Preparation Of Steel-Slag-Based Iron-containing Sulfoaluminate Cementitious Materials And Migration Of Heavy Metals

In recent years,China’s economy has developed rapidly,and the iron and steel industry is an important pillar to support China’s national economy.Steel slag is produced in the process of steelmaking,and its output is 15%～20%of that of crude steel.As a big country in steel industry,China will produce a large amount of steel slag every year.The storage of steel slag not only occupies a lot of land,but also pollutes the environment.The chemical composition of steel slag is similar to that of cement,which has potential cementitious activity and can be used as cement admixture or concrete admixture.However,the problems of poor volume stability,low added value and high content of heavy metals restrict the development of steel slag in this field from three aspects of technology,economy and environment.In order to break the shackles of steel slag properties and realize the large-scale resource utilization of steel slag,this paper proposes the research idea of "using steel slag to cooperate with other industrial solid wastes to prepare steel slag based iron-containing sulfoaluminate cementitious materials" by referring to the experience of industrial solid waste collaborative complementary preparation of sulphoaluminate cementitious materials.At the same time,in view of the high content of iron（Fe）,manganese（Mn）and chromium（Cr）in steel slag,the existing form and influence of iron in steel slag based iron-containing sulfoaluminate cementitious materials were studied,and the migration characteristics of heavy metals in the whole process of preparing steel slag based iron-containing sulfoaluminate cementitious materials were discussed.The main work of this paper is as follows:（1）The way to prepare high performance and high value-added steel slag based iron-containing sulfoaluminate cementitious materials was explored by using steel slag,desulfurized gypsum,aluminum ash and limestone tailings as raw materials.The influence of calcination temperature on steel slag based iron-containing sulfoaluminate cementitious materials was discussed.The mineral phase composition,compressive strength and hydration reaction of steel slag based iron-containing sulfoaluminate cementitious materials with different steel slag content were studied.Under the conditions of Cm=0.85,calcination temperature of 1220℃ and holding time of 30 min,the compressive strength of steel slag based iron-containing sulfoaluminate cementitious material prepared by 25 wt.%steel slag,22.39 wt.%aluminum ash,30.20 wt.%desulfurized gypsum and 22.38 wt.%limestone tailings can reach 81.4 MPa,86.6 MPa and 120.3 MPa after curing for 1,3 and 28 days.Compared with sulphoaluminate cementitious materials on the market,this material has better strength properties at the same hydration age.At the initial stage of hydration,a large amount of ettringite and aluminum glue are formed in the steel slag based iron-containing sulfoaluminate cementitious materials.After 28 days of hydration,the microstructure of the cementitious materials is more compact,so the steel slag based iron-containing sulfoaluminate cementitious materials have higher compressive strength.（2）The influence of the occurrence of iron on the mineral properties of steel slag based iron-containing sulfoaluminate cementitious materials was studied.Taking Fe₂O₃ and other chemical reagents instead of steel slag and cooperating with other industrial solid wastes to prepare iron-containing sulfoaluminate cementitious materials as control experiment,the influence of different existing forms of iron on iron-containing sulfoaluminate cementitious materials under the same low Cm was explored.Most of the iron in the steel slag based iron-containing sulfoaluminate cementitious materials still exists in the form of iron aluminum oxide,and a small amount of iron is dissolved in calcium sulphoaluminate;The increase of iron content in steel slag based iron-containing sulfoaluminate cementitious materials is beneficial to reduce the formation temperature of cementitious material minerals,and the mineral phase has been well formed at 1200℃;The compressive strength also increases with the increase of iron content.Further analysis of hydration characteristics shows that the increase of iron content in steel slag based iron-containing sulfoaluminate cementitious materials accelerates the hydration process,resulting in the formation of a large number of hydration products at the early stage of hydration,which improves the early strength.Under the same low Cm,the chemically pure Fe₂O₃ mainly form brownmillerite in the prepared iron-containing sulfoaluminate cementitious materials and there is residual Fe₂O₃,some iron is solid dissolved with gehlenite;The increase of Fe₂O₃ content indirectly promotes the formation of gehlenite,inhibits the formation of calcium sulphoaluminate,and delays the hydration process.（3）The migration characteristics of heavy metals in steel slag based iron-containing sulfoaluminate cementitious materials were studied,and the migration mechanism of heavy metals was analyzed.The research contents include the leaching concentrations of heavy metals in raw materials,raw mixture,clinker and hydrated cementitious materials,the distribution of existing forms of Mn and Cr,and the solidification mechanism of heavy metals in cementitious materials.It was found that the leaching concentrations of heavy metals in raw materials,raw mixture,clinker and cementitious materials with different hydration ages were lower than the TCLP limits;After calcination and hydration,the residual fraction of Mn and Cr in the steel slag based iron-containing sulfoaluminate cementitious material is maintained at more than 90%,and the effective solidification of Mn and Cr is realized;In steel slag,Mn and Cr coexist with Fe and are mainly solidified in RO phase;After calcination,Mn and Cr are solidified in FeAlO₄ in clinker.After hydration,FeAl₂O₄ in cementitious materials does not hydration,Mn and Cr are still solidified in it,thus reducing the potential risk of heavy metal migration to the environment.