| There is a large amount of mucky soft clay in the southeast of China.Soft clays have high porosity,high water content,low strength,and high compressibility,thereby requiring improvement for use as foundations,embankment and other filling materials.Ordinary Portland cement is a commonly used material for soft soil improvement.However,in the long-term application,it is found that Ordinary Portland cement production involves massive resources and energy,producing huge CO2 emissions,contrary to carbon peaking and carbon neutrality goals.Therefore,developing an eco-friendly and superior performance approach to replace ordinary Portland cement for soft soil treatment is urgently needed.Magnesium oxychloride cement（MOC）is an eco-friendly air-hardening cementitious material with high mechanical strength,rapid setting.Taking the soft clay in Wuxi as the main research object,The influence of MgO/Mg Cl2 molar ratio,H2O/Mg Cl2 molar ratio and curing time on the mechanical properties of MOC solidified soft clay were investigated by performing unconfined compressive strength（UCS）tests.On this basis,solidified effects of MOC solidified soft clay,MOC solidified silty clay and MOC solidified sand were evaluated.The water immersion test was used to evaluate the improvement effect of different modifiers on the water resistance of MOC solidified soft clay.Finally,X-ray diffraction（XRD）and scanning electron microscope（SEM）was performed to explore its mechanism and the improvement mechanism of MOC solidified soft clay.The main research contents and results are as follows:（1）The optimal H2O/Mg Cl2 molar ratio is found as 17–20 when MgO/Mg Cl2 molar ratio is 3:1～5:1,while it is 29～41 when MgO/Mg Cl2 molar ratios are 6:1 and 7:1.The main hydration products of MOC-solidified soft clay with MgO/Mg Cl2 molar ratio of 3:1～4:1 are5Mg（OH）2·Mg Cl2·8H2O（phase 5）and 3Mg（OH）2·Mg Cl2·8H2O（phase 3）,whereas phase 5and Mg（OH）2 are mainly produced when MgO/Mg Cl2 molar ratio is 5:1～7:1.MOC has the best solidification effect of sand,with the highest strength of 28MPa in 7d,followed by silty clay,with the highest strength of 9MPa in 7d.The highest strength of solidified soft clay is4MPa in 7d.（2）The UCS increases and then exhibits a decreasing trend with the curing time.Through microscopic test,it can be observed that the main product of MOC solidified soft clay are phase 5（5Mg（OH）2·Mg Cl2·8H2O）,phase 3（3Mg（OH）2·Mg Cl2·8H2O）and Mg（OH）2.The decrease of UCS is due to the gradual transformation of phase 5 into phase 3 and Mg（OH）2.（3）In the water immersion test,the sample of MOC solidified soft clay cracks within 24hours of water immersion,and the strength of the sample decreases significantly after 28 days of water immersion,and the softening coefficient is only 0.28,which is difficult to apply in practice.After adding silica fume,fly ash,mineral powder and brick powder,the dehiscence of soil samples has not been significantly improved,and the maximum softening coefficient can only be increased by about 0.04,which still cannot be applied in practice.The soluble phosphate contained in phosphogypsum can effectively improve the water resistance of MOC solidified soft soil,so this paper proposes a new idea of phosphogypsum modified MOC solidified soft soil.The water resistance of the specimens increases with the increase of phosphogypsum content,while the strength of the specimens decreases first and then increases before immersion.When the mass ratio of soft soil to phosphogypsum is 7:3,the strength of the specimens is similar to that of the unmodified sample,but the softening coefficient increases by 0.24 and the strength of specimens after water immersion increases from 1.2MPa to 2.2MPa.When the mass ratio of soft soil to phosphogypsum is 5:5,the strength of specimens after water immersion increases by 2.5MPa.A small amount of phosphogysum can effectively improve the water resistance of MOC solidified soft soil and ensure the application of MOC solidified soft soil.When the mass ratio of soft soil to phosphogypsum is 2:8,the strength and water stability of the specimens are optimized.The strength of specimens solidified soft soil before water immersion reaches 6.8MPa and the strength of specimens can still reach 5.2MPa after 28d water immersion,with a softening coefficient of 0.76.MOC solidified soft soil modified by SAP and phosphogypsum can reduce the MOC content and also ensure that the strength of specimens meets the actual engineering requirements,providing a new idea for the recycling treatment of solid waste. |
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