One of the most recent international concern in the field of building materials is the phenomenon of the deterioration of hardened concrete structures moist stored at room temperature after being subjected to heat treatment, such as, pre-cast steam cured concrete members. This phenomenon was mostly attributed to the “delayed ettringite formation (DEF)” which is a form of internal sulfate attack [1-5], figure 1. During the last decade many theories were proposed to explain the mechanism of the DEF and suggested to be caused either by uniform expansion of the cement paste [6] or by the crystal growth of a reformed ettringite germs at localized sites [7]. A new chemical aspect covering both ideas is recently proposed [8]. Recent researches have also considered the dependence of the expansion behavior of concrete upon the used types of cement and aggregate [9 and 10]. The present paper focuses on the definition of the DEF and the factors influencing the DEF process as well as local experience in this field