Controllable Preparation Of Insitu Grown NS/CSH On Iron Tailings And Its Action Mechanism On Cement-based Materials

To meet the needs of engineering construction and green sustainable development,new materials and technologies need to be developed to improve the utilization rate of solid waste while enhancing the performance of cement-based materials.The emergence of nanomaterials and nanotechnology provides unprecedented prospects for enhancing the performance of traditional cement-based materials.The collision of tradition and modernity has given cement-based materials new vitality.In recent years,research indicated that the application of nanomaterials in cement-based materials could achieve positive effects in terms of hydration properties,mechanical properties and durability.Although nanomaterials have excellent performance,the high price compared to cement limits their development and application in this industry.Therefore,the intersection of research value and application value is more realistic.Nano silica,as one of the most widely used and economically cost-effective nanomaterials in this industry,has a unique pozzolanic effect in addition to the filling and nucleation effects that most nanomaterials possess.However,the small size and large surface energy of NS make it prone to agglomeration,which limits its strengthening effect in cement-based materials.Based on the background discussed above,to fulfill the excellent properties of NS,it is necessary to find suitable dispersion and modification methods,and apply them reasonably to strengthen cement-based materials.This study proposes to use solid waste as a carrier for in situ grown nano-SiO₂（abbreviated as INS）,and use the excellent dispersion of solid waste to improve the dispersion of NS in cement-based materials,realizing the combination of nanotechnology and solid waste resource utilization.This article first explores the particle characteristics and activity of ITP,and obtains that ITP has good dispersibility in cement paste,which is also a prerequisite for in-situ growth matrix.Secondly,to address the issue of the dispersion,we studied the controllable preparation of in-situ grown nano SiO₂.On this basis,INS was used as a silicon source to prepare in-situ grown nano CSH（abbreviated as ICSH）by hydrothermal method;and the dispersion characteristics of INS/ICSH were studied to determine the optimal preparation process.Then,the influences and mechanisms of INS/ICSH on workability,rheological properties,mechanical properties,cement hydration,hydration products cement hydration,pore structure,interfacial transition zone and C-S-H gels were systematically investigated.The main research results obtained are as follows:（1）As the grinding time increases,the ITP particles tend to be refined.The refinement process increases the specific surface area of ITP,reduces surface energy,and improves activity;But there is a grinding limit.The pozzolanic activity of the ITP needs to be stimulated in a cement alkaline solution for a long time.The results indicate that appropriate grinding is beneficial for improving its activity,but it cannot compensate for the decrease in hydration activity of the system caused by low activity of ITP;it was found that the ITP is well dispersed in the cement paste,which is also a prerequisite for in-situ growth matrix.（2）In the preparation process of INS,when the mass ratio of surfactant CTAB to ITP is 1:1,uniform mixing for 24 h is the optimal substrate/surfactant ratio for NS growth;and by regulating the ratio of raw materials and preparation conditions,uniform growth of NS is obtained.The results of SEM,TEM,XRD and FTIR show that uniform NS particles have successfully grown on the surface of ITP and achieved control over their growth particle size and growth amount.At this time,NS particles have good dispersibility and show good binding properties in morphology.Through thermogravimetry,it was found that NS accounts for 38.67%by weight of INS composite material,and the pozzolanic activity of INS is significantly higher than NS.Additionally,INS shows a more stable dispersion state than NS in deionized water solution.（3）The morphology and structure of the synthesized pure CSH powder were determined as a prerequisite condition through testing methods such as XRD,TEM,and NMR.In-situ grown single-dispersed NS particles are used as silicon source to successfully grow nano-CSH on the surface of ITP through hydrothermal method,and its growth mechanism was revealed.The results of SEM,XRD,XPS and FTIR showed that nano-CSH was successfully wrapped and grown on the surface of ITP,showing core-shell structure characteristics in morphology.TG analysis showed that nano-CSH accounts for 56.36%by weight of ICSH composite material.ICSH has higher dispersion stability in water or saturated CH solution than pure CSH powder.（4）INS maintains good workability and has little effect on the yield stress and plastic viscosity of fresh paste.INS not only improve the early strength of cement paste（within 3 days）,but also did not lose the later compressive strength（28 days）.However,the agglomerated NS is mixed with air and water,which would become a weak area of the hardened paste,leading to stress concentration and weakening the strength enhancement effect.Compared to NS,INS provides a larger number of nanoparticles and a large relative value of total surface areas,thus providing more nucleation sites,accelerating the deposition of hydration products and the dissolution of cement minerals,further accelerating the hydration of cement.INS can more effectively reduce the crystal size of CH while increasing the MCL of Si-O chains in C-S-H gel,making Q1become Q2 and improving the degree of silicate polymerization.The microstructure indicates that the cluster of NS leaves large pores in the hardened paste;while INS can better refine the pore structure,while ITP as a nucleation can better compact the pores and increase the overall density of the hardened paste.Additionally,INS forms a certain adhesive force in the ITZ,hindering the microcrack extension between fine aggregates and paste,making the interface transition zone more uniform and denser.（5）Due to its good dispersion state,ICSH can be used for more nucleation growth,providing more additional nucleation sites in cement pores,so that the acceleration effect is more obvious;pure CSH powder can improve early strength but has little effect on later strength,while ICSH can improve strength at all ages;at the same time,fresh paste containing ICSH maintains good workability and rheological properties.With the extension of curing time,the side effects of agglomeration of pure CSH powder become more obvious,and the free water and gas wrapped by CSH powder gradually evaporate,leaving large pores exposed as weak areas of the hardened paste;however,ICSH as ITP-CSH shell structure produces a rigid system,and ITP matrix can better compact the pores and improve the overall compactness of the hardened paste.Finally,ICSH can effectively fill between slurry and aggregate with bonding and stability,hindering microcrack extension between fine aggregate and slurry,making ITZ more uniform and denser.Additionally,CSH nanoparticles have an autocatalysis acceleration process,and the comparison results show that ICSH has more obvious strengthening effect on cement hardened paste than INS.In summary,the improvement of the dispersibility of nanoparticles can be achieved through solid waste carriers.This paper broadens the idea of solving the dispersibility problem of nanoparticles in cement-based materials by growing NS on ITP surface in situ,and also provides theoretical basis and method guidance for preparing green high-performance cement-based materials.