| In many countries and regions, increase of waste concrete has caused serioussocial, economic and ecological problems, and preparation of recycled concrete is themost effective way to solve this problem. However, in the process of crushing,accumulated damage generated in aggregate internal resulted in large numbers ofmicro cracks, which lead to the low strength, high porosity and water absorption ofrecycled aggregates. Moreover, as compared with natural concrete, the complicateddouble interfacial transition zone (ITZ) structure in recycled concrete limit itsapplication in lower filed, such as, bedding embankment and foundation of backfillingetc. In this research, improvement of recycled aggregate surface structure anddevelopment of new mixing method are key techniques to enhance performance ofrecycled concrete and enlarge its application fields.Sulphoaluminate cement was innovatively used to modify recycled aggregatesand effects of slurry types and slurry thickness on recycled aggregate and concretewere investigated on the basis of establishment of wrapped slurry theoretical system;Influence of recycled fine aggregate replacement rate on mechanical properties ofmortar was studied. Furthermore, permeability resistance and sulfate attack resistanceof cement mortar were discussed; Besides that, mechanical property, frost resistanceand microstructure of TM-modified recycled concrete were inspected; In addition,comprehensive economic efficiency of recycling waste concrete was analyzed toprovide a theoretical basis for the development and utilization of recycled concrete.In this research, effects of aggregate modification, surface-coating with mineraladmixtures and mixing method on mechanical property, frost resistance andmicrostructure of recycled cement concrete were investigated. The test results showthat physical performance of pre-coated recycled aggregate was within the range ofspecification requirement (GB/T14685-2011). Surface-coated recycled aggregatewith paste thickness of0.035mm resulted in a higher physical performance,indicating that too much or less coating paste reduced aggregate crushing strength.Compressive strength of concrete increased with an increase on water to cement ratio of coating paste and56days compressive strength of RAC reached to70MPa withrespect to the coating paste of sulphoaluminate cement containing fly ash. In addition,compressive strength of mortar cured for28days increased by34.8%when theaggregate was treated by Sulphoaluminate cement and fly ash, as well as32.4%treated by sodium silicate impregnation. The sulfate attack resistance test results showthat strength loss of mortar significantly reduced after recycled fine aggregatemodified; A new triple mixing method (TM) was developed to realize surface-coatingof aggregate with pozzalanics materials for further improving microstructure ofinterfacial transition zone (ITZ) of recycled aggregate concrete. It was found that28dcompressive strength made with TM showed19%higher than that of concreteprepared with DM and strength decreased when GBFS was replaced by doublemineral additives (SF and GBFS); DTA results show that, endothermic peak area ofTM-modified recycled concrete was smaller, indicating that new triple mixing method(TM) and surface-coating with pozzalanics materials could decrease Ca(OH)2contentwhile increase C-S-H gel phase significantly; Moreover,28d compressive strength ofTM-modified recycled concrete showed25%higher than that of concrete preparedwith IM; Microstructure result showed that surface-coated with pozzalanic particlescombined TM could further enhanced ITZ of RAC as well as in situ strengthen theRA; For quantitative analysis, the proportion of Si and Ca in energy spectrum wasreduced, the accumulation of CH was reduced while more C-S-H gel and ettringitegenerated, thus increase the strength and interface cohesive force of recycled concrete. |
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