Study On The Properties Of Fully Recycled Concrete Prepared From Recycled Fine Powder-recycled Aggregate

With the development of urban construction,more and more construction waste is produced.The construction waste can be recycled to make a series of concrete products,and it can also be ground into recycled micro powder,which can be used as auxiliary cementitious materials for cement-based materials.The research contents of this paper are mainly divided into three aspects:（1）Study the influence of two different recycled micro powders:recycled concrete powder（RCP）and recycled brick powder（RBP）on the performance of fully recycled mortar;（2）Study the influence of RCP and RBP on the performance of fully recycled concrete;（3）Selecting three modification methods to study the effect of three different modification methods on the properties of fully recycled mortar Improve the performance of recycled concrete.In this paper,the following results are obtained through experimental study:（1）The mechanical properties of cement-based materials were improved by adding recycled micro powder.The results of powder activity test showed that RCP activity index was 75%,RBP activity index was 83%.Therefore,the addition of recycled micro powder reduces the early mechanical properties of cement-based materials,but with the continuous growth of curing age,due to its own micro aggregate effect and pozzolanic effect,under the effect of secondary hydration,the strength loss caused by adding recycled micro powder is gradually reduced,and the increase range of mechanical properties in the later stage is improved.（2）The results show that adding recycled micro powder can improve the water permeability and chloride ion penetration resistance of fully recycled mortar and recycled concrete.With the addition of two kinds of micro powders,the higher the replacement rate,the more obvious the improvement effect.Under 30%replacement rate,the water permeability resistance of fully recycled mortar decreased by 16.29～22.68%,the water permeability resistance of fully recycled concrete increased by21.36～27.78%,and the chloride ion permeability resistance of fully recycled mortar increased by 5.33～13.76%,the chloride ion penetration resistance of recycled concrete increased by 26.47～33.17%.（3）After adding recycled micro powder,the carbonation resistance of fully recycled mortar is reduced,which makes CO₂more easily penetrate into the specimen.The carbonation depth increases with the increase of carbonization time,but the growth rate of carbonization depth becomes slower in the later stage.The carbonation depth increases gradually with the increase of the replacement rate of recycled micro powder.After 14 days of carbonization,the carbonation depth of pure cement reference group was 4.4 mm,and the maximum carbonation depth was increased to13.1 mm at 30%replacement rate after adding recycled micro powder.（4）Three methods of secondary mixing,adding silica fume and adding Na OH were selected to modify the recycled concrete,and the mechanical properties and durability were tested.The results show that the compressive strength,splitting tensile strength,dynamic elastic modulus,chloride ion penetration resistance and water permeability resistance of recycled concrete increased by 9.41%,14.69%,18.70%,8.21%and 12.04%respectively;after adding silica fume,the compressive strength,splitting tensile strength and dynamic modulus of elasticity increased by 11.37～22.19%,22.03～26.22%and 25.54%,respectively～27.03%,chloride ion penetration resistance and water permeability resistance increased by 6.16～18.39%and 25.40～29.55%respectively;After adding Na OH,the compressive strength increased by13.14～14.50%,the splitting tensile strength increased by 20.63～24.13%,the dynamic modulus of elasticity increased by 22.01～23.83%,the resistance to chloride ion penetration and water permeability increased by 3.21～13.39%and 16.96～22.84%,respectively.In general,silica fume has the best effect,followed by Na OH,and the effect of secondary stirring is slightly insufficient.