Macro-micro Experimental Study On Mechanical Properties Of Concrete Under Freeze-thaw Cycles Coupled With Sulfate Solution

In the saline soil area of Northwest China,the temperature difference between day and night is large.There is a high concentration of sulfate ion in the soil.The local concrete suffers from the coupling effect of freeze-thaw cycle and sulfate erosion for a long time.Fly ash,as a mineral admixture,has been used in various types of concrete.A certain amount of fly ash concrete can meet the requirements of durability and bearing capacity in practical engineering,and can reduce carbon emissions and save energy.Therefore,it is of great scientific value and practical significance to study the durability of fly ash concrete under the coupling effect of freeze-thaw cycle and sulfate solution.This paper mainly studies the durability of fly ash concrete as follows.Firstly,Macroscopic mechanical experiments were carried out on concrete specimens with specified freeze-thaw cycles,and the mechanical properties of concrete under the coupling action of freeze-thaw cycles and sulfate solutions were analyzed.The results show that higher concentration of Na₂SO₄ solution can increase the number of freeze-thaw resistance of concrete;concrete with 10%fly ash has good freeze-thaw resistance;before 100freeze-thaw cycles,the relative dynamic modulus of elasticity of concrete specimens has been in an increasing state;the relative dynamic modulus of elasticity of concrete specimens with10%concentration of Na₂SO₄ solution can advance to a decreasing stage;The relative dynamic modulus of elasticity of concrete sample decreases fastest when the content of concrete is 20%.Secondly,Scanning electron microscopy（SEM）was used to observe and analyze the corrosion products of concrete samples from the micro-level.The pore size distribution and porosity of concrete samples under different freeze-thaw cycles were investigated by mercury intrusion method（MIP）.The corrosion products of concrete were quantitatively analyzed by TG-DSC comprehensive thermal analysis.It is found that the content of corrosion products is positively correlated with the erosion time;with the increase of freeze-thaw cycles,the bigger the ettringite crystal is,the larger the expansion stress of concrete is;with the increase of freeze-thaw cycles,the porosity of concrete shows an upward trend,and in the later period of freeze-thaw cycles,the proportion of harmful holes increases obviously;with 150 freeze-thaw cycles,the proportion of pulverized coal increases obviously.The porosity of concrete specimens with 20%lime content is obviously higher than that of concrete specimens with0%and 10%fly ash content;the content of ettringite is higher than that of gypsum,and its formation time is earlier than gypsum;the formation rate of corrosion products in the early stage of freeze-thaw cycle is faster than that in the later stage of freeze-thaw cycle.In the later stage of freeze-thaw cycle,the higher the concentration of salt solution,the more erosion products.Thirdly,At last,the damage degradation of concrete in different periods is analyzed by studying the rate of corrosion products formation in different periods under the coupling action of freeze-thaw cycle and sulfate solution,and the damage degradation mechanism of concrete under the coupling action of freeze-thaw cycle and sulfate solution is further revealed.