Experimental Study On Sand Erosion Of Cement Mortar Subjected To Multiple Factors Of Freeze-thaw And Dry-wet Cycles In Compound Salt Solution

Parts of northern China are dominated full year by frequent sandstorms,serious salinization of soil,temperature variations and many dry-wet types,which are important reasons for the durability of concrete material.In this paper,based on the study of soil salinization,sandstorm and cold environment in Midwest of Inner Mongolia,instead of concrete,cement mortar was used to study the damage mechanism of the concrete surface located in harsh environment.According to the results of field sampling,compound salt solutions were prepared,and rapid corrosion tests were carried out in multiple factors including compound salt,freeze-thaw cycles and dry-wet cycles.Sand-air injection method was used for erosion wear test by erosion test rig for simulating sandstorm environment.Erosion rate was employed to evaluate its damage degree and erosion law in different mechanical parameters.In order to figure out erosion damage mechanism of cement mortar from a microscopic view in every corrosion tests mentioned above,some methods such as SEM,LSCM and XRD were selected to measure morphologies and chemical composition of sample surfaces,which can provide a theoretical basis for concrete infrastructure protection and durability evaluation in this region.Researches contents are as follows.1.Based on the results of soil chemical composition obtained by field sampling in Midwest of Inner Mongolia,two types corrosion solutions of 8% and 10% compound salt of NaCl and Na2SO4 were prepared for corrosion test of freeze-thaw cycles,dry-wet cycles as well as freeze-thaw and dry-wet cycles in compound salt.2.After three types rapid corrosion,sand erosion test was carried out on cement mortar and the results are as follows:(1)Erosion rate of cement mortar increases with the increase of impact angle,and normal kinetic energy component plays an important role in the process of erosion damage of brittle materials;(2)Erosion rate increases as a almost exponential growth with the increase of erosion velocity,and it is the same trend as that in different concentrations and freeze-thaw cycles;(3)Erosion rate decreases with the increase of sand flux;(4)Erosion rate decreases firstly and then increases with increasing freeze-thaw cycles in compound salt solution and all of that reach minimum after 10 cycles in different concentrations;(5)Erosion rate decreases firstly and then increases with the increase of dry-wet cycle in compound salt solution.The higher the concentration is,the slower the erosion rate is,the longer the trough period is;(6)Erosion rate decreases firstly and then increases with the increase of freeze-thaw and dry-wet cycle in compound salt solution,and the higher the concentration,the later the rise period.3.Sample surface morphology was observed by scanning electron microscopy(SEM)and confocal laser scanning microscopy(LSCM),and chemical composition of sample surface corroded by salts was confirmed by X-ray diffractometer(XRD)to discuss its erosion wear law,which can explain the damage mechanism of concrete material subjected to sand particle erosion from a microscopic view:(1)Corroded by salts solution,the edges and corners of cement mortar is peeled off with varying degrees.Compared with cement slurry peeling on the top,the sides of sample is attached by white stuff;(2)SO2-4 and Clcan react with cement mortar to form ettringite and gypsum crystals,which can compact its internal holes and improve the erosion resistance ability;(3)Erosion mechanism is characterized by horizontal cutting and vertical impact;(4)Erosion wear degree of sample subjected to freeze-thaw cycle is influenced by some main factors including swelling stress of chemical reaction products and supersaturated crystallization in low temperature,as well as osmotic pressure and frost effect;(5)Erosion rate of sample subjected to dry-wet cycle is mainly affected by internal shrinkage,permeability and crystal expansion;(6)Anions can react with cement mortar in freeze-thaw and dry-wet cycle in compound salt solution,coupled with the effect of shrinkage and supersaturated crystallization,which make the internal structure can be failed easily.This is the result of a series of physical and chemical process affected together by multiple factors.