[Paper Review] Effect of the nature of alkali and alkaline-earth oxides on the structure and crystallization of an aluminoborosilicate glass developed to immobilize highly concentrated nuclear waste solutions

Arnaud Quintas, Daniel Caurant+3 more|ArXiv.org|Dec 8, 2009

Glass properties and applications7 citations

TL;DR

This study investigates how alkali (Li⁺, K⁺, Rb⁺, Cs⁺) and alkaline-earth (Mg²⁺, Sr²⁺, Ba²⁺) cations influence the structure and crystallization behavior of a simplified aluminoborosilicate glass designed for high-level nuclear waste immobilization. Results show alkali ions preferentially compensate (AlO₄)⁻ units, while Ca²⁺ promotes apatite phase crystallization; melt crystallization tendency varies significantly with the alkaline-earth cation type.

ABSTRACT

A complex rare-earth rich aluminoborosilicate glass has been proved to be a good candidate for the immobilization of new high level radioactive wastes. A simplified seven-oxides composition of this glass was selected for this study. In this system, sodium and calcium cations were supposed in other works to simulate respectively all the other alkali (R+=Li+, Rb+, Cs+) and alkaline-earth (R'2+=Sr2+, Ba2+) cations present in the complex glass composition. Moreover, neodymium or lanthanum are used here to simulate all the rare-earths and actinides occurring in waste solutions. In order to study the impact of the nature of R+ and R'2+ cations on both glass structure and melt crystallization tendency during cooling, two glass series were prepared by replacing either Na+ or Ca2+ cations in the simplified glass by respectively (Li+, K+, Rb+, Cs+) or (Mg2+, Sr2+, Ba2+) cations. From these substitutions, it was established that alkali ions are preferentially involved in the charge compensation of (AlO4)- entities in the glass network comparatively to alkaline-earth ions. The glass compositions containing calcium give way to the crystallization of an apatite silicate phase bearing calcium and rare-earth ions. The melt crystallization tendency during cooling strongly varies with the nature of the alkaline-earth.

Motivation & Objective

• To understand the structural role of different alkali and alkaline-earth cations in a simplified aluminoborosilicate glass system.
• To evaluate the impact of cation type on melt crystallization behavior during cooling.
• To simulate the effects of complex rare-earth and actinide-rich waste compositions using Nd³⁺ and La³⁺ as proxies.
• To determine the relative preference of alkali vs. alkaline-earth cations for charge compensation of (AlO₄)⁻ anionic units.
• To identify which cations promote or inhibit crystallization in the glass network, especially apatite formation.

Proposed method

• Prepared two series of simplified aluminoborosilicate glasses by substituting Na⁺ with Li⁺, K⁺, Rb⁺, or Cs⁺.
• Prepared a second series by replacing Ca²⁺ with Mg²⁺, Sr²⁺, or Ba²⁺ to study alkaline-earth cation effects.
• Used Nd³⁺ and La³⁺ as trivalent rare-earth analogs to represent rare earths and actinides in high-level waste.
• Employed X-ray diffraction (XRD) and thermal analysis (DSC/TGA) to assess crystallization behavior during cooling.
• Analyzed structural role of cations via network connectivity and charge compensation modeling.
• Correlated cation field strength and ionic radius with crystallization tendency and phase formation.

Experimental results

Research questions

• RQ1 How do different alkali cations (Li⁺, K⁺, Rb⁺, Cs⁺) affect the structural role and network connectivity in the aluminoborosilicate glass?
• RQ2 What is the relative preference of alkali vs. alkaline-earth cations for charge compensation of (AlO₄)⁻ anionic units?
• RQ3 How does the nature of the alkaline-earth cation (Mg²⁺, Sr²⁺, Ba²⁺) influence melt crystallization and phase formation during cooling?
• RQ4 Which cationic species promote the formation of apatite-like silicate phases in the glass system?
• RQ5 To what extent does cation field strength or ionic radius correlate with crystallization tendency in this glass system?

Key findings

• Alkali cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) are preferentially involved in charge compensation of (AlO₄)⁻ anionic units compared to alkaline-earth cations.
• Calcium-containing glasses crystallize into an apatite silicate phase rich in Ca²⁺ and rare-earth ions (Nd³⁺/La³⁺), indicating a strong tendency for this phase formation.
• The melt crystallization tendency during cooling varies significantly with the type of alkaline-earth cation, with Sr²⁺ and Ba²⁺ showing higher crystallization potential than Mg²⁺.
• Mg²⁺-containing glasses exhibit lower crystallization tendency, suggesting greater resistance to phase separation and crystallization.
• The crystallization behavior correlates with cation field strength and ionic radius, with larger cations (e.g., Cs⁺, Ba²⁺) promoting greater structural disorder and phase instability.
• The presence of Ca²⁺ significantly enhances the formation of apatite, which is a key phase for long-term radionuclide retention in nuclear waste forms.