Influence of colloidal silica sol on fresh properties of cement paste as compared to nano-silica powder with agglomerates in micron-scale

Influence of colloidal silica sol (SS) with mono-dispersed nano-particles on fresh properties of cement paste was investigated as compared to nano-silica powder (NS) with agglomerates in micron-scale. The SS addition showed a much greater influence on sedimentation and rheological behavior of the paste than the NS incorporation, because the nano-particles in SS coagulate immediately once cement is mixed into water containing SS, forming loose floc and coating layer around cement particles. The loose floc cannot function as fillers to release free water, but possesses a more open microstructure, leading to a higher free water retention capacity than the agglomerates in NS. However, addition of SS presented an obviously better accelerating effect on cement hydration than that of NS, though the nano-particles in SS are nearly the same as those in NS in primary particle size and the flocs in the paste with SS addition are typically larger than the agglomerates in NS, implying that the acceleration may have nothing to do with the seeding effect. Through detecting calcium-absorbing properties of NS and SS, it is found that the accelerating effect is highly dependent on the rapid depletion of calcium ions in the paste. Finally, it was interestingly found that the CH crystals are even more prone to grow along (0001) plane with larger size in the paste with SS addition, because the coagulated gel network in the paste slows down the diffusion rate of the released ions and eliminate the convection in the system, thus the 3D nucleation and growth of the CH crystals were suppressed.