The purpose of the thesis was to investigate the coal combustion products - CCP- from the brown coal burning thermo-power plant as a component for grouts for backfilling trenches and geo-sounds. The grout is a mixture of inert material, hydraulic binder, additives and water. It must fulfil the criteria of viscosity, volumetric stability, strength and others, for specific usage required properties, like heat permeability. The laboratory tests were performed on different mixtures of fly ash, boiler slag, cement and bentonite. Additionally, a commercial grout was investigated as a reference material. Index properties like the grain size distribution, the Enslin Neff water adsorption test, the Methylene blue test and the Atterberg limit tests were performed on dry row materials. On mixtures with water the volumetric stability and the viscosity were investigated. The compressive strength of hardened grout was measured after 28 days. The investigation shows that the water/dry mass ratio has a great influence to the mixture as well as to the hardened grout properties. Low addition of bentonite improves the stability of fresh mixture properties and the cement improves the strength of hardened fly ash grout. The boiler slag was found not to be suitable as a part of inert fill, because it is to coarse-grained and should be sieved before its use and it has not hydraulic properties.