Evolution of ettringite content and its effects on hydration properties of CaO/fluorgypsum-activated granulated blast furnace slag binders

Abstract

To investigate the influence of ettringite on the hydration properties of CaO/fluorgypsum-activated granulated blast furnace slag binders, the ettringite content in hydrated pastes with different amounts of fluorgypsum added was measured by selective dissolution in an ethylene glycol/methanol solvent system. The effects of ettringite content on the compressive strength development, the hydration products, the hydration degree of slag, the polymerization degree of calcium aluminum silicate hydrate (C-A-S-H), and the microstructure of hardened pastes were studied using a suite of characterization methods. The quantity of ettringite was found to increase with the amount of fluorgypsum added during the entire curing process. Only a moderate amount of ettringite was shown to improve the compressive strength due to the reinforcing effect of the fiber-like microstructure of ettringite. X-ray diffraction analysis revealed that hydrotalcite-like phases were strongly inhibited with the addition of fluorgypsum while ettringite formation was promoted. 29Si nuclear magnetic resonance spectroscopy and scanning electron microscopy analysis suggested that excessive ettringite caused a reduction in the hydration degree of slag, poor polymerization of C-A-S-H gels, and a loose microstructure of hydrated pastes. We concluded that an optimal strength was achieved by controlling the level of ettringite content to exploit the emergent reinforcing effect while avoiding both a poor microstructure and insufficient slag hydration. Moderate ettringite content improves the hydration properties of CaO-activated slag binder due to the fiber-like reinforcing effect.