Vitrification is a widely used technology for treating high-level radioactive wastes (HLW) [1]. Borosilicate glasses are the matrix internationally selected for the immobilization of HLW. However, the use of this glass is linked to melting at high temperatures or sintering the glass powders loaded with wastes, which poses a risk of evaporation of volatile radioactive elements [2]. 

An attractive technology that implies the use of boro-aluminosilicate glasses (BASG) to produce materials with interesting properties for the stabilization of contaminants is the alkali activation [3]. Alkali-activated materials (AAMs) are widely recognized as eco-friendly alternatives to conventional high-CO₂ binders. Alkali-activation technology also plays an important role in advancing the circular economy by effectively transforming inorganic waste streams into valuable products [4]. One of the promising areas for AAMs is the wastewater treatment. Various works addressed this topic. However, the use of these materials for immobilizing nuclear waste has been investigated less frequently, probably due to the intrinsic complexity of radioactive materials, that include a long half-life, high activity, solubility in water, and high volatility. One of the highly problematic radionuclides is cesium [5]. 

This work presents a new method of cesium immobilization, using alkali activation of BASG. This sustainable approach involves reused powdered glass from discarded pharmaceutical vials that are treated with a 2.5 M CsOH aqueous solution. The glass powders are suspended in the solution and consolidated at 40°C for 7 days, resulting in condensation reactions at hydrated surface layers. The products from partial glass dissolution combine with Cs⁺ ions to form a boro-pollucite (CsBSi₂O₆) solid solution. The immobilization process involves the formation of a stable gel that binds both the residual glass powder and the newly formed crystal phase. 

The main novelty lies in the use of a very low temperature to incorporate cesium into a framework mineral structure, which is recognized as one of the most promising options for storing radioactive cesium. Additionally, boro-pollucite represents the first example of an insoluble crystalline phase formed through the combination of activation by an alkaline compound (CsOH) and glass dissolution products. Cesium remains immobilized in blocks that can withstand immersion in boiling water. 

Further stabilization was achieved through the formation of glass matrix viscous flow sintered at temperatures as low as 700°C. The content of cesium remained almost constant before and after sintering. The effectiveness of the immobilization is confirmed by leaching test using the MCC-1 standard.