Determination of Chloride Ion Content in Concrete Admixtures by Ion Chromatography

Steel corrosion is a primary factor affecting the durability of reinforced concrete structures. The presence of chloride ions in concrete is one of the main causes of steel corrosion, and chloride ion erosion is the most significant trigger for such corrosion. Concrete is primarily composed of cement, aggregates, mixing water, and concrete admixtures. The chloride ions contained in concrete admixtures represent a major source of chloride ions in concrete. Therefore, detecting chloride ions in concrete admixtures is of great significance.

In this study, a Wayeal ion chromatograph equipped with NovaChrom HS-5A-P2 anion exchange column was employed to separate target ions from other interfering ions. A highly sensitive, precise, rapid, and accurate analytical method for chloride ions in concrete admixtures was established.

Keywords: Ion chromatography, Concrete admixtures, Chloride ions.

1. Experiment Method

1.1 Instrument Configuration

Table 1 Configuration List of Ion Chromatography System

1.2 Experiment Material and Auxiliary Equipment

Standard Chloride Ion Solution in Water: 1000mg/L

Nitric acid: GR

Sample: Concrete treatment agent

Water-based syringe filter (0.22μm)

Analytical balance with 0.0001g accuracy

The experimental water was prepared by Wayeal ultrapure water purification system, with a conductivity of 18.25 MΩ·cm (at 25°C).

1.3 Test Conditions

Table 2 Ion Chromatography Conditions

1.4 Sample Pretreatment

Weigh exactly 0.5g of the admixture sample, accurate to 0.1mg. Place it into a 50mL centrifuge tube, add 25mL of water and 5 drops of nitric acid to dissolve the sample. If the sample is water‑soluble, add an additional 25mL of water. Vortex to mix thoroughly and then allow to stand. Pass the sample through a pretreatment RP cartridge and a 0.22μm disposable syringe filter before introducing it into the ion chromatograph for analysis. Maintain the same analytical conditions and perform a blank test.

2. Result and Discussion

2.1 Linear Test

A series of chloride ion standard working solutions with concentrations of 1mg/L, 2mg/L, 4mg/L, 6mg/L, and 8mg/L were sequentially pipetted. Following the analytical conditions specified in Section 1.3, the overlap chromatograms for the standard curve as shown in Figure 1. The linear equation is showed in Table 3. Under these chromatographic conditions, the chloride ion calibration curve exhibited a correlation coefficient (R) greater than 0.999, indicating excellent linearity.

Fig 1 Overlap Chromatogram of Standard Curve

Fig 2 Chloride Ion Standard Curve

Table 3 Standard Curve Linear Equation

2.2 Detection Limit

According to the “GB/T 8077-2023 Test Method for Uniformity of Concrete Admixtures”, the detection limit of chloride ions by ion chromatography is 0.01mg/L. In this method, the signal-to-noise ratio for chloride ions at 0.01mg/L is significantly greater than 3, which meets the sensitivity requirements of the national standard. The signal-to-noise ratios for the chloride ion detection limit are listed in Table 4, and the corresponding chromatogram is shown in Figure 3.

Table 4 Chloride Ion Detection Limit Signal-to-Noise Ratio

Fig 3 Chromatogram of Chloride Ion Detection Limit

2.3 Sample Test

2.3.1 Sample Content Test

Under the analytical conditions specified in Section 1.3, parallel tests were performed on the pretreated samples described in Section 1.4. The results showed good reproducibility of the samples. The chromatograms of the parallel samples are shown in Figures 4 and 5, and the chloride ion content obtained in the samples is presented in Table 5.

Fig 4 Chromatogram of Chloride Ion Test for Parallel Sample 1 of Treatment Agent

Fig 5 Chromatogram of Chloride Ion Test for Parallel Sample 2 of Treatment Agent

Table 5 Sample Result Analysis

Notes: ① Test results have been corrected for blank values; ② Variability may exist between results obtained using different methods or across different laboratories.

2.3.2 Sample Repeatability Limit

According to the requirements of the national standard, the repeatability limit for sample testing shall comply with the specifications in Table 6 below:

Table 6 Sample Repeatability Limit Requirements

Based on the test data of the samples in Section 2.3.1, the repeatability limit for the treatment agent is calculated as |0.0487% – 0.0469%| = 0.0018%, which is less than 0.02% and complies with the requirements of the national standard.

2.3.3 Sample Repeatability Test

Under the working test conditions specified in Section 1.3, repeatability tests were performed on the pretreated samples described in Section 1.4. The overlap chromatograms obtained from multiple injections are shown in Figures 6 and 7. The test results are presented in Table 7, indicating that the relative standard deviation (RSD) of the peak area for chloride ions in the repeatability tests ranged from 0.238% to 0.243%, and the RSD of the retention time ranged from 0.015% to 0.028%. The results are reliable, demonstrating good repeatability.

Fig 6 Overlap Chromatograms of Chloride Ion for Treatment Agent Parallel Sample 1 (6 Injections)

Fig 7 Overlap Chromatograms of Chloride Ion for Treatment Agent Parallel Sample 2 (6 Injections)

Table 7 Sample Repeatability Results

3. Conclusion

This experiment referred to the ion chromatography method specified in the upcoming standard GB/T 8076-2025 "Concrete Admixtures." The experimental results indicate that for six consecutive injections of the sample solution, the retention time RSD ranged from 0.015% to 0.028%, and the peak area RSD ranged from 0.238% to 0.243%. The method demonstrated good injection detection limits and excellent linearity, with a correlation coefficient R² of 0.99911. All the above data meet the requirements of the national standard method for instrumental performance.An ion chromatography method was established for the determination of chloride ion content in concrete admixtures using a Waayeal ion chromatograph equipped with a conductivity detector. After sample pretreatment, separation was performed on an ion chromatographic column, and quantification was carried out using the external standard method. This approach enables both qualitative and quantitative analysis of chloride ions in concrete admixtures. The method is straightforward, shows good repeatability, and meets the required criteria for sensitivity, parallelism, and repeatability limits. It can be adopted for the determination of chloride ion content in concrete admixtures. The validation of the ion chromatography method for chloride ion determination specified in the newly released GB/T 8076-2025 "Concrete Admixtures" has been successfully completed.