Determination of Chloride in Concrete Admixtures by Ion Chromatography per GB/T 8076-2025

Background

Rebar corrosion is one of the main causes of reduced durability in reinforced concrete structures, and chloride is a leading driver of that corrosion. Concrete consists of cement, sand and aggregate, mixing water and chemical admixtures — and the chloride carried in by admixtures is a major source of total chloride in the mix. Reliable determination of chloride in admixtures is therefore essential for durability control.

On 31 October 2025, the Standardization Administration of China announced GB/T 8076-2025 «Concrete Admixtures», issued that day and effective from 1 August 2026. The new standard replaces GB/T 8076-2008 and specifies that chloride content shall be determined by the ion chromatography method it describes; potentiometric titration may still be used for in-house checks. In practice, potentiometric titration suffers from poor silver chloride stability, unstable electrode construction and strong sensitivity to environmental conditions, which all hurt repeatability. Most concrete admixtures are organic macromolecular compounds, or contain them: these organics form conjugate acid-base systems that interfere with the titration end-point, and long exposure leaves organic films on the electrode surface, causing passivation and delayed end-points. The result is a larger error, especially at low chloride levels where reproducibility and confidence are weakest.

This case reports a high-sensitivity, high-precision and fast IC method for chloride in concrete admixtures, using a Wayeal ion chromatograph with an anion-exchange column to separate chloride from other ionic species.

Instrument Configuration and Test Conditions

Sample Preparation

Approximately 0.5 g of admixture is weighed to 0.1 mg into a 50 mL centrifuge tube. 25 mL of water and five drops of nitric acid are added to dissolve the sample; once dissolved, a further 25 mL of water is added, and the tube is vortexed and left to stand. The solution is then passed through the RP pre-treatment cartridge and a 0.22 µm disposable syringe filter before injection. A blank is run under identical conditions.

Results

Linearity: chloride standards at 1, 2, 4, 6 and 8 mg/L were injected. The calibration curve was y = 21.76426x − 7.24929 with a correlation coefficient R of 0.99955, confirming good linearity over the working range.

Detection limit: GB/T 8077-2023 requires an IC detection limit of 0.01 mg/L for chloride. At 0.01 mg/L the measured signal-to-noise ratio was 30.729 — far above the required 3 — so the method comfortably meets the national standard sensitivity requirement.

Sample results: two parallel determinations on a concrete treatment agent gave 4.878 mg/L and 4.711 mg/L, corresponding to chloride contents of 0.0487% and 0.0469% with a mean of 0.0477% (blank already deducted).

Repeatability limit: for chloride in the 0.01% < W ≤ 0.1% band the standard allows a repeatability limit of 0.02%. The measured difference between parallels was |0.0487% − 0.0469%| = 0.0018%, well inside the requirement.

Repeatability: six consecutive injections of each parallel sample gave retention-time RSD of 0.015–0.028% and peak-area RSD of 0.238–0.243%, showing a stable chromatographic system and reliable quantification.

Conclusion

The method, referenced to the IC procedure in the forthcoming GB/T 8076-2025, uses a Wayeal ion chromatograph with conductivity detection, simple acid-assisted dissolution and cartridge clean-up, separation on an anion-exchange column and external standard quantification. Linearity, sensitivity, parallelism and repeatability all satisfy the national standard, and the work completes verification of the IC chloride determination described in GB/T 8076-2025.