When holding of the high-alumina-cement concrete at various temperatures the following hydrate phases create: amorphous or crystal AH 3 , crystalline hydrates CAH 10 , C 2 AH 8 , C 3 AH 6 . When developing or modifying of new fire-proof concrete's compositions it is necessary to take into account the influence of holding temperature on the concrete's structure forming and its service properties. In the article the properties of both conventional concrete modified by microsilica additions and medium-alumina-cement fire-proof concrete with fireclay aggregate Gorkal 70 were investigated at holding temperatures 10, 20 and 30 °C. The ultrasonic method was applied to define the concrete's structural differences formed at their hardening during 48 hours. It was established that the structural differences formed in course of concrete's hardening influence the concrete's properties after their burning at 1100 and 1200 °C. Ill. 4. Ref. 8. Tab. 1.