Effects of internally introduced sulfate on early age concrete properties: Active acoustic monitoring and molecular dynamics simulation

Abstract

Sulfate attack of concrete is still a “confused world” that deserves further study. An active acoustic method is employed to monitor the early-age evolution of fresh concrete mixed with soluble sulfate ions. The effects of introduced sulfate salt at different dosages on the setting and hardening process of fresh concrete as well as mechanical properties of hardened concrete up to the age of 28 days are examined and analyzed. The experimental results from the active acoustic monitoring clearly reveal the acceleration effect of introduced sulfate salt on the solid skeleton formation process of fresh concrete. Introduced sulfate salt leads to higher values of mechanical property indices at early ages (e.g., 1 day), but poorer mechanical properties of hardened concrete at the age of 28 days. The corresponding mechanism has been studied in the light of molecular dynamics simulation, scanning electron microscopy and X-ray diffraction, and it is concluded that introduced sulfate ions not only damage the cement matrix due to the overexpansion of ettringite formation, but also weaken the loading resistance of the ionic-covalent bonds in the C-S-H gel.