Effect and mechanism of colloidal silica sol on properties and microstructure of the hardened cement-based materials as compared to nano-silica powder with agglomerates in micron-scale

Influence of colloidal silica sol (SS) consisting of mono-dispersed nano-silica on properties and microstructure of the hardened cement-based materials was investigated as compared to the nano-silica (NS) powder with agglomerates in micron-scale through detecting the mechanical and durable properties of the hardened mortar and by using SEM observation, nano-indentation test and MIP analysis. The results showed that the strength enhancement of the mortar with SS addition revealed an overall more improvement but a higher discreteness than that with NS addition. After the calcium leaching test, it was found that the strength of the mortar with SS addition showed a clearly higher loss than that with NS addition, though the neutralization depth revealed a more reduction while incorporating SS and the residue strength still tends to increase with increasing the nano-silica addition. All these observations are resulted from the fact that the nano-particles in SS coagulate immediately once cement is mixed into water containing SS, forming loose flocs with a more open microstructure and a higher free water retention capacity than the agglomerates in NS, thus leading to a lower free water cement ratio in fresh cement paste and an overall more compact microstructure in hardened cement paste while incorporating SS. However, pozzolanic C-S-H gels from the gelled nanosilica can be observed to show a loose compaction, with nanopores and even micropores inside and also an abnormal growth of the larger CH crystals in the pozzolanic C-S-H gels but without nucleated growth of the C-S-H gels from cement hydration. The dissolution of the larger CH crystals in the coagulated gel network in the paste may leave large pores in the hardened cement mortar to cause a higher strength loss after the calcium leaching test. The MIP testing results indicated that the threshold pore size showed few differences for the HCP with and without 1.0% nano-silica addition, illustrating the permeability of the hardened cement paste may have nothing to do with the threshold pore size, but the total porosity and the micropores in the range of 100–10000 nm.