Engineering Properties And Mechanism Of High Sulfur Fly Ash-Cement-Bubble Lightweight Mixed Filler(HCBF)

Saving resources and protecting the environment are the basic national policies of our country.High sulfur fly ash is the product of flue gas desulfurization in thermal power plant.When mixed with cement,sulfur in fly ash reacts with hydration product of cement to produce expansion stress,which has an impact on volume stability,which is not conducive to the reuse of high sulfur fly ash solid waste resources.Based on the comprehensive utilization of high sulfur fly ash as the guide,to"flow filling material backfill engineering technical standards"to provide technology and theory support for the purpose,adhere to the resource conservation and the development of green ecological concept,to add high sulfur fly ash to Cement-Bubble Lightweight Mixed Filler,the formation of High Sulfur Fly Ash-Cement-Bubble Lightweight Mixed Filler(HCBF),use packing porosity to alleviate the expansion stress of high sulphur fly ash.In this paper,the phenomenological constitutive model and freeze-thaw cycle damage constitutive model with dry density as variable are established by studying the physical,mechanical,salt-resistant properties of HCBF and the macro and micro mechanism of action.Combined with MIP,the influence range of high sulfur fly ash on the aperture of HCBF was analyzed.The equivalent circuit model of HCBF was established according to EIS theory,and the prediction model of porosity based on solid-liquid interface transfer resistance was proposed.Based on the pore structure characteristics of HCBF,combined with XRD,SEM,EDS and strength variation law,the mechanism of salt corrosion resistance of HCBF is revealed,and the mechanism of mutual improvement between high-sulfur fly ash and HCBF is clarified.The main conclusions are as follows:(1)Through the research analysis of four content of high sulfur fly ash and five dosage of bubbles HCBF’s bibulous rate,dry density,thermal conductivity,the unconfined compressive strength and freeze resistance performance,considering the engineering claim to the dry density,strength and thermal conductivity,puts forward the reference HCBF proportioning scope:water cement ratio of 0.7,high sulfur fly ash content is 66.7%～75%of reference,the bubble of the reference content for 300 L/m~3～900 L/m~3(reference indicators are as follows:Dry density 820 kg/m~3～1200 kg/m~3,unconfined compressive strength3.2 MPa～8.3MPa,thermal conductivity 0.145 W/m·K～0.410W/m·K).(2)By analyzing the unconfined compressive strength and stress-strain curve of HCBF,it can be concluded that high-sulfur fly ash affects the unconfined compressive strength of HCBF from the aspects of material composition and pore structure.The stress-strain curve of HCBF can be divided into four stages:compaction stage,elastic deformation stage,yield stage and recession stage.Based on Gibson and Transom models,phenomenological constitutive models and dimensionless constitutive models with consideration of HCBF dry density parameters are proposed.By analyzing the freezing-thawing cycle characteristics of HCBF,it can be concluded that the freezing-thawing cycle grade of HCBF can reach F25 under the condition of high sulfur fly ash content of 66.7%～75%and low bubble content(0～600L/m~3).A freeze-thaw cycle damage model of HCBF with dry density as variable was proposed and its accuracy was verified.(3)HCBF through analysis on the appearance of the different concentration of salt solution after corrosion morphology characteristics,the unconfined compressive strength and XRD,SEM and EDS results:HCBF under sodium sulfate and compound salt solution corrosion products mainly for the AFt crystal and gypsum crystals,in sodium chloride solution under corrosion and corrosion products mainly for Cl-AFm crystal and gypsum crystals;Compared with AFt and gypsum crystals,Cl-AFM crystals are sparser and more dispersed,which makes it occupy less pores.It can be concluded that the severity of HCBF corrosion affected by different solutions is arranged from large to small as follows:sodium sulfate solution>compound salt solution>sodium chloride solution;It is suggested that the content of high-sulfur fly ash in HCBF should be 66.7%～75%in case of the corrosion of sodium sulfate or compound salt environment.For the corrosion of sodium chloride environment,the value of high-sulfur fly ash content can be relaxed appropriately.(4)By analyzing the MIP test results of HCBF,HCBF pores were divided into microscopic capillary(<50nm),macroscopic capillary(50nm～50μm)and macropore(>50μm)according to the pore size.It can be divided into primary pore(≤0.5μm)and artificial pore(>0.5μm).High-sulfur fly ash affects the artificial cellular pores in the macroscopic capillary range,and finally leads to the change of porosity.Based on EIS theory and the characteristics of HCBF,the equivalent circuit model of HCBF is established.The solid-liquid interface transfer resistance of the electrical element in the model can effectively reflect its pore characteristics,and the prediction model of porosity based on the solid-liquid interface transfer resistance is established.(5)Through the comprehensive analysis of the variation rules of MIP,SEM,EIS,EDS,XRD and strength,the interaction mechanism between HCBF and high-sulfur fly ash is revealed:An appropriate amount of high-sulfur fly ash(content<75%)can optimize the pore structure of HCBF,which is beneficial to the improvement of strength.Pores in HCBF are tolerant to expansion products,and samples with high porosity are more resistant to salt corrosion.Before the porosity reduction value reaches the threshold value,the generation of corrosion products will not have a negative impact on the strength and structure of HCBF.