Pore-scale swelling mechanism of magnesium oxide granules during hydration

Refractories are substances produced during high-temperature processes and operations in steel, glass, ceramic, and cement industries. Magnesium oxide (MgO), one of the main chemical components in refractories, becomes magnesium hydroxide (Mg(OH)₂) upon hydration with volume expansion. This study investigated the volume-expansion characteristics during MgO hydration and the pore-scale swelling mechanism. The packing of two types of MgO granules calcined at a low (i.e., dead-burned magnesia, DBM) and a high temperature (i.e., crystal-fused magnesia, CFM) was subjected to a stepwise hydration sequence, and the changes in weight and volume were measured in conjunction with three-dimensional (3D) X-ray computed tomography imaging at each step. The results showed that the DBM particles had a smaller grain size than that of the CFM particles, which induced a quicker hydration reaction. At the early stage of hydration, the DBM particles exhibited a noticeable volume expansion and weight increase, and the pore space was quickly filled with Mg(OH)₂. The CFM particles gradually cracked, with a lower rate of swelling toward the asymptotic values, similar to the DBM particles. The results of this study indicate that the direct reuse of refractories as an aggregate in cement concrete or as a backfill material may not be ideal owing to the volume expansion. However, the volume-expansion behavior of MgO hydration can be used to modify soil properties such as the hydraulic conductivity, or to mitigate ground settlement.