Research On Magnesium Chloride Corrosion Of Cement Mortar And Modification Mechanism Of Nano SiO₂/Al₂O₃ In Plateau Environment

The atmospheric pressure and humidity in the western plateau area are generally low,and the lake water and saline soil in some areas contain a large amount of soluble salt.It is found that the concrete structures such as bridge piers,tunnels and railway sleepers which have served for a long time in this multi-factor environment of low pressure,low humidity and salt corrosion have suffered serious corrosion damage.The main reason is that the evaporation rate of water on the surface of concrete members increases in this composite environment,which leads to the decrease of hydration degree of cement,the deterioration of pore structure,and finally the decrease of corrosion resistance and durability of concrete structures and the shortening of service life.Therefore,the durability of concrete buildings located in the western plateau area has become an urgent problem to be solved.This paper simulates the plain（atmospheric pressure-low humidity-magnesium chloride corrosion）and plateau（low pressure-low humidity-magnesium chloride corrosion）environments to conduct semi-immersion corrosion research on cement mortar for half a year,and explores the damage mode and corrosion mechanism of the components.The results show that the damage of components in plateau environment is earlier and more serious,which is mainly because the early low-pressure and low humidity curing deteriorates the pore structure of the specimen,resulting in the increase of the number of connected pores.In the later stage,the salt solution enters the test piece under capillary action,migrates to the components exposed to the air through adsorption and diffusion,and then evaporates to the environment under the action of low humidity and low pressure,causing the internal salt solution to prematurely saturate and cause physical crystallization and chemical erosion damage.Finally,it is proposed that the durability of concrete components in the plateau environment should be improved by reducing the porosity of the components,improving the microstructure,improving the uniformity of the structure and reducing the source of corrosion.In view of the excellent performance of nano-silica and nano-alumina in cement-based materials,this study proposes to modify cement-based materials by adding appropriate amount of nano-silica and nano-alumina.The porosity and pore size distribution of nano-modified cement paste in low pressure and low humidity-low humidity environments are studied by mercury intrusion method.The results showed that the porosity of cement paste cured in low humidity and low humidity-low pressure environment increased by at least 15%compared with standard curing environment.The critical pore diameter increased from 60 nm to 200 nm,and the most probable pore diameter increased from 30 nm to more than 50 nm.The number of pores and small pores decreases,and the number of harmful pores above 50 nm increases.This is very detrimental to the impermeability and corrosion resistance of cement-based components.However,nano-modification can effectively reduce the porosity of cement paste and the number of harmful pores.Among them,the introduction of 1.0%nano-silica has the best effect on improving the pore structure,followed by the composite nano-system.The corrosion research of nano-modified cement-based specimens in low humidity-magnesium chloride corrosion and low humidity-low pressure-magnesium chloride corrosion environments is carried out using full-immersion and semi-immersion methods.The modification effect of nano-Si O₂/Al₂O₃ on the properties of cement mortar under low pressure-low humidity-magnesium chloride corrosion environment is comprehensively analyzed by testing the apparent damage,strength,pore structure and microscopic products of the mortar.It is found that the capillary adsorption height of salt solution on the mortar surface of nano-modified mortar hardly changed from the beginning of soaking to six months of soaking,and only 20 cm higher than the liquid level,which indicates that nano-modified mortar could cut off the transmission channel of salt solution and improve the uniformity and compactness of mortar.It is found that nano modification can effectively reduce the risk of corrosion cracking of magnesium chloride in low humidity and low pressure-low humidity environment.This is mainly related to the reduction of porosity and the content of reactant calcium hydroxide by nano silica and nano alumina.The action mechanism of nano-silica and nano-alumina in cement paste under low humidity and low humidity-low pressure is studied by low field nuclear magnetic resonance,thermal analysis,scanning electron microscope and X-ray diffraction.The research shows that nano-silica and nano-alumina can be used as nucleation sites of hydration products in the hydration process,which can accelerate the hydration process of cement and improve the homogeneity of paste.In addition,the introduction of nano materials does not produce new products in standard curing,low humidity and low pressure environments,but changes the content of calcium hydroxide and bound water in the original hydration products,which is attributed to the secondary hydration of nano materials.Finally,the grain size is refined,the number of macropores is reduced,and the micro compactness of cement-based components in low humidity and low humidity-low pressure environments is improved,which is due to the nucleation,pozzolanic activity,filling effect and homogenization of the two nano-materials.