The information about concentrations of natural radionuclides in concrete mix and mineral raw materials used for concrete manufacture can be helpful for determination of concrete composition. The present research deals with the development of the experimental method based on gamma-ray spectrometry for controlling concrete composition of fresh and hardened mixes. Ordinary Portland cement CEM-I 52.5N, manufactured by Nesher - Israel Cement Enterprises Ltd, was used for all concrete mixtures. The coal fly ash was obtained from the Orot Rabin power station. The local aggregates of three categories were used - coarse, intermediate and fine (sand). Fresh concrete mixes were cast in Polyethylene Marinelli beaker with a volume of 1 liter and sealed immediately. Cast concretes were measured in different ages: from several hours to half a year. To compare the results of the radioactivity measurements, at the age of 28 days from the cast concrete cubes were crushed and sealed in Marinelli beakers. The activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the raw materials and concrete were measured using scintillation gamma-ray spectrometer equipped with the scintillation NaI(Tl) detector. The measurement results and their accuracy were compared for two types of gamma-ray spectrometers, with semiconductor high purity germanium (HPGe) and scintillation detectors, while the HPGe measurements were made in Ben-Gurion University. To study the effects of disequilibrium in the ²²⁶Ra decay chain and radon diffusion through the Marinelli beaker walls, which accompany the gamma spectrometry measurements, the additional tests were performed. In particular, ²²²Rn leakage from Marinelli beakers and different containers were determined.

The following conclusions are made:
• Scintillation NaI(Tl) detectors, in spite of their lower energy resolution, can be successfully used for quantitative determination of activity concentrations of the mixtures of few and known radionuclide composition, such as those containing natural  radionuclides only.
• To distinguish between different radionuclides in the complicated mixture and to determine their activity concentrations accurately, NaI(Tl) gamma spectrometry system should have a special software.
• Gamma spectrometry is sensitive to the presence of high concentration of ²³²Th in local concrete mixes containing coal fly ash and serves an efficient tool for determination of fly ash content in both fresh and hardened concrete.
• Contents of materials with lower radionuclide concentration (cement, aggregates) are difficult to determine precisely.
• Permeability of Marinelli beakers to ²²²Rn has a direct influence on the ²²²Rn equilibrium concentration and accuracy of ²²⁶Ra activity determination.