Tensile Creep Behavior And Crack Resistance Of Alkali-activated Slag Concrete With Recycled Concrete Aggregate

Alkali-activated slag recycled concrete(AASRC)prepared with alkali-activated slag and recycled coarse aggregate(RCA)has good environmental and economic benefits.However,the cracking induced by autogenous shrinkage is one critical factor limiting the application of AASRC in construction industry.Tensile creep could partially relax the tensile stress caused by the shrinkage of AASRC,thereby reducing the cracking risk.However,very few studies were conducted on the tensile creep and crack resistance of AASRC.Thus,this study investigated the effects of various activator types and RCA contents on the workability,mechanical properties,autogenous shrinkage,tensile creep,and cracking resistance of AASRC.Three activators were used:water glass(WG),combined WG and sodium carbonate(WNC),and combined calcium oxide and sodium carbonate(CNC),and three RCA contents were 0%,50%and 100%.Moreover,hydration heat,XRD,TG-DTG,MIP,SEM-EDS and microhardness were used to analyze the type and quantity of hydration products,pore structure distribution,C(N)-A-S-H gel properties and interfacial transition zone changes,and the mechanism was accordingly explored.Finally,the life cycle assessment(LCA)and life cycle cost(LCC)methods were used to evaluate the environmental and economic impacts of AASRC during raw materal and production phases.This study contributes to improve the cracking resistance of AASRC,increase the utilization of solid wastes and promote the application of AASRC.The main conclusions can be drawn as follow:1.With the same RCA content,compared with the WG concretes,the amounts of C(N)-A-S-H gels of WNC and CNC concretes decreased,while their nominal crystal-to-binder ratios increased,significantly reducing the autogenous shrinkage;the decreased tensile stress induced by shrinkage and the reduced Na~+content in the gels declined the contact creep modulus,resulting in the reduction of tensile creep.In addition,the cracking resistance of WNC and CNC groups improved compared to WG concretes since their circumferential tensile stress reduced and the creep/shrinkage ratioφ(t)increased.And CNC group obtained the best cracking resistance.With the increase in RCA contents,the cracking resistance of WNC and CNC groups reduced due to the decrease in splitting tensile strength and creep/shrinkage ratioφ(t).2.With the RCA contents increased,the microhardness of interfacial transition zone decreased,and the hindrance effect of aggregate on gels deformation decreased,increasing the autogenous shrinkage and tensile creep of the AASRCs regardless of any activator types.In addition,the best cracking resistance was found for the WG concrete with 50%RCA content due to its largest creep/shrinkage ratioφ(t),consequently delaying the cracking time.However,with the increase in RCA contents,the cracking resistance of WNC and CNC concretes reduced,which is attributed to the decrease in splitting tensile strength and creep/shrinkage ratioφ(t).3.Compared with the WG concretes,the life-cycle environmental impacts in terms of energy consumption and five impact categories and LCC of WNC and CNC concretes reduced,where CNC concretes achieved the best environmental(reduced by more than 39%)and economic benefits(reduced by more than 24%).With the increase in RCA contents,the energy consumption and environmental impact categories of AASRC slightly increased,while their LCCs decreased since the RCA cost is slightly lower than that of natural aggregate.