Experimental Studies On Conditioning Solidification And Environmental Characteristics Of Solidified Silt

Lake as an important water function area has important significance on security and development of city. However, the sediment deposition and serious pollution have deeply affected the function of drainage and water supply. Polluted silt is also the endogenous source leading to the bad water quality of lake. Dredging is one of the important means of lake treatment. Dredged silt has the features of high moisture content, small particle size, high compressibility, low bearing capacity, superfluous pollutants and so on. If silt can’t be managed timely and effectively, not only it will occupy many precious land resources, but also cause serious secondary pollution. Therefore, the treatment and disposal of dredged silt are the key problems to be solved. Practices prove that resource utilization is the ultimate way of silt treatment and disposal. Converting it to geotechnical materials which can meet the requirements of a certain bearing capacity used for garden filled soil, road foundation and embankment soil is an effective way of silt resource utilization. Solidification technology can improve the physical and mechanical properties of dredged silt. Taking silt samples from Swan Lake and South Lake in Wuhan city as the research object, the experimental studies on physicochemical characteristics, mechanical properties, microstructure characteristics and environmental characteristics of solidified silt were carried out based on conditioning solidification. The main contents of this paper are as follows:(1) The basic characteristics of silt in Swan Lake and South Lake were analyzed. Silt in Swan Lake contains much more silicon while silt in South Lake contains much more aluminum and iron. Swan Lake silt mainly contains quartz, albite, kaolinite and illite. South Lake silt also contains a certain amount of calcite. The moisture content of silt in two lakes is higher than liquid limit. That is to say, it takes on a flow state. The organic matter, nitrogen, phosphorus and heavy metal content of South Lake silt are higher than that of Swan Lake silt. Obviously, the pollution of silt in South Lake is more serious.(2) The influence of different species and dosage of fly ash on physical and chemical properties of silt in Swan Lake was studied under fixed dosage of cement. The water content, pH value, calcium ion concentration and cation exchange capacity of solidified silt were estimated. The results show that fly ash can lower water content of solidified silt, and the effect of high calcium fly ash is more outstanding. High calcium fly ash contributes to higher pH value and calcium ion concentration. The variation tendency is opposite when it comes to low calcium fly ash. The promotion effect of different high calcium fly ash on pH value and calcium ion concentration is mainly associated with the content and form of calcium in fly ash. Cation exchange capacity of solidified silt is varied with the type and dosage of fly ash, and the relationship is complicated. It is well related with the carbon content of fly ash, pH value and particle size of solidified silt. The moisture content, pH value, calcium ion concentration and cation exchange capacity of the solidified silt are decreased along with curing time.(3) The effect of different dosage and species of fly ash on silt strength characteristics was studied under the fixed cement dosage. At the same time, the effect of additives on silt strength characteristics under the fixed cement and fly ash dosage was made clear. The results show that fly ash can promote the development of strength, and the effect of high calcium fly ash is more obvious. Among the three additives, sodium silicate can increase the strength of solidified silt in each age stage. The strength decreases when the addition amount of sodium hydroxide is higher than0.7%during early curing age. The strength decreases when the addition amount of sodium sulfate is higher than0.7%during late curing age. After comparison of the three additives, it is found that sodium sulfate plays the most significant role in strength development during early curing time. And the difference between sodium hydroxide and sodium silicate is related to the addition amount. When it comes to late curing time, sodium silicate shows the most significant improvement in strength, sodium hydroxide takes second place, sodium sulfate is the worst.(4) The method of scanning electron microscopy was employed to study the microstructure of solidified silt. The quantities and types of hydration products were analyzed. As is seen from the scanning electron microscopy images, fly ash can improve the microstructure of solidified silt. And the effect of high calcium fly ash is more significant. During the late curing age, gels are observed when additives are added. In addition, needle-bar-like products are found when sodium hydroxide is added. These products are interlaced with each other. And it takes on a network shape. Plate-like hydration products as well as crack and large pores are detected when sodium sulfate is added. Needle-like hydration products as well as smooth and compact products are observed when sodium silicate is added. In a word, the uniform distribution of hydration products as well as the better integrity can promote the strength development.(5) The relationship between physicochemical characteristics and mechanical properties of solidified silt was explored. Specifically speaking, the relationships between unconfined compressive strength and moisture content, strength and pH value, strength and calcium ion concentration, strength and cation exchange capacity of solidified silt were investigated. The results show that the strength is increased with the deceasing of moisture content and it takes on a power relationship between strength and moisture content. A power relationship is also found between strength and pH value. The exponential relation between strength and calcium ion concentration is observed. The relationship between strength and cation exchange capacity of solidified silt is varied according to different kinds of fly ash.(6)The effect of different species and dosage of fly ash as well as different species and dosage of additives on environmental behaviour of solidified South Lake silt was studied. The variation of unconfined compressive strength of solidified silt, the pH value, COD value, total phosphorus concentration and copper concentration of leachate from solidified silt were taken into consideration. The results show that unconfined compressive strength has an effect on leaching behavior of pollutions to some extent. The pH value of leachate is highly related to the pH value of solidified silt. Fly ash can stabilize organic matter, phosphorus and copper when using cement as the base material. The effect of fly ash on the fixation of organic matter and copper depends on the carbon content of fly ash. The effect of fly ash on phosphorus fixation is mainly related to the content and form of calcium in fly ash. The effect of three additives on the fixation of pollutants is different. Sodium hydroxide can stabilize organic matter only when the addition amount is low and the case is opposite when the dosage is too high. Sodium hydroxide is bad for the fixation of phosphorus and copper. Sodium sulfate can stabilize organic matter and copper. Sodium sulfate is effective on phosphorus fixation when the addition amount is low, and the effect becomes worse when the addition amount is high. Sodium silicate is helpful for the stabilization of organic matter. However, it is adverse to phosphorus fixation. Sodium silicate can stabilize copper when the dosage is low. However, it plays the opposite role when the dosage is too high.