Experimental Study On Stabilised Soil With Metakaolin-Based Geopolymer Excited By Complex Alkali

Peaty and red clay are widely distributed in Yunnan plateau,peaty has the characteristics of extremely high organic matter content,ultra-high water content and great pore ratio,The red clay has the characteristics of crack development,large shrinkage,upper hard and lower soft layer distribution and so on,which together determine that the construction structures associated with it usually have great safety hazards.Therefore,it is necessary to solidify the two to improve their engineering applicability in order to reduce the safety risk.However,the effect of strengthening peaty soil with ordinary silicate cement is not ideal.When strengthening red clay,the cement content is too high,which is extremely uneconomical and poses a threat to the environment.Therefore,according to the characteristics of peaty and red clay,on the premise of ensuring the strength,to explore the new curing agent and the availability of red clay suitable for different soils,it is of great theoretical significance and engineering application value for the construction and development of a large number of peaty or red clay areas.In this paper,a series of laboratory tests on peaty and red clay were carried out to study the mechanical properties and microscopic mechanism of pure cement,supplementary cementitious system（cement+metakaolin）and compound alkali activated system（cement+metakaolin+sodium silicate）.The main research contents are as follows:1.The relevant physical and chemical indexes of peaty soil and red clay in Kunming,Yunnan are determined,and the results show that:（1）The water content of the peaty is as high as 284.02%,the organic matter content is 64.91%,the porosity ratio is 5.48,and the fiber content Wf is only 3.07%.The peaty is composed of quartz,kaolinite and Muscovite,etc.,and its microstructure has many pores and the overall structure is loose.（2）The water content,porosity ratio,liquid limit and plastic limit of red clay are 33.92%,1.43%,59.8%and 32.2%,respectively.The main oxides Si O2,Al2O3 and Fe2O3 account for more than 80%.The main phases are quartz,kaolinite,hematite and so on.2.The indoor solidification test of peaty was carried out,and the results showed that:（1）The strength of the solidified soil of the supplementary cementitious system（cement+metakaolin）is improved to a certain extent,and the strength gain is significant when the content of metakaolin is less than 7%.The strength of peaty solidified by compound alkali activation system（cement+metakaolin+sodium silicate）decreases at first and then increases with the increase of sodium silicate,because the oligomer is combined or even eroded by humus and can not form high polymer gel;and humus adsorption water and conservation water have an inhibitory effect on geopolymerization.（2）After curing with different systems,the moisture content of the samples decreased,the dry density and p H value increased,and all the samples showed ductile failure during shear,and there was a positive correlation between vertical pressure and shear strength.（3）The micro-morphology of the solidified soil in the supplementary cementitious system is relatively dense,and the hydration products produced can effectively improve the internal structure of the solidified soil.The micro-morphology of the solidified soil of the composite alkali-activated system can see the amorphous geopolymer gel with white flocculation and agglomeration,which is different from the supplementary cementitious system.（4）The phase composition of the solidified soil of the compound alkali activation system was basically unchanged,but the characteristic peaks decreased in varying degrees,and the steamed bread dispersion peak appeared in the range of 2θangle 20～40.3.The indoor curing test of red clay was carried out.The results show that:（1）The strength of different samples after standard curing is basically higher than that of immersion curing,and the immersion condition is not conducive to the occurrence of alkali-activated geopolymerization.（2）The stress-strain curve of the solidified soil of the supplementary cementitious system shows that the linear elastic stage of the failure process is prolonged and the plastic stage decreases,while the solidified soil of the compound alkali activated system is closer to the brittle material.（3）When the solidified soil of the supplementary cementitious system is mixed with 9%,12%and15%cement,the threshold value of kaolin is 3%,and the strength beyond the threshold is reduced.（4）The strength of solidified soil with compound alkali activation system is much higher than that of pure cement soil,and the strength increases greatly at first and then decreases and tends to be stable with the continuous increase of sodium silicate.The strength improvement coefficient is the highest when12%cement is added,indicating that the calcium content should be kept in a suitable range.And there is an optimal n（Ca O）/n（Si O2）,which makes the strength of the sample reach the maximum;with the increase of n（Si O2）/n（Al2O3）,the strength of the sample increases rapidly,and then the change of strength tends to be stable;the hydration of cement produces part of the negative charge in the Ca²⁺energy balance system,and the strength of the sample is greatly improved when n（Na2O）/n（Al2O3）is small.（5）All solidified soils show brittle failure during shear,and there is a significant peak value in the stress-strain curve.（6）Compound alkali excited metakaolin-based polymer solidified red clay contains whitening amorphous polymeric gel（N,C）-A-S-H and C-（A）-S-H,and the peak value of the main phase composition of the sample decreases,which proves that the strength increase is mainly due to the effect of geopolymeric gel,and the microscopic action mechanism model is established.（7）When the composite alkali activation system meets the same strength requirements,each cubic meter of solidified red clay needs only 0.083 tons of cement（mixed with 5%metakaolin and 3%sodium silicate）,which can reduce carbon dioxide emissions by 38.76%.4.The laboratory test of solidifying peat soil with red clay as external admixture was carried out.The results show that:（1）Adding red clay can improve the strength of pure cement and supplementary cementitious system solidified soil,and the effect is the best when the mixing ratio reaches 30%.（2）The solidified soil strength of the composite alkali activated system mixed with red clay is lower,and its strength decreases with the increase of sodium silicate content,and the decrease is the largest at 7%,due to the complexation of humus to Ca²⁺,Al³⁺,etc.,and the release of humic acid will neutralize part of OH^-,and continuously erode the solidified structure.（3）The addition of clay can reduce the moisture content of solidified soil and increase the dry density of solidified soil,but has little effect on p H value.（4）The addition of red clay can also improve the shear strength and increase the angle of internal friction.（5）After the addition of red clay,the pores of cement soil are refined and ettringite growth is promoted,and the solidified soil of supplementary cementitious system has the densest microstructure;There are many cracks in the solidified soil of the compound alkali excitation system,and the gel fragments are distributed among the soil particles,and the peak of primary minerals in the XRD spectrum decreases,but no steamed bread-like dispersion peak is observed in a specific range,indicating that humus hinders the formation of geopolymerized gel in the high aggregation state.