Experimental Research On Modified Saline Soil Of Coal Bearing Metakaolin-Mineral Powder-Cement

Saline soil refers to a kind of soil in which salt exceeds a certain content.In engineering application,it is defined as the soil containing more than 0.3%of salt and showing adverse engineering characteristics including salt expansion,karst collapse,and corrosion.Hazards related to saline soil have caused serious adverse effects on engineering facilities.To meet the demand for infrastructure constructure,researchers have carried out numerous research on treatment of saline soil and gained remarkable effects in treatment of saline soil in northwest and coastal regions in China.However,due to significant differences in engineering characteristics of saline soil in different regions,Less research on corrosion engineering facilities and modification and curing of saline soil in Shanxi Province.The research is part of a horizontal project“Modification test and application of backfilling saline soil”.Saline soil collected from the south of Yuncheng City,Shanxi Province was used as the research object.Mixed coal metakaolin,mineral powder,and cement improved the poor engineering properties of saline soil.The compressive strength,permeability coefficient,consolidation content,and resistivity of the modified saline soil were tested under different dosages of modifiers to analyze the influence of the dosage of modifiers on the mechanical properties and durability of saline soil,and found the best optimal dosage.Through the change of resistivity of modified saline soil under pressure,it was analyzed the dynamic change law of resistivity and stress with strain,and was explored the feasibility of resistivity characterizing the compressive strength of modified saline soil under pressure.Moreover,X-ray diffraction（XRD）and scanning electron microscopy（SEM）were conducted to observe microstructures and material compositions of the modified saline soil and then the mechanism of action of the modifier was expounded.The main research contents of this paper were as follows:1)Saline soil modified by compound additives such as coal metakaolin,mineral powder,and cement was used in the research.On this basis,unconfined compressive strength,permeability coefficient,and content of consolidated materials of modified saline soil added with different amounts of modifiers after being cured for 7,28,and 90 d were tested.Then,influences of compound addition of coal metakaolin,mineral powder,and cement on the strength and impervious performance of the saline soil were investigated.Results indicate that the compound addition can significantly enhance the unconfined compressive strength and impervious performance of the saline soil.When the total amount of the modifiers is 10%,in which coal metakaolin,mineral powder,and cement show a ratio of 1:3:1,the modification effect is the optimal.Comparison of unconfined compressive strength of and content of consolidated materials in the saline soil modified by different amounts of modifiers and then cured for 7,28,and 90 d suggests that compound modifiers enhance the strength of the saline soil in the early period.2)Electrical resistivity of saline soil modified by different amounts of modifiers after curing periods of 7,28,and 90 d was measured using a LCR meter.Then,influences of compound addition of coal metakaolin,mineral powder,and cement on electrical resistivity of modified saline soil were studied.The feasibility of using the electrical resistivity to characterize compressive strength of modified saline soil under compression was studied.Results show that electrical resistivity of saline soil modified by each amount of modifiers decreases with the increasing current frequency.When the current frequency is less than 50000HZ,the resistivity of the modified saline soil decreases rapidly with the increase of the current frequency;when the current frequency is greater than 50000HZ,the resistivity of the modified saline soil decreases slowly and gradually stabilizes with the increase of the current frequency.At the same current frequency,resistivity of modified saline soil declines with the increasing addition of the coal metakaolin.For modified saline soil under strain lower than 1%,the stress grows slowly while electrical resistivity declines rapidly.Under strain in the range of 1%～5%,the stress and electrical resistivity of the modified saline soil remain basically unchanged.As the strain further grows to exceed 5%,stress of modified saline soil begins to rapidly grow while the electrical resistivity shows a decrease rate slightly faster than that under strain of 1%～5%.After a same curing period,modified saline soil added with modifiers exhibits the electrical resistivity changing at a slower rate compared with saline soil not added with modifiers.3)The X-ray diffraction（XRD）analyzer was used to test compositions of natural saline soil and saline soil modified using different amounts of modifiers.By doing so,the compositions of hydration products of saline soil modified by compound addition of coal metakaolin,mineral powder,and cement were explored.Results indicate that modified saline soil added with different amounts of modifiers share the same diffraction peaks on the XRD patterns,which suggests that crystalline types of hydration products do not change with the increasing addition amount of coal metakaolin.The height of the SiO₂ diffraction peak of the modified saline soil is lower than that of the natural saline soil,indicating that the chemical reaction in the modified saline soil consumes SiO₂.4)Morphological detection and quantitative analysis were performed for natural saline soil and modified saline soil added with 0%,2%,and 4%of coal metakaolin using the scanning electron microscope（SEM）.In this way,influences of addition amounts of coal metakaolin on internal structures of modified saline soil were investigated.Results indicate that The internal structure of natural saline soil is loose,with many pores and poor structural stability.after compound addition of modifiers,modified saline soil has compact internal structures,less pores,and enhanced structural stability.The optimal modification effect is obtained when adding a total amount of 10%of modifiers,in which coal metakaolin,mineral powder,and cement are set in a ratio of 1:3:1.In summary,the coal-based metakaolin,mineral powder,and cement compounding can improve the internal structure of the saline soil,increase the strength and impermeability of the saline soil.The modification effect is the best when the coal series metakaolin,mineral powder,cement=1:3:1...