Effects of sodium gluconate on early hydration and mortar performance of Portland cement-calcium aluminate cement-anhydrite binder

Abstract

Effects of sodium gluconate on early hydration of the Portland cement-calcium aluminate cement-anhydrite (PC-CAC-CS¯) ternary binder were investigated through isothermal calorimetric, X-ray diffraction analysis (XRD), differential scanning calorimeter analysis (DSC) and scanning electron microscope (SEM). Additionally, mortar performances of the ternary binders, including mechanical strength, fluidity as well as setting time were studied. The results show that sodium gluconate firstly delays ettringite (AFt) formation slightly and then promotes the AFt formation once it begins to form. Sodium gluconate postpones the hydration of silicate and reduces its hydration rate slightly. The hydration of silicate is delayed less in calcium aluminate cement (CAC) rich ternary binder at the same sodium gluconate dosage. High dosage of sodium gluconate will cause significant delay of silicate hydration in CAC poor ternary binder and lower hydration rate. Sodium gluconate impedes CA hydration and inhibits the dissolution of gypsum synchronously, which may lead to the slight delay of AFt formation. Moreover, sodium gluconate influences the growth orientation of AFt, which shows better defined as well as larger shape. Sodium gluconate is not an ideal retarder for fluidity loss control in ternary binder, which increases initial mortar fluidity but cannot control fluidity loss.