Mechanical Property And Micromechanism Of Dredged Sludges Solidified By A Novel Combined Technique Of CO₂ Carbonation And Reactive MgO-Fly Ash/Slag

To achieve the goal of beneficial reuse of solid wastes,energy saving and emission reduction,reactive MgO-fly ash/slag blends combined with CO₂carbonation technique was introduced in the solidification of dredged sludge and CO₂ mineral storage under the context of sustainable development.Through a large number of experimental tests,the mechanical properties,durability and microscopic mechanism of carbonated solidified sludge provide a theoretical basis for CO₂-reactive MgO-fly ash/slag combined solidification technique.The main findings are presented as follows.（1）On the properties of reactive MgO-fly ash carbonated-solidified sludge:Through unconfined compression strength（UCS）,scanning electron microscopy（SEM）and mercury intrusion porosimetry（MIP）tests,the effect of CO₂ carbonation on the mechanical properties and microstructure was investigated under different carbonation time and dosage of binding agent.The test results indicate that the strength of solidified sludge is evidently increased due to CO₂ carbonation.The sludge specimens with different binding agents have different optimal pressurization modes,and affects the strength gain of carbonated samples.The swelling and cementation of magnesium carbonates（hydromagnesite,dypingite and nesquehonite）are the main reason why the compressive strength is significantly enhanced by the combined carbonation-solidification technology.（2）On the properties of durability evolution of carbonated-solidified sludge:Through unconfined compression strength,XRD,SEM and MIP tests,the effect of CO₂ carbonation on the mechanical properties and microstructure was investigated under water immersion,dry-wet cycles and freeze-thaw cycles.The test results indicate that reactive MgO-activated fly ash solidified sludge subjected to CO₂carbonation has higher strength and better water stability.After 20-day water immersion,the compressive strength of carbonated samples is about 32%higher than that of non-carbonated solidified samples.The strength of carbonated samples rises slowly with wet-dry cycles,while the strength of non-carbonated samples first increases followed by an observable decrease.The evolution of compressive strength of carbonated and non-carbonated solidified samples agrees well with each other as the freeze-thaw cycles extend,i.e.the strength increases initially,then decreases and finally keeps constant.（3）On the properties of reactive MgO-slag solidified sludge by CO₂mineralization:The results obtained from UCS and carbonation tests indicate that the strength of CO₂ carbonated slag-solidified sludge sample tends to increase,but the compressive strength and absorbed CO₂ amount were quite limited,and increase with increasing slag content and CO₂ carbonation pressure.Provided that reactive MgO replace the same amount of ground slag,the compressive strength of carbonized MgO-slag-solidified sample is significantly improved,and the CO₂absorption and storage capacity was significantly enhanced.Microscopic analysis shows that CO₂ is transformed to carbonates inside the sample and this helps realize the permanent stable storage of CO₂ gas.（4）On the properties of durability evolution of reactive MgO-slag-solidified sludge by CO₂ mineralization:Series of tests including drying-wetting cycle,freeze-thaw cycle,water immersion and microstructural analysis were carried out to evaluate the durability of carbonated-solidified sludge and associated mechanisms.The results indicate that the elongated prismatic hydromagnesite（skeleton construction）,flower/bone-like and flaky dypingite and nesquehonite（filling and cementing）are the main carbonation products,and the combined skeleton-filling-cementing effect makes the carbonated samples with high strength,good water stability,and strong resistance to dry-wet and freeze-thaw cycles.The continuous water immersion leads to an increasing macropores and transformation of dypingite and nesquehonite to hydromagnesite.The dry-wet cycles induce the transformation of dypingite and nesquehonite to hydromagnesite,but there is no obvious phase transformation in carbonated samples under freeze-thaw conditions.