Study on Frost Resistance Durability and Interface Microstructure of Recycled Concrete in Sulfate Environment

In order to study the durability performance of recycled concrete under the coupling effect of both sulfate attack and freeze-thaw cycle,the freeze-thaw cycle test in sulfate environment was designed,the mass loss and relative dynamic elastic modulus of concrete with replacement rates of recycled aggregate of 0,30% and 60% were measured,and to investigate the effect of aggregate replacement rate on the durability damage process and microstructure development pattern of recycled concrete using nuclear magnetic resonance (NMR) and scanning electron microscope (SEM) techniques.The results show that in the freeze-thaw cycle test,the frost resistance and sulfate resistance of concrete with 60% replacement rate of recycled aggregate and ordinary concrete are similar.The addition of recycled aggregate increases the number of pores in small holes (d<10nm),medium holes (10~50nm) and large holes (50~1 000nm),and the number of favorable pores increases.The order of the number of favorable pores before freeze-thaw is R60>R30.When the replacement rate of recycled aggregate is 60%,it can effectively reduce frost heave cracks,and the expansion of internal pore structure is not obvious.The internal pore expansion of concrete with 30% replacement rate of recycled aggregate is more significant,and the total area change of its T₂ spectrum curve is 162.46%.The SEM results show that the interface transition zone and cracks are the weaker links in the recycled concrete.The ettringite crystals and gypsum crystals formed in the pores of concrete under sulfate freeze-thaw environment are the direct reasons for the development of concrete cracks in the later stage.