Influence of calcination temperature on the structure and hydration of MgO

Abstract

The performance of magnesium oxide-based cements and expansion agent closely relates to the reactivity of MgO. This paper systematically investigates the structure and hydration properties of MgO calcined from amorphous magnesite at a wide range of temperatures from 400 to 1150 ℃. XRD, SEM, BET and PSD tests were carried out to determine the structure and particle properties of MgO. The hydration reactivity of MgO was indexed with isothermal calorimeter, DTG, XRD and TEM test. Increasing temperature augments the crystallinity and size of MgO nanograins from about 23.85 nm to 86.37 nm, decreases the specific surface area but makes the particles agglomerate into smaller size. The increasing temperature prolongs the induction period and retards the hydration rate of main peak. The total cumulative heat from complete hydration of MgO is with a similar value of about 800 J/g. The hydration reactivity of MgO can be accurately indexed by its heat release. MgO calcined at 700 ℃ has high reactivity and it dissolves quickly leading to the precipitation of needle Mg(OH)2 initially. The needle brucite has poor crystallinity but it transforms into flake and clavate shape. MgO calcined at 1150 ℃ has very low dissolving rate. The dissolution starts from the (110) truncation at <100> edges, and then the cuts at (111) occur to create an octahedron shape. Some clavate brucite initially precipitates instead of needle brucite during the hydration of low reactive MgO.