| Under the unfavorable geological conditions of cold climate and high sulfate content in Xinjiang region of China,concrete is often damaged by sulfate and freeze-thaw cycle,which greatly reduces the durability of concrete and results in the decline of service life of concrete.At present,in the freezing-thawing mechanism of concrete,most researchers use fast freezing method for experimental research.Even if one side freeze-thaw method is adopted,sodium chloride solution is also used as freeze-thaw medium.However,when concrete is subjected to freezing-thawing cycle,many concrete structures are exposed to the external environment on one side,which is accompanied by strong sulfate erosion.Therefore,the process and mechanism of concrete degradation under single side freezing-thawing in sulphate environment need to be further studied.In this paper,by controlling the air content of concrete to be 5(±1)%,the law of mass loss per unit area and ultrasonic relative dynamic elastic modulus of concrete with four different mineral admixtures after 8,16,28,56,84,112 single-sided freeze-thaw cycles in 2%Na2SO4 solution,5%Na2SO4 solution,5%NaCl solution.The pore size ratio,bubble spacing coefficient and erosion products in concrete after 112 freeze-thaw cycles were investigated.The freeze-thaw degradation mechanism of concrete in sulphate environment was analyzed synthetically from macroscopic and microscopic aspects.The following conclusions are drawn:(1)Under the combined action of sulfate erosion and freeze-thaw cycle,a certain amount of erosion products enhance the compactness of concrete and reduce the permeability of concrete,which is beneficial to the frost resistance of concrete;When the erosion products continue to increase,the crystallization pressure and freezing pressure act on the concrete to accelerate the failure of concrete.(2)When the controlled gas content is 5(±1)%,after 112 freeze-thaw cycles in 5%Na2SO4 solution environment,the unit area mass loss of single 40%mineral powder concrete is 2.3 times that of single 20%fly ash concrete;According to the test results,the sulfate freeze-thaw resistance of 20%fly ash is the best.(3)After freeze-thaw cycle in 5%Na2SO4 solution environment,the four different mineral admixtures have the tendency of less harmless pores,more harmful pores,less specific surface area,more average pore size and more bubble spacing coefficient.This trend seriously affects the pore structure inside the concrete,resulting in poor durability of the concrete.(4)According to SEM and energy spectrum analysis,after freeze-thaw cycle in 5%Na2SO4 solution environment,the amount of gypsum and ettringite produced in 56 cycles before freeze-thaw is less,and the main factor of concrete failure is freeze-thaw failure;After 112 single-sided freeze-thaw cycles,the content of ettringite in concrete increased,and the content of ettringite in concrete without any mineral admixture was higher than that in single-doped 20%fly ash concrete.It shows that fly ash has good inhibitory effect on sulfate erosion and freeze-thaw property of concrete. |
