Efficient Utilization And Action Mechanism Of Desulfurization Ash In Solidification/Modification Of Highway Engineering

Iron and steel industry is an important basic industry of China’s national economy,as well as the source industry of high pollution and high emission.It is the key area to realize green environmental protection development.Pollution in the iron and steel industry mainly comes from SO₂ produced in the sintering process,which accounts for about 40%～50%of the production system of the whole steel industry.With the proposal of SO₂ emission reduction target and emission reduction plan of the"13th Five-Year"iron and steel industry,control of SO₂ emission of sintering flue gas has become the focus of pollution control in the steel industry.At present,the semi-dry desulfurization technology is used for flue gas desulfurization in China,which produces a large number of desulfurization by-products--desulfurization ash,which will become the third major solid waste in the steel industry after both furnace slag and steel slag.There are few researches on the comprehensive utilization of desulfurization ash in China,and most of them adopt landfill treatment,which not only occupies land,but also wastes a large amount of resources.Therefore,desulfurization ash needs a scientific disposal method and resource utilization approach.Based on desulfurization ash from steel works as the research object,it is used as modifier for subgrade filling and admixture of cement-solidified muck soil for foundation,systematically reveals the desulfurization ash to the amelioration of soil physical properties and mechanical properties,cement-desulfurization ash composite microcosmic mechanism of the strength of the solidified mud and explores the feasibility of recycling desulfurization ash in road engineering,the main research content is as follows:（1）The optimal content of road filler modified with desulfurization ash is 6%,and the liquid plastic limit and mechanical properties of the modified soil can be better improved.The content of desulfurization ash was set as 2%,4%,6%and 8%,and the liquid plastic limit,compaction,CBR and unconfined tests were carried out on the modified soil.The results show that with the increase of the content of desulfurization ash,the liquid limit and plastic index of soil greatly decrease,while the plastic limit slightly increases,the hydrophilicity and viscosity of soil samples decrease,the water stability is enhanced and it is easy to form stably,and it is easy to control the water content of the fill and the mechanical compaction treatment of the fill.The addition of desulfurization ash smoothens the peak of compaction curve,increases the optimum moisture content of soil and decreases the maximum dry density of soil,which can reduce the difficulty of field rolling construction and is beneficial to air drying and compaction of fillers.The CBR value of the modified soil was significantly improved by desulfurization ash.When the content of desulfurization ash was 8%,the CBR value of the modified soil was increased by 135%,which greatly met the requirements of the specification.The desulfurization ash can significantly improve the 7d unconfined compressive strength of the modified soil,and the optimal content of desulfurization ash is 6%,and the 7d compressive strength of the modified soil is increased by 167%.The optimal content of desulfurization ash modified soil is 6%,the strength meets the requirements of the specification,and it is feasible for road use.（2）The optimal mixing amount of cement-desulfurization ash solidified muck soil is 8%cement,and desulfurization ash is 20%cement.Combined with vacuum pumping,the strength of cement soil can be significantly improved.Combined with vacuum pumping,the strength of solidified soil can be significantly improved.The cement content was set as 4%,6%,8%,and the desulfurization ash content was 10%,20%,30%,50%,75%,100%of the cement content.After curing for 7d,14d,and 28d under standard curing conditions,the unconfined compressive strength was measured to analyze the change rule of the strength of solidified soil under different desulfurization ash content,different cement content,and different ages.The test results show that the optimal content of desulfurization ash is 20%when the cement content is constant.When the content of desulfurization ash is constant,the higher the cement content is,the higher the strength is.The optimal ratio of desulfurization ash and cement solidified muck soil（C8A1.6）28d strength reaches 0.38MPa,which can meet the requirements of design foundation bearing capacity.Combined with the decrease of hydrophilicity of clay particles caused by desulfurization ash and the decrease of strength caused by excessive desulfurization ash,the reason is considered to be the change of strength caused by the change of soil moisture caused by desulfurization ash.In order to explore the influence of water,vacuum pumping test of soil-cement was conducted.The results show that vacuum pumping has a significant improvement effect on the strength of soil-cement,increasing the strength by 70%on average.The addition of desulfurization ash can improve the effect of vacuum pumping,and the strength of sample C8A1.6 with the optimal content reaches 0.55MPa on 28d after vacuum pumping,which increases by 45%.There is a linear positive correlation between cement slump and initial water content,and a negative correlation between cement content and desulfurization ash content.The slump of sample C8A1.6 with the optimal content is 152mm.（3）Using scanning electron microscope,energy spectrometer,mercury injection test three kinds of microscopic examination method to explore the mechanism of desulfurization ash composite curing.The test results showed that:The desulfurization ash added a certain amount of Ca²⁺and increased the basicity,make up for the consumption of organic matter（rich acid,humic acid）on Ca²⁺,so that cement hydration and hardening reaction can be normal;Ca SO₃·1/2H₂O in desulfurization ash can react with aluminate minerals in cement to form sheet monosulfur hydrated sulfoaluminate（AFm）.The solid phase volume of this reaction increases by 115%,which can produce a certain expansion.At the same time,flak-like hydration products AFm grow in clumping inside the soil void and interlock with each other to fill the pores in the silt.It interweaves with C-S-H to form a more compact structure,which makes the structure of the solidified soil more compact,thus improving the strength of the solidified soil.Desulfurization ash ion exchange reaction with the soil makes the clay particles combined water film thinning,combined water,free water more,reduced clay minerals content of hydrophilic and water-holding ability,improve the dehydration properties of sludge,cooperate with the vacuum pump can reduce the free water in the stabilized soil,under the condition of invariable in cement dosage,reduce the water cement ratio,improve the strength of the stabilized soil;Compared with the traditional gypsum,the hydration products of desulfurization ash are in the form of sheet,and the expansion of desulfurization ash is less,and it will affect the soil moisture.（4）Relying on the Shuyang-Suyu section of Lianyungang-Suqian Expressway,the field test was carried out for river pond dredging project.the field test was carried out with the optimal ratio of the laboratory test,and the in-situ solidification technology was selected.The relevant process construction steps,quality control indexes and inspection and acceptance standards were summarized.Through the light dynamic penetrating,drilling and coring test and static sounding,proved that cement-desulfurization ash composite solidified mud for packing with the engineering application feasibility of foundation.Compared with the road foundation dredging technology,the in-situ solidification technology with in situ use,resource intensive,low cost,time limit control and other significant advantages,can reduce the project cost,realize the recycle of resources,can provide certain reference for subsequent similar projects and using for reference.