During combustion of coal in fluid bed boilers solid waste become into existence in the forms of slug and ash, contaminated with reactant residues. Economical applicability of these chemically active by-products is still a subject of research and new ways of their utilization are searching. Within the last few years a range of methods of activation of fluid bed ashes have been invented, however offered technologies are characterized by high operational and investment costs. Continuous research and tests took place in order to extend the range of industrial application of power generation waste, including waste from desulphurization circuits and fluid bed boilers. Proposed solutions are oriented both on processing of these waste - being considered as raw materials - into valuable products that can be used in different branches of economy or their usage in construction and engineering works [1]. Presently, combustion by-products from fluid bed boilers (UPSF) can be consumed, between others, by mining industries for grouting of underground voids and sealing of coal waste dumps within spontaneous coal ignition control measures [2]. Also tests are carrying out on use of slug, which is the main component of UPSF, as a backfill material in hydraulic backfill technology in underground mining. Materials to be used in this technology must meet several requirements given by the relevant standard and their physical and chemical properties must be known. The paper presents results of research on possibilities of use of slugs from fluid bed boilers as a component of hydraulic backfill, which is used for filling of underground voids resulted from extraction of coal in longwalls in Polish coal mines. Testing procedures are compliant with relevant PN-G/11010 standard and embraced slugs from two large hard coal fired power plants, marked in the paper with "X" and "Y".