Experimental Study Of Carbon-modified Recycled Coarse Aggregate And Its Interfacial Properties With New Mortar

The surface of recycled coarse aggregate is often adhered to old mortar,which is porous and loose in structure,resulting in poorer performance of recycled aggregate compared to natural aggregate and thus lower mechanical and durability performance of recycled concrete.In this paper,the performance of recycled coarse aggregate and the interface between old and new mortar in the prepared recycled concrete are studied by carbonation modification of recycled coarse aggregate,and the main research contents and conclusions are as follows:（1）Carbonation treatment was carried out on the recycled coarse aggregate soaked in water and clarified lime water respectively,and the effect of carbonation treatment on the modification of recycled coarse aggregate was analyzed by measuring the macroscopic properties of apparent density,water absorption and crushing index of recycled coarse aggregate.The results showed that with the increase of carbonation time,the apparent density of recycled coarse aggregate would be improved,and its water absorption and crushing index would be reduced.The apparent density,water absorption and crushing index of the recycled coarse aggregate soaked in clarified lime water are better than those soaked in water under the same carbonation time.（2）XRD and TG tests were used to determine the compositional changes of the old mortar on the surface of recycled coarse aggregate before and after carbonation,and SEM tests were used to observe the microscopic morphology of the aggregate-old mortar and the transition zone at the interface of old and new mortar in the recycled concrete prepared from recycled coarse aggregate.The results showed that the CaCO₃content inside the old mortar increased with the increase of carbonation time,and the specimens soaked in clarified lime water at the same carbonation time had higher CaCO₃content than the specimens soaked in water.The carbonation products generated by the carbonation reaction filled the pores and cracks of the old mortar,making the structure of the old mortar more dense.The carbonation treatment improved the microstructure of the aggregate-old mortar and the transition zone of the old-new mortar interface of the recycled concrete.（3）New and old mortar interface model specimens were designed to construct the transition zone of old and new mortar interfaces with different roughness by secondary casting method.The splitting tensile strength of the recycled concrete with different carbonation days,different interfacial roughness and different loading rates were considered to analyze the bonding performance of the old and new interfaces.The results show that the roughness changes the damage mode of the recycled concrete,with the increase of roughness,the damage mode of the specimen notch changes from end pull-out to root pull-out.The roughness has a greater effect on the bonding performance of the old and new mortar interfaces,the carbonation time of the recycled concrete has the second highest effect on the bonding performance of the old and new mortar interfaces,and the loading rate has a smaller effect on the bonding performance of the old and new mortar interfaces.