This paper presents the heat treatment's effect, on the effective thermal conductivity (k e ) at room temperature, of the flying dust ashes, from the Termotasajero S.A's coal combustion thermoelectric. The above mentioned thermo-physical parameter, was measured using the Thermal Properties Analyzer KD2Pro® system, which works with the linear transitory heat flow's physical principle. The samples' heat treatment, were performed using an electrical muffle Ney® Vulcan D-130 for the 1050, 1100, 1130, 1200, 1250 and 1300 °C's temperature ranges. The porosity and the apparent density were found using the standard NTC 4321-3.The samples' superficial morphology was analyzed using a scanning electronic microscopy (SEM). Our results show that as the temperature increased in the heat treatment, the samples changed color from a dark brown to a beige gray. It also decreased the porosity percentage, due to the vitrification process, increasing the density too, and consequently the thermal conductivity effectiveness. The k e values found are very important for the thermal transmittance calculations' coefficient (U-value), in the heat transfer processes, where these ashes are used.