Study On Solidification/Stabilization Of Cement On MSWI Fly Ash And The Leaching Mechanism Of Pb And Cd

The paper presents results of an investigation carried out to analyze the leaching behavior mechanisms of heavy metals and interaction of cement-MSWI fly ash-heavy metals on principle of cement chemistry and solidification/stabilization of cement on heavy metals,and high-efficient technical approach and methodology for solidification on MSWI fly ash has been investigated and applied to optimize the solidification/stabilization of hazardous waste containing heavy metals especially of MSWI fly ash.Investigation on the rules of adding method,cement hydration,particle size of the solidified form,leaching time/medium on cement solidified heavy metal Pb and the mechanism of Pb's transfer behavior has been conducted and a curing-dissolution model was established.The results showed that the C-S-H gel or silicic acid sol formed during the hydration process of original cement powder or unhydrated cement in acid medium can absorb Pb readily.The particle size of the solidified form had double effects on Pb leaching behavior.On the one hand,The hydrated product erosion process and Pb diffusion became rather faster with the decrease of particle size.On the other hand,with the increase of pH,the hydration process of unhydrated cement became faster,enhancing the secondary solidification of Pb.Therefore,we detected the lowest Pb dissolution concentration when the leaching particle size of cement-Pb solidified form was about 0.6¹.18mm,a 60.47%decrease compared with that of 4.75⁹.5mm group.The Pb concentration increased subsequently at the initial time and then decreased gradually as a result of secondary solidification of partial Pb absorbed by newly formed silicic acid sol and C-S-H or precipitated into Pb?OH?₂during the process of solidified form leaching in acetate solution,Experimental research on the influence of cement content,forming process,curing condition on MSWI fly ash solidification was conducted systematically.The results showed that mechanical strength and the pH value presented a tendency of straight climb with the increase of cement content,while the leaching concentration of Pb and Cd showed different decreasing ways and the solidified degree of unit cement decreased significantly and reached to 95%when cement constituted a proportion of 10%,a 50%increase compared with that of 20%group.It showed that as water cement ratio increased,the Cd dissolution concentration decreased significantly,while Pb dissolution concentration decreased initially and reached to a minimum value when water cement ratio increased to 0.35 and then increased.Hydration and strength of solidification were improved and the leaching concentration of Pb and Cd increased while the leaching solution balanced pH decreased with the prolonging of curing period.The samples of cement solidification/stabilization?S/S?to MSWI fly ash should be maintained in sealed or moist condition instead of being submerged in water.This paper investigated cement-MSWI fly ash hydration and solidification in different curing temperatures and solidification effects of quantity and classification of the hydration products and strength and alkalinity of the cement-MSWI fly ash solidified form on heavy metals.Based on"grey relational algorithm"and"effect index analyzing method",we concluded that the alkalinity of the solidified form was the key factor for the stabilization/solidification of heavy metals and the heavy metals'leaching concentration reached to a lowest value with the balanced pH value ranging from 9 to 10.5.However,the hydration products also played a role in the stabilization/solidification of cement on heavy metals and the AFm,C₃S₂H₃,C₅S₆H₅were superior to that of AFt,C₃S₂H₈,C-S-H in stabilization/solidification process.Experimental study of solidification of magnesium oxychloride cement on MSWI fly ash and its influencing factors was carried out for the first time and greatly desired solidification results were achieved.The leaching concentration of Pb was almost 0 when the Magnesium oxychloride cement was in proportion of 20%and the MgO/MgCl₂ molar ratio was 10:1,while 0.433mg/L and 0.292mg/L when the proportion of P·O52.5 was 20%and 40%respectively under the same condition.And the results showed that caustic-burned magnesia powder activity had a remarkable influence on solidification of cement on MSWI fly ash and the activity of MgO as a main component curing agent should be higher than 70%.A new efficient curing agent MC whose main components were cement and magnesium oxide was developed.MC achieved marked effects on various types of MSWI fly ash with the content of 15%and MC-MSWI fly ash solidified form displayed long-term environmental stability based on the results of"the rapid test method"and"the long-term immersion method".