Mechnical Performance And Microstructure Characteristic Of Ultra-deep Shaft Lining Concrete In Coastal Environment

More than 80%of the substances used by human come from the mining industry,and mineral resources are the important material basis of the national economic development.However,after hundreds of years of exploitation,the resources on the surface of earth are almost exhausted.In theory,the mineralization spatial distribution of the earth is 10,000 m from the surface to the ground.Marching deep into the earth is a strategic and technological problem that we must solve.However,the environmental characteristics of high stress,high osmotic pressure,high temperature and strong corrosion in the deep underground pose an unprecedented challenge to the engineering.As the throat of mine safety,the selection of shaft lining concrete material is crucial.Focusing on the Shaling metal mine under construction in the coastal areas of eastern China,the paper designed a type of high strength and toughness cementitious composites(HSTCC),which contains quartz sand,microfilament fiber and nanometer silica fume according to the underground environment.Firstly,the bursting liability of different kinds of concrete were explored.Secondly,the failure pattern,energy characteristics and damage of typical types of concrete under static and dynamic loads were clarified.Thirdly,the change of phases and the failure mechanism of hardened pastes under the temperature and compound salt were revealed.Finally,the structural morphology and nano-mechanical properties of C-(A)-S-H in typical hardened pastes affected by environment were detected to show the macroscopic performance characteristics of concrete.The innovative achievements are following:(1)Concrete can accumulate deformation energy and produce impact damage,which can be called the bursting liability of concrete.The compressive strength,splitting tensile strength,elastic energy index,impact energy index and dynamic failure time of different strength grades,contents and kinds of fiber reinforced concrete are measured.The results show that the higher the compressive strength is,the stronger the bursting liability of normal concrete is.Meanwhile,the addition of fiber can effectively reduce the bursting liability of concrete.The related bursting liability parameters of HSTCC are the best,followed by steel wire end hook steel fiber reinforced concrete.(2)The failure patterns and energy evolution of normal high strength concrete(NHSC),steel fiber reinforced concrete(SFRC)and HSTCC subjected to static and dynamic loading were investigated by uniaxial hydraulic servo machine,acoustic emission(AE),split hopkinson pressure bar(SHPB)and ultrasonic wave analyser.The results indicate that the confluence of multiple cracks forms a penetrating cross section in NHSC under the static loading,while the elastic energy that surges out at failure can cause tremendous damage subjected to dynamic loading.A single crack was split into multiple propagation directions due to the presence of fibrers in SFRC,and adding fibers to concrete should be an effective way to dissipate energy.HSTCC exhibits a strong ability to resist impact.Meanwhile,it can store cracks,dissipate energy and ensure its integrity through its own structural characteristics.(3)The phase contents,morphology,and pore structure characteristics of the hardened pastes were investigated by means of a series of quantitative methods,such as x-ray diffraction(XRD),thermogravimetric(TG)and scanning electron microscope(SEM).The degree of hydration(DoH)and the degree of pozzolanic reaction(DoPR)were calculated to show the performance change of SFRC and HSTCC in coastal ultra-deep mine environment.The experimental results indicate that compared with high performance hardened paste(HPHP),the early DoH and DoPR of high strength and high toughness hardened paste(HSTHP)were very low,but the increase of DoH and DoPR is obvious under the influence of deep high temperature,which is beneficial to the impermeability and durability of HSTCC.Meanwhile,the invalidation of SFRC in coastal ultra-deep mines may be mainly caused by the crystallization pressure of Friedel's salt.(4)The molecular structure,element changes and nano-mechanical properties of C-(A)-S-H in hardened paste were studied by 29Si and 27Al solid nuclear magnetic resonance(NMR),SEM and nano-indentation technique.The results show that when HSTHP was affected by 60? and compound salt(HSTHP-sE2),the average main chain length of C-(A)-S-H reached 7.19,Ca/Si decreased greatly,and the content of high-density and ultra-high-density gel increased.The microstructure of HSTHP-SE2 was the densest and its macroscopic properties are further improved.Through the comprehensive tests,HSTCC was applied in the ingate of Shaling gold mine at-1,120 m,and the research is of great significance in ensuring the safety of workers in deep underground engineering environments.