Experimental Studies On Dredged Sediment Carbonated And Stabilized By Reactive Magnesia

Rivers,lakes and reservoirs will produce a large amount of sediment in dredging engineering.Due to the bad characteristics such as low strength,high water content and large porosity,the direct utilization value of dredged sediment is not high,and it needs to be improved to meet the engineering application.Chemical curing is a common way to improve the engineering properties of soil through a series of chemical and physical reactions between curing agent and soil.This method has the advantages of small environmental impact,large strength improvement,and simple construction.In the field of chemically reinforced soil,cement is widely used as a traditional curing agent.However,the production of cement is accompanied by a large amount of resource consumption and environmental pollution,and the curing cycle of solidified soil is long and the strength growth is slow.Reactive magnesia is a kind of green and environmentally friendly cementing material,with the characteristics of high activity,small energy consumption in the production process,and small environmental pollution.It can carbonize with CO₂,and combined with carbonization technology,the strength of soil can be rapidly and significantly improved.In this thesis,the basic physical and chemical indexes of the dredged sediment of Xiaoqing River are determined through the basic physical and chemical indexes test.From the aspects of reactive magnesia content,moisture content,compaction degree and carbonization time,the unconfined compressive strength test of the solidified dredged sediment was carried out to explore the influence of single factor on the solidification effect,and the appropriate solidification condition was determined.By means of X-ray diffraction and scanning electron microscopy,the composition,particle distribution,pore distribution,and the morphology and occurrence state of the carbonized minerals produced during the carbonization process were studied,which revealed the mechanism of reactive magnesia carbonization and solidification of dredged sediment.Combined with the test of modulus of rebound and splitting strength,the road mechanical properties of the carbonized solidified dredged sediment are obtained.In addition,the immersion test,the coupling test of wet and dry cycle and salt solution erosion,and the freeze-thaw cycle test were carried out to study the long-term service performance of the carbonized solidified dredged sediment,so as to provide theoretical basis for the application of reactive magnesia carbonized solidified dredged sediment technology.The key results from tests were as follows:（1）The natural moisture content of the dredging sediment is 17.9%,the optimal moisture content is 14.4%,and the maximum dry density is 1.93g/cm³.The contents of gravel,sand and fine grain in the bottom soil samples were 13.7%,62.7%and 23.6%,respectively,and the liquid limit and plastic limit were 29.30%and 22.90%,respectively.The compressive strength,resilience modulus and CBR of natural dredged-sediment are 0.39MPa,38MPa and 16.8%,respectively.When the dredged sediment is used as subgrade packing,its compressive strength and modulus of rebound cannot meet the requirements,so it needs to be improved.（2）Microscopic experiments showed that the carbonation products of MgO were spheroid magnesite,hydromagnesite and magnesite trihydrate.On the whole,the uncarbonized sample shows that the pores between the unearthed particles are large and interconnected.The pores of the carbonized sample are less and small,the soil particles are compacted,and the microstructure integrity is good.The carbonized products themselves have high strength,and the products are interwoven and coated with soil particles,and the intergranular pores are filled to form a dense and complete skeleton structure,which is the main reason for the improvement of the strength of the carbonized solidified dredged sediment.（3）Under the single factor condition,the compressive strength of carbonized specimens was positively correlated with the content of MgO.Since the MgO-H₂O-CO₂ reaction was promoted by the increase of reactive magnesia content,the compressive strength of the specimens was continuously improved with the increase of reactive magnesia content.The compressive strength of carbonized specimens decreased gradually with the increase of water content,and the optimal water content for carbonization was 12%-15%.After carbonization,the strength of 15%moisture content specimen was 9.31MPa,which was 1.58 times of that of the uncarbonized specimen after 28 days curing.The compressive strength of carbonized specimens increased first and then decreased with the increase of compaction degree and carbonization time.The optimal compaction degree and the optimal carbonization time were95%and 2h,respectively.When the compaction degree is greater than 95%,the soil is denser and has fewer pores,so it is difficult for carbon dioxide to diffuse through the pores between soil particles,and the carbonation reaction inside the specimen is blocked,the carbonation products are reduced,and the effect of carbonation on improving the strength of the specimen is weakened.When the ventilation time is too long,the carbonation products decompress and expand,and the original soil structure is squeezed to produce cracks.In addition,the internal disturbance of the sample is caused by the CO₂ airflow,and the strength of the sample is reduced.Therefore,it is necessary to control the carbonation time reasonably.（4）The carbonized solidified dredged sediment has good long-term water-immersion stability.The compressive strength of the specimens immersed in water for 14 days and 90days was 4.20MPa and 3.32MPa,and the strength loss rate was 34.79%and 48.21%,respectively.The outer surface of the specimens was complete and smooth,and the health was good during the whole immersion process.In the coupling test of wet-dry cycle and salt solution erosion,the change trend of compressive strength of specimens in water and sodium chloride solution was relatively consistent,which could be roughly divided into two stages:rapid decline and slow loss.The trend of strength change in sodium sulfate solution can be divided into three stages:rapid decline,slow decline and rapid decline.After 20 cycles in water,sodium chloride solution,and sodium sulfate solution,the strength of the specimens was 2.39MPa,1.89MPa,and 1.12MPa,respectively,and the strength loss rates were 62.71%,70.51%,and 82.53%,respectively.The appearance deterioration of the specimens in sodium sulfate solution was the most serious,and the carbonized solidified dredged sediment had poor resistance to wet and dry corrosion of sodium sulfate solution.In the freeze-thaw cycle test,the alternating action of freezing and melting,the periodic erosion of water molecules and the destruction of frost heaving force cause the gradual dissolution of carbonized products,and the continuous expansion and development of pore structure,resulting in the loss of surface particles and the decrease of strength of the specimen.After 1 and 20 freeze-thaw cycles,the strength of the specimens was 4.76MPa and 2.62MPa,respectively,and the strength loss rate was 25.74%and 59.13%,respectively.The specimens maintained good health throughout the freeze-thaw cycle,and the carbonized solidified dredgy sediment had good freeze-thaw resistance stability.