Study On The Effect Of Curing Methods On The Properties Of Cement-Based Materials Under Plateau Environment

In recent years,with the development of infrastructure in western China,attention has been paid to the influence of plateau environment on the properties of cement-based materials,which is characterized by low atmospheric pressure,low humidity and large temperature difference between day and night.In this condition,the strength of the concrete will be reduced,the temperature difference between the inside and outside will become large,and cracks will appear inside the concrete.It is a serious threat to the durability,safety and reliability of cement-based materials in the plateau environment,but appropriate curing method can improve the impact of the plateau environment on the performance of cement-based materials.At present,the common curing methods include plastic film curing,geotextile curing,sprinkler curing,curing film curing and natural curing.In order to study the influence of different curing methods on the properties of cement-based materials in the plateau environment,low pressure and low humidity environment was simulated with empty chamber,and the following contents were studied:(1)The effects of curing cement-based materials on their properties in different curing methods(plastic film curing,curing film curing and natural curing)under the environment of relative humidity of 30% and air pressure of 101 k Pa and 60 k Pa were studied.Combined with the pore structure and microstructure results of nuclear magnetic resonance(NMR)and scanning electron microscope(SEM),the influence mechanism of curing method on the properties of cement-based materials under low pressure and low humidity environment was explained.The results showed that: The lower the air pressure,the lower the compressive strength and flexural strength of cement-based materials.Among the three curing methods,the curing film has the function of releasing water and preventing water loss,which will improve the strength of cement-based materials and make cement-based materials more hydrated,and the curing effect is better than that of natural curing and plastic film curing.Under the same relative humidity,the chemical bound water content of cement-based materials under low pressure curing is less than that under normal pressure curing,and the gel pore development of cement-based materials under low pressure curing is not as full as that under normal pressure curing.Electron microscope(SEM)microstructure analysis found that the hydration product morphology of cement-based materials under low pressure curing is relatively wide compared with that under normal pressure curing,and the compactness is relatively poor.(2)On the basis of study(1),the influence degree and rule of curing time(7d,14 d,28d)on the properties of cement-based materials in the plateau environment were studied,and the influence of curing time on cement-based materials was analyzed through testing and analyzing the pore structure and microstructure of cement-based materials.The results showed that: Under the same curing method,with the extension of curing time,the curing effect of cement-based materials is better,and the compressive strength and flexural strength gradually increase.(3)The water loss of cement-based materials in different transport times(1h,1.5h,2h)was studied when the relative humidity was 30% and the air pressure was different.The results showed that: The reduction of air pressure accelerates the rate of water loss from the cement-based material to the environment.Compared with the atmospheric environment,the water loss is faster in the low-pressure environment.With the extension of transportation time,the more water loss.At the same time,the fluidity of cement-based materials after standing is tested.The longer the standing time,the greater the fluidity loss value of cement-based materials,that is,the loss of water will affect the workability of cement-based materials.(4)According to the contents studied in(1)and(2),grey correlation method was used to judge the influence of curing age,curing mode and environmental pressure on the mechanical properties of cement-based materials.The results showed that the influence of environmental pressure was the lowest.The relationship model between compressive strength and pore structure,and the relationship model between chemically bound water and compressive strength were established.It was analyzed from the model that in the plateau environment,the porosity was inversely proportional to the compressive strength,the proportion of gel pores was proportional to the compressive strength,and the content of chemically bound water was proportional to the compressive strength.