Study on the mechanisms of retardation of cement hydration by zinc and acceleration of hardening by sodium aluminate from crystallographic phase analysis

Zinc was focused on as an element causing hardening retardation due to delayed hydration of Ordinary Portland Cement (OPC), and its hardening inhibitory effect and the mechanism of hardening enhancement by sodium aluminate were analyzed by X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectroscopy. The addition of zinc hydroxide retarded hardening at more than 0.3 mass%, as previously reported, and zinc produced calcium zincate hydrates (qatranaite) and inhibited the hydration of tricalcium silicate (C₃S), the main component of hydration of OPC. After the zinc had been fully consumed in qatranaite formation, C₃S started normal hydration. The addition of sodium aluminate caused earlier re-hydration of C₃S, which was inhibited by hydration, as the hydrates produced appeared to consume zincate anions. The rapid setting effect of sodium aluminate addition was also delayed by zinc hydroxide, but it was estimated that the addition of the same amount of zinc would ensure initial strength through early formation of hydrocalumite. In the longer term, strength was considered to be more enhanced by the onset of C₃S hydration.