Experimental Study On The Effect Of Fly Ash On The Mechanical And Durability Properties Of Recycled Concrete

Large amounts of construction waste are generated during the process of buildings being constructed and demolished.Fly ash is a by-product of thermal power generation,and huge amounts.Construction waste is not only wasted resources,but also polluted the environment by simple disposal.Construction waste is crushed into recycled coarse aggregates,natural aggregates are replaced by recycled aggregates and cement is partially replaced by fly ash in the preparation of concrete,fly ash recycled concrete was prepared.Carbon emissions are reduced,in line with the"carbon neutral,carbon peak"development requirements.The maximum temperature difference of 47℃in a year in the central and western regions of Inner Mongolia Autonomous Region,and concrete buildings in saline areas suffer from coupled salt and freeze-thaw erosion all year round.Therefore,the mechanical properties and durability of fly ash recycled concrete are studied for their scientific value.The mechanical properties and durability of fly ash recycled concrete are investigated in this thesis as follows.（1）The effects of fly ash replacement rate（0,15%,25%and 35%）and age（3d,7d,28d,56d and 90d）on the compressive and splitting tensile properties of recycled concrete were investigated.Tests show that fly ash has a double effect on compressive strength,At the age of 90d,the compressive strength of fly ash replacement rate 25%is 11%higher than that of unmixed fly ash.The compressive strength of fly ash substitution rate 35%group is6.5%lower than that of unadulterated fly ash group.At 3d,7d,28d and 56d curing ages,the compressive strength of fly ash replacement rate 25%was 86.3%,94.6%,96.0%and 102%of that of unadulterated fly ash,respectively.Fly ash on splitting tensile law and compressive law like.At 90 d of maintenance age,the compressive strength of fly ash replacement rate of 25%is 8.4%higher than that of unadulterated fly ash.The test data were used to establish the relationship between age and compressive strength and the relationship between compressive and splitting tensile strength,and other scholars’test data were used to verify the relationship.（2）The effect of fly ash replacement rates（0,15%,25%and 35%）on the full uniaxial compressive stress-strain curve of recycled concrete was investigated,The test results showed that fly ash increased the elastic modulus,peak stress,peak strain and ultimate strain by 8.8%,4.5%,4.3%and 18.4%at maximum,but the trend of the curve was not changed.The log-normal distribution function is based on the established random damage principal structure relationship for fly ash recycled concrete.（3）Fly ash replacement rates（0,15%,25%and 35%）,number of freeze-thaw cycles（30,60 and 90 cycles）on the durability performance of recycled concrete were investigated and the changes in compressive strength loss rate,mass loss rate and relative dynamic elastic modulus were analyzed.The"quick-freeze"method was adopted for testing.The results show that:the frost resistance of ordinary concrete is better than that of recycled concrete,and after 90 times of compound salt freezing and thawing,the compressive strength loss rate of ordinary concrete is 6.74%lower than that of the recycled concrete group;among the recycled concrete,the frost resistance of the 25%fly ash replacement rate group is the best,and the mass loss rate of the 25%fly ash replacement rate group is 36.0%lower than that of the unadulterated fly ash group;after freezing of ordinary water in the 25%fly ash replacement rate group The relative dynamic elastic modulus was 12%higher than that of salt frozen.Compressive strength loss rate and relative dynamic modulus of elasticity were modeled based on the post-freeze-thaw damage of fly ash recycled concrete respectively,and other scholars’experimental data were verified with the model curves.Fly ash recycled concrete life is predicted.（4）Fly ash replacement rates（0,15%,25%and 35%）on the chloride diffusion coefficient of recycled concrete were studied.The RCM method was used to do the chloride ion penetration resistance test.The results showed that the chloride ion diffusion coefficients of fly ash recycled concrete with 15%,25%and 35%fly ash replacement rate were 23.43%,42.63%and 17.08%lower than those of recycled concrete without fly ash,respectively,and the fly ash recycled concrete with 25%fly ash replacement rate had better chloride ion permeability resistance than ordinary concrete,and the chloride ion diffusion coefficient was 19%lower than that of ordinary concrete.（5）In the previous period,the phenomenon that the amount of hydrated calcium silicate produced by the recycled concrete in the fly ash-doped group was lower than that in the non-fly ash group was observed by SEM technique.At a later stage,Ca（OH）₂ is consumed by the hydration of fly ash to produce C-S-H,the internal pores of concrete are filled,and the properties of concrete are improved.A large number of calcium alumina and gypsum crystals are generated inside the concrete,and cracks are promoted to develop through,resulting in the concrete performance being reduced.