Study On Preparation And Performance Of MSWI Fly Ash Polymerized Solidified Body

Municipal Solid Waste Incineration fly ash（MSWIFA）is included in the national hazardous waste list due to the leaching toxicity of heavy metals,and it needs to undergo harmless treatment before it can be landfilled.At present,the commonly used solidified methods are cement solidified method and chelating agent method.The cement solidified method has the disadvantages of poor long-term durability and low solidified efficiency of heavy metals;the chelating agent method has the problems that the effect of solidified heavy metals cannot be maintained for a long time and is expensive.In contrast,geopolymers have the advantages of environmental protection,energy saving,pollution-free,and good corrosion resistance.Therefore,explore the application of geopolymers in solidified MSWI fly ash,and study the mechanism and influence of geopolymer solidified MSWI fly ash is of great significance.Based on the experiment of the research group,this thesis focuses on the following four different systems under the condition that the fly ash content is 50%and the raw material ratio remains unchanged:MSWI fly ash-metakaolin-slag（MMS）,MSWI fly ash-metakaolin-slag-silica fume（MMSS）,MSWI fly ash-metakaolin-slag-fly ash（MMSF）and MSWI fly ash-slag-fly ash（MSF）.The effects of alkali concentration,water-binder ratio and curing conditions on the compressive strength of each system were studied separately.The experimental study of n（CaO）/n（SiO₂）showed:With the increase of n（CaO）/n（SiO₂）,the compressive strength of the solidified bodies of different systems showed a trend of first increasing and then decreasing.The strength of the MMS solidified body was the highest when n（CaO）/n（SiO₂）was 1.00,and its 56d compressive strength was 16.7MPa;The strength of MMSS solidified body was the highest when n（CaO）/n（SiO₂）was 0.95,and its 56d compressive strength was 18.0MPa;MMSF solidified body had the highest strength when n（CaO）/n（SiO₂）was 1.00,and its 56d compressive strength can reach 20.8MPa;MSF solidified body had the highest strength when n（CaO）/n（SiO2）was 1.00,and its 56d compressive strength can reach 19.7MPa.It showed that under the condition of different types of raw materials,the chemical composition of the solidified body of different systems still plays a major role in the compressive strength of the system.The experimental study of the alkali concentration showed that:in the MMS solidified body,when the alkali concentration was 11.6%,the compressive strength growth rate of curing 7d to 28d was 114%,and when the alkali concentration was 16.0%,the compressive strength growth rate of curing 7d to 28d was 37%.,The compressive strength of the cured MMS solidified body is 16.8MPa when the alkali concentration is 16.0%;in the MMSS solidified body,when the alkali concentration was 11.6%,the compressive strength growth rate of curing 7d to 28d is 156%,and when the alkali concentration is 16.0%,the curing rate was 7d.The compressive strength growth rate of curing at 28d is 44%.When the alkali concentration of MMSS solidifying body was 16.0%,the compressive strength of 28d was16MPa;in MMSF solidified body,when the alkali concentration is 11.6%,the growth rate of compressive strength from curing 7d to 28d was 30%,when the alkali concentration was16.0%,the curing 7d and 28d compressive strength growth rate was 23%,the 28d compressive strength of the MMSF solidified body was 15.8MPa when the alkali concentration was 16.0%;in the MSF solidified body,the alkali concentration was 11.6%and curing for 7d to The 28d compressive strength growth rate was 49%.When the alkali concentration was 16.0%,the compressive strength growth rate of curing 7d to 28d was 4%,and the 28d compressive strength of the MSF solidified body was 16.4MPa when the alkali concentration was 16.0%.It shows that the amount of alkali has a great influence on the early strength of MMS and MMSS solidified bodies,but has little effect on the later strength.The amount of alkali has little effect on the early and late strength of the MMSF solidified body,and it had a greater impact on the early strength of the MSF solidified body,and the late strength was little affected by the amount of alkali.The experimental study of water-binder ratio shows that the best water-binder ratio of MMS system,MMSF system and MSF system is 0.54,and the best water-binder ratio of MMSS system is 0.52.The experimental study of curing conditions shows that the strength of the test block under the standard curing condition is greater than the test block under the natural curing condition of the same age,and the MSS solidified body is most affected.The intensity of the standard curing 28d and natural curing 28d is 8.9MPa and 14.8 MPa.It has the least impact on the MMSF solidified body,and its standard curing 28d and natural curing 28d strength are18.6MPa and 20.2MPa.By changing the ratio of different curing systems to establish the relationship between the mechanical properties of different curing systems and the oxide composition,the following conclusions are obtained:When the compressive strength of the solidified body was less than 10MPa,the content of CaO was 20%～36%,the content of SiO₂was 41%～60%,the content of Na₂O was 12%～27%;when the compressive strength of the solidified body was between 10MPa～15MPa,the content of CaO was 29%～41%,SiO₂content was 40%～58%,Na₂O content was 8%～22%;when the compressive strength of the solidified body was greater than 15MPa,the CaO content was 34%～57%,the SiO₂content was 33%～50%.The Na₂O content was 10%～19%.It showed the compressive strength of the solidified body was mainly determined by the oxide composition of the raw material,and the type of raw material only affects the reaction process,and has little effect on the result of the reaction.According to the ternary system,the optimal ratio of each system was screened out,and the heavy metal leaching concentration experiment was carried out.The results are as follows:the Cd leaching concentration of the MMS solidified body is 0.044mg/L,and the Pb leaching concentration is 0.01mg/L;MMSS Cd leaching concentration of solidified body is 0.01mg/L,Pb leaching concentration is 0.01mg/L;MMSF solidified body Cd leaching concentration is0.01mg/L,Pb leaching concentration is 0.02mg/L;MSF solidified body Cd leaching concentration is 0.07mg/L,the leaching concentration of Pb is 0.02mg/L.All are lower than the national standard limit.In the experiments on the softening coefficient and sulfate corrosion resistance of the solidified body,the water resistance and corrosion resistance of each system performed well,and the softening coefficient and corrosion resistance coefficient were all above 0.85.Under the conditions of long-term water immersion and sulfate immersion,the leaching concentration of heavy metals did not change much.This thesis analyzes the microstructure and chemical mineral composition of the solidified body by XRD,FT-IR and SEM methods,and compares it with the original MSWI fly ash and pure geopolymer.The result of XRD can prove that MSWI fly ash is not simply fixed in the structure as a waste material.The solidified body had a normal polymerization structure.FT-IR results showed that heavy metal ions participate in the coordination of silicon-oxygen tetrahedrons and aluminum-oxygen tetrahedrons to form part of the polymeric structure.SEM results showed that MSWI fly ash partly participate in the reaction to form a solidified body,but there will also be many impurities,which will be fixed in the formed cage structure or laminated sheet structure as the polymerization reaction progresses.