Leaching of magnesium potassium phosphate cement pastes under alkaline conditions

• Published in: Applied geochemistry Vol. 170; p. 106067
• Main Authors: Diaz Caselles, Laura, Cau Dit Coumes, Céline, Antonucci, Pascal, Rousselet, Angélique, Mesbah, Adel, Montouillout, Valérie
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.09.2024; Elsevier
• Subjects: Alkaline leaching; alkalinity; aluminum; Applied geology; cement; Chemical Sciences; corrosion; domain; Durability; Earth Sciences; fly ash; geochemistry; Hydroxyapatite; magnesium; Magnesium phosphate cement; magnesium silicates; Material chemistry; Modeling; potassium phosphates; radioactive waste; Sciences of the Universe; spectroscopy; Alkaline leaching; Durability; Hydroxyapatite; Magnesium phosphate cement; Modeling; hydroxyapatite; solid state NMR; alkaline leaching; modeling; magnesium phosphate cement; Leaching Durability
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2024.106067

Some legacy radioactive waste containing aluminum (Al) metal need to be stabilized and solidified before their final disposal. Currently, Portland cement (PC) is extensively used for conditioning low- or intermediate-level radioactive waste. However, the high alkalinity of PC leads to strong corrosion of Al metal, which is associated with significant dihydrogen release. Therefore, it is important to investigate alternative binders that show better chemical compatibility with Al metal. Magnesium potassium phosphate cements (MKPCs), comprising equimolar amounts of MgO and KH2PO4, are interesting candidates since their pore solution pH may fall within the passivation domain of Al metal. The understanding of their long-term durability, especially under alkaline conditions, is however incomplete. Hence, MKPC paste samples (with fly ash as a filler) were submitted to semi-dynamic leaching tests using an alkaline solution under well-controlled conditions. The leachates were analyzed over time using ICP-AES, and the leached solids were characterized by XRD, SEM/EDS, and 31P MAS-NMR spectroscopy. Leaching induced a decrease in the content of crystalline K-struvite (MgKPO4·6H2O), the main hydrate of the paste samples, as well as the precipitation of calcium-deficient hydroxyapatite (CDHA), brucite (Mg(OH)2) and possibly magnesium silicate hydrates, OH-LDH or PO4-LDH phases. The experimental data were then used as an input for numerical simulations using the reactive transport code HYTEC. •Magnesium phosphate cement (MKPC) paste was fabricated using fly ash as a filler.•MKPC paste was leached under alkaline condition, showing a decrease in K-struvite.•Hydroxyapatite and brucite precipitated as secondary phases.•Brucite was a good indicator of MKPC degradation.•Reactive transport modeling helped to highlight the degradation mechanisms.