| Durability of reinforced concrete structure in marine and hydraulic engineering projects has been common concerns at home and abroad. In recent years, many studies have shown that the microstructure and thus the physical mechanics performance and durability of the hardened concrete can be significantly improved through incorporating nano-particles in the concrete mix.At present, three sources of nano-silica, i.e. the powder, dispersion and sol were mainly used as raw materials in the studies. However, the nano-particles in the powder and dispersion tend to agglomerate together. Despite the sol showed excellent dispersion, its stability is greatly affected by electrolyte. It is not reported at home and abroad yet whether the sol doped in paste can still maintain its high dispersion degree.On the basis of the related research at home and abroad, this thesis mainly focused on the agglomeration characteristics of the nano-silica powder and silica sol in the cement paste and their influences and mechanism on the properties of the fresh and hardened cement-based materials by using precipitated (PS) and fumed (NS) silica powder together with silica sol (SS) as raw materials. The results showed that:(1) The agglomeration degree of PS is far greater than that of NS. The featured grain size D90is even far greater than the cement. Despite silica sol itself showed mono-disperse, the sol gelled and agglomerate quickly in cement paste due to the rapid ionic strength increase with the hydration of cement.(2) The agglomeration and amount of the nano-silica have obvious influence on rheological property of the fresh cement paste. The dope of NS led to a slight decrease first then slight increase in viscosity of the paste with increasing the nano-silica addition within1.5%. As compared to NS, the paste with PS showed the same trend when the addition was below1.0%, but showed an obvious increase with further increase of the addition. For the paste with SS addition, the paste showed the same trend as that with NS addition below0.6%, but the viscosity showed a sharp increase with further increase of the addition. The main reason is that all the agglomerates in NS can act as fillers but just the small ones can in PS. In the paste with SS addition, the agglomerated nano-silica can act as fillers as NS when the addition was low, but the gelled nano-silica even coagulated the cement particles together, thus resulted in a sharp increase of the viscosity.(3) Within1%of the nano-silica addition, the strength of the mortar increases gradually with PS and SS concentration increasing. The strength of the mortar decreases obviously after accelerated corrosion in6.0M NH4Cl. The neutralization depth decreases, but the residual strength increases obviously with increase of the PS and SS dosage. The chloride ion penetration resistance of the mortar also increases. All properties were improved more significantly while incorporating SS than PS.(4) The SEM observation showed that the microstructure of the hardened cement pastes with both PS and SS addition was improved markedly as compared to the blank paste. However, it seems that the pozzolanic C-S-H gel can not act as binder. The large PS agglomerate can be easily found and there even exists interfacial zone between the agglomerate and the bulk paste. In the cement paste with NS doped, similar interface can be found, but the probability is greatly reduced. For the paste with SS addition, no interface can be found. MIP analysis showed that the pore structure of the HCP with nano-silica addition was refined apparently as compared to the blank paste. However, it seems that the addition of PS led to a better refinement in microstructure than that of SS, but worse than that of NS. The reason why the properties of the mortar with PS was improved less significantly than that with SS was probably due to that there exists obvious interfacial zone between the agglomerate and the bulk paste. |
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