Study on the Repair Performance and Carbon Emission Analysis of Magnesium Phosphate Cement Prepared from Lithium Extraction Residue

Magnesium phosphate cement (MPC), as a rapid-hardening and early-strengthening cementitious material, is commonly used for repairing concrete defects. To enhance the repair efficiency of MPC and its mortar, MPC mortar and paste with the addition of 40% fly ash and 20% slag were studied and designed to evaluate the performance of MPC in repairing different types of defects such as concrete cracks and surface spalling. The test results for flexural bond strength, splitting tensile bond strength, pull-off strength, and fracture toughness indicate that MPC neat paste exhibits the highest efficiency for repairing concrete cut surfaces and cracks, with bond strength and splitting tensile strength reaching 2.8 MPa and 2.39 MPa, respectively, at 28 days. For surface spalling repairs, MPC mortar with mineral admixtures demonstrates the highest repair efficiency. The pull-off strength of MPC repair mortar with fly ash reaches 0.62 MPa after 28 days, and the fracture toughness of MPC repair mortar with slag is 0.614 MPa/m1/2. Additionally, this study explores the effectiveness of using calcined magnesium oxide and fly ash to produce MPC mortar in reducing carbon emissions during the production stage, comparing it with traditional Portland cement mortar. The research results reveal that, compared to MPC paste made solely with magnesium oxide containing boron, MPC paste with slag addition reduces carbon emissions by 38.95%. However, compared to traditional Portland cement mortar, the carbon emissions of MPC paste with slag addition still increase by 27.67%. Compared to MPC mortar without reactive mineral powder addition, MPC mortar with slag addition reduces carbon emissions by 39.24%, and compared to traditional Portland cement mortar, it reduces carbon emissions by 25%. The addition of reactive mineral powder and sand can effectively reduce the carbon emissions during the application of MPC, with slag addition showing the best carbon reduction effect.