Carbonation Activation Of Recycled Concrete Fines And Their Effect On The Hydration And Hardening Properties Of Portland Cement

The process of preparing recycled concrete aggregates from waste concrete generates large amounts of recycled concrete fines(RCFs)containing fully or partially hydrated cement paste,fine aggregates and fillers.Accelerate carbonation is a promising method that is technically,environmentally and economically feasible to convert RCFs into active supplemental materials.Due to the complexity of RCFs components and the different carbonation methods and parameters,the calcium carbonate crystals and quantities,the components and surface properties of amorphous silica-alumina gel of complete carbonated recycled concrete fines(C-RCFs)vary widely.The influence of the diversity reactivity of C-RCFs on the hydration and hardening properties of cement-based materials remains unclear.In this paper,a three-component composite cement paste of fly ash-slag-Portland cement prepared by the triangular mass center design method,was crushed and ground finely as a simulated RCFs after high-temperature curing,and was activated by incubator curing,autoclave carbonation and wet carbonation.The changes in the physical phase composition and pore structure of the carbonation products of C-RCFs were compared,and their effect on the hydration of silicate cement was also investigated.The influence of C-RCFs on the hydration and hardening properties of Portland cement was investigated.The main results obtained in this paper are as follows.(1)The content of carbonation products of C-RCFs mainly depends on the material reactivity,porosity and carbonation approaches.The fly ash and slag reduced the CaO content in RCFs,but the fly ash increased the porosity of RCFs.Compared with autoclave carbonation or incubator curing,wet carbonation could form more better crystalline calcium carbonate in a shorter time.The formation of silica-alumina gel significantly increased the BET surface area of C-RCFs,especially for RCFs prepared with 25%fly ash,and the BET surface area was as high as 72.25 m~2/g,64.21 m~2/g,and 42.50 m~2/g after incubator curing,wet carbonation,and autoclave carbonation,respectively.All the carbonation methods significantly reduced the average pore size of RCFs.Autoclave carbonation and wet carbonation increased the cumulative porosity of C-RCFs,while incubator curing decreased their cumulative porosity.(2)The mechanisms of the effects of incubator curing(C-RCF1),autoclave carbonation(C-RCF2)and wet carbonation(C-RCF3)on the properties of RCFs and the hydration-hardening properties of composite cements were revealed.The hydration rate and exotherm of composite cement doped with C-RCF1 were significantly higher than those of C-RCF2 and C-RCF3 due to the dilution effect and nucleation effect.at the later stage(after 7 d),C-RCF2 was more effective than other carbonated recycled powders in improving the compressive strength of the composite cement paste,which might be due to the higher Q~2content in C-RCF2 than C-RCF1and C-RCF3.Both C-RCFs improved the compressive and flexural strengths of the recycled mortars compared with the untreated-RCFs,and also reduced their water absorption and chloride ion permeability coefficients.The most significant reduction in chloride ion permeability coefficients of C-RCF3 for recycled mortars of different ages may be due to the higher content of silica-alumina gels(Q~3and Q~4)in C-RCF3.(3)The higher specific surface area C-RCFs substantially reduced the fluidity of the composite cement,which can be attributed to the increased adsorption of free water in the fresh cement slurry by amorphous silica gel and calcium carbonate.Wet-RCFs significantly increased the dissolution peak and almost all C-RCFs increased the second exothermic peak of the cement.The Auto-RCFs and Incub-RCFs improved the strength of composite cement better than Wet-RCFs.The Wet-RCFs and Incub-RCFs were significantly more efficient than Auto-RCFs in improving the compressive and flexural strength of the recycled mortar.The modification efficiency of Wet-RCFs on water absorption and chloride ion permeability coefficient of recycled mortar was significantly higher than that of Incub-RCFs and Auto-RCFs.The research results of this paper are expected to promote the recycling of RCFs in engineering.Calcium carbonate rich materials can be used as active fillers or supplementary cementing materials in the field of construction materials.The accelerate carbonation of RCFs has great environmental potential for the recycling of construction waste and the capture of greenhouse gases.There are 74 figures,31 tables and 191 references in this paper.