mortars, this paper studied the influence of Gypsum, Bassanite and clinker sulfate on the cement paste expansion and the formation of delayed ettringite. The investigation covers the mortar cured at 20 °C and 80 °C and stored in water. In contradiction to many studies, the results showed that the cement produced with high sulfur clinker had lower expansion than that produced with classical gypsum or high soluble Bassanite. In addition to that, we have detected that the percentage of SO3, in the cement, could increase the risk of DEF if it surpasses the level of 3.5%. Also, the curing temperature at 80 °C affects the microstructure of concrete by generating cracks in the cement paste.