The paper presents an analysis of the effect of elevated temperatures on concrete, with an
emphasis on microstructural changes in the hydrated cement paste. The physical and chemical
processes in the cement paste due to the increase of temperature are described. Research has shown
that the first changes in the cement matrix occur at a temperature of 300oC, and that the microcracks
appear at temperatures of 500oC. Intensive microcrack development occurs at 700oC, while at
temperatures of about 900oC some particles of hydration products completely cracked. Reducing the
degree of degradation of cement paste is possible by applying certain types of addition.
