Modification Of Strontium Carbonatewasteresidues Cementitious Material And Solidification Of Incineration Fly Ash

This paper aims to improve the performance of strontium carbonate waste residues as cementing material by the alkali-activated method,then to investigate feasibility and effectiveness of alkali-activated strontium carbonate waste residues on immobilizing municipal solid waste incineration(MSWI)fly ash with Portland cement.Strontium carbonate waste residues are low-grade,with less possibility of being reused.However,there are certain amount of potential active elements in residues,such as silicon and aluminum.According to relevant references,alkaline activators could alter the activity of strontium carbonate waste residues to be raw cementing material.The parameters of tested MSWI fly ash samples include water content,elemental composition,heavy metal content,leaching toxicity and heavy mental speciation.The determinations of strontium carbonate waste residues consist of chemical composition,elemental composition and leaching toxicity.In this paper,such alkaline materials as sodium hydroxide,water glass and potassium carbonate are selected as activators.The compressive strength of activated strontium carbonate waste residues was determined with different diameters of residues and mixing ratios of activators.Also,the dry shrinkage rate was calculated to obtain the optimum alkaline activator and modification parameters.On this basis,multifactor experiment was conducted by combining sodium hydroxide,water glass and potassium carbonate,thereby optimizing the experiment.Finally,the cementing material of alkali-activated strontium carbonate waste residues was introduced to stabilize MSWI fly ash by adding Portland cement.The compressive strength of solidified blends at different curing time was determined.The leaching toxicity of heavy metals in blends was also measured.The speciation of heavy metals in both blends and MSWI fly ash were compared through sequential chemical extraction to assess the solidification effect of alkali-activated cementing material.The speciation,phase composition and organization structure of alkali-activated cementing material were analyzed by way of XRD and SEM techniques.Meanwhile,this paper discussed both the reaction mechanism of alkaline activators and strontium carbonate waste residues and the curing mechanism of alkaline cementing material.The results are as follows.1)the optimum activator is potassium carbonate,followed by water glass and sodium hydroxide.The optimum modification parameters are 100-mesh strontium carbonate waste residues,potassium carbonate as activator and 15% ratio of activator to residues.2)The maximum compressive strength,after 28 d curing,is 2.44 MPa,with 20% : 10% : 70% ratio of cement,residues and fly ash.The compressive strength increases with increasing curing time.3)After 14 d curing,leaching concentrations of heavy metals in 3#(20% cement,30% residues and 50% fly ash)and 4#(20% cement,40% residues and 40% fly ash)are lower than limit values based on the Chinese Standard for Pollution Control on the Landfill Site of Municipal Solid Waste(GB 16889–2008),thereby betraying that the solidification of modified cementing material is favorable.4)The main reaction principle of strontium carbonate waste residues and alkaline activator is that such potential active elements in residues as Si-O,Al-O,etc.are hydrated under alkaline conditions(with the presence of OH-).The solidification is realized under the influence of water media.5)The hydration products of alkaline cementing material contain C-S-H gel and similar matter of ettringite,of which C-S-H gel is the main part.The curing mechanisms of alkaline cementing material blends include physical encapsulation,physical absorption,ion exchange,chemical absorption precipitation,and so forth.