Determination of Mix Composition of Concrete Containing Fly Ash Using Gamma Spectrometry
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Determination of Mix Composition of Concrete Containing Fly Ash Using Gamma Spectrometry Konstantin Kovler and Zakhar Prilutsky National Building Research Institute – Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, Haifa, 32000, Israel
ABSTRACT 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. INTRODUCTION Most building materials of terrestrial origin contain small amounts of NORM, mainly radionuclides from the uranium (238U) and thorium(232Th) decay chains and the radioactive isotope of potassium, 40K [1]. Gamma emitters in building materials include natural radionuclide 40K, daughter nuclides of 226Ra (214Pb, 214Bi) and of 232Th (228Ac, 212Pb, 212Bi and 208Tl). If a material does not undergo a chemical treatment/separation process, 228Ac is under equilibrium with its daughters, 212Pb, 212Bi and 208Tl, and their activity should be the same as that of all the radionuclides belonging to the 232 Th series. However, activities of 214Pb and 214Bi in building materials differ from the activity of their parent nuclide 226Ra, because of 222Rn emanation. In order to achieve a radioactive equilibrium the samples should be placed for at least 3 weeks in a hermetically closed container prior to the measurements. The presence of the natural radioactive components in mineral building materials, such as concrete contribute to an indoors radiation dose. At the same time, the information about concentrations of natural radionuclides in mineral building products and raw materials used for their manufacture can be helpful for determination of the composition
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