Study On Solidification Efficiency Of Municipal Solid Waste Incineration Fly Ash By Alkali-activated Slag Cement

Municipal solid waste incineration fly ash （MSWI Fly Ash） is defined as ahazardous waste because of the soluble heavy metals in it, which must be treated beforelandfill. Cement solidification has been widely adopted today, but solidificationefficiency of heavy mental ions is low and long-term volume stability of solidificationbody is poor when traditional cement is used for solidification of MSWI fly ash.Compared to traditional cement based materials, alkali-activated slag cement （AASC）is a new kind of low-carbon cement material, which has a special potential forsolidification of heavy mental ions owe to calcium silicate hydrates with a low Ca/Siratio as the main AASC hydration products. Therefore, the investigation ofsolidification efficiency and solidification mechanism of heavy mental ions byalkali-activated slag cement stone has a great significance to promote the application ofAASC used for the solidification of MSWI fly ash.On the basis of the physic-chemical properties of MSWI fly ash, the volumestability of solidification body, the effect of main factors such as the treatment forMSWI fly ash, Na₂O equivalent, modulus values of water glass and the content ofactive mineral admixtures on solidification efficiency of MSWI fly ash by AASC stonewere studied in this paper, as well as the solidification mechanism of heavy mental ionsby AASC stone with the help of XRD, SEM, EDS analysis and so on. The main resultsof the study are as follows:The leaching procedure affects leaching concentration of heavy metal ions inMSWI fly ash. The leaching concentration of heavy metal ions has little differencebetween the Roll over leaching procedure and Sulphuric acid&nitric acid method. Theleaching concentration of heavy metal ions tested by Acetic acid buffer solution methodexceeded Roll over leaching procedure and Sulphuric acid&nitric acid method, exceptPb and Cr⁶⁺/Cr.The main reason for swelling of fresh cement-fly ash paste, a treatment for MSWIfly ash could settle, is the presence of metallic aluminum（Al⁰）in MSWI fly ash, whichproduced hydrogen gas by the oxidation reaction of metallic aluminum. The treatment issoaking for72h with a water-fly ash ratio2/3using alkali of MSWI fly ash, as well as0.1mol·l^-1sodium hydroxide solution soaking for24h. And the treatment can improvethe compressive strength of solidification body, reduce the leaching concentration and surface leaching rate of heavy metal ions from solidification body, improve thesolidification content of fly ash from30%to near70%curing for28d.The long-term volume stability of MSWI fly ash solidification body is good.There is a relation between Na₂O equivalent and modulus values of water glasswith solidification efficiency of heavy mental ions by the alkali-activated slag cementstone. solidifying70%treated fly ash, while the Na₂O equivalent is3～6%, with theimprovement of Na₂O equivalent, the leaching concentration of heavy metal ions fromsolidification body decreased. While the modulus values of water glass is1.0².42, withthe improvement of modulus values, the leaching concentration of heavy metal ionsfrom solidification body firstly decreased, then increased.The replacement of slag with some active mineral admixtures has effects onsolidification efficiency of heavy mental ions by the alkali-activated slag cement stone.When Na₂O equivalent is4%, modulus values is1.5, content of treated MSWI fly ash is70%, in the range of0～9%, while the content of metakaolin increases, leachingconcentration of heavy metal ions increases. While the content of fly ash and silicafume increases, leaching concentration of heavy metal ions firstly decreases, thenincreases. While the content of zeolite powder increases, leaching concentration ofheavy metal ions decreases.Solidification efficiency of heavy metal ions in MSWI fly ash by AASC stone ismuch better than Portland cement system, mainly because of the calcium silicatehydrates with a low Ca/S ratio and a small amount of zeolites form as the hydrationproducts of AASC, which have a strong adsorption and solid solution for heavy metalions in MSWI fly ash.