Properties of ion leaching of alkali-activated blast furnace slag at early hydration

Efforts are being made to reduce carbon dioxide formation in the cement industry, and studies continue to show the effectiveness of blast furnace slag (BFS) as a cement substitute. BFS is well known for its hardening mechanism in ordinary Portland cement or for alkali activation owing to its latent hydraulic property. This study accurately analyses the mechanism of initial hydration delay caused by the latent hydraulic property of BFS, with the aim of establishing a basic theory to extend the use of BFS based on this initial hydration delay mechanism analysis. The concentrations of sodium hydroxide (NaOH) solution, used as an alkali actuator, are 0, 0·01, 0·1, and 1·0 M. To understand the early hydration, the leaching concentrations of calcium, aluminium and silicon ions are measured by inductively coupled plasma atomic emission spectroscopy. To confirm the hydration products formed by the leached ions, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric differential thermal analyses are conducted. For the BFS, calcium ions are leached in very high amounts when the sodium hydroxide concentration is ≤ 0·1 M. At sodium hydroxide concentration > 1·0 M, the leaching of calcium ions decreases noticeably, and aluminium and silicon ions begin to leach. Ettringite and calcium–silicate–hydrates (C-S-H) are confirmed as reaction products.