Expansion Efficacy Of MgO Mortar And Its Durability Under The Salt-freezing Environment Of Magnesium Sulfate

Magnesia expansion agent has many excellent properties compared with the traditional expansion agent,and it has been used more and more in practical engineering.At present,its theoretical research has yielded fruitful results,but there are still many problems to be solved in terms of its expansion efficacy under different conditions and the durability of its expansive concrete in different environments.In this subject,the limited expansion law,mechanical strength,durability in magnesium sulfate salt-freezing environment of magnesia mortar and the pore structure characteristics of magnesia paste hardening slurry under different environmental conditions were stud ied,and the influence of MgO content,activity,temperature,humidity and other factors on macroscopic properties of cement mortar was analyzed.The subject also tries to explain the relative mechanism from the microscopic angle of pore structure.The test results show that:The expasion law of magnesia expansive mortar is different under the condition of water bath curing,drying curing,curing in changing environment and curing in magnesium sulfate solution.Under the condition of water bath curing,the limited expansion rate and expansion velocity of MgO mortar increase with the increase of MgO content and temperature.The temperature sensitivity and final expansion rate of MgO were negatively correlated with their activity.Turning high-temperature curing to low-temperature curing is not conducive to MgO expanding.Under the condition of drying curing,the expansion drop of MgO mortar increases with the increase of MgO content,activity and early curing temperature,which means that the humidity sensitivity of MgO mortar is positively correlated with dosage of MgO,activity of MgO and the early curing temperature.Under the changing curing environment,MgO mortar has strong shrinkage reversibility,its expansion performance fluctuates with the constant coupling change of environment temperature,humidity and other factors,and the expansion law has a certain degree of deviation from the law in environment of water bath curing and drying curing.Under the condition of erosion curing,the expansion efficiency of MgO mortar is almost unaffected,which indicates its adaptability is good in 5% magnesium sulfate corrosion environment.The strain of blank group in the same environment is almost unchanged,which indicates that the material with low aluminum content in engineering can improve the ability of resisting erosion of sulfate.With the increasing of Magnesia expansion agent and temprature,the compressive and flexural strength of expansive mortar has a slight downward trend,but the addition of MgO will not cause too much adverse effect on the strength,and sometimes even improves the strength slightly.Compared with the free state,the maintenance mode of restricting curing can increase the strength obviously.Magnesia expansive mortar and blank group mortar can still maintain a complete appearance after 200 times of freeze-thaw cycle in the environment of magnesium sulfate erosion,the quality of which is almost unchanged.The compressive strength of magnesia expansive mortar in process of freeze-thaw cycle is lower than that of blank group,and the deterioration speed is faster than that of blank group.The deterioration speed of flexural strength of magnesia expansive mortar and blank group is not obviously different,the flexural strength of former is lo wer than that of latter in the early stage of freeze-thaw cycle,but the gap narrows in later stage.The influence of MgO expansion agent on the compressive strength of mortar is greater than that on flexural strength.For the group at higher temperature in early stage,its strength is less than the group at lower temperature in the process of freeze-thaw cycle.Adding magnesia expansion agent will make the total porosity of cement paste hardening slurry,the most possible aperture and threshold aperture increase,and the number of pores above 50 nm increases,so the compressive and flexural strength of MgO expansive mortar under free-water environment and magnesium sulfate salt-freezing environment is lower than that of blank group.At the same time of curing age,the increase of temperature will make the volume of pore larger than 50 nm of cement hardening slurry increase.In the 40? curing environment,the large pore volume over 50 nm of cement hardening slurry will increase with the extension of age,while the change trend of this kind of pore volume is opposite when the curing temperature is 20?.Compared with the total porosity,po re distribution can reflect the hydration cement slurry more comprehensively and accurately in strength,durability and other aspects of the macroscopic characteristics,but the distribution of pores below 50 nm of the hardened slurry is not a single linear relationship with the activity of MgO,the dosage of MgO and the curing temperature,this test result and the dry shrinkage behavior of the macroscopic performance of MgO expansion mortar do not exactly match.