Influence of nano-silica agglomeration on fresh properties of cement pastes

• Effect of nano-silica on fluidity depends on whether the agglomerates can act as fillers or not.
• The small agglomerates fill in the void space to release free water.
• The large agglomerates absorb free water originally contributing to fluidity in paste.
• The acceleration of cement hydration may have nothing to do with the seeding effect.
• The acceleration depends on the calcium-absorptive ability of the agglomerates.

The influence of nano-silica agglomeration on fresh properties of the cement paste was investigated by using precipitated nano-silica (PS) with very large agglomerates and fumed nano-silica (FS) with much smaller ones as nano-strengthening admixtures. The rheologic tests revealed that addition of PS showed a greater influence on rheological behavior of the paste than that of FS, because the very large agglomerates in PS cannot act as fillers to release free water in the void space originally contributing to fluidity, but absorb free water originally contributing to fluidity in paste. Through monitoring the heat evolution, it is interestingly found that addition of PS accelerated cement hydration more significantly than that of FS though the latter provides much more seeds than the former, implying that the acceleration may have nothing to do with the so-called seeding effect. The calcium-adsorption tests confirmed that the accelerating effect is probably caused by the rapid calcium adsorption of nano-silica, which can keep a low under-saturation of calcium ions in paste, enabling a higher dissolution rate of calcium and thus an increase of heat evolution.