Determination of mix composition of concrete containing fly ash using gamma spectrometry

The information about concentrations of natural radionuclides in concrete mix and mineral raw materials used for concrete manufacture, supplementary cementitious materials (SCM) including, can be helpful for determination of concrete composition. The paper deals with the novel approach to determine concrete mix composition - using gamma-ray spectrometry. In order to determine concrete composition, the content of naturally occurring radioactive materials (NORM) was determined in cement, FA and aggregates. Concrete compositions of both fresh and hardened mixes were determined by solving an over-determined system of four algebraic equations. The over-determined system consists of three equations, which represent activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in concrete mix as a function of activity concentrations of the same radionuclides in cement, fly ash and aggregates, and the fourth conditional equation representing a sum of volumetric concentrations of cement, fly ash, aggregates and water in concrete mix as 100%. An over-determined system of linear equations was solved by the method of Lagrange multipliers, which provides a strategy for finding the maxima and minima of a function subject to constraints. Gamma spectrometry was found very sensitive to the presence of FA in both fresh and hardened concrete, while ²³²Th activity concentration - well correlated with the FA content in the mixes. On the contrary, accurate determination of the rest of concrete composition was difficult.