Co speciation in hardened cement paste: a macro- and micro-spectroscopic investigation

Area of Science:

• Materials Science
• Environmental Science
• Nuclear Engineering

Background:

• Cement-based materials are critical for multi-barrier systems in hazardous and radioactive waste disposal.
• Cement is utilized for waste conditioning, stabilization, and constructing engineered barriers in repositories.
• Understanding heavy metal immobilization in cement is vital for safe waste management.

Purpose of the Study:

• To investigate cobalt (Co) uptake by hardened cement paste (HCP) at a molecular level.
• To elucidate the immobilization mechanisms of heavy metals in cementitious materials.
• To understand the role of oxygen in the speciation and immobilization of cobalt.

Main Methods:

• X-ray absorption spectroscopy (XAS) on bulk powder samples to determine the local Co environment.
• Micro-X-ray fluorescence (micro-XRF) for spatial distribution analysis of Co.
• Micro-XAS for microscale Co speciation.
• Preparation of Co-doped HCP samples under aerobic and anaerobic conditions over various hydration times (1 hour to 1 year).

Main Results:

• In aerobic conditions, Co(II) in HCP is oxidized to Co(III) within 1 hour of hydration, with increasing Co(III) amounts over time.
• Co(II) predominantly forms hydroxide-like or phyllosilicate phases, while Co(III) incorporates into OOH-like or phyllomanganate phases.
• In the absence of oxygen, only Co(II) was detected, confirming oxygen's critical role in cobalt oxidation within cement.

Conclusions:

• Oxygen significantly influences the oxidation state and speciation of cobalt in cementitious materials.
• Co(III) species and phases are important considerations for assessing cobalt release from stabilized waste under oxidizing conditions.
• This research enhances the understanding of heavy metal immobilization in cement for radioactive waste management.