Study On The Effect Of Internal Curing Of Recycled Coarse Aggregate Adhered Mortar On Different The Performance Of Cements Recycled Concrete

The resource utilization of construction waste has become a social focus.Using recycled aggregate(RA)to replace natural aggregate(NA)to prepare recycled concrete is one of the ways to efficiently utilize construction waste.How to improve the performance of recycled concrete has become an urgent problem to be solved.In this paper,under the support of the National Natural Science Foundation of China(52078261)and(51978353),the content and water absorption of adhesive mortar on the surface of class Ⅰ and class Ⅱ RCA were studied.A series of recycled concrete was prepared by using two different cementitious material systems(ordinary Portland cement mixed with fly ash,fast hardening sulphoaluminate cement),three water-binder ratios(0.26,0.32,0.42),and two RCAs to replace natural coarse aggregate in a 25%gradient.The effects of RCA type,pre-wetting degree and replacement rate on the workability,mechanical properties,durability,shrinkage and internal relative humidity of recycled concrete with different cement materials were studied.The internal microstructure of concrete was studied,and the effects of RCA aggregate defects and internal curing effect of attached mortar on the properties of recycled concrete with different cement materials were analyzed.The main research contents and conclusions are as follows:(1)The content of mortar on the surface of type Ⅰ and type Ⅱ RCA was determined by calcination-grinding method,and the water absorption rate of mortar on the surface of type Ⅰ and type Ⅱ RCA was calculated by the difference of water absorption capacity before and after grinding.The results show that the optimal calcination temperature range of type Ⅰ and type Ⅱ RCA is 500℃～600℃,and the grinding time is controlled at 30 min and 35 min respectively to ensure the complete grinding of the adhesive mortar.Finally,the content of attached mortar on the surface of type Ⅰ and type Ⅱ RCA was 9.11%and 17.17%respectively,and the water absorption of attached mortar was 24.08%and 23.06%respectively.(2)RCA has a negative effect on the workability,mechanical properties and durability of recycled concrete,and the degree of influence increases with the increase of the amount of attached mortar.After pre-wetting treatment of RCA,the internal curing effect of adhesive mortar can promote the hydration of internal cement and make up for some performance loss.The OPRC and SARC with a water-binder ratio of 0.26 have excellent workability.The 28d compressive strength can reach more than 69MPa and 74MPa,respectively.The 28d electric flux is maintained within 1000C,and the number of freeze-thaw cycles is more than 200 times,which can meet higher performance requirements.The internal curing effect of adhesive mortar on recycled concrete increases with the decrease of water-binder ratio,and the internal curing effect of recycled aggregate adhesive mortar on SARC is better than that of OPRC.(3)Under the condition of water shortage inside the concrete,the internal water storage of the attached mortar maintains a high level of relative humidity inside the concrete,and can play a moisturizing role under single-sided drying and sealing conditions.The autogenous shrinkage and drying shrinkage of OPRC are large,and the internal curing effect of attached mortar effectively reduces the autogenous shrinkage of OPRC within 7d,but has no obvious effect on the drying shrinkage at 90d.The internal curing of the attached mortar has a good effect on the autogenous shrinkage of SARC within 7d and the drying shrinkage reduction within 90d and its autogenous shrinkage and drying shrinkage are small.The shrinkage reduction effect increases with the increase of the amount of adhesive mortar,and the internal curing effect of adhesive mortar is better in recycled concrete with lower water-binder ratio.(4)The microstructure of the interface of recycled concrete was analyzed by XRD,SEM,TG-DTG and BSE.The results show that the water release of the attached mortar promotes hydration,and the degree of cement hydration at the interface is high.The porosity distribution of the interface transition zone between the attached mortar and the new mortar is uniform,and the porosity is less than that of the interface transition zone between the natural coarse aggregate and the new mortar.