Study On Properties And Microstructure Of Composite Cement-Based Materials Modified By Nano Silica

With the development and exploitation of marine resources in China,higher standards of marine engineering materials are required to reduce the economic and social losses.In this thesis,the research is conducted by using Nano-SiO₂ to improve the mechanical properties of composite cement-based materials like ordinary portland cement,cement-based material of slag,cement-based material of fly-ash and cement-based material of marine engineering,and the performance of chloride-penetration resistance.To begin with,the impact of Nano-SiO₂ on compressive strength of various cement-based materials is studied.The study shows that there is a positive correlation between the mixture amount of Nano-SiO₂ and the compressive strength of composite cement-based materials,which means the strength of composite cement-based materials are enhanced gradually as the amount of Nano-SiO₂ increase.The influence of Nano-SiO₂ on the compressive strength of composite cement-based materials is mainly reflected in the early stage whereas that of Nano-SiO₂ on cement-based material of fly ash is more significant in the later stage.Then,the research combined the chloride diffusion coefficient of cement with the artificial seawater immersion test is conducted to explore the effect of Nano-SiO₂ on chloride-penetration resistance in different cement-based materials.The study shows that there is a positive and negative effect on the chloride penetration resistance of various cement-based materials by adding Nano-SiO₂.According to the chloride diffusion coefficient of cement,adding appropriate amount of Nano-SiO₂ can improve the chloride penetration resistance of composite cement-based materials.Even if we add more than the most appropriate amount of Nano-SiO₂,the chloride-penetration resistance of test pieces is still better than that of the test pieces without adding Nano-SiO₂.In addition,what we can learn from the artificial seawater immersion test that there is an adverse impact on the chloride-penetration resistance of cement-based material of fly ash,in the short term,by adding small amount of Nano-SiO₂,but in the long term the impact is still favorable.If adding more than the optimal amount of Nano-SiO₂,the adverse effect in the long-term is significant.Moreover,the cement-based material of the marine engineering have better performance of the chloride-penetration resistance that of the blank control group after 28-day,56-day,even 90-day immersions.Overall,the results from the chloride diffusion coefficient of cement with the artificial seawater immersion test are in good agreement,which means they are complementary and verified.Lastly,X-ray diffraction（XRD）,thermogravimetric analysis（TG-DTG）and mercury porosimetry（MIP）are adopted to analyze the hydration mechanism and pore structure of composite cement-based materials.The research shows that Nano-SiO₂ can react with Ca（OH）₂ in the slurry at the early stage to generate a large amount of C-S-H gel and to promote hydration of cement clinker.It contributes greatly to the optimization of pore structure and the improvement of porosity of the early slurry.The pore structure gets more compact,and the compressive strength and the chloride-penetration resistance of the test pieces are enhanced.