Binding And Leaching Of Heavy Metal Elements In Cementitious Materials: A Case Study On Metallurgical Slags

Reusing metallurgical slags as cementitious materials can efficiently utilize them.However,before applying the prepared materials to engineering,it is necessary to scientifically evaluate the leaching risks of heavy metal elements（HMEs）.The key for scientific evaluation is the understanding of the binding and leaching mechanisms.Based on them,the stabilization effect of hardened pastes on HMEs can be quantified,and the leaching behaviors can be predicted.The obtained quantitative result provides a basis for the scientific evaluation of HMEs’leaching risks.In this study,the existence forms of HMEs in metallurgical slags,as well as their leaching behaviors were systematically characterized.Then,according to the characteristics of the metallurgical slags,slag-containing hardened pastes,including cement-based composite materials and alkali-activated materials,were prepared.Their reaction processes and mechanical properties were subsequently investigated.Afterwards,the binding of HMEs in hardened pastes were thoroughly investigated.Based on the binding mechanism,a thermodynamic modelling was performed to quantify the stabilization capacity of the hardened pastes on HMEs.Finally,the leaching risks of HMEs were scientifically evaluated,when the slag-containing hardened pastes serve in different environments.The main conclusions are listed as follows.（1）The HMEs in metallurgical slags mainly exist as solid solution,Si-Al glassy phase or soluble salts.When the slags suffer from the acidic environment,the HMEs can leach from the slags and pose great environmental risks.（2）Copper slag（CS）,lead-zinc slag（LZS）,blast-furnace ferronickel slag（BNS）and electric-furnace ferronickel slag（EFS）exhibit latent hydraulic reactivity and pozzolanic reactivity.They can be used as supplementary cementitious materials.Among them,BNS can also serve as the precursor for the alkali-activated materials.Electrolytic manganese residue（EMR）was an inert solid waste with high gypsum contents.It can be used as a sulfate activator to prepare alkali-activated hardened pastes(EG^CH).（3）In cement-based and alkali-activated hardened pastes,Cu²⁺,Mn²⁺and Ni²⁺can be totally stabilized by precipitating as hydroxide;As O₄^3-is totally stabilized by precipitating as Ca₃（As O₄）₂;Most of Zn²⁺,Pb²⁺and Cr³⁺can be stabilized by precipitating or being absorbed by hydration products,but a small fraction of them remains soluble;Cr O₄^2-is hard to be stabilized.（4）Regarding cement-based hardened pastes prepared with30%a-type（storage in a standard way）CS,LZS and EFS,they can be safely used in the damp environment and the water-corrosion environment,but may pose great environmental risks in the acid-corrosion environment.Regarding cement-based hardened pastes prepared with 30%a-type BNS,alkali-activated BNS（a-type）and EG^CH（a-type）,they can be safely used in the damp environment,but may pose environmental risks in the water-corrosion and acid-corrosion environments.Regarding the hardened pastes(other than EG^CH)prepared with b-type（storage in a nonstandard way）slags,they may pose great environmental risks even in the damp environment.They are not recommended to be used in engineering.