Preparation And Performance Of Low Carbon Recycled Concrete

Concrete contains a large amount of Portland cement,which is one of the main sources of greenhouse gas emissions and easily leads to global warming and climate change.Most of the construction waste is composed of waste concrete and waste bricks.Usually dumping construction waste in landfills not only requires a lot of space,but also causes serious environmental problems.The rational use of cement mineral admixtures and recycled aggregates in concrete not only reduces carbon emissions and improves some properties of concrete,but also helps to solve the problem of resource utilization of construction waste.In this paper,the waste clay brick powder is taken as the research object,and the cementitious material with high activity brick powder is prepared by combining mechanical excitation and chemical excitation.The cementitious material and waste concrete aggregate are used to prepare low-carbon recycled concrete(LCRC).The working and mechanical properties and carbon reduction effect of LCRC are explored,and the mechanism of brick powder activity enhancement is characterized by XRD and SEM.The excitation test of brick powder shows that the method of mechanical grinding can effectively reduce the fineness of waste clay brick powder and improve its activity index.However,with the prolongation of grinding time,the activity index gradually increases and tends to be stable.Lime has a certain influence on the compressive strength of brick powder mortar.With the increase of lime content,the strength of mortar increases first and then decreases.Appropriate amount of sodium silicate and sodium sulfate can promote the hydration reaction of cement,improve the early strength and cohesion of mortar,and excessive amount will reduce the strength of mortar.Among them,sodium silicate and lime have a significant effect on the compressive strength of mortar.The best ratio of brick powder activator determined by Taguchi method is 5 % lime,12 % sodium silicate and 15 % sodium sulfate.The concrete test shows that the use of recycled concrete aggregate can reduce the fluidity of concrete,and the fluidity decreases obviously with the increase of dosage.The activated brick powder can improve the low early compressive strength of recycled concrete at 10 %,30 % and 50 % coarse aggregate replacement levels.In the recycled concrete with activator,the compressive strength of concrete with 10 % recycled concrete aggregate and 20 % brick powder at 7d and 28 d is better than that without activator.The early and late strength of concrete is significantly improved by the activated brick powder than the unexcited.The carbon dioxide emission analysis of concrete shows that the carbon dioxide emission of recycled concrete is less than that of ordinary concrete.The microscopic analysis shows that the activator can significantly accelerate the hydration rate of the cementitious material and improve its compressive strength,but does not change the type of hydration products.XRD results show that the hydration degree of the specimen with activator is higher and the effect is better.TG-DSC analysis showed that the test block had obvious mass loss in the range of 85 °C-200 °C,400 °C-480 °C,600 °C-710 °C,and the chemically bound water of the test block added with the activator was equivalent to that of pure cement,indicating that the activator promoted hydration;the addition of brick powder can promote the pozzolanic reaction and consume more cement hydration products.The total porosity and total pore area of the sample with activator are lower than those without activator,which makes the structure denser.The results of SEM and EDS show that the hydration products of the sample are mainly C-(A)-S-H gel,which fills the pores.The brick powder with activator improves the microstructure and interface structure of the sample,which is beneficial to improve its performance.