Performance Optimization And Principle Of Steel Slag-based Carbonated Cementitious Material And Artificial Aggregate

Steel slag is the industrial by-product of steel manufacturing,characterized as highly calcareous,ferrous and siliceous.Untreated slag possesses significant amount of free calcium oxide that may induce volumetric instability and its reactivity on exposure to water is found to be low,both of which result in the limited applications of steel slag.However,recent investigations justified the feasibility of steel slag activation by accelerated carbonation.The calcium and magnesium bearing phases in steel slag are able to quickly react with carbon dioxide and form reaction products with binding properties,which provides the clue for producing construction materials.In this thesis,two sections of experiments are conducted.The first section uses ground basic oxygen furnace slag as the raw material to prepare dry-mixed steel slag compacts.Accelerated carbonation is employed as the activation method and the main purpose of this experiment is to identify the main contributions and interaction effects of compaction pressure,water to solid ratio and carbonation duration on the compressive strength and CO₂ uptake of the compacts.Response surface models are used to define the coupling effects and optimal parameters values.Apart from the effect identification,reaction kinetics in terms of CO₂ diffusion coefficient and mineralogy evolution are also discussed using shrinking core model,X-ray diffraction,etc.The second section uses ground basic oxygen furnace slag to prepare the artificial aggregates by pelletization and accelerated carbonation.CO₂ is introduced in different stages of aggregate production and the physical,mechanical and micro-structural properties of the resulting aggregates are investigated.The purpose of this experiment is to find the alternative binder and raw materials for artificial aggregate industry to identify the feasibility of solving the main bottleneck in the development of artificial aggregates in terms of performance.The results show that:（i）The influences of compaction pressure,water to solid ratio and carbonation duration on the properties of dry-mixed compacts can be modelled by reduced cubic equations.The equations indicate the strong interactions of the three factors.At the low level of water to solid ratio（e.g.0.1）,compaction facilitates strength growth but barely affects the CO₂ uptake.While,at 0.18 water to solid ratio,the compaction pressure causes a decrease in the compressive strength.Based on the result of shrinking core model,the difference in CO₂ diffusion rate could be more than 30times.The optimal parameter combinations are 5MPa-0.13 and 15MPa-0.12,the CO₂uptake and compressive strength of which reach 9.56%and about 44MPa respectively after carbonation for 10h.Based on the experimental results,preconditioning for the dry-mixed compacts at 0.10-0.14 is found to be not necessary.Besides,portlandite,brucite and larnite are the most active phases in carbonation;hatrurite and brownmillerite are less active and magnesioferrite and wuestite are inert.（ii）Carbonation curing and synchronized carbonation can activate the reaction of steel slag but they influence the properties of aggregates differently.Carbonation curing can effectively improve the aggregate strength by up to 190%compared with the uncarbonated group and reach the highest value of 5.23MPa.The porosity of the aggregate shell is comparable with the reference group while the microhardness value improves by 40%,reaching 111.9MPa.Carbonation curing does not alter the water absorption and bulk density.Synchronized carbonation is regarded as harmful for aggregate strength development and the aggregate core is identified as the main weakness with the porosity of～48%.But synchronized carbonation is favorable for decreasing the bulk density of the aggregates.Thus,it is considered potential for lightweight aggregate production.