Cementitious materials are commonly used in nuclear repository sites to immobilize intermediate-level radioactive wastes. This is due to the large surface area of the calcium silicate hydrate (C-S-H) gel, the main hydration product of ordinary Portland cement, which provides many sorption sites in which the contaminants can be adsorbed. The retention capacity of these materials is strongly dependent on the composition, the water content, the pH or the presence of additives. Likewise, it is also known that the durability and performance of cement and concrete are adversely affected in chloride and/or sulfate environments. In this work, atomistic simulations have been employed to analyze the effect of the presence of chlorides and sulfates in the retention and transport of ¹³⁷Cs, one of the most hazardous radioisotopes, in calcium silicate hydrate. The simulations suggest that the presence of a moderate amount of chlorides does not alter significantly the Cs uptake in C-S-H gel, while a moderate content of sulfates enhances substantially the retention of Cs ions and reduces their migration throughout the pore. This behavior is attributed to the ability of the sulfates to pull Ca out the high-affinity sites from the C-S-H surface, allowing Cs ions to occupy them.