The Effects Of Nano-SiO₂on The Performance Of Cement-based Materials And The Study Of The Mechanism

Since the ninety’s of the last century, nano materials have been added to the cement-based materials to improve the performance, so as to meet the technology demand of increasing economic and social development.At present, the research of scholars at home and abroad mostly focuse on the impact of nano materials on the traditional mechanics and durability of ordinary cement-based materials, the mechanism are also inferred according to nano materials properties and microstructure analysis, and lack of analysis from the angle of cement hydration, also the research of the impact of nano-materials on different types of cement-based materials under different environmental conditions are not reported.In this study, the effect of different sizes of nano SiO2on cement paste fluidity, mechanical and durability and microstructure were investigated. Combined with Krstulovic-Dabic cement hydration model and the results of cement hydration heat test, the dynamic model of the effects of nano SiO2with different particle size on cement hydration process was first suggested. Furthermore, the effects of nano SiO2on concrete with different strength grade were also studied.The data presented in this study showed that:1Nano-SiO2has significant effects on the cement paste fluidity, and it is more effcective when the particle size is smaller.2Nano-SiO2can increase the early strength. of cement paste, but with litte influence on the later strength. For example,2%content nano-SiO2with the diameter of15nm and50nm, can increase the strength of cement paste by38%and57%respectively at1d, but show no significant increase cement paste strength at28d.3.The effect of nano-SiO2on the cement hydration degree and hydration time of different hydration stages were analyzed, and the early hydration model of cement with nano-SiO2was initially established. Nano-SiO2can improve the hydration degree of cement at the stage of crystal nucleation and growth(NG), and can make the time of diffusion reaction stage(D) earlier, and nano-SiO2can reduce the hydration rate of diffusion reaction stage(D), which leads to slow growth of strength in the late.4.Nano-SiO2can reduce the total porosity of the cement paste, and so as to the average pore size and critical aperture, and15nm-SiO2has a better effect on the pore refinement than50nm-SiO2at the same dosage.5. Nano-SiO2can reduce the orientation index of Ca(OH)2in the cement paste, and it can thinning the size of Ca(OH)2.15nm-SiO2has a better effect on reducing the orientation of Ca(OH)2than50nm-SiO2.6. There is an optimum value for the dosage of nano-SiO2in concrete, and the optimum value of high strength concrete is less than ordinary concrete. For C30comcrete, even with the dosage of nano-SiO2up to7%, the strength of concrete are still increasing, but the rate of increase become smaller. For C60concrete, when the dosage of nano-SiO2is up to3%. the strength begin to decrease.7.For C30concrete, nano-SiO2can siginificantly reduce the average pore size, and the percentages of harmful holes also significantly decreased at the same time, but for C60concrete, nano-SiO2has no significant influence on the average pore size or the distribution of pore size.