Experiments on Chloride Binding and Its Release by Sulfates in Cementitious Materials

Area of Science:

• Materials Science
• Civil Engineering
• Environmental Science

Background:

• Chloride ingress is a major cause of reinforcement corrosion in concrete structures.
• Understanding chloride binding and release mechanisms is crucial for predicting concrete durability.
• Sulfate attack can influence the stability of bound chlorides within cementitious matrices.

Purpose of the Study:

• To experimentally investigate chloride binding in hardened cement paste particles (HCPs).
• To examine the sulfate-induced release of bound chlorides from HCPs.
• To correlate chloride binding and release with material properties and sulfate exposure conditions.

Main Methods:

• Hardened cement paste particles (HCPs) with varying water-to-cement (w/c) ratios (0.35 and 0.45) were prepared.
• Long-term immersion tests in sodium chloride and subsequent sodium sulfate solutions were conducted.
• Silver nitrate titration measured free chloride concentrations, while quantitative X-ray diffraction (QXRD) determined bound chloride content.

Main Results:

• HCPs with a higher w/c ratio (0.45) exhibited greater chloride-binding capacity.
• Bound chloride content correlated with the formation/decomposition of Friedel's and Kuzel's salts.
• Sulfates readily released chlorides from Friedel's salt but not completely from Kuzel's salt.
• Physically bound chlorides were released more easily than chemically bound chlorides.

Conclusions:

• The water-to-cement ratio significantly impacts chloride binding in cementitious materials.
• Sulfate presence induces differential release of bound chlorides, primarily affecting Friedel's salt.
• Physically bound chlorides are more susceptible to sulfate-induced release, with release rates influenced by w/c ratio and sulfate concentration.