Chemical changes and reaction mechanism of hardened cement paste-(NH4)2SO4-H2O system

Physical performance and chemical composition of some hydration products in hardened cement paste are changed considerably in presence of NH4+ ions, which is closely related to long term durability of cement concrete. The dissolution of some hydration products and the newly formed reaction products in such system were thus studied. The results showed that the concentration of dissolved Ca2+ ions were increased with the concentration of (NH4)2SO4 solution in external environment; when dissolution process was reached equilibrium, the main product was gypsum; while koktaite was also detected in condition of higher concentration of (NH4)2SO4 solution, which was usually ignored or mistaken as ettringite. In this case, gypsum crystals experienced significant morphology change with the deceasing of particle size when the concentration of (NH4)2SO4 solution was increased. Moreover, by the dissolving of Ca2+ ion, [SiO4]4− tetrahedral chains polymerized into network structure and NH4+ ions were supposed to be doped into C-S-H network structure.