Kinetic study of the magnesium oxychloride cement cure reaction

Abstract

Magnesium oxychloride (MOC) is a ceramic material with significant fire-resistant properties and growing potential as an alternative building material for passive fire protection systems. The present study examined the magnesium oxychloride 5-phase cure reaction at temperatures from 35 to 55 °C using time-resolved quantitative X-ray diffraction and differential scanning calorimetry to monitor kinetics. The reaction was characterized as a two-step process: dissolution of magnesium oxide followed by crystallization of magnesium oxychloride from the solvated state. With stoichiometric proportions, 37% of the MgO dissolves before the onset of crystallization at a critical amorphous concentration. A maximum crystallinity of 82–84% was achieved for each temperature. Assuming first-order kinetics for both MgO dissolution and MOC crystallization, a kinetic model predicts 42.4 and 26.1 kJ/mol for dissolution and crystallization activation energies, respectively. This model was applied to pilot-scale production and accurately predicts the cure reaction time as a function of cure temperature. In an alternative approach to modeling the cure reaction, the Avrami nucleation and growth model was fit to calorimetric measurements. This model predicts diffusion-controlled, one-dimensional growth with an activation energy of 72.4 kJ/mol, which accounts for both dissolution and crystallization.