Study On Properties Of UltraFine Cement-Based Micro-Crack Grouting Material Modified By Nano-Calcium Carbonate

With the depth of coal mining into kilometer level,micro-fissure seepage and permeability become new water plugging problems.Under the action of high in-situ stress and high osmotic pressure,the seepage effect of many micro-fissures less than 0.1mm increases,which becomes the main reason for the increase of wellbore water inflow.Ultra-fine cement slurry is the main grouting material for water plugging in micro-fissures because of its green,low cost and small size.However,it is difficult to achieve ideal grouting effect because of its high viscosity and high water precipitation rate.The addition of ultra-fine fly ash to ultra-fine cement slurry can enhance the fluidity of slurry.Nano-calcium carbonate can play the role of crystallization nucleus of nano-materials in cement-based composites,change their layered structure,promote the hydration rate of cement,and complement the advantages of ultra-fine fly ash.Polycarboxylic acid superplasticizer has high water reducing properties,which can reduce slurry viscosity and improve fluidity.Therefore,in this paper,superfine fly ash,nano-calcium carbonate,polycarboxylic acid water reducer and other additives are used to improve the performance of pure superfine cement.The superfine cement-based slurry is studied from the following aspects:(1)Based on the analysis of the basic physical properties of ultra-fine cement,ultra-fine fly ash and nano-calcium carbonate,the requirements of particle size,water-cement ratio,viscosity and water evolution rate for grouting water shutoff in deep well with 0.1 mm opening micro-fissures were obtained.(2)Two kinds of pure ultra-fine cement slurries were tested in terms of particle size,water cement ratio and water reducing agent content,and their water analysis rate,compressive and flexural strength were compared.The effects of water cement ratio and Superplasticizer on the initial viscosity,flexural and compressive strength and time-varying viscosity of superfine cement slurry were obtained(3)The influence of nano-calcium carbonate on the water evolution rate,viscosity,diffusion size,compressive and flexural strength of ultra-fine cement-based composite slurry(nano-calcium carbonate,ultra-fine fly ash,polycarboxylic acid water reducer)was obtained by single factor test of nano-calcium carbonate.(4)Through orthogonal test,the water release rate,viscosity,diffusion size,compressive and flexural strength of slurry were studied.The time-varying curves of 9 groups of grout viscosity in orthogonal test were obtained.The optimum proportion of nano-calcium carbonate modified ultra-fine cement micro-crack grout was nano-calcium carbonate 1%,ultra-fine fly ash 40%,superplasticizer 0.5%,water-cement ratio 1.5 by using regression analysis method.(5)The micro-fracture grouting with 0.1 mm opening was studied by numerical simulation,and different void ratio,different elastic modulus and different time were selected for comparison.The variation of grouting pressure with diffusion distance,stress and deformation of surrounding rock and micro-fissure displacement,and the temporal and spatial variation of micro-fissure boundary displacement were obtained respectively.