Effect of Na2O on the crystallization behavior, structure, and chromium immobilization in CaO–MgO–Al2O3–SiO2–Cr2O3 glass–ceramics

The harmless treatment of Cr-containing solid waste remains a critical environmental challenge. Herein, Cr 2 O 3 -doped CaO–MgO–Al 2 O 3 –SiO 2 glass-ceramics with varying Na 2 O contents were prepared via a melting method. The study focuses on how Na 2 O regulates the glass network, crystallization behavior, and Cr immobilization efficiency. With increasing Na 2 O content, the degree of depolymerization (DOD) of the parent glass increases from 1.76 to 1.99, while the crystallization activation energy decreases accordingly. The optimal formulation is achieved at 1 wt% Na 2 O combined with a two-stage heat treatment (nucleation at 720 °C for 2 h, followed by crystallization at 850 °C for 2 h). Under these conditions, the glass-ceramic exhibits the lowest Cr leaching concentration of 0.031 mg/L, far below the regulatory limit of 5 mg/L. The dominant crystalline phases are diopside and a spinel solid solution, where Cr 3+ is stably incorporated into the spinel. These findings demonstrate that controlled Na 2 O addition provides an effective compositional lever for optimizing both crystallization kinetics and heavy metal immobilization, offering a sustainable pathway for valorizing Cr-containing wastes into high-performance glass-ceramics.