Study On The Carbonation Mechanism Of Refined Slag To Prepare Geopolymer Based On Microstructure

Refining slag is a high-calcium-based waste slag produced during the refining process outside the steelmaking furnace.For a long time,a large amount of refining slag has been stacked in the open air,which not only takes up the land,but also causes serious pollution to the groundwater and atmospheric environment.The mineral carbonation technology can permanently seal CO₂ in natural minerals or solid wastes containing calcium-based substances,which can not only achieve the purpose of mitigating the greenhouse effect,but also achieve efficient resource utilization of refined slag.Geopolymers have the characteristics of low energy consumption,low cost,good mechanical properties,and environmental friendliness.They have been widely used in the fields of building materials,refractory insulation materials,high-strength materials,and heavy metal solidification,especially in the recycling of solid waste,it has high potential for utilization.The main research results of this paper are:taking the refined slag as the research object,exploring the conditions of different temperatures(20?,40?,60?,80?)and different particle sizes(96?m-180?m,48?m-96?m,38?m-48?m,<38?m)changes in the microstructure of the refined slag during carbonation,using X-ray diffraction(XRD),scanning electron microscope(SEM),thermogravimetry(TG-DTG),Fourier infrared(FT-IR),specific surface area tester(BET),²⁹Si solid-state nuclear magnetic resonance and other test methods to analyze the microstructure changes after the hydration carbonation reaction of the refined slag.Single-factor experiments were used to explore the effect of temperature and particle size on the microstructure of carbonation.BET analysis showed that a deeper ion leaching channel was formed after hydration and carbonation of the refining slag.When the temperature is 40? and the particle size is less than 38?m,the active substances C₂S and C₃S in the refining slag are more likely to be leached to the surface of the reaction phase;TG-DTG analysis shows that the C₂S and C₃S on the surface of the refining slag particles undergo hydration reaction in the early stage to form C-S-H gel.Under the action of carbonation,the C-S-H gel decalcifies and condensation phenomenon produces CaCO₃ and calcium modified gel;SEM analysis shows that the reaction product CaCO₃ is filled in the voids,and the calcium modified gel adheres to the surface of the reaction phase,preventing further contact between the refining slag and CO₂;FT-IR analysis shows that the temperature at 20?,more C-S-H gel participates in the carbonation reaction in the later stage,reducing the particle size is favorable for C-S-H gel to participate in the reaction;²⁹Si NMR analysis shows that the degree of decalcification of C-S-H gel gradually decreases with the increases of temperature and particle size.When the temperature is 20? and the particle size is less than 38?m,the degree of decalcification reaches 13.Using carbonated refining slag as raw materials,sodium hydroxide and NaOH as alkali activators,the carbonated geopolymer of refined slag was prepared,and the influence of NaOH,water glass,water-cement ratio,curing temperature and other factors on the mechanical properties of geopolymers was explored.Through single-factor experiments to optimize the process parameters,it is concluded that the process parameters are NaOH 45g,water glass 120 ml,water-cement ratio 0.2,and curing temperature 60?,showing the best mechanical properties.On the basis of the optimal process parameters,four types of carbonated refined slags with different particle sizes(<38?m,38?m-48?m,48?m-96?m,96?m-180?m)are selected,combined with XRD,SEM,TG-DTG,FT-IR,²⁹Si NMR and other characterization methods to explore the mechanical properties and microscopic properties of geopolymers.It is found that geopolymers are mostly amorphous SiO₂ and C-S-H gels,which increase the gelling properties of geopolymers.When the particle size is 48?m-96?m,more calcium silicate phases participates in the reaction of geopolymers.As the particle size increases,the content of Ca(OH)₂ gradually increases,that is,more Ca(OH)₂ is generated after the geopolymers reaction;²⁹Si NMR characterization analysis found that under the action of the alkali stimulator NaOH and water glass,the active material in the refining slag has almost reach a fully hydrated state.When the particle size increases to48?m-96?m,the gelation and compactness of the sample reaches the best effect,and the mechanical properties of the sample are gradually reduced when the particle size is increased.This paper explored the changes in the microstructure mechanism of the refined slag at different temperatures and particle sizes,and determined the optimal reaction temperature and particle size.Through XRD,TG-DTG,FT-IR,SEM,and ²⁹Si NMR analysis,it is concluded that the refining slag undergoes hydration reaction in the early stage of the reaction to form Ca(OH)₂ and C-S-H gel,and then under the action of carbonation reaction,decalcification and condensation occurred to form calcium carbonate and calcium modified gel,since the active substances in the carbonated refining slag are excited,the gelation of the refining slag is improved,and the mechanical properties of the geopolymer are effectively enhanced.