Solid solutions between CrO4- and SO4-ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)(1-x)]3*26 H2O

Area of Science:

• Environmental Chemistry
• Materials Science
• Geochemistry

Background:

• Chromate is a toxic environmental contaminant, particularly soluble in alkaline conditions.
• Cementitious systems may incorporate chromate into ettringite, a mineral phase, potentially reducing its environmental mobility.

Purpose of the Study:

• To investigate the formation and solubility of chromate-ettringite solid solutions.
• To quantify the impact of chromate incorporation on ettringite stability and solubility.

Main Methods:

• Synthesis of ettringite with varying sulfate (SO4(2−)) and chromate (CrO4(2−)) compositions.
• Characterization of solid phases using X-ray diffraction.
• Solubility product determination after 3 months of equilibration.

Main Results:

• A regular solid solution formed between sulfate-ettringite and chromate-ettringite, with a miscibility gap observed between 0.4 ≤ XCrO4 ≤ 0.6.
• The solubility product for chromate-ettringite was determined as log K(S0) = −40.2 ± 0.4.
• The solid solution formation led to stabilization of ettringite and increased chromate uptake.

Conclusions:

• Sulfate- and chromate-ettringite form a stable solid solution, enhancing chromate sequestration in cementitious materials.
• The miscibility gap influences the extent of chromate incorporation and environmental risk mitigation.
• Understanding these solid solutions is crucial for managing toxic contaminants in construction and environmental remediation.