Experimental Study On The Strength Characteristics And Micro-mechanism Of Lime-metakaolin Modified Silt Soil

Most of the earthen sites in Henan Province are rammed with silt or silt,which is exposed to natural conditions for a long time and has strong water sensitivity.The soil of the sites is prone to deterioration,cracks and collapse,the Preventive Protection of Earthen Ruins is very urgent.The key to protect earthen sites is whether the reinforcement materials are used properly,so the research and development of earthen sites restoration materials plays a vital role.In order to develop hydraulic lime substitute material and improve the improvement effect of traditional lime soil in the restoration of earthen sites,four kinds of lime and four kinds of metakaolin modified soil samples are prepared with typical silty sand soil in Henan Province as the research object,after curing,the unconfined compressive strength test and cracking tensile strength test are carried out,and the results are compared with those of hydraulic lime soil.At the same time,some soil samples were tested by SEM and XRD to clarify the internal micro-mechanism of strength characteristics.The study can provide theoretical and technical support for Preventive Protection of earthen sites.The main findings and conclusions are as follows:?1?The compaction tests of lime-metakaolin and hydraulic lime with different content were carried out.The results show that with the increase of metakaolin content,the optimum moisture content increases and the maximum dry density decreases.The addition of metakaolin can effectively inhibit the water sensitivity of lime soil,and the lime-metakaolin material has better effect than hydraulic lime in inhibiting the water sensitivity of lime soil,and is more favorable for field construction.?2?Through unconfined compressive strength test,the axial stress-strain curve is drawn and analyzed.The results show that the stress-strain curve of the modified soil belongs to softening type,and both of them present two stages of strain hardening and strain softening.When the curing age and lime content are fixed,with the increase of metakaolin content,the failure strain increases gradually and is obviously larger than that of lime soil.It shows that the lime metakaolin modified soil has good"toughness".Compared with hydraulic lime,it is found that when 6%lime+4%metakaolin and8%lime+4%metakaolin are added,the failure strain of L-MK improved soil is equivalent to that of 8%and 10%hydraulic lime respectively.?3?In addition,the failure modes of lime soil and hydraulic lime modified soil are cone-shaped.However,the fracture characteristics of splitting tensile strength specimens basically show typical splitting failure characteristics.The main feature of the fracture is that the specimen produces a main crack which is approximately 90°.After the failure,the specimen breaks from the middle into two halves,and the failure surface appears uneven.?4?It is found by analyzing unconfined compressive strength.The splitting tensile strength increases with the increase of metakaolin content and age,and is higher than that of lime soil.Compared with hydraulic lime,when 6%lime+4%metakaolin and 8%lime+4%metakaolin are added after 28 days of curing,the strength value of hydraulic lime reaches 8%and 10%,and meets the requirements of specification test code for inorganic binder stabilized materials of Highway Engineering?JTG E51-2009?.The compressive strength of the modified soil also shows a law of increasing with the increase of age and the amount of admixture,and the strength grade requirement of hydraulic lime NHL2 in Building lime?EN459-1/2010?.The results show that the unconfined compressive strength of lime metakaolin is about 7.63 times of the tensile strength,and the compressive strength of hydraulic lime is about 6.91 times of the tensile strength.?5?It is concluded that the unconfined compressive strength and splitting tensile strength have obvious"segmentation"when 4%metakaolin is added,that is to say,with the increase of lime and metakaolin,the strength growth rate first increases and then decreases.Therefore,6%?8%lime and 4%metakaolin are recommended for strength value and strength growth rate.?6?According to the XRD test results,a series of hydrates,including Ca?OH?₂and C₄AH₁₃,are produced in the lime metakaolin reaction.The qualitative and quantitative analysis of the modified soil by SEM test shows that there is an inseparable relationship between the macro strength characteristics and the micro pore structure.That is to say,with the increase of the amount of admixture,the soil structure becomes more and more dense,and the soil strength also increases gradually.Further quantitative analysis of SEM images shows that the pore content and the average equivalent diameter of soil particles decrease gradually with the increase of the amount of admixture,and the pores show a nondirectional arrangement.In conclusion,due to the cementitious material generated by hydration reaction,the soil is denser,thus enhancing the strength characteristics of the soil,which also corresponds to the compressive strength and tensile strength of lime metakaolin soil modified soil.?7?Based on the comprehensive analysis of the failure strain,unconfined compressive strength,strength growth rate,splitting tensile strength and the strength requirement of NHL2?2?5MPa?of Euro standard BS.EN459-2010.It is suggested that 6%L+4%MK and 8%L+4%MK should be used instead of 8%NHL and10%NHL respectively for the restoration of earth sites.