Cold reaction sintering of geopolymers for enhanced Cs⁺ immobilization: Synthesis, characterization, and leaching behavior

Geopolymers with zeolite-like structures can effectively immobilize radioactive ions through their three-dimensional cage-like framework. This study investigated the synthesis of geopolymers via cold reaction sintering using metakaolin, anhydrous and nonahydrate sodium metasilicate for Cs⁺ immobilization. Optimizing the Si/Al molar ratio to 1.5:1 and water content to 20 wt% yielded the best mechanical properties and near-complete reaction. The transient liquid phase formed by the dehydration of sodium metasilicate nonahydrate facilitated densification. CsCl incorporation resulted in a predominantly amorphous phase with the formation of NaCl crystals. Toxicity leaching tests demonstrated Cs⁺ immobilization rate exceeding 91 % for CsCl content below 12 wt%. Short-term leaching tests confirmed low leaching rates (< 4 × 10⁻³ cm/day after 4 days), and leachability indices well above the typically required value of 6 for cement-based matrices. These results establish the viability of cold reaction sintered geopolymers for Cs⁺ immobilization, highlighting their promise for nuclear waste management applications.