Study On The Modification Enhancement And Carbonation Performance Of Alkali-activated MSWI Bottom Ash

Massive accumulations of municipal solid waste incineration（MSWI）bottom ash occupy high amounts of land,resulting in severe environmental pollution due to lack of good recycling technology.However,it is impossible to apply MSWI bottom ash in cement base materials due to compatibility issues between Portland cement and MSWI bottom ash.In this paper,the high-strength,durable and environment-friendly MSWI bottom ash alkali-activated materials were prepared at room temperature with the MSWI bottom ash as the raw material as well as improving curing mechanism.The substitution of Portland cement is gradually realized and demanding for it is also alleviated,which reduce the high energy consumption and high pollution problems caused by its production.Finally,large-scale safety absorption and resource utilization of MSWI bottom ash will be realized.Explorative and innovative works have been done in the following aspects:（1）Study on the mechanism of pre-aging defoaming of MSWI bottom ash alkali-activated materialsThrough the design of the foam expansion test when MSWI bottom ash in reaction with alkali,the phenomenon of foam expansion was observed and the destruction mechanism of foam expansion was also analyzed,which to eliminate the influence of foam expansion and achieve the preparation of MSWI bottom ash alkali-activated materials.The results showed that the residual aluminum in MSWI bottom ash was the main reason for the formation of foam expansion.The reaction between aluminum and strong alkali gave off hydrogen,resulting in the expansion and deformation of MSWI bottom ash alkali-activated samples,which forming the"cavernous"structure.The alkali defoaming pretreatment could completely remove expansion,the pretreatment cost is not additionally added and the activity of MSWI bottom ash is also not affected.（2）Study on modification and enhancement of MSWI bottom ash alkali-activated material（Increasing silicon and calcium content）Researching the role of active silicon and calcium in the polymerization reaction and strength development of MSWI bottom ash alkali-activated material,the method of increasing content of active silicon and calcium as well as adjusting the ratio of calcium and silicon was proposed to achieve the preparation of high strength MSWI bottom ash alkali-activated material.The results showed that increasing the content of active silicon and calcium could promote the formation of polymerization products and significantly increase the compressive strength of MSWI bottom ash alkali-activated materials.The effect of liquid sodium silicate is superior to that of solid sodium silicate.The active silicon in liquid sodium silicate does not need to undergo the process of dissolution,which improved the absorption of active silicon and avoided the formation of solid precipitation.Increasing calcium content through adding Ca（OH）₂ could solve the problem of active calcium"dissolution time difference",and overcome the sudden drop of active calcium dissolution rate caused by slag.However,the excessive incorporation of Ca（OH）₂（>2%）will cause the aggregation of Ca（OH）₂ and result in dry shrinkage and cracking in MSWI bottom ash alkali-activated samples,which is not conducive to the development of strength.（3）Study on the strength development and alkali conversion mechanism of NaOH dosage（Transformation law from free alkali to union alkali）The relationship between the NaOH dosage and the development of strength was established by analyzing the influence of NaOH dosage on the internal pH value and the transformation law from free alkali to union alkali in MSWI bottom ash alkali-activated material.The results showed that increasing the amount of NaOH could promote the transformation from free alkali to union alkali,facilitate the formation of polymerization products and enhance the compressive strength.However,NaOH adding excessive（>5.3%,24 g）will adversely affect the development of strength.This is because excessive Na⁺will compete with active calcium for active silicon and active aluminum,which result in a decrease in the production of C-A-S-H and C-S-H gels.（4）Study on the curing mechanism of MSWI bottom ash alkali-activated material（Eliminate the effect of OH^-and free alkali precipitation）The strength development law of MSWI bottom ash alkali-activated samples was studied under different curing methods.The effects of free alkali and OH^-precipitation on polymerization reaction and the formation mechanism of temperature crack and dry shrinkage crack were analyzed by means of microscopic analysis.The results show that the curing method of concrete is not suitable for MSWI bottom ash alkali-activated samples.Sealing curing can effectively prevent the leaching of free alkali and OH^-,maintain the high alkalinity environment inside the samples,improve the polymerization reaction rate,promote transformation from free alkali to union alkali,and further enhance the compressive strength.Meanwhile,sealing curing can also prevent excessive temperature difference inside and outside the samples,maintain the humidity balance and effectively prevent the formation of temperature cracks and dry shrinkage cracks,which enhanced the integrity,stiffness and durability.（5）Study on the carbonation mechanism of MSWI bottom ash alkali-activated concrete（Ca（OH）₂ self-generated）The purpose of Ca（OH）₂ self-generating was achieved by mixing Ca（OH）₂ or Portland cement in MSWI bottom ash alkali-activated concrete,which increased the content of substance resistant to carbonation and enhanced the resistance against carbonation.The results show that both Ca（OH）₂ and Portland cement could enhance the carbonation resistance and the improvement effect of Portland cement is obviously better than Ca（OH）₂.Due to the Ca（OH）₂ absorption effect of polymerization,it does not enhance the ability of carbonation resistance when the Ca（OH）₂ dosage is less than 30%.The residual Ca（OH）₂can be used to enhance the carbonation resistance only when the dosage of Ca（OH）₂exceeds 30%.The addition of Portland cement not only ensures the self-generating of Ca（OH）₂,but also overcomes the absorption of the polymerization reaction,which significantly improves the carbonation resistance.（6）Study on carbonation anti-diffusion mechanism of MSWI bottom ash alkali-activated concreteIt was found that the MSWI bottom ash alkali-activated material has the ability of anti-diffusion behavior by means of accelerated carbonation test.The results showed that the ability of anti-diffusion behavior could significantly improve the alkalinity in complete carbonation area of MSWI bottom ash alkali-activated material.The dissolution of magadiite under low alkalinity condition could release a large amount of OH^-and increase the alkalinity in complete carbonation area,which is caused the phenomenon of anti-diffusion behavior.