Preparation And Mechanical Properties Of Magnesium Phosphate Cement Concrete For Rapid Repair Construction In Severe Cold Environment

In the event of major natural disasters,timely and rapid repair of important infrastructure,especially important bridges and roads,is of great significance to traffic safety and the smooth implementation of disaster relief work.The rapid repair of concrete roads and bridges at normal temperature can be carried out with more repair materials and easy construction.However,in severe cold environment,rush repair and construction projects face many difficulties,especially the lack of repair materials that can quickly set and harden and have higher early strength,which seriously hinders the smooth implementation of rush repair and construction.Compared with other inorganic cementitious materials,magnesium phosphate cement(MPC)has many excellent properties such as fast hardening,early strength,easy control of hydration heat release,low water cement ratio and so on,and is considered to be the most suitable cementitious material for rapid repair in severe cold environment.MPC and MPCC for rapid repair in severe cold environment were prepared by using moderate amount of light burnt magnesia(LBM)partially replaced dead burnt magnesia,and applied to practical engineering.The effects of ambient temperature and LBM content on the physical and mechanical properties of MPC were systematically studied.The early strength mechanism of MPC was studied by means of XRD,SEM and TG-DSC.On this basis,the influences of slurry collection ratio,sand ratio,mold material,specimen size and other factors on the physical and mechanical properties of MPCC are investigated.The test results provide strong support for practical engineering application.The MPC prepared in severe cold environment can still be solidified and hardened normally with 2-hour compressive strength reached 30 MPa due to the proper addition of LBM.When preparing MPC,phosphoric acid antifreeze is used to reduce the freezing point of mixed water.When the temperature is-10℃ and-20℃,the appropriate LBM/M ratio is 2%～4% and 6% ～ 8%,respectively.MPC is prepared by replacing mixed water with ice and snow.When the temperature is 0℃,-10℃ and-20℃,the appropriate LBM/M ratio is 2%～4%,4%～6% and 8%～12%,respectively.When the ambient temperature returns from negative temperature to normal temperature,the amount of struvite produced by MPC hydration will continue to increase due to secondary hydration,and the 28 d compressive strength of MPC can return to 70% of that prepared at normal temperature.After boiling test and Reyes test,no volume stability problem was found in MPC.In severe cold environment,the influence of slurry concentration ratio on setting time of MPCC is more significant than that of normal temperature,but the influence on slump and expansion degree is smaller than that of normal temperature environment.The change of sand ratio in the range of(40%～50%)has little influence on the early strength of MPCC.W/C has great influence on the early strength of MPCC,and the appropriate W/C is 0.16～0.18;The compressive strength of MPCC specimens molded with adhesive film is close to the rebound strength of the project site,so the compressive strength value of concrete specimens molded with adhesive film can be used to evaluate the actual strength development of MPCC in severe cold environment.The compressive strength of MPCC cured at negative temperature for 28-day is 73% of that prepared and cured at normal temperature.By adding a proper amount of LBM and using phosphoric acid antifreeze,MPCC with good working performance and high early strength can be prepared for severe cold environment.The 2-hour compressive strength of MPCC under no curing condition exceeds 30 MPa and 1d compressive strength reached 50 MPa,which can meet the high early strength requirements of rapid repair materials for rush repair and construction projects.