The narrow fractions of microspheres with a size of −0.4 + 0.2 mm with a low bulk density in the range of 0.34–0.64 g/cm³ were separated from cenosphere concentrates of fly ashes and fly ash from local selection obtained from pulverized coal combustion. The main components of the chemical composition in obtained narrow fractions are SiO₂ - 55–66 and Al₂O₃ - 21–40 wt. % The part of amorphous glass phase are 63–93, the mullite phase are 1–34, the quartz phase are 2–8 and the calcite phase are 0–1 wt. %. It is found that a narrow fraction of microspheres, characterized by maximum values of bulk density 0.64 g/cm³ and apparent thickness of the shell 20 μm, has the best strength properties. After successive exposure to compressive loading, the particles by increased strength were selected, for which the values of bulk density and apparent shell thickness increased to 0.86 g cm³ and 29 μm, respectively. It is shown that the spheres with the single-ring structure are primarily subjected to destruction; the particles with net-work and solid structure provide improved strength characteristics of narrow fractions of microspheres.