Effect of sodium sulfate activation on the early-age matrix strength and steel fiber bond in high volume fly ash (HVFA) cement mortar

Abstract

Retarded hydration and delayed microstructure development could reduce the early-age mechanical performance of fiber reinforced, High Volume Fly Ash (HVFA) cement composites. In this study, the effects of the pH-neutral Sodium Sulfate (SS), as a chemical activator on the early-age matrix mechanical strength development and bond performance of a steel macrofiber in HVFA cement mortar mixtures were examined. Mortar mixtures containing 0–10% Silica Fume (SF), 30–40% Ordinary Portland Cement (OPC) and 4–6 wt% SS were tested. Specifically, changes in matrix compositional structure, mechanical strength, bond strength and absorbed energy of two End-deformed (ED) steel macrofiber were evaluated. The microstructure of the fiber–matrix Interfacial Transition Zone (ITZ) was also studied through Scanning Electron Microscopy-Backscattered Electron (SEM-BSE) imaging. Results showed that the SS accelerated pozzolanic reaction in HVFA mixtures, causing a rapid depletion of portlandite and a moderate enhancement of the 7th day mechanical strength properties. The positive influence of the SS increased considerably with respect to the pullout test whereby bond strength and pullout energy higher than those of the fiber embedded in 100% OPC (Mix 1) mortar were measured. SEM-BSE imaging showed that a denser fiber–matrix ITZ, a more efficient fine aggregate-fiber interlocking, and enhanced stress transfer from the cement paste to the solid inclusions in the matrix were responsible for the superior pullout performance of steel macrofiber embedded in SS-activated HVFA mortar mixtures. Based on these results, applications such as rigid pavements and repair overlays where accelerated deployment is an important consideration can be constructed using steel macrofiber reinforced HVFA cement mixtures.