Sulfate Resistance of Nanosilica Contained Portland Cement Mortars

摘要

Soils, sea water and ground water high in sulfates are commonly encountered hostile environments that can attack the structure of concrete via chemical and physical mechanisms which can lead to costly repairs or replacement. Sulfate attack is a slow acting deteriorative phenomenon that can result in cracking, spalling, expansion, increased permeability, paste-to-aggregate bond loss, paste softening, strength loss, and ultimately, progressive failure of concrete. In the presented research study, Portland cement (PC) mortars containing 1.5% to 6.0% nanosilica (nS) cement replacement by weight were tested for sulfate resistance through full submersion in sodium sulfate to simulate external sulfate attack. Mortars with comparable levels of cement replacement were also prepared with microsilica (mS). Three cement types were chosen to explore nS’ effectiveness to reduce sulfate expansion, when paired with cements of varying tricalcium aluminate (C3A) content and Blaine fineness, and compare it to that of mS. Mortars were also made with combined cement replacement of equal parts nS and mS to identify if they were mutually compatible and beneficial towards sulfate resistance. Besides sulfate attack expansion of mortar bars, the testing program included investigations into transport and microstructure properties via water absorption, sulfate ion permeability, porosimetry, SEM with EDS, laser diffraction, compressive strength, and heat of hydration. Expansion measurements indicated that mS replacement mortars outperformed both powder form nS, and nS/mS combined replacement mixtures. A negative effect of the dry nS powder replacement attributed to agglomeration of its nanoparticles during mixing negated the expected superior filler, paste densification, and pozzolanic activity of the nanomaterial. Agglomerated nS was identified as the root cause behind poor performance of nS in comparison to mS for all cement types, and the control when paired with a low C3A sulfate resistant cement.Testing the effects of mixing methodology and nS dispersion (mechanical blending vs. ultrasonic dispersion vs. aqueous solution) on sulfate resistance became a separate focus of the study. Use of the aqueous form of nS resulted in a more sulfate resistant and impermeable mortar than all other tested methods of mixing and dispersing dry form nS. At 6% replacement, aqueous nS contained mortars were more resistant to expansion than those with mS. Excessive ultrasonic dispersion of dry nS in the mixing water was shown to likely cause further agglomeration that harmed permeability and sulfate resistance.Overall, nS proved effective at improving sulfate resistance of mortars provided good dispersion could be achieved, otherwise mS remained the more effective, reliable, and economic choice. Parts of this study, a testing phase exploring the effectiveness of aqueous form nS on mortar resistance to physical sulfate attack via partial submersion, is still ongoing.