Laboratory Experimental Study On Soft Soil Stabilized By Fly Ash-slag Based Geopolymer

Soft soil usually has lots of shortcomings,such as poor compressibility,small permeability,large disturbance,low strength,obvious time effect,and uneven thickness distribution,etc,which is often difficult to meet the actual engineering conditions.Therefore,it needs to be stabilized.Cement is often used as solidifying agent to stabilize soft soil.While the traditional cement deep mixing has several disadvantages:large dry-shrinkage,poor water stability,easy cracking,poor curing effect in the soil with high plasticity index and high organic matter.In order to avoid these problems,present paper used fly ash-slag based geopolymer,a green cementing material,to replace the traditional Portland cement in soft soil stablization,a large number of experimental studies had been carried out on the mechanical properties,water stability and microscopic mechanism of soft soil stbalization.The main contents and research results are list as follows:(1)Study on the mechanical properties of fly ash-slag based geopolymer.Firstly,selecting compressive strength as the evaluation index of mechanical properties,single factor tests were carried out on the main controlling factors affecting the mechanical properties of geopolymer,such as slag content,activator modulus,alkali equivalent and water-binder ratio.We can find that,with the increase of slag content,the compressive strength of the geopolymer increases continuously,while when the activator modulus,alkali equivalent,and water-binder ratio change within a certain range,the compressive strength of the geopolymer specimens show a trend of first increase and then decrease.Based on the results of single factor tests,response surface experiments were carried out to investigate the interaction among the major regulatory factors.It is found that the short-term strength of geopolymer slurry is mainly related to alkali equivalent and activator modulus,and the interaction between activator modulus and water-binder ratio is significant.(2)Study on mechanical properties of fly ash-slag based geopolymer and soft soil stabilized by fly ash-slag based geopolymer.The main factors affecting the properties of soft soil stabilized by fly ash-slag based geopolymer are slag content,activator modulus and alkali equivalent.Based the single factor test,it was found that the higher the slag content was,the higher the strength of the solidified soil was.When the activator modulus was 0.7 and the alkali equivalent was 10%,the compressive strength of the solidified soil was the best.On this basis,orthogonal design tests were carried out.Compressive strength,cohesive force and internal friction angle were selected as evaluation indexes to obtain the primary and secondary order of each factor,and the optimal mix ratio.The results showed that when the activator modulus was 0.6,the alkali equivalent was 10%,and the slag content was 50%,the mechanical properties of the geopolymer stabilized soft soil were the best.Based on study of the effect of fly ash-slag based geopolymer on soft soil stabilization,it is found that the increase of curing age and geopolymer content can improve the compressive strength of geopolymer stabilized soil,while the higher the moisture content of soft soil is,the worse the effect of geopolymer stabilized soil is.In addition,geopolymer has an important effect on the water stability of soft soil and can significantly improve the ability of soft soil to resist the weakening effect of flooding.(3)Study on micro-mechanism of the stabilized soft soil by fly ash-slag based geopolymer.Through XRD and SEM tests,the microscopic mechanism of fly ash-slag based geopolymer and stabilized soft soil was studied and analyzed.It can be seen that the improvement of mechanical properties of geopolymer is mainly due to the formation of C-S-H and N-A-S-H from geological polymerization reaction,which jointly promote the solubility-condensation reaction.The main reason for the improvement of macroscopic mechanical properties of soft soil stabilized by fly ash-slag based geopolymer is the formation of C-S-H and N-A-S-H,which together gather loose soil particles and is filled in the void,so that the soil particles are connected more closely,which improves the integrity of soil.In addition,the remaining unreacted fly ash and slag particles play a filling role among the soft soil particles,which also reduces the porosity and improve the strength.The research results of present paper can provide basis and reference for the application of fly ash-slag based geopolymer materials in the field of soft soil stablization instead of cement.