Mechanical Characteristics And Micro-mechanism Of Sludge Solidified With Reactive Magnesia/Alkali-activated Fly Ash

Cement and lime are the most commonly used binder for solidification of dredged sludge.However,there are significant environmental impacts associated with the production of cement and lime in terms of high consumption of energy and non-renewable resources as well as high CO2 emissions.Therefore,the low-carbon and green binders based on industrial byproducts should be developed to replace completely or partly cement and lime in sludge solidifciation.By using reactive MgO/alkali-actived fly ash,the unconfined compressive strength,durability and micro-mechanism are systematically investigated on the dredged sludge from East Lake,China.This study can provide a theoretical reference for the development of green soil stabilizer and its practical application in the solidified sludge reused as geotechnical materials and soft ground improvement.The main research findings are presented as follows.(1)On the properties of reactive MgO-fly ash pastes:The characteristics of strength,durability and micro-mechanism are experimentally investigated by series of tests including compressive strength,sulphuric acid attack,X-ray diffraction,scanning electron microscopy etc.The obtained results indicate that the compressive strength and durability of MgO-fly ash pastes can be evidently enhanced when the reactive MgO amount and curing time increase and the water-cement ratio decreases.The production of cementitious Mg(OH)2 and M-S-H gles is the main reason why the strength performance of studied materials is obviously improved.The hydration products and microstructure of reactive MgO-fly ash pastes is largely influenced by reactive MgO amount and water-cement ratio.(2)On the properties of alkali-activated fly ash solidified sludge:Based on the unconfined compressive strength tests,the stabilization effect of solidified sludge with alkali-activated fly ash is evaluated and compared for different alkali-activators,i.e.NaOH,Na2CO3,Na2SO4 and Na2SiO3·9H2O.The chemical reaction products and micro-mechanism of alkali-activated fly ash solidified sludge are clarified by a series of minerological and microstructural tests including XRD,SEM,TGA and MIP.The results demonstrate NaOH-and Na2SiO3·9H2O-activated fly ash solidified sludges have higher compressive strength than those solidified with Na2O3-and Na2SO4-activated fly ash.The strength value of Na2CO3-and Na2SO4-activated fly ash solidified sludge posess reaches only 100 kPa even at curing time of 360d,and this indicates the activation efficiency of Na2CO3 and Na2SO4 for fly ash is quite limited.The unconfined compressive strength of alkali-activated fly ash-solidified sludge increases with alkali-activator amount,fly ash amount and curing period.The formation of N-A-S-H gels is the intrinsic reason why the strength performance of alkali-activated fly ash solidified sludge is evidently improved.(3)On the properties of reactive MgO-fly ash solidified sludge:Series of tests including drying-wetting cycle,freeze-thaw cycle and water immersion were carried out.The test results indicate that the compressive strength of reactive MgO-fly ash solidified sludge increases with curing time,MgO-fly ash content and mass ratio of MgO to fly ash prior to 180 days,followed by a decrease in compressive strength at 360 days.The strength performance is evidently weakened due to the destructuration of drying-wetting cycles,freeze-thaw cycles and water immersion,and the durability of reactive MgO-fly ash solidified sludge is greatly improved with reactive MgO-fly ash content and mass ratio of MgO to fly ash.The XRD,SEM,TG and MIP results indicate that the formation of Mg(OH)2 and M-S-H gels is the main reason why the strength performance of solidified sludge is evidently improved.