Characterization Of Solidified/Stabilized Heavy Metal In Calcium Carbide Slag-ground Granulated Blast Furnace Slag Based Cementitious Materials

In recent years,with the rapid development of economic construction,the stock of various industrial solid wastes has been increasing year by year,occupying a large amount of land space.The heavy metals contained in these wastes can leach out and migrate to the environment through processes such as leaching,dissolution,and weathering,ultimately endangering the ecological environment and human health.Utilizing industrial waste residues to prepare alkaline-activated cementitious materials is not only an important way to recycle solid waste resources,but also has excellent advantages in the solidification/stabilization of heavy metals and has the potential to treat heavy metal pollution.This article uses calcium carbide slag（CS）and ground blast furnace slag（GGBS）as raw materials to prepare cementitious materials that immobilize heavy metals Cr³⁺,Ni²⁺and Pb²⁺,explores the effects of heavy metal species,dosage,and compound forms on the properties of cementitious materials,and analyzes the leaching characteristics and solidification rate of heavy metals Cr³⁺,Ni²⁺and Pb²⁺.The solidification mechanism of the hydration products of CS-GGBS cementitious materials on heavy metals Cr³⁺,Ni²⁺and Pb²⁺is revealed,and a suitable CS-GGBS cementitious material mixture ratio for heavy metal solidification is designed.The main research results are as follows:（1）This research conducted a systematic analysis of unlimited compressed strength（UCS）,microstructure,and hydration product development of CS-GGBS cementitious materials.When CS content exceeded 15 wt%,the 28d UCS remained stable at above15MPa,the fluidity of the fresh mortar decreased continuously with the increase of the CS powder content,and the decrease in fluidity was as high as 20.43%when CS content exceeded 25 wt%.The main hydration products of CS-GGBS cementitious materials are C-S-H,C-A-S-H,semicarbonate,and hydromagnesite.After CS content exceeded 15%,residual Ca（OH）₂ appeared and the residual amount continued to increase,which inhibited the development of UCS of the cementitious materials.Based on this,the idea of utilizing residual Ca（OH）₂ and C-（A）-S-H for synergistic solidification of heavy metals is proposed.（2）Using 20 wt%CS and 80 wt%GGBS to prepare CS-GGBS cementitious material for heavy metal solidification,the effect of water-cement ratio,curing time,as well as heavy metal species,dosage,and compound form on UCS of the solidifiers was studied.The results showed that UCS of the solidifiers increased first and then decreased when Cr³⁺and Ni²⁺were added in the form of nitrate,and the optimal dosage for UCS of the solidifiers was 1 wt%.However,UCS of the solidifiers continuously decreased when Pb²⁺was added in the form of nitrate.When Cr³⁺and Ni²⁺were added in the form of chloride,UCS of the solidifiers decreased,but adding Pb²⁺in the form of chloride can improve UCS of the solidifiers.Adding heavy metals Cr,Ni,and Pb in the form of oxide were all detrimental to the development of UCS of the solidifiers.（3）The toxicity leaching test was adopted to analyze the leaching characteristics and solidification rate of heavy metals Cr³⁺,Ni²⁺and Pb²⁺.The leaching concentration of Cr³⁺,Ni²⁺and Pb²⁺is inversely proportional to UCS of the solidifiers.The 28d leaching concentration of Cr³⁺is below 27.6 ppb,the leaching concentration of Ni²⁺is below 10^-3ppb,and the solidification rate of both Cr³⁺and Ni²⁺is up to 99.99%.The 28d leaching concentration of Pb²⁺is below 3.37 ppb.After the solidification of heavy metal oxides,the leaching concentrations are all less than 1.06 ppb.The leaching of the above heavy metals meets the requirements of"GB5058.3-2007 Identification Standards for Hazardous Waste-Leaching Toxicity Identification".Heavy metals Cr,Ni,and Pb can be effectively solidified.（4）The solidification mechanism of heavy metals Cr³⁺,Ni²⁺and Pb²⁺was dissected using test method such as XRD,SEM-EDS,FTIR and XPS.When the Cr³⁺doping is below1.5wt%,Cr³⁺was mainly immobilized with OH-forming Cr（OH）₄^-,when the Cr³⁺doping was between 1.5wt%and 2.5wt%,Cr³⁺was mainly immobilized with Ca²⁺exchanged into the gel structure of C-（A）-S-H,Ni²⁺was mainly immobilized in the form of Ni（OH）₂precipitation or Ni（OH）₃^-.Pb²⁺is mainly immobilized in the form of Pb Si O₃ formation and exchange with Ca²⁺in C-（A）-S-H into the gel structure.CS-GGBS based cementitious materials mainly immobilize the heavy metals Cr,Ni and Pb by precipitation or complex formation with OH^-in residual Ca（OH）₂ and ionic exchange with Ca²⁺in C-（A）-S-H.