Influence of nano-silica agglomeration on microstructure and properties of the hardened cement-based materials

Influence of nano-silica agglomeration on microstructure and properties of the hardened cement-based materials was investigated by using precipitated silica (PS) with very large agglomerates and fumed silica (FS) with much smaller ones as nanoscale additives. The results showed that the addition of either PS or FS refines pore structure of the hardened cement paste. However, the SEM observation revealed that the pozzolanic C–S–H gels from the agglomerates cannot function as binder. There even exists interfacial transition zone between the agglomerates and the bulk paste. The nano-indentation test indicated that the large agglomerates may become weak zones due to their low strength and elastic modulus. It is proposed that the microstructure improvement have nothing to do with the seeding effect, but result from the water-absorbing, filling, and pozzolanic effects. Through PS addition, compressive strength of the mortars and their resistance to calcium leaching and chloride penetration were enhanced. However, these improvements were less significant than those with FS addition. The reason is that much more fillers are provided whereas much fewer weak zones are introduced in the mortar with FS addition than that with PS.