Research On The Failure Mechanism And Control Of The Key Bearing Structure Of Mining Coal Pillar Dam

Nanometer SiO₂ mineral powder has enormous potential to enhance the performance of cement-based materials.However,the addition of nanometer SiO₂ mineral powder to cement tends to cause agglomeration,which prevents the full exploitation of the nanoscale effect and introduces defects into the cement concrete.Existing methods such as high-speed mixing,ultrasonic dispersion,and surface modification have achieved some success in reducing the agglomeration of nanometer particles,but complete dispersion remains a challenge.Therefore,the question of how to fully exploit the nanoscale effect when nanomaterials are mixed with cement is an urgent problem to be solved internationally.To address this issue,this study is based on research on the preparation of nanometer SiO₂mineral powder using liquid-phase methods both domestically and internationally.We propose a method that replaces nanometer SiO₂ mineral powder with a precursor solution and allows the nanometer particles to uniformly precipitate in the cement paste.This approach is referred to as"nano-internal generation"in cement-based materials.Firstly,we investigate the influence of synthesis parameters such as silicon raw materials and acidic media on the stability of the nanometer SiO₂ precursor solution and successfully synthesize a stable solution where nanometer particles have not yet precipitated.Based on this,we mix the nanometer SiO₂precursor solution with cement to prepare nanometer internally generated cement-based materials.Meanwhile,using SEM,TEM,and NMR techniques,we establish a multidisciplinary and multi-perspective approach to elucidate the mechanism of nanometer particle internal generation in freshly mixed cement slurry.We analyze the role of internally generated nanometer particles in the mechanical and workability properties of cement-based materials and provide a profound understanding of the reasons behind the full exploitation of the nanoscale effect by internally generated nanometer particles.To achieve these objectives,this study explores and innovates in the following areas:（1）Research on the Long-term Stability Mechanism of Nano-SiO₂ Precursor SolutionThe instability reasons of the nanometer SiO₂ precursor solution were investigated using various testing techniques,including video microscopy,scanning electron microscopy,transmission electron microscopy,X-ray diffraction,infrared spectroscopy,and nuclear magnetic resonance.The evolution mechanism of the precursor solution’s macroscopic,microscopic,and nanoscopic structures during the instability process was studied.A long-term stabilization mechanism with a saturated small molecule layer was proposed to eliminate the detrimental effects of gelation instability on the precursor solution’s stability.The research findings revealed that the initial synthesized nanometer SiO₂ precursor solution is a true solution,with silica existing as silicic acid oligomers in the solution before precipitation.However,the presence of a large number of silanol groups on the surface of silicic acid promotes the aggregation,cross-linking,and agglomeration of silicic acid oligomers,leading to overall gelation instability of the precursor solution.By adjusting the formulation of the precursor solution and controlling the system’s p H value to≤4,acetic acid molecules can attach to the surface of silicic acid oligomers through hydrogen bonding,forming a saturated small molecule adsorption layer.This layer effectively maintains the long-term stability of the precursor solution.（2）Research on the Internal Generation Mechanism of Nano-SiO₂ in Fresh Cement PasteTo identify and confirm the presence of internally generated nanoparticles,the cement paste in the early hydration stage was diluted into a low-concentration anhydrous ethanol suspension.Differential settling was then employed to separate particles with different densities from the suspension.Subsequently,various testing methods such as video microscopy,scanning electron microscopy,and transmission electron microscopy were utilized to observe the morphology and analyze the composition of the different settling layers.Based on parameters such as the morphology,particle size,and uniformity of the internally generated nanoparticles,together with the analysis of the acidity/alkalinity of the cement slurry during the nanoparticle precipitation process and thermodynamic reactions,the mechanism of internal nanoparticle generation was investigated.The research results indicate that the precursor solution of nano-SiO2 is a weak acidic solution in which nanoparticles have not yet precipitated.Upon mixing with cement,rapid interaction occurs.As the acetic acid protective layer is consumed,the destabilization and precipitation of silicic acid oligomers lead to the formation of nanoparticles.These nanoparticles exhibit a spherical shape with a particle size distribution ranging from 15-80 nm,and are uniformly distributed around and on the surface of cement particles.（3）Research on Strengthening Mechanism of Nano-internal Generation on Mechanical Properties of Cement-based materialsBy analyzing the effects of the formulation,dosage,components,and storage time of the nanometer SiO₂ precursor solution on the mechanical properties of cement-based materials,in conjunction with scanning electron microscopy,mercury intrusion porosimetry,and CO₂adsorption techniques,the role of internally generated nanoparticles in the microstructure construction process of hardened cement paste was investigated.Additionally,the enhancement mechanism of nanometer internally generated nanoparticles on the mechanical properties of cement-based materials was analyzed.The research findings revealed that internally generated nanoparticles eliminate the agglomeration issues of powdered nanoscale particles and fully exploit the nucleation and filling effects of nanomaterials.This promotes the densification of hardened paste and enhances the mechanical properties of cement-based materials.（4）Research on the Enhancement Mechanism of Nano-internal Generation on the Rheological Properties of Cement PasteBy analyzing the composition,dosage,and the influence of the addition of superplasticizers to the nanometer SiO₂ precursor solution on the flowability and rheological properties of cement slurries,in conjunction with SEM,FSEM,and ZETA potential measurements,the enhancement mechanism of the precursor solution on the rheological properties of cement slurries was revealed.The research findings demonstrated that the nanometer SiO₂ precursor solution increased the early dissolution rate of cement particles and increased the ion types and concentrations in the cement slurry,leading to a redistribution of the charge composition in the double electric layer structure of cement particles.During the precipitation process of nanometer particles,the charge carrying capacity of cement particles significantly increased,resulting in electrostatic repulsion between cement particles that exceeded attraction forces,thereby enhancing the rheological properties of the cement paste.This dissertation includes 78 figures,21 tables and 153 references...