Internal sulfate ions are the main cause of delayed ettringite formation, which leads to expansive damage of cement concretes. On the other hand, the exposure of cementitious systems to chloride ions causes corrosion of reinforcement bars that finally seriously shortens the service life of reinforced concrete structures. It has been found that, when concomitantly present in cementitious systems, chlorides interact with sulfate ions. Particularly, for a given concentration of sulfate ions present, low to moderate concentrations of chloride ions bring about serious deterioration of concretes due to high amount of ettringite formed, while higher contents of chlorides tend to reduce and even completely eliminate ettringite formation. The objective of the present project is to mathematically formulate the interaction between chloride and sulfate ions in cementitious materials. Such knowledge is useful for accurate consideration of the action of chlorides on concretes used in sea environments, thus predicting more precisely their service life (this being a far important step in design).