Influence And Mechanism Of Nanoparticle Dispersion On The Properties Of The Cement-Based Materials

Currently, the durability of concrete structures has become a hot issue of common concern to researchers at home and abroad. Numerous studies showed that the strength and durability of concrete can be obviously improved by introducing nanoparticles. However, the nanoparticles in powder are often in an aggregated (firmly-held clusters) or agglomerated (loosely-held clusters) form with final grain size from submicron to as high as100μm due to their very high specific surface area and energy, which can not fully play its role in the improvement of concrete performance.In this thesis, influence and mechanism of nano-silica dispersion on properties of fresh and hardened cement-based materials were investigated through incorporating nano-silica with different degree of dispersion by ball-milling the precipitated nano-silica with large agglomerates for different hours and also by adding superplasticizer as dispersant during ball-milling. The results showed that,(1) The particle size of the agglomerates in nanosilica was greatly reduced after ball milling. By adding superplasticizer as dispersant during the ball-milling, the milling efficiency was gradually enhanced with a prolonged milling time, but not significant with a short milling time. (2) The penetration depth of the mortar was significantly decreased by adding1.0%PS in cement mortar admixture. However, it gradually increased by incorporating the milled PS with milling time and reached nearly the same as that of the blank mortar when the PS was milled for15.0h. The reason is that the longer the milling time, the smaller the particle size of the agglomerates in PS, and the more obvious filling effect of the agglomerates is. It seems that the addition of superplasticizer as a dispersing agent during milling time showed an adverse effect on improving fluidity of the mortar, probably because more superplasticizer molecules were absorbed by the nano-particles, thus the dispersion behavior of the cement partciles in mortar was weakened under the same amount of total superplasticizer in mortar.(3) The cement hydration was all more greatly accelerated while introducing PS with increased dosage, increased milling time, or with superplasticizer during milling time. The reason is that the adsorption of the calcium ions by nano-particles can be enhanced in all the above cases, thus help to promote the hydration of C3S in cement.(4) The strength and durability of cement mortar was all more greatly improved while incorporating PS with increased dosage, increased milling time, or with superplasticizer during milling time. The SEM and MIP analysis showed that the microstructure of the hardened cement paste and mortar was significantly improved with increasing the dispersion of the agglomerates in nano-silica.