It was determined by verification by in situ analysis that combustion products of firewood in fireplaces cause carbonation of alumina cement-bonded conventional refractory castable and accelerate its destruction. The carbonation of hydrated cement (Gorkal 40) paste was modeled in a laboratory conditions: having been fired at various temperatures (300 ºC – 950 ºC) the specimens were curing in humid conditions for two days. It was determined by X-ray analysis that the hydrates CAH10, C2AH8 and C3AH6 change into a mineral C12A7 after the cement paste are fired at 300 ºC – 800 ºC temperatures. The amount of the mineral depends on the firing temperature. In humid conditions C12A7 hydrates and carbonizes, consequently calcium carboaluminate hydrate forms. In the composition of cement and microsilica (9 %) carbonation of cement hydrates noticeably slow down at the same conditions. Carbonized minerals are not identified when the specimens are fired at 500 ºC –850 ºC temperatures
