Experimental Study On Durability And Composite Modification Of Magnesium Phosphate Cement Mortar

Magnesium phosphate cement（MPC）,as an air hardening cementitious material,is usually composed of magnesium oxide,potassium dihydrogen phosphate,retarder,and water in a certain proportion.After acid alkali reaction and physical action,it forms a magnesium phosphate cement stone with potassium struvite（MKP,MgKPO₄·6H₂O,Struvite）as the main hydration product,which has the characteristics of fast setting,early strength,high bonding strength,good wear resistance,and dry shrinkage.Widely used in civil engineering fields such as roads and bridges,airport runways,and housing construction.However,controlling the balance between the setting time and mechanics of MPC has become the main problem limiting its promotion and use.Adding mineral powder has a certain improvement effect,and the significant strength shrinkage of MPC when exposed to corrosive environments also limits its application.Therefore,exploring the durability corrosion behavior and mechanism analysis of MPC mortar,as well as obtaining high-performance mineral powder composite modified MPC mortar,has certain theoretical and engineering application value.In this paper,based on the previous research completed by the research group,the optimal content of modified MPC mortar mixed with silica fume,fly ash and metakaolin is selected by comprehensive comparison for water resistance,salt corrosion resistance（5%NaCl solution and 5%Na₂SO₄ solution）,freeze thaw erosion resistance tests,and the effects of different mineral powders,soaking age,and cycle times on the strength,mass loss,and appearance of MPC mortar are analyzed.Study the effects of composite material types and different dosages on the setting time,flowability,and strength of MPC mortar using three mineral powder pairwise composite modification methods.The mechanism of mineral powder composite improvement was studied by combining phase analysis（XRD）and scanning electron microscopy（SEM）methods.The research results indicate that:（1）The fly ash or metakaolin modified MPC mortar shows good water erosion resistance.The MPC mortar system mixed with silica fume has filling effect and wrapping effect.The compressive strength of MPC mortar at each age is significantly improved after immersion,which is conducive to improving the ion penetration resistance of MPC mortar.The performance in sodium chloride solution is better than that in sodium sulfate solution.The salt corrosion resistance of MPC mortar modified with fly ash or metakaolin is poorer than that of silica fume,but the compressive strength after immersion for 90 days is equivalent to that of MPC mortar without mineral powder.（2）Under freeze-thaw cycling conditions,the addition of silica fume has a good reinforcing effect.The loss of flexural strength of metakaolin after 100 freeze-thaw cycles is large,and the loss rate of flexural strength of MPC mortar mixed with fly ash is similar to that without fly ash.Overall,the durability of modified MPC mortar specimens under four erosion conditions ranges from strong to weak:5%NaCl≥100 freeze-thaw cycles≥5%Na₂SO₄≥purified water.（3）Composite mineral powder can significantly prolong the setting time of MPC mortar,and the optimal flowability of composite modification ranges from 10%to 20%.The composite of 20%silica fume and metakaolin can effectively improve the early compressive strength of MPC mortar,but has no obvious effect on the early flexural strength.When the content of silica fume and fly ash is within 30%,it is beneficial for the development of compressive strength.Composite fly ash and metakaolin will slightly reduce the flexural strength and compressive strength of MPC mortar,but its working performance will be significantly improved.