The Research On Modification Of Recycled Coarse Aggregate Of Building Waste And Performance Of Recycled Concrete

The particles protruding edges and corners and a large amount of hardened cement mortar is attached to the surface of the recycled coarse aggregate.And there are microcracks formed inside the recycled coarse aggregate.So,there are still many gaps between the natural coarse aggregate and recycled coarse aggregate in the crushing index,water absorption,bulk density and other properties.Recycled coarse aggregate is generally used in the preparation of medium and low strength recycled concrete.The application of recycled coarse aggregate engineering is relatively small.In order to obtain high quality recycled coarse aggregate and high strength recycled concrete,it is needed to modify the recycled coarse aggregate.The impact and grinding of the ball milling technology can efficiently remove the hardened cement mortar and the protruding edges attached to the surface of the recycled coarse aggregate to enhance the performance of the aggregate,especially to reduce the crushing index.But the modification effect is quite different at different ball mill operating conditions.In this paper,the crushing indexes and yield were used as evaluation criteria to research the effect of ball filling rate（v/v）,ball ratio（m:m）and residence time（s）on the modification of recycled coarse aggregate by doing single factor test and three-factor and three-level response surface test to explore the better operating condition of ball mill technology.After the ball milling,the water absorption of recycled coarse aggregate is still large and can not meet the performance requirements（<3%）of Class I recycled coarse aggregate.In this paper,the dilute HCl,cement slurry（mixed with fly ash or silica fume）and water glass solution were used as chemical surface treatment for modifying the recycled coarse aggregate.And the water absorption was used as evaluation criteria to choose the appropriate chemical surface treatment by doing single factor test.On the basis of the above,it was used four kinds of recycled coarse aggregates（EP,QP,EPS and QPS）with different quality and replacement rates（0%,30%,50%,70%和 100%）to produce recycled aggregate concrete.In this paper,the effects of different substitution rate and quality on the performance and mechanical properties of recycled aggregate concrete were analyzed by doing single factor test.The main findings and conclusions of the test were as follows:（1）The better parameters of the factors obtained by ball milling crushing single factor test were as follows: the steel ball filling rate was 19.3%,the ball ratio was 4:1,the retention time of materials was 240 s.The ball milling crushing response surface test obtained the order of influence of each factor on the modification effect was the retention time of materials>ball ratio>ball filling rate.And it obtained the optimal combination of process parameters were as follows: the steel ball filling rate was 19.8%,the ball ratio was 4:1,the retention time of materials was 220 s.In this case,the crushing index was 9.7% which was 29.7% lower than the untreated and the yield was 9.9g/s.The quality of recycled coarse aggregate belonged to Class II.（2）When the treatment was carried out with 2% water glass solution for 5 h,the modification effect was relatively good.At this time,the water absorption of recycled coarse aggregate was 2.7%,which was 37.2% lower than that of untreated,and reached the requirements the Class I for water absorption（<3%）.After treated with water glass solution,the quality of EPS belonged to class II and the quality of QPS belonged to class I.（3）When the mixing ratio of recycled aggregate concrete was same,the slump decreased with the increasing of the ratio of recycled coarse aggregate and increased with the better quality of recycled coarse aggregate.（4）When the mixing ratio of recycled aggregate concrete was same,the f_cu,kEP、f_tsEP、f_cpEP and Ec EP decreased,the f_cfEP first rose and fell with the increasing of the ratio of recycled coarse aggregate.The f_cu,kQP、f_tsQP and E_cQP decreased,the f_cfQP first rose and fell,the fcp QP increased with the increasing of the ratio of recycled coarse aggregate.The fcu,k EPS decreased,the fcf EPS andE_cEPS first rose and fell,the f_cpEPS increased,the f_tsEPS changed was not obvious with the increasing of the ratio of recycled coarse aggregate.The f_cu,kQPS、f_tsQPS and f_cpQPS increased,the f_cfQPS decreased,the E_cQPS changed was not obvious with the increasing of the ratio of recycled coarse aggregate.The mechanical properties of recycled aggregate concrete were enhanced with the improvement of the recycled coarse aggregate quality.