<p>Cement, widely used for radioactive waste confinement, contains ettringite, a calcium-aluminium-sulphate mineral crucial to concrete performance and radionuclide (RN) adsorption. The aim of this study is to analyse the sorption behaviour of ⁶³Ni, ¹³³Ba, ²³³U, ¹⁵²Eu, ²³⁸Pu, and ⁷⁵Se on ettringite under alkaline conditions and evaluate the results by geochemical modelling, accounting for their speciation under the experimental conditions. A one-site surface complexation model effectively reproduced the sorption isotherms for all RNs, while a double-site exchange model was proposed to explain Se sorption behavior.</p>