Solidication Of Cr?Cd?Pb In Portland Cement/Sulphoaluminate Cement

Technology of cement rotary kiln co-processing living garbage and solid waste has been widely used.The flow of heavy metals of municipal solid waste has been payed attention in application of cement.Research about the solidification of heavy metal ions Cd²⁺,Cr³⁺and Pb²⁺in different cement pastes and the leaching behavior of heavy metal ions in C-S-H is meaningful for the solid waste in cement industry.Revealed solidification mechanism and leaching behavior of heavy metal ions Cd²⁺,Cr³⁺and Pb²⁺in Portland cement,sulphoaluminate cement,and Portland-sulphoaluminate blended cement by XRD,calorimetry,ICP,AAS,SEM-EDS and other testing methods.Cr³⁺and Cd²⁺decreased the initial and final setting time of three kinds of cement;Pb²⁺increased the initial and final setting time of Portland cement and blended cement,but decreased the initial and final setting time of sulphoaluminate cement.Lead-containing compound formed a dense impervious coat around cement particles when they deposited on their surface,thus hinder the reaction between cement and water.Adding Cr³⁺in sulphoaluminate cement,there was a competition between chromium-oxygen octahedron,NO^3-and SO₄^2-,aluminium-oxygen octahedron,which led to the formation of chromium-substituted ettringite,nitrate-substituted ettringite and ordinary ettringite.The addition of Pb²⁺and Cd²⁺improved the hydration of C₄A₃S and results in the increment of the amount of formed AFt.C-S-H gel with heavy metals was leached by acidic.With the increase of Cr³⁺decalcification became strong,and proton first decalcified Q¹ into Q²,resulting in more amorphous C-S-H gel and a three-dimensional network like silica gel.The Cr?OH?₃·3H₂O generated by Cr³⁺in the C-S-H gel was eroded,while the Cr³⁺solidfied in the C-S-H gel chain was less affected.The dissolution of Cd²⁺weakened the interaction between atoms in the calcium oxide layers and increased the interatomic force between C-S-H layers,thus reducing the interlayer spacing.Cd²⁺had a limited effect on the Si-O bond,weakening the influence of hydrogen proton on C-S-H gel and resulting stable stuacture.The CaCd?OH?₄ generated by Cd²⁺in the C-S-H gel was eroded,while the Cd²⁺solidfied in the C-S-H gel chain was less affected.The Pb₁₃O₈?OH?₆?NO₃?₄ generated by Pb²⁺in the C-S-H gel was eroded,while the Pb²⁺solidfied in the C-S-H gel chain was less affected.Pb²⁺replaced Ca²⁺in low content and produced bond energy with the silicon oxygen tetrahedron resulting stable C-S-H gel.