Durability Of Fine Lightweight Aggregate Concrete Exposed To Sulfate Erosion Coupled With Drying-wetting Attack

Sulfate erosion is one of the important environmental factors affecting the durability of concrete pavements.Soil,groundwater,industrial wastewater and acid rain contain sulfate ions,which penetrate into the interior of the pavement and chemically react with cement hydration products to form gypsum,calcium alumina and other swelling erosion products leading to concrete spalling and cracking.In addition,due to rainfall-evaporation and water level rise and fall,the moisture content of the pavement structure is in a cyclical change(drying-wetting cycle),accelerating salt erosion.Therefore,the research to optimize the design of pavement concrete materials to improve their durability in the sulfate erosion coupled with drying-wetting attack environment is to be carried out in depth and has important theoretical value and practical significance.Pre-wetted fine lightweight aggregates can improve the hydration and permeability of concrete by releasing their own stored water,thus providing the possibility to improve the durability of concrete in complex environments.In view of this,this paper designs concrete specimens with different fine lightweight aggregate admixtures and water-cement ratios,and systematically analyzes the effects of light and fine aggregate admixtures,water-cement ratios,mineral admixtures,drying-wetting cycle regimes and sulfate erosion coupled with drying-wetting attack days on the apparent morphology,mass loss rate,relative dynamic elastic modulus and mechanical properties of fine lightweight aggregate concrete,and explores the effects of these design parameters on the microstructural evolution of concrete and The effects of these design parameters on the microstructure evolution and erosion product composition of concrete were investigated to reveal the mechanism of sulfate erosion resistance of fine lightweight aggregate concrete,with emphasis on evaluating the effects of different design parameters on the resistance of fine lightweight aggregate concrete to sulfate erosion coupled with drying-wetting attack.Through the gray correlation theory,the durability of fine lightweight aggregate concrete with different mix ratios under erosion environment is evaluated with small data samples to provide a new evaluation method by comparing durability advantages and disadvantages visually with correlation values.The results show that the mass,relative dynamic modulus of elasticity and flexural strength of concrete show a trend of increasing and then decreasing under sulfate erosion coupled with drying-wetting attack.The expansive erosion products such as gypsum and calcium alumina generated at the early stage of erosion improve the performance of concrete by filling the pores and making the concrete structure more compact.With the increase of erosion time,the erosion products accumulate and lead to concrete damage,causing spalling and cracking.Pre-wetted fine lightweight aggregate can give full play to its internal curing effect,and the formation of a dense interface transition zone between the cement matrix,thereby improving the resistance of concrete to sulfate erosion coupled with drying-wetting attack,but its improvement effect decreases with the increase in the amount of fine lightweight aggregate.When the fine lightweight aggregate admixture was 30%,the loss rate of mass,relative dynamic elastic modulus and flexural strength after 210 days of erosion were the smallest,respectively,the loss rate of mass of mass concrete at 210 days of erosion age was the smallest,0.10%,and the relative dynamic elastic modulus and flexural strength were still in a positive growth rate,4.72%and 4.44%,respectively.When the water-cement ratio was increased from 0.40 to 0.55,the durability of fine lightweight aggregate concrete decreased,and the loss rates of mass,relative dynamic elastic modulus and flexural strength were as high as 0.37%,4.07%and 24.57%.The durability of fine lightweight aggregate concrete under sulfate erosion coupled with drying-wetting attack was significantly improved by compounding mineral admixtures(slag and metakaolin)when the fly ash admixture was less than 20%.The durability of fine lightweight aggregate concrete was best when the admixtures of fly ash,slag and pozzolan were 20%,10%and 10%,and the loss rates of mass,relative dynamic elastic modulus and flexural strength were the smallest,0.03%,respectively,and the relative dynamic elastic modulus and flexural strength were still higher than the initial state,with growth rates of 4.07%and 10.44%,respectively.In addition,changing the drying temperature in the wet and dry cycles significantly affects the sulfate attack resistance process of fine lightweight aggregate concrete.Compared to room temperature conditions,when the drying temperature was increased to 40 ~oC,the deterioration of concrete properties could be delayed by improving the hydration process,and when the mass,relative dynamic modulus of elasticity and flexural strength were still higher than the baseline values after 210 days of erosion.And when the drying temperature is 80 ~oC,it will significantly accelerate the damage process of concrete and reduce the durability.The results of gray correlation theory analysis showed that the trend of correlation coincided with the evolution of durability of fine lightweight aggregate concrete under sulfate erosion coupled with drying-wetting attack,and the results of the theoretical analysis were consistent with the pattern of experimental data.The durability of fine lightweight aggregate concrete is visually characterized by numerical values,and it is easy to visually compare the differences of each mix ratio.