Alteration of Sulfur-Bearing Silicate-Phosphate (Agri)Glasses in Soil Environment: Structural Characterization and Chemical Reactivity of Fertilizer Glasses: Insights from ‘In Vitro’ Studies

Vitreous carriers of essential nutrients should release elements in response to plant demand, minimizing over-fertilization risks. This study focused on designing and characterizing sulfate-bearing slow-release fertilizers based on four glass series (41SiO₂∙6(10)P₂O₅∙20K₂O-33(29)MgO/CaO/MgO + CaO) with increasing sulfate content. Structural analysis identified a network dominated by QSi2 units, with some QSi3 species and isolated QP0 units. This fragmented structure resulted in high solubility in acidic environments while maintaining water resistance. Such dual behavior is a direct consequence of the delicate balance between depolymerized silicate chains and isolated orthophosphate units, which ensure rapid ion exchange under acidic conditions while preventing uncontrolled leaching in neutral media. Nutrient leaching depended on SO₃ content, affecting matrix rigidity, and on the type of alkaline earth modifier and P₂O₅ content. Dissolution kinetics showed an initial rapid release phase, followed by stabilization governed by silicate hydrolysis. Thermal analysis linked network flexibility to dissolution behavior-CaO promoted an open structure with high SiO₂ release, MgO increased rigidity, while their co-addition reduced ion diffusion and silica dissolution. The thermal behavior of the glasses provided indirect insight into their structural rigidity, revealing how compositional variations influence the mechanical stability of the network. This structural rigidity, inferred from glass transition and crystallization phenomena, was found to correlate with the selective dissolution profiles observed in acidic versus neutral environments. These results reveal complex interactions between composition, structure, and nutrient release, shaping the agricultural potential of these glasses.