Suppression processes of anionic pollutants released from fly ash by various Ca additives

Area of Science:

• Environmental Science
• Materials Science
• Geochemistry

Background:

• Coal fly ash contains harmful trace elements that can leach into the environment when exposed to water.
• Leaching poses a long-term risk in structures built with coal fly ash.
• Calcium additives can mitigate anionic pollutant release, but mechanisms are not fully understood.

Purpose of the Study:

• To investigate the effectiveness of various calcium additives in suppressing anionic pollutant release from coal fly ash.
• To elucidate the detailed suppression mechanisms, including phase transformations and precipitation pathways.

Main Methods:

• Systematic investigation of anionic pollutants (B, F, S, As, Cr) with different Ca additives.
• Analysis of solution chemistry and solid-state characterization.
• Evaluation of the role of MgO and Mg(OH)2 in additive performance.

Main Results:

• 60% hydroxylated calcined dolomite (HCD 60) demonstrated superior suppression of diverse anionic pollutants.
• HCD 60 functions beyond simple alkalinity, providing MgO and Mg(OH)2.
• Key suppression mechanisms involve phase transformation from Ca(OH)2 to ettringite via hydrocalumite and precipitation of Ca salts.

Conclusions:

• HCD 60 is the optimal Ca additive for immobilizing anionic pollutants in coal fly ash.
• MgO and Mg(OH)2 enhance ettringite and hydrocalumite formation, respectively, crucial for pollutant suppression.
• Understanding these mechanisms improves the safe utilization of coal fly ash in construction.