Sodium Sulfate Resistance of Mortars Containing Combined Nanosilica and Microsilica

In this study, the effect of combined nanosilica (nS) and microsilica (mS) on sulfate resistance of portland cement mortars was compared with cement-only control mortars and mixtures with equivalent contents of only one form of silica. Silica-contained mortars had 6% cement replacement of either nS, mS, or 3% of each. An additional mixture with 3% mS was also tested. The series of mortars were prepared with both a moderate ${\mathrm{C}}_3\mathrm{A}$ (7.2%) and a low ${\mathrm{C}}_3\mathrm{A}$ (4.1%) cement to evaluate the effectiveness of each silica replacement paired with a chemically sulfate and nonsulfate-resistant cement. The mortars in this study were subjected to a 1.5-year period of full-submersion sulfate attack in a 5% sodium sulfate (${\mathrm{Na}}_2{\mathrm{SO}}_4$) solution. The mortars tested were measured for expansion and compressive strength. Additional testing for absorption, rapid sulfate penetration, and mercury porosimetry of select mortar mixtures, paired with laser diffraction particle analysis of sample silica particles suspended in water, supplemented the interpretation and explanation of the results. The expansion measurements indicated that mS replacement mortars outperform both nS-only and $\mathrm{nS}+\mathrm{mS}$ combination replacement mixtures. A negative effect of the dry nS powder replacement attributed to agglomeration of its fine-sized silica particles during mixing negated the expected superior pozzolanic activity of the nanomaterial. In light of the results, most of the beneficial contribution from the cement replacement with the combination mixtures could be attributed to the mS proportion given that the combination mixtures’ expansion performance was comparable to that of the 3% mS-only mortars.