Preparation And Performance Study Of Magnesium Phosphate Cement-based Repair Materials In A Negative Temperature Environment

The Sichuan-Tibet Highway is located in the Sichuan and Tibet regions with an average altitude of 4000 meters.The road is long and the terrain is steep,and it is susceptible to damage from ice,snow,and frost.The harsh environment can cause varying degrees of damage to concrete roads,resulting in a shortened lifespan and seriously affecting road traffic.Therefore,to respond to the need for rapid repair and reuse of the road,the road repair material must meet the requirements of rapid hardening and high early strength.Compared with other inorganic cementitious materials,Magnesium Phosphate Cement(MPC)has the advantages of rapid setting and rapid strength development,and can adapt to environmental temperatures,making it suitable as a fast repair material for low-temperature environments.Under negative temperature conditions,MPC repair materials still have a relatively fast setting time,but their workability is difficult to meet the requirements for rapid road repair in harsh environments.Therefore,optimizing the workability and mechanical properties of MPC repair materials under negative temperatures is a key technical problem in their practical application.To improve the workability and mechanical properties of repair materials,this study first modified magnesium oxide with sodium silicate and investigated the effects of modified magnesium oxide under negative temperature conditions on the setting time,fluidity,hydration characteristics,mechanical strength,and microstructure of magnesium phosphate cement.Then,through optimized proportioning design,the effects of different phosphate components,magnesium-phosphorus ratio,boron-magnesium ratio,and bentonite factors on MPC performance at negative temperatures(-5℃)were further studied.The hydration mechanism of MPC was studied using microscopic testing methods such as X-ray diffraction,scanning electron microscopy and energy spectroscopy,thermogravimetric analysis,and mercury porosimeter.Finally,durability performance studies were conducted using the optimized proportion.Surface modified magnesium oxide can effectively improve the performance of magnesium phosphate cement(MPC)repair materials at negative temperatures.When the amount of sodium water glass modification is 7.5%,the setting time is extended by 42 minutes,and the fluidity is good.At the same time as optimizing the MPC pore structure,the later strength growth rate can be improved,which is 39.4% higher than the reference group.Through ratio optimization research,the addition of sodium dihydrogen phosphate and the optimization of MPC pore structure can advance the hydration heat release of MPC and produce secondary heat release.At the same time,more dense hydrated products,sodium potassium struvite,are generated,and the pore structure is refined,increasing the volume fraction of 0-50 nm pores from 0 to more than 40%,improving the overall strength of MPC repair materials.When the sodium dihydrogen phosphate dosage exceeds 40%,the 3-hour strength of MPC exceeds 20 MPa.The addition of bentonite further optimizes the pore structure of the repair material,solves the problem of material bleeding,and improves the bonding strength.Finally,the ratio of the two phosphates is determined to be: 40% sodium dihydrogen phosphate and 60%potassium dihydrogen phosphate;M/P ratio is 3:1,B/M is 0.1,water-cement ratio is 0.2,and bentonite content is 0.5%.Under the optimized ratio,the setting time is 49 minutes,the fluidity is greater than 180 mm for 20 minutes,and the compressive strength at 3 hours,3days,and 28 days is 24.7 MPa,43.5 MPa,and 58.7 MPa,respectively.The bonding strength at 1 day is 1.53 MPa,which meets the basic performance indicators of the JC/T 2537-2019 "Magnesium Phosphate Repair Mortar" standard.In addition,the repair material has good freeze-thaw resistance,with a mass loss rate of less than 2% after 125 cycles of freeze-thaw cycles,and short-term water curing helps to enhance the strength development of the repair material,with a strength retention rate of more than 75% at 28 days.The volume shrinkage rate of the repair material after 28 days under negative temperature is below 0.4mm/m,and the volume stability is good.Overall,these properties meet the application requirements of repair materials.