Nine million tonnes a year of ash-slag wastes generated in Poland by power plants is a serious problem. On the other hand, it is a serious resource that may be used in earthworks, particularly in road engineering. Several mechanical and chemical properties of the mixture were estimated. The results are as follows: the properties classify it as useless for earthworks purposes. Norm requirements [PN-S-02205:1998] concerning usage of furnace waste were not fulfilled with reference to linear swelling value. What is more, frost heaving criteria categorised it as questionable soils, which in combination with high values of linear swelling can cause a decline in the stability structure, which was formed from a tested mixture with lack of proper isolation from water. Concerning fulfilling requirements of the norm, shear strength and load capacity, an attempt to reduce swelling should be made, for example through improving graining of ashslag mixture by adding some sand. However there are no doubts in matters pertaining to macro and trace elements’ content in the potential leachate from the structure, which was formed from the tested mixture.