Recycling of waste magnesia refractory brick powder in preparing magnesium phosphate cement mortar: Hydration activity, mechanical properties and long-term performance

Waste magnesia refractory bricks (MRBs) that remain from industrial production are landfilled in large quantities or recovered to prepare recycled bricks with low added value. In consideration of the significant level of magnesia in waste bricks, this work proposes to extract and recover magnesia from spent bricks for manufacturing magnesium phosphate cement mortar (MPCM). The essential attributes of recycled magnesia (RM) and its influence on the workability, hydration temperature rise, mechanical properties, phase evolution, long-term performance and microstructure of MPCM were analyzed. The results reveal that RM significantly prolongs the setting time and reduces the overall hydration heat release, which is beneficial to the application in large-scale engineering. The compressive strength of MPCM prepared by RM (MPCMRM) initially grew and then decrease with the increase of magnesia to phosphate ratio (M/P ratio). The optimal performance was observed at M/P = 4, and the compressive strength cured for 3 h and 28 d were 39.1 MPa and 68.5 MPa respectively. While the mechanical properties of MPCMRM were marginally inferior to those of dead-burned magnesia (DM), it was found to satisfy the early and long-term strength requirements of engineering repair materials. In addition, MPCMRM did not generate new mineral phases, and the crystal morphology of hydration products (struvite) was worse than that of mortar prepared by DM. MPCMRM also exhibits excellent water resistance and volume stability due to the crystal characteristics of low-activity RM. It seems that RM can completely replace DM to prepare MPCM and achieve satisfactory performance, which not only reduces the cost of repair materials, but also provides a fresh approach for the recycling of waste MRBs.