Investigations on phase constitution, mechanical properties and hydration kinetics of aluminous cements containing magnesium aluminate spinel

Aluminous cement containing magnesium aluminate (ACMA) spinel was prepared in situ by sintering mixtures of limestone, magnesia and bauxite. The phase constitution, micro-morphology, and hydration kinetics of ACMA were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and isothermal calorimeter measurements, respectively. Magnesium aluminate (MA) spinel, calcium monoaluminate (CA) and calcium bialuminate (CA2) are the primary phases of the obtained ACMA. In addition, trace amount of calcium silicoaluminate (C2AS) and residual alumina (Al2O3) were detected. MA, mainly exists in the form of octahedron on the order of 2–5 μm and forms agglomerates with the tabular or flaky-shaped CA in the clinker. The compressive strength of the ACMA castables increased remarkably as a function of CA content. The measurement of hydration heat demonstrated a five stage hydration process, namely, pre-induction, induction, acceleration, deceleration and stabilization. The N value was calculated to be lower than 1 in acceleration stage which indicates a nucleation controlled hydration process. On the other hand, it was higher than 1 in stabilization stage showing a diffusion controlled process.