Effect of boron impurity on the expansion behavior and mechanical performance of cement with MgO expansive agent calcined at different temperatures

This study employs a co-calcination method, using Mg(OH) 2 and H 3 BO 3 to simulate the behavior of boron impurity (B) during the calcination of MgO expansive agent (MEA). The investigation included systematic monitoring of the characteristics and micro-morphologies of B-containing MEA (BMEA), the hydration process of cement pastes containing BMEA, and the compressive strength and expansion behavior of mortar specimens. The results indicate that, under the light-burned regime (LBR), B accelerates the hydration of MgO while delaying the hydration of cement, leading to reduced compressive strength, and enhanced expansion. Conversely, under the high-burned regime (HBR), B reduces the reactivity of MgO, thereby decreasing the expansion of the specimens during the first 35 days while increasing the expansion in the later stages. Through analyses of pore structure and micro-morphologies in BMEA-containing cement pastes, this study elucidates the differential impacts of BMEA on microstructural evolution under varying calcination temperatures, as well as the underlying mechanisms responsible for distinct expansion rates in specimens. These findings establish a theoretical foundation for the application of boron-containing magnesium slag in MEA production.