Research On Impermeability Of Nano-modified Cement-based Materials And Its Mechanism

Impermeability is the first line of defense for the durability of cement-based materials.This paper studied the effect and mechanism of nano materials on the impermeability of cement-based materials,and established the impermeability model and prediction method of nano-modified cement-based materials,which is of great significance to the improvement of the durability of concrete structures.Cement-based materials have different initial microstructures due to different compositions and ratios,and have an influence on the modification effect of nanomaterials.Therefore,this article focused on the effect and mechanism of nanomaterials on the impermeability improvement of different cement-based materials,and adopted the research methods of macro performance testing,microstructure observation and analysis,theoretical calculation and simulation to systematically study the impermeability and mechanism of nano-modified cement paste,concrete and fiber concrete.Finally established the impermeability model and prediction method considering the difference of initial pore structure of cemented-based materials.The main contents include:First,the improvement effect and mechanism of nano-SiO₂ on the impermeability of cement paste were studied.Quantitative analysis of nano-SiO₂’s promotion for cement hydration and optimization of cement pore structure revealed the micro-mechanism of nano-SiO₂ to improve the impermeability of cement paste.The results shows that the optimization effect of nano-SiO₂ on the pore structure of cement paste increases with the decrease of water-cement ratio,revealing the mechanism of the impermeability improvement effect of nano-SiO₂ modified cement paste influencing by the water-cement ratio.Based on the generalized effective medium theory,the evolution rule and influencing factors of the impermeability of nano-SiO₂ modified cement paste were analyzed,and the relationship between the impermeability improvement effect of nano-modified cement paste and its initial microstructure was verified.Secondly,the improvement effect and mechanism of nano-SiO₂ on concrete impermeability were studied.Through the method of backscattering and kernel density estimation,the changes in the microstructure of concrete matrix and aggregate interface area were observed and analyzed,and the microscopic mechanism of nano-SiO₂ modified concrete with different water-cement ratio and aggregate gradation was revealed.Based on the recursive matrix method,the permeability model of nano-modified concrete was constructed,the change rule of permeability probability caused by the decrease of permeable unit was studied,and the mechanism of improving the impermeability of nano-SiO₂ modified concrete was revealed.The results shows that under the condition of low water-cement ratio and close packing of aggregates,nano-SiO₂ has the most significant effect on improving concrete impermeability.The reason is that when the permeable unit decreases but the connectivity remains unchanged,the degree of reduction in the penetration probability of concrete increases with the decrease in the penetration probability of the permeable unit,based on which the significant optimization of nano-SiO₂ for the microstructure of low water-cement ratio concrete further improves the impermeability of concrete.Then,the improvement effect and mechanism of nano SiO₂/Ca CO₃ on the impermeability of carbon fiber concrete were studied.Nano-SiO₂/Ca CO₃ compounding can significantly improve the uniformity of carbon fiber dispersion in concrete,thereby inhibiting the increase in porosity caused by fiber lap and the enhancement of pore structure connectivity,and effectively reducing the critical pore size of carbon fiber concrete.The theoretical calculation results of the permeability probability show that the decrease of the permeability probability caused by the weakened connectivity increases with the increase of the permeability probability of the permeable unit,revealing the impermeability mechanism of nano-SiO₂/Ca CO₃ modified carbon fiber concrete.Finally,by constructing the pore structure model of the nano-modified cement-based material,the permeability parameter distribution characteristics of the cement-based materials with different pore structures were simulated and studied,and the relationship between porosity,connectivity and impermeability of the nano-modified cement-based material were obtained.A coupling relationship model among the impermeability improvement rate,the initial porosity of cement-based materials and its reduced value was established,thereby settled for the characteristic pore structure parameters that control the evolution of the impermeability improvement effect.A theoretical model of nano-modified cement-based materials was proposed.