Properties Of Alkali Activated Cementitious Material Modified By Nano SiO₂ And Its Application In Shield Slag Solidification

With the rapid development of subway traffic in my country,how to effectively dispose of shield muck has become one of the most prominent problems in subway construction.If the undisturbed shield muck is directly made into a non-burning product under the condition of controlling the finished product,it will save a lot of muck disposal costs for the construction enterprise.The previous research of this research group used alkali-induced mineral powder pressure forming to solidify shield slag,and the effect was good.However,the pressure forming preparation process has high energy consumption and limited product size,which affects large-scale application,and there are certain limitations.Amorphous nanomaterials have been widely used in the modification of cementitious materials due to their high chemical activity and large specific surface area.Studies have shown that the addition of amorphous inorganic nanomaterials such as silica into alkali-activated gelling materials can effectively improve the mechanical properties and durability of the system.However,the existing studies mainly use powdered nanomaterials,which have problems such as increased water demand and difficulty in ensuring uniformity.Commercially available nano-silica emulsions have problems such as easy agglomeration,short shelf life,and difficult quality assurance.The preparation of amorphous nano-silica by sol-gel method has the characteristics of simple process,high activity,easy mixing and uniformity.By studying its preparation process,the time and method of adding it to the alkali excitation system,it is expected to obtain high strength and high workability alkali excitation cementitious materials,realize the pouring and forming process of solidified shield slag,and solve the current shield slag Handling limitations.For this reason,this paper is based on the research and development project CSCEC-2018-Z-1 of China Construction Corporation Limited—“Research on Key Technologies and Industrialization of Subway Residue Treatment”,combined with the actual shield construction project of Changsha Metro,to study the preparation of sol-gel method.The properties and microstructure of nano-SiO₂modified alkali-excited slag-fly ash cementitious materials and solidified shield slag,in order to provide a new way for the resource utilization of shield slag,fly ash and mineral powder.The main research work and results of this paper are as follows:（1）The single factor experiment was used to explore the workability and compression resistance of the nanomaterial modified alkali-excited mineral powder-fly ash system in the process of preparing nano-SiO₂by sol-gel method,such as catalyst concentration,mixing time,aging time and other factors.Influence of strength and shrinkage.The results show that the 7d compressive strength of nano-SiO₂prepared by adding 1%nitric acid concentration of 1.8M,mixing time of5h,and aging time of 1h increases by 30%-40%,and the 7d compressive strength can reach up to 86.8MPa.The shrinkage value at 27 days decreased by 43.62%.The effects of nano-SiO₂content,alkali equivalent,water-cement ratio,and activator modulus on the fluidity,compressive strength and shrinkage of nano-SiO₂modified alkali-excited slag-fly ash cementitious materials were investigated by orthogonal experiments.The mineral composition and microstructure of the system were obtained.The research found that the optimal ratio is:the content of nano-SiO₂is0.5%,the modulus of alkali activator is 1.4,the alkali equivalent is 7%,and the water glue is 0.38.The fluidity of the nano-modified composite obtained by shuffling the distribution ratio is 231mm,the 28d compressive strength is about114.5MPa,and the shrinkage is-8012.5με.The microscopic test characterization results of the structure show that more and denser amorphous gel substances are formed inside the alkali-activated gelling material with an appropriate amount of nanometers,and the alkali-activated gelling material is more dense.The results of microstructure analysis are basically consistent with the trend of compressive strength and bulk density.（2）Based on the optimal ratio obtained above,the nano-SiO₂modified alkali-induced cementitious material solidified shield plot sample was prepared by pouring and molding,and the amount of shield slag,the amount of shield slag,the amount of The influence of water content and water reducing agent content of shield muck on the compressive strength,shrinkage and buckling ratio of solidified shield muck was analyzed.At the same time,the effect of nano-SiO₂content on the properties of the solidified soil was studied by single factor experiment.The study found that the addition of 0.5%nano-SiO₂can improve the 28d compressive strength of the solidified shield slag by 30%-40%compared with the control group without nano-materials.When the dosage of superplasticizer is 1%,the 28d compressive strength can reach 67.1MPa.The solidified soil has good resistance to sodium sulfate attack,but poor resistance to sulfuric acid and hydrochloric acid.Microscopic experiments show that the addition of nano-SiO₂optimizes the pore structure of the solidified soil.The experiments and analysis in this paper show that the nano-SiO₂prepared by the sol-gel method can modify the mineral powder-fly ash alkali-induced cementitious material,and obtain high fluidity and high strength,thereby realizing the solidification shield.The pouring and forming process of the muck provides the basis for the wider engineering application of the solidified shield muck.