The effect of incorporation of pozzolanic additives on the immobilization of Cesium and Strontium ions in cementitious pastes was investigated. Pastes containing Portland cement together with ground granulated blast furnace slag (50%, 75%), metakaolin (10%, 20%) or silica fume (20%), either in its densified or raw form, were prepared. The transport properties of the immobilized ions through the paste were evaluated using leaching tests. Single differential thermal analysis was used to estimate the extent of the pozzolanic reaction and the pozzolanic reactivity of the different formulations. For the Strontium ion, the best immobilization system was the 20% Raw Silica Fume (RSF) paste, characterized by the highest relative pozzolanity. For Cesium ions however, the most effective additive was the Densified Silica Fume (DSF), reducing the apparent diffusion coefficient by two orders of magnitude compared to the unblended paste.