Effect of nano-silica agglomeration on hydration and hardening of cement

Effect of nano-silica agglomeration on the hydration and hardening of cement was investigated by using precipitated silica (PS) with much larger agglomerates and nano-silica (NS) powder with much smaller ones as additives. The results reveal that the PS exhibits a higher pozzolanic reactivity at early ages and a better accelerating effect on the cement hydration though it possesses some larger agglomerates and a little lower pozzolanic reactivity at late ages rather than NS. The observation by scanning electron microscopy indicates that the cementitious property of the pozzolanic C-S-H gels from the agglomerates was limited. There even existed an interfacial transition zone between the reacted agglomerates and bulk hardened cement paste (HCP). The MIP results show that the NS addition can reduce the capillary and the gel pore of the HCP in the range of 20 nm-10 μm more effectively, compared to the PS addition. It is suggested that the effect of nano-silica addition on the microstructure improvement of the HCP could be resulted from the filling and water adsorbing effects of the agglomerates in the powder rather than the seeding effect.