Understanding the retardation of cement hydration caused by zinc

Heavy metals can be introduced into cement as impurities during its manufacturing or intentionally added to make the firing process of clinker more effective. Waste solidification/stabilization is another route through which heavy metals come in contact with cement paste or concrete. It has been reported extensively in the literature that some of the heavy metals can have detrimental effects on the early and late-age properties of cement paste/concrete even at very small dosages. Zinc is one of the heavy metals known to retard the setting and hardening of cement paste/concrete, and its effects can be very severe depending on the dosage. Research in the past has primarily focused on understanding the effect of zinc on the mechanical and late-age properties of concrete. Very limited research has been conducted with the objective of understanding the mechanism behind the retardation and proposing solutions to mitigate it. This project was undertaken to develop a better understanding of the cement-zinc system and contribute to the narrow pool of research. The cement-zinc system was studied at very early ages (first 48 h) as it provided sufficient time for the system to transition from the period of retardation and reach a similar state as a pure cement system. The following characterization techniques were used through the project: isothermal calorimetry, pore solution analysis through x-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM). This study sheds more light on the chemistry of the cement-zinc system, and a hypothesis to explain the observed effects has been proposed. The complexity of the chemical environment of a cement paste, and an incomplete understanding of even a pure cement system necessitates a deeper investigation into the cement-zinc system. However, this study will provide researchers a starting point to continue the efforts on understanding not only the effects of zinc but also the underlying processes in a pure cement paste