Experimental Study On The Physical And Mechanical Properties Of Red Mud Magnesium Phosphate Cement Repair Mortar

Magnesium phosphate cement（MPC）repair mortar is a repair material consisting of dead-burned magnesium oxide,acid phosphate,retarder,water and fine aggregates in the right proportions.It has early and fast hardening,high heat of hydration,low expansion and high interfacial bonding strength.It is commonly used for emergency repairs to roads,airport runways,bridges,tunnels and factory floors,as well as in cold conditions,but the application is limited by the high heat of hydration and high cost.Preliminary research by our group has shown that pretreated red mud can effectively reduce the heat of hydration of MPC and improve the water resistance and economy to a certain extent,and the mechanism of the influence on MPC repair mortar needs to be systematically studied.This paper used red mud MPC as the cementitious material to prepare repair mortar.Through experimental research and microscopic analysis,the influence of raw material composition,water-cement ratio,sand-cement ratio,fine aggregate type and polymer fibres addition on the hydration products,hardening mechanism,microstructure and physical and mechanical properties was elucidated.And the modification mechanism of water resistance and interfacial bonding strength of MPC repair mortar was further explored to provide basic data for promoting the engineering application.1)Research on the preparation technology and mechanism of red mud MPC repair mortar.The mechanism of the effect of red mud admixture,water-cement ratio,sand-cement ratio and fine aggregate type on the fluidity and mechanical properties of MPC repair mortars was investigated by XRD,SEM,heat of hydration analysis and capillary absorption tests.The results showed that the heat of hydration of MPC repair mortar decreased and water resistance increased after red mud was incorporated.The compressive strength,flexural strength and water resistance of red mud MPC repair mortar showed a trend of increasing and then decreasing with the increase of red mud admixture and sand-cement ratio,and increased with the decrease of water-cement ratio.When the red mud admixture was 15wt%,the water-cement ratio was 0.20,the sand-cement ratio was 1:1 and the aggregate type was quartz sand,the 28d compressive strength of the red mud MPC repair mortar reached 49.7 MPa,the strength retention coefficient reached 0.96,the capillary porosity and capillary absorption coefficient were the lowest,the microstructure of the hardened structure was dense and a large number of short columnar struvite were wrapped between the red mud particles.2)Study on the physical and mechanical properties of polymer fibre reinforced red mud MPC repair mortar.The effect of polymer fibres type and dosing on the physical and mechanical properties of red mud MPC repair mortars was studied in comparison.The results showed that the incorporation of polymer fibres significantly reduced the fluidity and setting time of the red mud MPC repair mortar and adversely affected the compressive strength,but was able to slightly increase the flexural and interfacial bonding strength,and effectively inhibited the dry shrinkage during the hardening process.Compared to polypropylene fibres and polyester fibres,polyvinyl alcohol fibres improved the interfacial bonding strength of red mud MPC repair mortars most significantly.However,the higher the polyvinyl alcohol fibre length to fineness ratio,the less beneficial the development of the physical and mechanical properties of the red mud MPC repair mortar.When the length of polyvinyl alcohol fibre was 6 mm and the volume admixture was 1.6v%,the flexural strength and interfacial bonding strength of the red mud MPC repair mortar reached 10.6 MPa and 4.9 MPa respectively,and the dry shrinkage rate was reduced to 2.25×10^-4.