Influence of Dispersion Methods on Sulfate Resistance of Nanosilica-Contained Mortars

Abstract

This study evaluates the influence of various dispersion methods on the sulfate attack resistance of nanosilica (nS)-contained mortars. Multiple mechanical or ultrasonic dispersion methods, high-range water-reducing admixtures (HRWRA) dosing procedures, and both dry and aqueous solution forms of nS were used to prepare a series of mortars with 0, 3, and 6% replacement of portland cement with nS. Mortars were subjected to 6 months of exposure in a 5% sodium sulfate solution. Expansion, compressive strength, water absorption, rapid sulfate ion permeability, and porosity were measured. Results indicated that use of the aqueous form of nS results in a more sulfate resistant and impermeable mortar than all other tested methods of dry form nS. High-range water-reducing admixtures dosage proved most effective when added directly to the mixer after all water, binders, and fine aggregate were combined. Excessive ultrasonic dispersion of dry nS in the mixing water may cause further agglomeration of the nS that proved deleterious to permeability and sulfate resistance. In terms of compressive strength, mortars with 3% nS content performed similarly to those with double the nS content. Increasing the nS content seemed to have the least influence on the compressive strength of the better dispersed aqueous nS mixtures.