Effect of atmospheric water vapor on independent-parallel thermal dehydration of a compacted composite of an inorganic hydrate: sodium carbonate monohydrate grains comprising crystalline particles and a matrix

Abstract

The effect of atmospheric water vapor on the thermal dehydration of sodium carbonate monohydrate (SC-MH), which was characterized as cubic grains of a compacted composite comprising columnar SC-MH crystals and a matrix, was systematically assessed using a humidity-controlled thermogravimetry system at various atmospheric water vapor pressures (p(H₂O)). The thermal dehydration of the SC-MH compacted composite occurred via an induction period (IP) and partially overlapping two-step mass loss steps due to the thermal dehydration of the SC-MH matrix and columnar crystals. All component reaction steps were retarded with an increase in the p(H₂O) value. The kinetics of individual reaction steps were universally described over different temperatures and p(H₂O) values based on a kinetic equation that considered p(H₂O) and the equilibrium pressure of the thermal dehydration. Additionally, the physico-geometrical consecutive surface reaction (SR) and subsequent phase boundary-controlled reaction (PBR) model was employed to describe the first mass loss step. The difference between the effects of atmospheric p(H₂O) on SR and PBR processes was parameterized via an advanced kinetic analysis. The kinetic behavior of the second mass loss step was discussed based on a three-dimensional contracting geometry model with accelerating reaction interface advancement, where the changes in the rate behavior with atmospheric p(H₂O) were explained by the total effect of atmospheric and self-generated p(H₂O) on the kinetics. The present results provide additional insights into the independent-parallel thermal decomposition kinetics of composite materials by considering the effects of atmospheric and self-generated gases.