This paper focuses on studying the effect of the delayed heating of cementitious materials on the phase assemblage variations in three types of cement paste. Two heating temperatures of the order of 65 °C and 80 °C were used to thermal treat the samples. Our results show that the thermal treatment induces the modification of the phase assemblage (particularly degradation of AFt and AFm phases) as well as the polymerization of the C-(A)-S-H. The observed variations depend mainly on the initial sulfate, aluminate and carbonate content in each cement. Finally, it is shown that the thermal treatment-related phenomena such as adsorption of aluminates on C-(A)-S-H are almost completely reversible after the temperature drop and re-immersion in water for 3 months, meanwhile other phenomena such as the formation of the hydrogarnet and hydrotalcite are only partially reversible. This modifies the free elements proportion and influences the probability of the DEF occurrence.