Study On Inhibition Mechanism Of Chemical Treatment On Volume Expansion Of Steel Slag And Its Influence On Asphalt Mixture Performance

Effective resource utilization of solid waste steel slag has good social,environmental and economic benefits.The hydration reaction of free calcium oxide（f-Ca O）in steel slag leads to its volume expansion,which is the main reason that hinders its use as road material.In this paper,a steel slag with a particle size of4.75～16 mm was taken as the research object,and the steel slag was soaked by preparing acid and salt solutions,so as to digest part of f-Ca O in steel slag and effectively prevent the volume expansion of steel slag when it meets water.Firstly,the effective acidic and saline solutions were screened by the free total calcium content in steel slag.And the optimum concentration,time and solid-liquid ratio were determined by using the free total calcium content and the p H value of the treatment solution.After comparative analysis,the main physical properties of steel slag and limestone are different before and after treatment,including density,water absorption,Los Angeles abrasion value,adhesion and crushing value,etc.Oxalic acid solution is selected as the best treatment agent ultimate.In order to better study the influence mechanism of oxalic acid solution on the microstructure and phase composition of steel slag,various micro-analysis techniques such as SEM,XRD,XRF and BET were adopted to explore it.The treated steel slag was used as the aggregate of AC-13c asphalt mixture,and compared with untreated steel slag and limestone asphalt mixture.After a long period of water immersion,the properties of steel slag and its asphalt mixture were verified in detail to ensure its stability.Finally,substances such as ethanol and ammonia water were added to the treated waste liquid,and the treated waste liquid was treated and recovered by ultraviolet catalysis and evaporation crystallization.Through the above research,the following conclusions are obtained:（1）The optimal acidic solution is oxalic acid,and the optimal concentration,treatment time and solid-liquid ratio are 0.4 mol/L,84 h and 1:1,respectively.The optimal saline solution is sodium citrate,and the optimal concentration,treatment time and solid-liquid ratio are 0.2 mol/L,108 h and 1:1,respectively.Both solutions inhibit the volume expansion of steel slag aggregate,and the content of f-Ca O is reduced to 2.33%and 2.87%respectively,and the expansion rate is reduced to 1.69%and 2.67%,respectively.（2）After the treatment with oxalic acid solution,a square block of calcium oxalate（Ca₂C₂O₄）is formed on the surface of steel slag,which effectively blocks most of the pore on the surface of steel slag,reduced the water absorption,porosity,specific surface area,roughness and average pore size of steel slag,and greatly improved the compactness of the surface of steel slag.At the same time,part of calcium silicate is also hydrated,which corrodes the weak particles on the surface of steel slag,and steel slag aggregate has stronger crushing resistance.（3）After oxalic acid treatment,the road performance of steel slag asphalt mixture has also been significantly improved,and its marshall residual stability and freeze-thaw splitting strength ratio are both over 90%,which is far superior to limestone asphalt mixture,while its volume expansibility is also reduced from 1.8%to 0.4%.However,after 10 days of water immersion,its high-temperature stability and low-temperature crack resistance decreased by 37%and 43.2%respectively,which are higher than those of untreated steel slag asphalt mixture,while water immersion has little effect on the crushing resistance of steel slag aggregate.（4）The waste liquid after treating steel slag with oxalic acid is mainly calcium and iron plasma.Specific recovery process are designed:adding ethanol and ammonia water to the waste liquid,and treating the waste liquid by ultraviolet catalysis,evaporation and crystallization,standing and centrifugation,etc.,so as to effectively obtain calcium oxalate,ferrous oxalate,iron hydroxide and ammonium oxalate,and achieve high treatment and recovery efficiency.The oxalic acid solution can effectively inhibit the volume expansion of steel slag,so that steel slag can be used in asphalt mixture instead of limestone.The basic performance of the treated steel slag and its asphalt mixture and its immersion stability are verified.The recovery of the treated waste liquid further confirms the feasibility of treating steel slag with oxalic acid.