Ordinary Portland cement (OPC) paste specimens and compacted C-S-H (I) powders were immersed in distilled water and aqueous salt solutions of varying concentration to study their volume change behavior. Immersion resulted in ionic interaction leading to various degrees of expansion, leaching, softening and dissolution of the test samples. In all cases relatively rapid expansion occurred. The expansion of C-S-H (I) in aggressive media was found to be large and fast in MgCl2, MgSO4, LiOH, LiNO3 and calcium (magnesium) acetate (CaMgAc) solutions. LiCl, CaCl2 and NaCl were moderately aggressive towards C-S-H (I) depending on the solution concentration. Trends in the length change behavior of OPC paste are similar to that of synthesized C-S-H (I). Similarities were observed between the length change behavior of compacted C-S-H (I) and the swelling of smectite clays in contact with these osmotic media. The similarities are compatible with explanations of expansion provided by both the osmotic and the electrical double layer (EDL) theories. The relationship between the expansive behavior of C-S-H and both Na and Ca Montmorillonite in contact with aqueous salt solutions is discussed extensively in the context of its significance in cement science.