Influence of pH on the formation of gypsum in cement materials during sulfate attack

Gypsum is usually considered to be one of the main corrosion products during sulfate attack and its negative role has been widely investigated. However, conditions necessary for the formation condition of gypsum are limited and many structures are located at neutral pH. This paper investigates the influence of pH on the formation of gypsum. The system of CaO-SO₃-Na₂O-H₂O was designed to analyse the precipitation of calcium ions with sulfate ions as gypsum in solutions at different pH conditions. Meanwhile, cement mortars were immersed in sulfate solutions at high pH and neutral pH. The products formed in the system of CaO-SO₃-Na₂O-H₂O were examined by using X-ray diffraction (XRD) and a laser scanning confocal microscope. Besides appearance and compressive strength, products formed in mortars during sulfate attack were also investigated with XRD and scanning electron microscopy. The results showed that low pH is beneficial for gypsum formation and promotes the growth of gypsum crystals. Leached portlandite or decalcification of calcium-silicate-hydrate at neutral pH produced more free calcium ions and caused the formation of much more gypsum in interfacial transition zones or matrix, which resulted in the serious mortar damage.