The aim of this investigation is to develop ultra high performance concrete (UHPC) for multipurpose - higher mechanical and ionizing radiation protection. The effect of nano-silica replacement (2 or 5%) on the properties of UHPC was compared with the referent concrete by testing compressive and flexural strength. As nano-silica influences cement hydration and modifies the pore structure, qualitative and quantitative analysis of pores was done using a device RapidAir 457. The second objective of this paper is to evaluate the influence of different aggregates on concrete properties. Two types of aggregate were used: quartz and barite. One of the most important characteristics of the concrete for protection against gamma and X radiation is its Total Attenuation Coefficient (µ/ρ)tot
