Study On Mechanical And Permeability Properties Of Modified Fiber Reinforced Geopolymer

In deep underground engineering,the problem of large deformation caused by high geostress and the problem of tunnel seepage caused by high osmotic pressure are becoming more and more serious.Ordinary concrete is brittle and can hardly meet the demand of seepage prevention and support.There is an urgent need for a new composite material with tight internal structure,excellent crack control ability,and resistance to large deformation.This article uses fly ash,slag,and PVA fibers as the basic raw materials,and Ca SO₄ whiskers and nano Si O₂ as modified materials to prepare modified PVA fiber reinforced geopolymers.The effects of Ca SO₄ whiskers and nano Si O₂ on the microstructure,mechanical properties and permeability of fiber geopolymer were studied through physical experiments,and the modification mechanism of the modified materials was discussed;Through numerical simulation,the variation characteristics of seepage and stress fields in deep underground engineering of tunnels supported by modified fiber geopolymer with increasing tunnel burial depth were studied.The main content and conclusions of the thesis are as follows:（1）Determine the basic silicon aluminum material ratio of fiber based geopolymer,select Ca SO₄ whiskers and nano Si O₂ as the influencing factors of material modification ratio,prepare geopolymer specimens,and conduct nuclear magnetic resonance testing to analyze the changes in porosity,pore size ratio,and other factors of the material with changes in Ca SO₄ whisker and nano Si O₂ content.Research has shown that Ca SO₄ whiskers and nano Si O₂ can effectively reduce the porosity of geopolymers.Compared with unmodified fibrous geopolymers,the porosity is reduced by 19.14%and 14.78%,with a smaller proportion of harmful and multi harmful pores in the matrix.（2）The effects of CaSO₄ whiskers and nano SiO₂ on the strength,deflection,number and width of cracks in the specimens were studied through uniaxial compression and four point bending tests.The results indicate that within a certain range,whiskers and nano Si O₂ can effectively enhance the strength of the matrix.Adding Ca SO₄ whiskers and nano Si O₂ can increase the initial and ultimate loads of geopolymers,delay the generation of microcracks,and reduce the width of cracks,micro cracks ranging from 40μm to 60μm account for over 70%.（3）The permeability performance of fiber toughened polymer at cracks after loading was studied using a variable head permeability device.The results indicate that the combined effect of fibers and modified materials endows geopolymers with an excellent ability to control microcracks,and the permeability of the material at the crack site remains at a low level after being loaded.Using quadratic polynomial fitting,obtain the relationship between the average permeability coefficient of each crack in each group of specimens and the variation of crack width.（4）Through numerical simulation methods,a tunnel model with modified fiber geopolymer as support structure was established to study the changes in seepage and stress fields of the tunnel at different burial depths.The results show that the water seepage and deformation of the surrounding rock in the tunnel continue to increase with the increase of burial depth.The EGC support with Ca SO₄ whiskers and nano Si O₂ has less water seepage as well as displacement than those without the modified materials The improvement effect of modified materials gradually becomes significant with the increase of burial depth,and is significantly better than that of ordinary concreteThere are 71 figures and 30 tables in this thesis.