Study On Mechanical Properties And Gelation Mechanism Of Steel Slag Cement Composite Solidification Dredged Sludge

With the development of Marine economy in China,a large amount of dredged sludge is produced every year,which needs to be solidified because of its high moisture content and poor mechanical properties.Ordinary Portland cement is currently the most commonly used inorganic soft soil curing agent,but the cement production process has high energy consumption and large CO₂ emissions.Therefore,industrial waste is often used to replace part of the cement to prepare a composite curing agent.Steel slag is the main by-product of the steelmaking process.Its mineral composition is similar to ordinary Portland cement clinker,and it has certain cementitious activity in alkaline environment.Therefore,using steel slag as a curing agent to solidify dredged sludge has a good development prospect.This paper is based on the concept of"using waste to treat waste",using different pre-treated steel slag to replace part of the cement for dredging sludge solidification,while meeting the strength requirements,achieving the purpose of reducing the amount of cement and the elimination of industrial wastes,for the steel slag to solidify the soil.Providing theoretical guidance and data support for the application of steel slag in the field of solidified soil.（1）In order to explore the influence of steel slag fineness and pre-carbonization degree on the performance of steel slag as a cement substitute cementitious material,the steel slag was ground and carbonation pretreated respectively,and the basic physical properties and composition of the steel slag with different pretreatment methods are analyzed through particle size analysis,X-ray diffraction test（XRD）,thermogravimetric analysis（TG）,and stability test.Different pre-treated steel slags were replaced with 10%,20%,30%,and 50%wt.%of cement respectively to prepare steel slag cement composite cementitious materials,and the effect of steel slag on the workability,mechanical properties and stability of cement was studied.The research results show that the particle size distribution of steel slag is more uniform after pretreatment,and the particles above 60μm are significantly reduced.The calcium hydroxide（CH）in the carbonized steel slag for 2 hours basically reacted completely,and the carbonized product is calcium carbonate（C?）.The carbonization rate decreases with the increase of carbonization time.The substitution of steel slag for part of the cement has a retarding effect on cement-based materials,and the replacement ratio with the best retarding effect is between 20%and 30%.Compared with the cement paste specimens,the replacement of part of the cement by steel slag has the greatest impact on the strength growth of the composite paste specimens from 7d to 28d.（2）Steel slag is used to replace 10%,20%,30%,and 50%wt.%of cement to prepare composite cementitious materials to solidify dredged sludge.The influence of steel slag replacement ratio and curing age on the mechanical properties and deformation characteristics of composite solidified sludge was studied through unconfined compressive strength and stress-strain curves.Explore the relationship between unconfined compressive strength and secant modulus E₅₀ and p H.Test the long-term stability of the composite solidified sludge through high temperature water bath curing.The hydration products and microstructure of the solidified dredged sludge were analyzed by TG and SEM tests,and the steel slag cement composite solidified dredged sludge（SCSs）and pure cement solidified sludge（S-C）were compared and analyzed.The test results show that when a small proportion of steel slag replaces cement,the strength growth rate of solidified sludge from 7d to 28d decreases,and the strength growth rate increases after 28d.When FSS-C-18H replaces 10%cement for curing for 60d,the unconfined compressive strength of solidified silt is the highest,which is109%of the strength of solidified sludge.The main hydration products of the solidified sludge are calcium hydroxide（CH）,hydrated calcium silicate（C-S-H）and ettringite（AFt）.The steel slag does not change the type of solidified sludge hydration products,and the AFt cluster generated in the system with carbonized steel slag Stronger sex.（3）The sulfate erosion resistance of different pre-treated steel slags instead of 30%cement-solidified dredged sludge was studied and compared with S-C.By measuring the size,quality and unconfined compressive strength of solidified dredged sludge specimens with different erosion ages,the effect of steel slag on the sulfate corrosion resistance of SCSs is studied.Through XRD,TG and SEM tests,the anti-sulphate corrosion mechanism of steel slag cement composite solidified dredged sludge was analyzed.The test results show that the steel slag cement composite solidified sludge generates a large amount of AFt during the sulfate erosion process.In the early stage of erosion,AFt plays a role of physical filling,increasing the compactness of the solidified sludge system and increasing the strength of the solidified sludge.As the erosion age increases,the content of AFt increases,and expansion stress is generated in the solidified sludge.When the expansion stress exceeds the bonding force between the particles,the specimen will expand and fail.Steel slag can reduce the rate of formation of hydration products during the erosion process of sodium sulfate solution and inhibit the formation of part of AFt.The 18h carbonized steel slag solidified dredged sludge has the strongest sulfate erosion resistance.