Study On Damage And Deterioration Of Cement-based Materials Under Dry Cold,Dry Hot And Sulfate Composite Environment

With the advancement of The Belt and Road Initiative and China’s Western Development Policy,a large number of cement and concrete infrastructures have been constructed and operated in the harsh environment,including the dry cold and dry hot natural climate and the sulfate media such as salt lakes and saline soils.It is a serious challenge to the durability design and service life of cement-based materials.Based on the natural climate of dry cold and dry hot,this paper proposes a set of temperature and humidity system that can be used in the laboratory to simulate the typical service environment in the west of China.Deterioration law and damage mechanism of cement-based materials partially immersed in sulfate solution under dry hot and dry cold/dry hot environment were systematically studied.This paper provides scientific guidance for life prediction and durability evaluation of cement-based materials in the west of China.The main results are as follows:（1）In dry hot environment,obvious damage occurred in the area below the immersion liquid level of cement-based materials.The erosion type was chemical erosion.In Na₂SO₄solution,the main erosion products were gypsum and ettringite,while in Mg SO₄ solution,the main erosion product was non-cohesive M-S-H.There was no obvious deterioration in the evaporation area.Because of the high saturation of sulfate at 40℃,it was difficult to crystallize in the evaporation zone.In addition,the evaporation rate of water was fast,which reduced the height of capillary adsorption.（2）In dry cold/dry hot environment,the cement-based material evaporated obviously and the erosion type was mainly physical erosion.The spalling degree of samples soaked in Na₂SO₄solution was more serious than that in Mg SO₄ solution.The saturation of Na₂SO₄ was greatly affected by temperature,and the dry cold/dry hot environment accelerated the salt crystallization and physical erosion rate.The saturation of Mg SO₄ was less affected by temperature,and it was still up to 18%at 0℃.The dry cold/dry hot environment had a low acceleration effect on its salt crystallization.At the same time,a large number of erosion products generated in the soaking area blocked the pores and reduced the height of capillary adsorption.（3）Reducing the w/c ratio and the concentration of the erosive medium could effectively improve the sulfate corrosion resistance of cement-based materials in dry cold and dry hot environment.The addition of 30%fly ash and 60%slag improved the sulfate erosion resistance in the area below the immersion liquid level,but aggravated the salt crystallization in the evaporation area above the liquid level.The addition of 10%limestone had the effect of diluting cement,increasing w/c ratio,increasing porosity,and reducing the sulfate resistance of cement-based materials.（4）Gypsum and ettringite in the immersion zone tended to fill in pores with a diameter of50-1000 nm in dry cold and dry hot environment.With the accumulation of erosion products,the expansion stress on the pore was generated,which led to the cracking of cement-based materials,thus increasing the volume of large pores above 1000 nm.The salt solution in the evaporation zone tended to crystallize in macropores with a pore diameter of more than 1000nm,and with the increase of the erosion age,the crystallization pressure generated by the crystal growth caused microcracks and increased the content of harmful pores.