Synthesis and hydraulic activity of novel Sr2+-doped Ca7ZrAl6O18 cement at 50 °C

This paper describes the design, synthesis and hydraulic activity of novel Sr2+-doped Ca7ZrAl6O18 cement in respect of potential applications as: matrix used in nuclear waste immobilisation, components of X-rays and gamma radiation shielding concretes or utilized in refractories technology. The obtained material was characterized by X-ray diffraction (XRD), scanning electron microscopy combined with energy-dispersive spectrometry (SEM/EDS) and heat flow calorimetry techniques. It was well established that the addition of Sr2+ cations to form solid solution with Ca7ZrAl6O18 can alter the hydraulic behaviour of this phase. A microcalorimetric study (heat flow calorimetry technique) of Sr2+-doped Ca7ZrAl6O18 cement hardening was combined with the phase characterisation by XRD and SEM/EDS. The thermal stability and mechanism dehydration formed hydrates were assessed by simultaneous thermal analysis i.e. thermogravimetry (TG) and differential thermal analysis (DTA) at different hydration times. The detailed scheme of thermal decomposition of the hydrated Sr2+-doped Ca7ZrAl6O18 cement has been clarified based on evolved-gas-analysis data with a quadrupole mass spectrometer (QMS). Two hydrated phases C4AH19 and C2AH8 persisted at the early stage of hydration, whilst recrystallisation to Ca-rich C3AH6 was inhibited. Hydration of Sr2+-doped Ca7ZrAl6O18 resulted in the formation of two hydrogarnet phases, one Sr-rich and the other Ca-rich.