Characteristic Of Recycled Coarse Aggregates And Its Effect On Concrete Properties

With the deeply advance of the urbanization construction,the demolition and reconstruction of a large number of buildings lead to the increase of construction waste in the number of billions of tons per year,which results in an environmental load that can not be ignored.At the same time,with the rapid development of infrastructure makes the demand of commercial concrete increase steadily,but the tightening of environmental protection policy leads to the serious shortage of natural sand aggregate.Therefore,the replacement of natural aggregate after recycling of construction waste not only meets the needs of sustainable development strategy,but also alleviates the situation of environmental load.The complex components of recycled aggregate in construction waste and the mortar attached to the performance of aggregate and limit the application of regeneration in concrete.In this paper,the composition,mortar content,geometric characteristics,aggregate hardness and basic physical properties of three different recycled coarse aggregate are studied.And the effects of recycled coarse aggregate characteristics differences of recycled concrete have been explored.The influence mechanism of construction waste composition and aggregate attached mortar content on the performance of recycled concrete is revealed.Lastly,the recycled aggregate RA-E with the highest content of attached mortar was calcined and ground to realize the high efficiency separation of attached mortar.And the mortar aggregate is modified by sodium salt containing hydroxypolysaccharide immersion and soaking slurry.The effects of recycled coarse aggregate on working,mechanical,volume stability and impermeability of recycled concrete was investigated.The results showed:（1）Compared to RA-E,the length-diameter ratio of RA-C1 and RA-C2 decreased by9.55%and 11.46%.The irregularity decreased by 9.35%and 10.79%.The content of mortar decreased by 6.65%and 7.72%,the water absorption decreased by 12.40%and 22.35%,respectively.Compared with RA-E,the RA-C1 showed lower crushing index due to the decrease of the content of attached mortar.The content of RA-C2 mortar is the lowest,but its crushing index is the highest,which is mainly caused by the R-C2 containing more red bricks and ceramics.（2）Generally,recycled aggregate is composed of concrete blocks,bricks,ceramics,etc.The decrease of attached mortar content on the surface of concrete block surface can improve the mechanical properties of recycled concrete,while the red brick and ceramics will greatly reduce the mechanical properties of recycled concrete.The mortar attached to the surface of recycled aggregate can provide internal curing effect in the early stage of cement concrete hydration,thus reducing the shrinkage of concrete.As a result,early hydration show RA-E<RA-C1<RA-C2.However,the shrinkage of recycled concrete will be affected by the elastic modulus of recycled aggregate,and the lower the elastic modulus of aggregate,the greater the shrinkage of concrete.The dry shrinkage for 90 d showed RA-C2>RA-E>RA-C1.The durability of recycled aggregate concrete is related to the content of recycled coarse aggregate adhesion mortar,and the higher the content of attached mortar,the worse the durability.The durability is show RA-C2>RA-C1>RA-E.（3）During the calcination process that re-treatment of recycled coarse aggregate by calcination-grinding,the recycled coarse aggregate attached mortar is effectively separated with the increase of calcination temperature.However,if the calcination temperature is too high,it will lead to a significant reduction in the strength of the recycled coarse aggregate and the crushing index.The particle morphology of mortar aggregate treated by SA on the basis of calcination-grinding is well improved,and obvious water retention due to the formation of gel in mortar pores.（4）Based on the weak short plate of recycled aggregate attached mortar,the internal and external cooperative treatment of the separated mortar particles is proposed.As for the mechanical properties,the recycled aggregate concrete treated by soaking respectively with Ca（OH）₂+SA and rewrapping shows the highest mechanical strength,and the compressive strength for 28 d reaches 92.15 MPa.At the same time,the resistance to chloride penetration is improved,the electric conductance is only 1 494 C,and the shrinkage strain is less than293.55×10^-6。...