The behaviour of cement composites containing rice husk ash, which have previously undergone high-temperature thermal curing, was experimentally investigated. A specific initial thermal curing up to 85°C was designed to trigger delayed ettringite formation, as was a specific exposure environment, by water immersion at 38°C, over the course of 1 year. Expansion measurements and microstructural analyses were performed to evaluate the level of attack and the integrity of mortars and concretes. To complement the study, the mechanical properties of concretes were assessed to detect the level of damage by expansion from delayed ettringite formation. Mortar performed differently from concrete, bringing risks of mistaken conclusions about admixture performance. Tests on concrete have indicated that the addition of rice husk ash can reduce the level of expansion; however, the amounts used were not sufficient to mitigate the expansion completely in order to avoid delayed ettringite formation and, consequently, the associated damage.