Effects of Al in C–A–S–H gel on the chloride binding capacity of blended cement paste

Abstract

This study quantitatively examines the impact of aluminum (Al) in C–A–S–H gel² on the chloride binding capacity of blended cement paste containing supplementary cementitious materials (SCMs). The experimental results show that Al incorporation does not influence the physical chloride binding of C–A–S–H gel, which is governed by its Ca/Si ratio. However, in the presence of Cl⁻ ions, the OH⁻ ligands that stabilize high-coordination Al^VI species³ (specifically [AlO₂(OH)₄]⁵⁻) can be replaced by Cl⁻ ions. This replacement leads to the leaching of Al^VI species from the C–A–S–H gel and an increase in OH⁻ ion concentration in the solution. The released Al³⁺ ions then interact with Ca²⁺ and Cl⁻ ions in the pore solution to form Friedel's salts (Fs), which further promotes the leaching of Al^VI species. Consequently, the chemical chloride binding capacity of the cement paste increases as the Al/Si ratio of the C–A–S–H gel rises, along with refinements in paste microstructure.