Test of SO₄^2– in Cement-Based Materials Based on Conductivity Titration

It is essential for understanding sulfate-induced deterioration mechanisms to quantify sulfate ions (SO₄^2–) in cement-based materials accurately. Reliable sulfate measurements are also required for developing corrosion prediction models and for supporting durability-oriented design and maintenance strategies. Conventional analytical methods, including gravimetric analysis and ethylenediaminetetraacetic acid (EDTA) titration, have limitations in highly alkaline and turbid cement extracts that contain multiple interfering ions such as Ca²⁺, Mg²⁺, OH^–, Na⁺, and Cl^–. These characteristics complicate endpoint identification and reduce measurement precision. The difficulty becomes more pronounced when sulfate concentrations are low, which is typical in the early and intermediate stages of sulfate ingress. Therefore, a rapid, accurate and interference-resistant technique is required. This study was to establish conductivity titration as a reliable method for quantifying sulfate in cement-based materials.

The conductivity titration system used was constructed with a DJS series conductivity electrode, a DS18B20 temperature compensation module, an automated peristaltic pump and a microcontroller unit. Barium nitrate (Ba(NO₃)₂) was selected as a titrant. Standard sulfate solutions were first tested to establish the characteristic stages of the titration process. The titration curve of SO₄^2– conductivity could be divided into unsaturated section, crystallization nucleation section, crystal growth section, quasi-reaction end section, reaction end section and infinite titration section. The endpoint was identified by a slope transition between the crystal growth and completion stages.

Optimal operational parameters were identified through repeated tests. These included the appropriate titrant concentration, droplet rate, sample volume and working temperature. Interference-removal procedures were then developed to address the complex ionic environment of cement-based systems. Carbon dioxide (CO₂) was used to eliminate OH^– and Ca²⁺ during free sulfate measurements. A treatment involving silver oxide(Ag₂O), carbon dioxide (CO₂) and hydrochloric acid (HCl) was applied to eliminate H⁺, Ca²⁺ and Cl^– during total sulfate analysis. The accuracy, resolution, and repeatability of conductivity titration were validated using standard SO₄^2– solutions and compared with gravimetric and EDTA titration methods.

The optimized conductivity titration system achieves a minimum SO₄^2– resolution of 0.1 mmol/L, indicating sufficient sensitivity for detecting sulfate ingress in cement-based materials. Clear and reproducible inflection points appear on conductivity-volume curves even at low concentrations, ensuring accurate endpoint identification. For a solution containing 0.01 mol/L SO₄^2–, a titrant concentration of at least 0.033 mol/L Ba(NO₃)₂ produces a slope change greater than 60°, confirming a distinct crystallization transition. The results of repeatability tests verify the stability of the method, and 20 consecutive titrations yield an average titrant consumption of approximately 5 mL with a standard deviation of 0.41.

Interference removal proves an essential for maintaining measurement accuracy. CO₂ and HCl can be used to remove some interfering ions such as OH^– and Ca²⁺ in the test of free sulfate ions. In the acid-soluble sulfate test, HCl is used as a dissolution solution. Ag₂O, CO₂ and HCl can be used to remove a large number of interfering ions such as H⁺, Cl^– and Ca²⁺, and improve the stability of the titration curves. The results of comparative experiments show that conductivity titration provides more reliable results than gravimetric analysis and EDTA titration when samples are turbid, contain multivalent cations or exhibit a unstable color, or when SO₄^2– concentrations vary in a wide range. The gravimetric analysis produces large errors at SO₄^2– contents of below 0.5%, and the EDTA titration is strongly affected by Ca²⁺, Mg²⁺ and turbidity. In contrast, the conductivity titration accurately reflects changes in ionic composition regardless of solution appearance.

The results of tests on sulfate-exposed cement paste confirm that the method can resolve SO₄^2– profiles and diffusion gradients within the material. This ability demonstrates its potential for evaluating the progression of sulfate attack and for supporting durability assessment and corrosion monitoring.

The conductivity titration could provide an accurate and stable quantification of both free and total SO₄^2– in cement-based materials. The optimized titration parameters produced clear and reproducible inflection points on conductivity-volume curves, enabling reliable endpoint identification even at low sulfate concentrations. The pretreatment procedures designed for alkaline and ion-rich cement extracts effectively eliminated OH^–, Ca²⁺, H⁺ and Cl^–, ensuring a high measurement precision. The results of comparative tests showed that conductivity titration exhibited significantly better adaptability than gravimetric analysis and EDTA titration, especially for turbid or multi-ion systems. Application of the method to sulfate-exposed cement paste confirmed its capability to resolve sulfate profiles and identify diffusion gradients. This study demonstrated that conductivity titration could be an effective analytical tool for sulfate determination in cementitious materials and offer a promising potential for improving durability evaluation and corrosion monitoring in concrete structures.