Quantification of the re-agglomeration of carbon nanofiber aqueous dispersion in cement pastes and effect on the early age flexural response

The effects of the chemical environment of cement composites on the quality and stability of carbon nanofiber (CNF) aqueous dispersion and changes in relative agglomeration state of CNFs from the aqueous solution to the hydrated cement paste were studied. Several dispersion methods and CNF loadings (0.02–1.0% per mass of cement) were examined. Evidence of secondary agglomeration of formerly dispersed CNFs and growth of non-dispersed primary agglomerates in the hydrating and hydrated cement paste was shown, regardless of the dispersion method, dispersing agent to CNF mass ratio, and CNF loading. The polycarboxylate-based high range water reducer (P-HRWR) dispersing agent resulted in a smaller number of large agglomerates (>200 μm) and a smaller agglomerate-to-agglomerate average distance (0.79–0.55 mm for CNF loadings of 0.2–1.0%) when compared to the other DAs (>1 mm). The flexural performance of the CNF/cement pastes was influenced by the size and spatial distributions of the CNF agglomerates and the nanoscale reinforcing behavior of the agglomerates. The cement pastes prepared with the P-HRWR and a CNF loading above 0.2% per mass of cement showed improvements in the flexural strength and toughness despite the presence of microscale agglomerates (65% and 125%, respectively at the 1.0% CNF loading).