Investigation Of Carbonated Artificial Aggregates Produced With Waste Concrete Powder And Its Modification

Waste concrete powder(WCP)is a byproduct with a particle size smaller than 0.15 mm generated from the recycling process of producing recycled concrete aggregate.Turning WCP into artificial aggregates(AAs)by pelletization techniques is a promising method to reutilize a large volume of this solid waste.However,previous studies claimed that the production of AAs usually required 10 wt.%-30 wt.%cement to achieve satisfactory properties.In addition,the crushing strength of AAs was low even cement was used regardless of curing methods.Basic oxygen furnace slag(BOFS)generated from steel-making process is considered as a potential alternative binder due to the presence of CO2-reactive phases(calcium oxide and silicates),and the carbonated products such as carbonates and hydrates can act as binding agent to offer solid compound matrix and dense microstructure.Moreover,polyvinyl alcohol(PVA)is a high-molecular polymer with excellent film-forming properties,which can used to fill the pores and enhance the crushing strength of AAs.On this basis,this project investigated the feasibility of using BOFS as alternative binder for AAs production with WCP and adopting PVA as a modifier to enhance the crushing strength of AAs.The impacts of BOFS dosage,curing methods,treatment methods and different PVA dosage/concentration on the properties of AAs were systematically studied.In addition,the influence of PVA-treated aggregate on the properties of incorporated concrete was also explored.Following results could be drawn based on this project:(1)The feasibility of using BOFS as binding agent to manufacture AAs containing low-value WCP was demonstrated and the properties of the obtained AAs were superior as compared to most aggregates reported in the literature.Every 10 wt.%BOFS increment resulted in crushing strength increase(by 3.2%-42.9%)and porosity decrease(by 7.4%-31.2%).CO2 curing could further improve the crushing strength(by 30%-109%)and decrease the water absorption(12%-22%)due to the formation of rich CaCO3 and C-S-H.In addition,the use of BOFS as binder resulted in 25%global warming potential reduction compared with conventional AAs containing 20 wt.%cement.(2)PVA could be used to enhance the crushing strength of AAs and either mixing or coating treatment could considerably improve the strength by up to 99%and 69%,respectively.Adopting mixing treatment led to the introduction of air voids in the AAs and increased the porosity due to the intrinsic properties of PVA.However,the formed polymer film in the AAs could bridge the aggregate matrix and enhance the bond between particles.The coating treatment resulted in the formation of polymer film on the aggregate surface and enabled the PVA penetrated into the aggregate,which exhibited pore filling and bridging effect and improved the crushing strength of the AAs.Considering the treatment cost and operation difficulty,the mixing treatment with a PVA dosage of 0.5 wt.%was recommended for AAs’ strength improvement.(3)The PVA-mixed aggregate showed good compatibility with cement,which resulted in 25%increase of compressive strength compared with the concrete containing untreated aggregates.By contrast,the PVA-coated aggregate led to a reduction of 67%in terms of compressive strength due to the presence of polymer film on the aggregate surface.However,it should be noted that the PVA-mixed and PVA-coated aggregate could densify the interfacial transition zone.This might be associated with the high water absorption of the PVA-mixed aggregate and the polymer film on the PVA-coated aggregate surface,which resulted in better internal curing and enhanced the hydration of the surrounding cement.