Six concrete mixtures were prepared with 0%, 20%, 30%, 40%, 50%, and 60% of flyash replacing the cement content and having constant water to cement ratio. The testing specimens were casted and their mechanical parameters were tested experimentally in accordance with the Indian standards. Results of mechanical parameters show their improvement with age of the specimens and results of radiation parameters show no significant effect of flyash substitution on mass attenuation coefficient. 1. Introduction With the increasing applications of radioactive isotopes in several fields of science and technology, there arises the need of using them with extreme care, only after having proper shielding. Efficiency and cost of the material are the two factors that are primarily taken care of, for selecting any material for field applications. Concrete is the most widely used material for shielding gamma radiations satisfying the guidelines. It is used in abundance, particularly due to its good radiation shielding and mechanical properties [1]. Concrete has structure heterogeneous in nature and the origin of its peculiar characteristics is its internal structure. The structure/microstructure variation for a material is a very important factor which plays a crucial role in determining the mechanical properties and its deformation behaviour. The knowledge of complex relation between properties of concrete ingredients and its structure assists in preparing concrete mixture according to the requirements as it has lasting effect on workability, early strength, shrinkage of hardened concrete, and permeability characteristics of concrete. To make cost effective concrete, research has been carried out to use minimum amount of cement and aggregates and utilise by-products in as much quantity as possible. Ground blast furnace slag, silica fume, foundry sand, cement kiln dust, and flyash are the major materials that have been tested as an alternative of cement. Among the various admixtures, flyash is most suitable due to its pozzolanic nature. Not only does flyash play the role of filler but it also has the properties of a binder. It reacts with free lime liberated during the hydration of cement resulting in positive effect on late age strength of concrete. It is particularly suitable to use in mass concrete applications where the cement requirement is large. The properties of fresh and hardened concrete have been studied in detail by various workers. Workability, slump loss, setting time, bleeding, segregation, and a number of related practical issues are addressed among the
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