The key technology for the production of aerated concrete, using sedimentary fly ash to replace the new fly ash, is of enormous interest to the concrete industry. The chemical composition, mineral composition and microstructure of the sedimentary fly ash in Qian-Xi, Gui-Yang and Qing-Zhen power plants are presented. The activity of the sedimentary fly-ash-aerated concrete was tested in detail by employing grinding, mineral admixtures and chemical admixtures. The results show that the sedimentary fly ash is essentially low-calcium fly ash. The mineral composition of the sedimentary fly ash is basically the same as that of the new fly ash. The compressive strength and flexural strength of the ground fly ash specimens are larger than that of untreated fly ash. The use of mineral admixtures SH1 and SH2 for sedimentary fly-ash-aerated concrete can increase the compressive and flexural strength. The excitation effects of SIII activator, SIV activator and SV activator are better than those of other activators. The best chemical admixture is calcium oxide (CaO)–SIV activator. These studies are of significance for the use of sedimentary fly ash in aerated concrete. This study provides a new way to eliminate sedimentary fly ash.