Resistance to Sulfate Attack of Mortars Containing Colloidal Nanosilica and Silica Fume

Presented is a direct-comparison sulfate resistance study of mortars containing 3% or 6% cement replacement with either colloidal nanosilica (nS) or microsilica (mS), exposed to 6 months of full submersion in a 5% sodium sulfate (${\mathrm{Na}}_2{\mathrm{SO}}_4$) solution. Mortar bar samples measured for linear expansion per current standards indicated that at 6% cement replacement, colloidal nS exhibited on average 75% of the expansion of the microsilica-containing counterpart. At 3% replacement, either form of silica reduced the sulfate attack–related expansion to a similar degree (by $35\%\pm 2\%$) in comparison with the control mixture. Supplemental rapid sulfate permeability (RSPT) and absorption testing current standards supported the expansion results. The 6% nS mortar mixture exhibited the least charge passed and smallest permeable pore volume, which indicated that the nS-containing mortars were physically more impermeable and more resistant to ion transport. Increasing nS replacement (i.e., from 3% to 6%) decreased a mixture’s permeable pore volume, whereas increasing mS had an inverse effect. This countered the benefits of increasing the pozzolanic content in the sulfate attack test. Mercury intrusion porosimetry (MIP) testing also revealed evidence of paste and paste-to-aggregate interfacial zone densification, as well as pore-size refinement, which was further supported by higher increases in compressive strength in mixtures with nS over those with mS.