# Exploration of the Chloride Binding Behavior of Anhydrous Calcium Sulfoaluminate Under Dual Chloride Ingress Modes

## TL;DR

This study examines how anhydrous calcium sulfoaluminate binds chloride under two different exposure conditions and how gypsum affects this process.

## Contribution

The study reveals the dual mechanisms of chloride binding and the critical role of gypsum in regulating chloride binding efficiency in SAC.

## Key findings

- External chloride binding increases with solution concentration and immersion time due to ion exchange and physical adsorption.
- Internal chloride binding increases with curing age and involves direct Cl− participation in hydration reactions.
- Gypsum dosage significantly affects chloride binding efficiency, with lower gypsum ratios leading to higher chloride binding.

## Abstract

This study explored the chloride binding characteristics and mechanisms of sulphoaluminate cement (SAC) by isolating its principal mineral component, anhydrous calcium sulphoaluminate (C4A3S-), as the research object. Chloride ingress was investigated under external penetration and internal incorporation conditions, with gypsum dosage varied at molar ratios of 1:0, 1:1, and 1:2 relative to  C4A3S-. Through chloride binding experiments and hydration product analysis performed by XRD and TG, the following findings were obtained: under external chloride exposure, the binding capacity increased with rising solution concentration and immersion time. External chloride binding was attributed to SO42−/Cl− ion exchange in AFm to generate Friedel’s salt and was complemented by physical adsorption of chloride in AH3 gel. Under internal chloride incorporation, binding capacity increased progressively with curing age. Internal chloride binding involved the direct participation of Cl− in hydration reactions to form Friedel’s salt in addition to the chemical reaction of AFm and the physical adsorption of AH3. Gypsum dosage critically regulates the AFm/AFt ratio, which in turn governs chloride binding efficiency under both external and internal chloride scenarios (e.g., after immersion in 1 mol/L NaCl solution, the bound chloride content for C4A3S-/gypsum ratios of 1:0, 1:1, and 1:2 was 50.94, 27.28, and 13.47 mg/g, respectively).