Effect of combined nanosilica and microsilica on resistance to sulfate attack

Abstract

In this study, the effect of combined nanosilica (nS) and microsilica (mS) on sulfate resistance of Portland cement (PC) mortars was evaluated against all cement 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 C3A (7.2%) and a low C3A (4.1%) cement to evaluate the effectiveness of each silica replacement paired with a chemically sulfate and non-sulfate 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 (Na2SO4) 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 the suspended silica particles supplemented the interpretation and explanation of the results. The expansion measurements indicated that mS replacement mortars outperform both nS only, and nS+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 the case of the low C3A sulfate resistant cement, the dry nS replacement of 6% exhibited more expansion than the control. The nS+mS combination mortar mixtures for both cement types performed better than those with nS only but not better than the mS only mortars. Combining both silica types did not merge the strengths of both forms of pozzolan admixtures as hypothesized. 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.