Reaction Process And Mechanical Properties Of Magnesium Phosphate Cement In Severe Cold Environment

After natural disasters and sudden accidents,the rapid repair of damaged infrastructure,especially bridges and roads,is of great significance for the restoration of traffic safety and emergency rescue.At present,the research on repair materials has become a hot spot in the field of construction materials,and a large number of repair materials that can achieve rapid repair of concrete structures under normal temperature environment have been developed.However,the commonly used quick repair materials are more difficult to apply in severe cold environments.The rush repairs in the severe cold areas in winter will face the dilemma of lack of repair materials suitable for severe cold environments.Therefore,in-depth research on the application of rapid repair materials for severe cold environments is very important for the smooth implementation of repair and construction projects in severe cold areas.Magnesium phosphate cement(MPC)is considered to be the most potential rapid repair material under negative temperature because of its characteristics of fast hardening and early strength,good adhesion to the old interface,and good environmental adaptability.However,in negative temperature environment,the hydration and hardening of MPC will be significantly inhibited.Therefore,it is necessary to investigate the basic performance of MPC under negative temperature environment and regulate its mechanical properties.This paper mainly shows the hydration reaction process of MPC under negative temperature environment,especially under-20 ℃.By exploring the factors that influence the hydration rate and strength development of MPC,the strength development law of MPC affected by it at-20 ℃ is clarified,two ways to improve the early mechanical properties of MPC at-20 ℃ are proposed.First,the influence of changes in M/P,B/M,and W/C on the workability,setting time,and mechanical properties of MPC in a negative temperature environment were investigated.When the ambient temperature is lowered,the MPC fluidity is slightly reduced.At 0℃and-10℃,the setting time is not much different,but at-20℃,the setting time is significantly extended.Under the condition of-20℃,M/P increases,its fluidity does not change significantly,the setting time is shortened,the early compressive strength is improved,but the later strength development tends to be gentle.With the increase of B/M,the MPC fluidity and setting time are greatly improved,and the early and later strengths are significantly reduced.When the W/C increases,the MPC setting time does not change much,and the early strength will decrease significantly.In terms of the influence of the three factors on the setting time and compressive strength of MPC,B/M is the largest,M/P is the second,and W/C is the smallest.The hydration temperature rise test shows that the changes of M/P and B/M have a greater influence on the hydration rate and temperature peak of MPC,and the changes of W/C have no obvious effect on them.Under the condition of-20℃,the best experiment ratio is M/P=4,B/M=0.05,W/C=0.14.By adjusting the fineness of Mg O,the early compressive strength of MPC at-20 ℃can be improved.The results show that with the extend of the grinding time of Mg O,both the fluidity and setting time of MPC show a downward trend,and the compressive strength increases.The best fineness range is when Mg O grinding time is5～10min.The compressive strength of MPC 2h can reach 24 MPa,which is about 20%higher than that of the blank group.The incorporation of Mg O whiskers improves the early mechanical properties of MPC at-20 ℃.The results prove that with the improve of whisker content,the fluidity of the slurry decreases,the setting time is shortened,and the early compressive strength and flexural strength are significantly improved.When the content of Mg O whiskers is 3% to 5%,the workability and mechanical properties of MPC at-20 °C are the best,and the 2h compressive strength of MPC can reach 27 MPa,which is 33%higher than the blank group.The above results show that MPC is suitable for rapid repair of concrete structures under severe cold environment.