Multi-scale insights into cobalt immobilization mechanisms of basic magnesium sulfate cement: Bridging macro/microstructural experiments to molecular dynamics simulations

Abstract

Immobilization of heavy metal Co²⁺ constitutes a critical challenge in treating cobalt-containing industrial waste to mitigate its environmental toxicity. This study investigates the effects of Co²⁺ incorporation on the mechanical properties and leaching toxicity of basic magnesium sulfate cement (BMSC). The immobilization mechanisms were systematically explored through microstructural characterization and molecular dynamics simulations. Results demonstrate that Co²⁺ introduction accelerates MgO hydrolysis and enhances the hydration exothermic rate during the induction period. However, it inhibits the nucleation and growth of 5·1·7 crystalline phases, thereby suppressing BMSC hydration and prolonging setting time with increasing Co²⁺ content. A 28-day compressive strength of approximately 50 MPa is attained by Co²⁺-incorporated BMSC matrices, accompanied by positive immobilization capacity. In addition, remarkable acid and water resistance are also observed. The Co²⁺ immobilization by BMSC arises from: (1) physical encapsulation within the dense matrix; (2) Co(OH)₂ precipitation during the induction period; 3) physical adsorption by the hydration products (Mg(OH)₂ and 5·1·7 phases). Notably, Mg(OH)₂ demonstrates superior initial adsorption capacity and binding strength for rapid immobilization, while 5·1·7 phases exhibit enhanced structural stability post-ion incorporation for long-term retention. This work reveals BMSC's potential as a high-performance immobilization material and proposes an innovative strategy for efficient immobilization of cobalt-laden solid wastes.