Design And Performance Study Of Magnesium Phosphate Cement-based Repair Materials For High Temperature And Low Humidity Environment

The Sichuan-Tibet Railway is a significant rail route that forms part of China’s thirteenth five-year plan for the advancement of transport infrastructure and a crucial endeavor for fostering progress in border areas.The Sichuan-Tibet Railway has a highly variable and harsh environment,with some areas experiencing low precipitation and high temperatures.Temperature differences between day and night are also significant.These conditions can cause the surface moisture of concrete exposed to this environment to evaporate rapidly,resulting in shrinkage and cracking and significantly reducing the material’s strength,which can seriously affect its practical performance.Therefore,this article proposes the development of a fast repair material that can adapt to high-temperature and low-humidity environmental changes to timely repair concrete damage and meet the demand for rapid repair materials for structures in special environments.Magnesium phosphate cement（MPC）has the advantages of rapid hardening,high early strength,high temperature resistance,and good bonding properties with old concrete.It also exhibits stable strength growth in low-humidity environments,making it an environmentally adaptable fast repair material.Therefore,it has a wide range of applications in the maintenance and reinforcement of many concrete structures.However,due to its rapid hardening,MPC sets even faster in high-temperature environments,which poses challenges for on-site construction.This research project aims to investigate how to regulate different factors in a reasonable way in order to ensure that MPC can balance both workability and mechanical performance.This will enable it to meet the performance requirements of fast repair materials for high-temperature and low-humidity environments.As the hydration products of magnesium phosphate cement will begin to decompose above60℃,this paper first explores the performance variation of MPC under a maximum operating temperature of 50℃.The paper focuses on analyzing the changes in factors such as the magnesium-phosphate mass ratio（M/P）,boron-magnesium mass ratio（B/M）,and water-cement mass ratio（W/C）.Based on these findings,the paper investigates the optimal mixing ratio for 30℃and 40℃ambient temperatures and a more environmentally adaptable formula.The paper further examines the effects of different temperatures on the workability,mechanical performance,hydration temperature,phase composition,hydration product formation,pore structure,microstructure,and bonding performance of magnesium phosphate cement using the selected formula.Finally,durability is evaluated.The research results show that:At 50℃,the MPC specimen with an M/P ratio of 2:1 exhibits the best workability and highest strength,with rapid early strength growth,meeting the requirements for rapid opening to traffic.Boric acid is an important factor for regulating the workability and mechanical performance of MPC,with a B/M ratio of 17.5%being optimal under this temperature and humidity environment.Increasing the amount of water can effectively resist the decrease in flow caused by rapid evaporation of moisture in MPC slurry under high temperature and low humidity conditions,but it can also lead to an increase in pores inside the hardened MPC structure,resulting in a decrease in compressive strength and bonding strength.At 50℃and30%relative humidity,the best ratio for MPC fast repair material that meets technical specifications is M/P=2:1,B/M=17.5%,and W/C=0.18.The setting time of the MPC is 28minutes,the initial flowability is 180mm,the flowability after 15 minutes is 145mm,and the strength after 3 hours is 18.4MPa.The optimal formula for 30℃and 40℃environments are M/P=2:1,B/M=15%,W/C=0.18,and M/P=2:1,B/M=17.5%,W/C=0.16,respectively.The formula that can simultaneously meet the technical specifications of high-temperature and low-humidity environments of 30-50℃is M/P=2:1,B/M=17.5%,and W/C=0.18.The increase in ambient temperature can lead to a decrease in the working performance of MPC.While the early mechanical properties are improved,insufficient hydration reaction in the later stages affects the crystallinity and yield of the hydration product K-Struvite,resulting in slow strength growth.The formation of rod-shaped hydration products containing sodium ions in MPC samples cured at 30-40°C can fill the pores,making the structure denser and stronger in the later stages.The bonding strength of MPC repair materials decreases with the increase in curing temperature.In a high-temperature and low-humidity environment,MPC repair materials quickly lose water in the early stages,leading to accelerated volume shrinkage.However,the volume shrinkage rate of MPC gradually stabilizes in the later stages,with a volume shrinkage rate of below 4.5×10^-4 at 28 days,indicating that the repair materials have good volume stability under high-temperature and low-humidity conditions.Short-term immersion in water is beneficial for improving the density and strength development of the structure.Even after 28 days of curing,the samples at different temperatures still retain relatively high strength,indicating that MPC repair materials can be used to repair water-immersed concrete structures in high-temperature environments.Lower curing temperatures lead to faster strength reduction of MPC after freezing,so increasing the curing temperature is beneficial for improving the frost resistance of MPC.Repair materials cured at 40-50℃have better frost resistance.