For many years, coal combustion by-products from Polish professional power generation sites (CCB) have been successfully applied in many industries as a material component, or after re-cycling as a ready-made product. Most frequently, CCBs are used in building and road construction engineering, as well as in mining technologies. Currently in underground mining, finely grained power generation waste (fly ash) is commonly used for grouting of cavings as a fire retardant. Taking into consideration that some CCBs possess binding properties, it seems necessary to search for new fields of their application as alternatives to the materials already in use. As an example of this approach, technology for solidifying backfill should be mentioned, consisting of the filling of empty, separated voids with materials in order to protect the ground surface above. Within this field, the dominant application is the technology of hydraulic backfill, where generally sand is applied, sometimes with other materials - most frequently crushed waste rock of a grain size above 0.1 mm. Alternatively, solidifying backfill could be adopted, which mainly employs materials of a grain size below 0.1 mm, including fly ashes from power generation waste and flotation tailings. This paper presents the results of research on the physical-mechanical properties of mixtures being made on the basis of a fly ash with the code number 10 01 02, and sand which represents bottom residue from a fluidal vessel with code number 10 01 24. In order to achieve shorter binding times and higher compressive strength Rc, Portland cement in amounts of 5,10, and 15% has been added to the mixture. Laboratory tests were conducted according to the standard PN-G/11011:1998. On the basis of the conducted research on the physical-mechanical properties of these mixtures, it should be concluded that: - The spread test result value maintains a range from 178 mm to 237 mm, with the density of the mixture in the range from 1.51 Mg/m3 to 1.72 Mg/m3. - Excess water amount is between 6.6% and 13.9%. - Compressive strength after 28 days of cure is between 3.7 MPa and 6.2 MPa. - Soak resistance is from 2.2% to 16.2%. Considering the requirements of the standard, it should be concluded that a mixture fully applicable for solidifying backfill is prepared of 75% fluidal bed ash, 25% bottom sand, with an addition of 10% cement.