Damage Characteristics Of Metakaolin-fly Ash Base Polymer Concrete Under Salt-alkali-freeze-thaw Cycles

Geopolymer Concrete is more and more popular in the field of building materials because of its good mechanical properties,high temperature resistance,and chemical stability.It is a new type of environmentally friendly building materials.In addition,the unfavorable environment in the saline-alkali areas in the west of China,the deicing salt areas in the north,and the coastal areas in the east cause a large amount of saline-alkali and freeze-thaw damage to the concrete structure every year.The cumulative damage caused by the salt-alkali-freeze-thaw environment to the concrete material is irreversible.Long-term exposure to this environment will gradually accelerate the destruction of the concrete material and affect the normal use of the concrete structure.In order to further study the damage characteristics of geopolymer concrete in saline-alkali-freeze-thaw environment,this article uses metakaolin and fly ash as the main raw materials to prepare geopolymer concrete.Characteristic test research,and set ordinary concrete as a comparative test.It provides some theoretical basis for the promotion of geopolymer concrete in severe cold areas in practical applications.(1)The mechanical properties of ordinary concrete and geopolymer concrete under saline-alkali corrosion were studied.The results show that:under the action of saline-alkali erosion,with the increase of the erosion cycle,there is no obvious damage on the surface of the specimen;The mass,relative dynamic elastic modulus,and compressive strength all showed a trend of slowly increasing and then gradually stabilizing.Under the same erosion cycle,the increase rate of ordinary concrete mass,the increase rate of relative dynamic elastic modulus,and the increase rate of compressive strength are higher than those of geopolymer concrete.The relative dynamic elastic modulus and compressive strength of geopolymer concrete specimens soaked in compound salt and chloride salt were lower than those of sulfate and water.(2)The damage characteristics of ordinary concrete and geopolymer concrete after different freeze-thaw cycles in saline solution were studied.The results show that:under saline-alkali environment,as the number of freeze-thaw cycles increases,the surface damage of ordinary concrete specimens becomes more and more obvious,and the mass,relative dynamic elastic modulus,and compressive strength all show an increasing trend before decreasing;The surface damage of the geopolymer concrete specimens was gradually serious,the mass showed a trend of increasing first and then decreasing,and the relative dynamic elastic modulus and compressive strength showed a trend of stabilizing and then decreasing.After 75 freeze-thaw cycles,the degree of freeze-thaw damage of ordinary concrete changes from strong to weak in order of compound salt freeze,sulfate freeze,chloride salt freeze,and water freeze.Under the same freezing and thawing times,the freeze-thaw damage degree of geopolymer concrete is from strong to weak in order of compound salt freeze,chloride salt freeze,water freeze,sulfate freeze.Based on the test data,the damage and deterioration model of concrete in saline-alkali-freeze-thaw environment is also proposed.(3)The uniaxial compressive stress-strain curve and constitutive relationship of geopolymer concrete and ordinary concrete under salt-alkali-freeze-thaw cycles were studied.A stress-strain constitutive model considering the peak stress,peak strain,control parametersα_d,β_d of concrete and the number of freeze-haw cycles under different salt-alkali erosion was established.The results show that:under saline-alkali environment,with the increase of freezing and thawing times,the peak stress of geopolymer concrete tends to stabilize first and then accelerates to decrease,and the peak strain increases first slowly and then rapidly.The peak stress of ordinary concrete increases first and then decreases,and the peak strain increases first and then increases rapidly.Under the conditions of the same salt solution and the same number of freeze-thaw cycles,the ascending parametersα_d of geopolymer concrete is less than ordinary concrete,and the descending parametersβ_d is larger than ordinary concrete.It shows that the ductility and plastic deformation properties of geopolymer concrete are not as good as ordinary concrete.