Due to the very important role played by the construction industry, the production of cement and concrete experiences continuous development in terms of product diversification simultaneous with a reduction in cost through the use of industrial waste.

A solution of waste utilisation is the use of fly ash for obtaining new composite cements. A problem, however, considered by the new regulations of years 2003 – 2004, is the radioactivity of this fly ash and based on the knowledge thereof to evaluate the degree of radiological risk in relation to the population living in constructions of which a component is fly ash.  

Through OMS Nr. 381/2004 the radioactivity of the final product used in the construction of concrete is limited to a maximum of 0.5q/kg. This is the radioactivity index and is calculated using the concentrations of Ra226, Th232 and K40 obtained from the gamma spectrometry analysis. The paper consists of the results of the research carried out on fly ash originating from thermal power stations, regarding the radionuclide concentrations of Ra226, Th232 and K40 in determining the radioactive index.