Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using Si-29 MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450degreesC, to a modified C-S-H gel. For temperatures above 200degreesC to a progressive formation of a new nesosilicate. At 750degreesC, the transformation of C-S-H is complete into the nesosilicate form with a C2S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200degreesC. A progressive increase of calcite formation up to 450degreesC is noticed. The ettringite disappearance below 100degreesC is confirmed and the portlandite and calcite are converted to lime at 750degreesC. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750degreesC) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena. (C) 2004 Kluwer Academic Publishers.