The properties of autoclaved aerated concrete (AAC) formed with the utilization of modifi ed raw materials - new generation of fl y ash from coal power plants (fl uidized fl y ash) were studied. The fl uidized fl y ash is a mixture of bed material and fl y ash in the ratio 2:1. The high content of CaSO4, CaO and clay minerals (illit, diaspor) has been proved during the chemical and phase composition investigations. The fl uidized fl y ash was used as 0%, 30% and 100 % component of the raw mixture respectively. The AAC produced commercially (with fl y ash from classical combustion) was used as reference material. The results of long-range experiments, aimed at the durability characteristics within the three years tests, are comprehensively discussed. The strength of samples decreases with the addition of fl uidized fl y ash, the lowest values are exhibited by samples with 100 % fl uidized fl y ash content in the raw mixture. The X-ray analysis, TG and DTA, IR spectroscopy, electron microscopy and EDX spectroscopy have been employed. Both phase analysis and strength data show the higher tendency of samples with fl uidized fl y ash towards the thaumasite formation, being the result of partial conversion of hydraulic C-S-H and tobermorite-like phases to nonbinding thaumasite. However, the expansion of tested samples resulted from the formation of thaumasite, as well as gypsum and ettringite in AAC, does not imply the deterioration on such a scale as observed in the concretes. The difference is due to the porosity and pore structure in AAC, developed as a result of air drying. These data contribute also to the discussion of topics of thaumasite formation vs. thaumasite sulfate attack. The results represent a basis of the proposals of modifi ed production of AAC.