Key Technologies On Reuse And Reactivation In Building Materials Of All Components Of Waste Concrete

The need of large-scale infrastructures causes huge consumptions of aggregates and cement.Meanwhile,demolitions and reconstructions of old concrete buildings and engineering structures,together with further development of infrastructure constructions,result in increasing amounts of waste concrete accumulated.To mitigate the issue that cement and concrete is facing serious deficiency in raw material resources and solve the environmental,social problems caused by waste concrete,it is necessary to resourcefully reutilize waste concrete.In this paper,experimental investigations are adopted to implement resourceful reutilization of waste concrete in accord with construction waste treatment principles of reduce,reuse and recycle?3R?.Components separation of waste concrete is firstly conducted.Both coarse and fine aggregates are effectively separated from hardened cement paste.Then the special“double interfacial transition zone”in the interface transition zone?ITZ?of recycled coarse aggregate concrete is emphatically analyzed and the influence of internal curing of recycled fine aggregate on shrinkage resistance capability of concrete is emphatically studied.At last,cementitious property reactivation of hardened cement paste?HCP?is studied,then a new reactivated cementitious material?RCM?is prepared.The hydration and hardening properties of RCM are further studied by experiments of workability,mechanical strength,rheology to reveal the mechanism.In this way,reuse and reactivation in building materials of all components of waste concrete can be implemented.The exploratory and innovative work of this paper lies in the following aspects:?1?Separations of adhesive mortar from recycled coarse aggregate and researches on double ITZ in the interface transition zone of recycled coarse aggregate concreteA peeling device is designed to efficiently separate recycled coarse aggregate from adhesive mortar in waste concrete.Then recycled coarse aggregate with higher apparent density and lower water absorptivity and porosity is prepared,and its properties are close to natural aggregate.Moreover,the double ITZ in the interface transition zone of recycled coarse aggregate concrete is studied from microscopic view.The double ITZ contains the interface of fresh paste and old hardened cement paste and the interface of old hardened cement paste and recycled aggregates.The mechanism of double ITZ on mechanical strength of recycled aggregate concrete is studied by comparing with interface transition zones of natural aggregate concrete.The study shows that interface of old hardened cement past and recycled aggregate is produced due to long-term mechanical interlink of old hardened cement past and recycled aggregate.Therefore,tight chemical bonds are formed between old hardened cement past and recycled aggregate.In the interface of fresh paste and old hardened cement past,old hardened cement paste has already involved in cement hydration process.Therefore,surface of old hardened cement paste is with relatively high activity and chemical reaction exists in the interface of old hardened cement paste and fresh cement paste.As a result,the interface of fresh cement paste and old hardened cement paste is formed due to chemical reaction of fresh cement paste and hydration products of old hardened cement paste.Moreover,the interface is with tighter chemical bonds rather than mechanical interlink.Therefore,the interface transition zone of recycled aggregate concrete is enhanced with the formation of double ITZ formed due to chemical reaction.?2?Influence of internal curing of recycled fine aggregate on shrinkage resistance of concreteThe recycled fine aggregate is used as concrete internal curing agent due to its characteristics of high porosity and absorptivity.Thus,the amounts of recycled fine aggregate to affect concrete mechanical strength and shrinkage resistance is studied.Meanwhile,characteristics and advantages of internal curing of recycled fine aggregate is analyzed by mechanical strength and shrinkage resistance of concrete,compared with commonly used internal curing agent of superabsorbent polymer?SAP?.The research results reveal that early mechanical strength of concrete obviously increases with increasing amount of recycled fine aggregate,which differs from that of concrete with internal curing agent of high water-absorbing resin that has negative influence on concrete early strength.However,excessive amounts of recycled fine aggregate are disadvantageous for later strength of concrete.Internal curing area of recycled fine aggregate gradually expands with increasing amount of recycled fine aggregate,which can greatly lower concrete autogenous shrinkage.Compared with SAP,recycled fine aggregate is characterized by larger range of particle size and better dispersibility.Therefore,recycled fine aggregate is less affected by ion concentrations of cement paste,and internal curing performances of recycled fine aggregate is better than that of SAP.?3?Component design of dehydrated cement paste?DCP?composite reactivated cementitious materials?DCRCM?and researches on hydration mechanismComponent design of DCRCM is implemented due to deficiency in properties of DCP cementitious system.Moreover,mechanical strength,hydration process and micro morphology of DCRCM are studied,together with the hydration and hardening mechanism of DCRCM.The component design is demonstrated to be reasonable according to changes of macroscopic properties of DCRCM.There are some problems when DCP is separately grinded.For example,micro agglomerations among DCP particles will occur,and DCP particles are likely to stick to the balls and mill.The research results indicate that the above problems can be solved by using GGBFS due to its micro-ball mill effect.Meanwhile,effective dense network structure is formed in DCRCM cementitious system due to the micro-aggregate effect of GGBFS.And the dense network structure is the skeleton of the whole cementitious system,then DCRCM with good mechanical strength is prepared.Therefore,the problem that utilization requirement of cementitious material with good mechanical strength cannot be met because no effective skeleton is formed in cementitious system due to high activity and loose structure of DCP can be solved.?4?Regulation mechanism of retarders on early hydration of dcrcmInfluence of different retarders,including gypsum,sodium citrate,sodium gluconate and borax,on setting time and compressive strength of DCRCM is studied to find the best retarder with good retarding effect and mechanical strength.Hydration and hardening process of DCRCM is studied with different amounts of retarder with better comprehensive performances.Therefore,the mechanism of the retarder to affect early hydration process and initial structure formation is revealed.The research results show that different retarder is with different retarding performance.Of all the retarders,sodium gluconate is of the best retarding performance,but early strength of DCRCM is quite low.Besides sodium gluconate,borax is with significant retarding performance.It can effectively lower the initial hydration rate of DCRCM and thus greatly increase setting time and improve workability.Moreover,borax can accelerate early hardening process of DCRCM,promoting establishment and development of early strength,also helping to increase later strength.Borax hydrolyzes in water,producing borate.Borate can react with hydration products of DCRCM to form complex film layer and wrapped layer of borate,which can effectively prevent water from entering inside of cementitious material particles.Moreover,crystallization of Ca?OH?₂ is also effectively restrained because initial ion concentration of the paste is changed and calcium borate is produced.Therefore,the setting process of DCRCM is retarded.?5?Effect of Borax on Rheological Properties of DCRCMRheology test of DCRCM is conducted to study influence of borax on yield stress and plastic viscosity of DCRCM paste.Ways to improve rheology of DCRCM is also studied to optimize the mix proportion design of mortar and concrete based on rheology.Research results reveal that single-mixing of water reducer or borax will not help to improve rheology of fresh DCRCM paste,while double mixing can help to significantly improve rheology of fresh DCRCM paste.With increasing amount of borax,particles of DCRCM uniformly disperse in the paste,and forces among particles are thus greatly reduced.As a result,yield stress of fresh DCRCM paste is greatly lowered,and rheology of the paste is thus greatly improved.Moreover,increasing amount of borax causes lower yield stress of DCRCM paste with same shear rate.The complex film layer and wrapped layer reduce flocculent structures in cementitious system.Therefore,particles can uniformly disperse in DCRCM paste,and force among particles thus significantly decreases,leading to lower yield stress and plastic viscosity,which is the basic reason for borax to improve rheology of DCRCM paste.