The crush test and acid resistance of microsphere narrow fractions with average diameter d_av of 5, 8 and 25 μm of the SiO₂–Al₂O₃–FeO system and with d_av of 4 and 10 μm the CaO–SiO₂–Al₂O₃–FeO system separated from fly ash from pulverized combustion of Ekibastuz and Irsha-Borodinsky coals were studied. It has been established that all investigated ash fractions of both raw materials are characterized by high strength: microspheres of a larger narrow fraction with d_av = 25 μm are not destroyed by compressive loading at pressures up to 51.7 MPa, dispersed narrow fractions of microspheres with d_av ≤ 10 μm – up to 68.9 MPa. Microsphere narrow fractions with aluminosilicate composition have satisfactory acid resistance; the weight loss after treatment with 15% hydrochloric acid at 65°C for 30 minutes was 10–15 wt. %.