Comparison of commercial nitrate-based accelerators and their pure constituents on hydration kinetics, composition, and hydration degree of zinc oxide blended Portland cement

Abstract The rising concentration of zinc in ordinary Portland cement has a concerning impact on the behaviour of cement-based mortars by drastically prolonging setting time and decreasing early mechanical properties. The current theory behind the mechanism of the effect of this contaminant is the depletion of Ca 2+ ions from pore solution and the formation of a semipermeable layer of CaZn 2 (OH) 6 ·2 H 2 O on the surface of silicate cement phases, thus retarding their hydration. The cement industry employs various additives to counteract these problems in the form of water-soluble salts of alkali metals such as sodium nitrate. The induction period, which describes the time interval before the setting of cement, can be easily determined by various calorimetric methods. One of the simplest methods is isoperibolic calorimetry, which gives information about the natural behaviour of samples in a set environment by measuring temperature changes in the sample placed in a stable ambient environment. This factor can be determined using differential thermal analysis, indicating the degree of retardation. X-ray diffraction and scanning electron microscopy can also study composition changes. These methods are commonly used to determine phase changes in materials. Pure nitrates proved to be more effective at reducing setting time than commercial additives. Calcium and sodium nitrates reduced the setting time of zinc-contaminated Portland cement from ~ 200 h to about 60 h. The commercial accelerators reduced setting time to about 120 h. Every accelerator used to mitigate retardation of setting reduced setting time without compromising mechanical properties. The highest doses of pure nitrates slightly increased setting time and decreased compressive strength, probably due to the destabilisation of ettringite in a high alkali environment and the displacement of sulphate ions by nitrate ions. This indicates a hard limit for the dose of nitrates. This phenomenon was not observed in high doses of commercial accelerators.