Investigation of Calcium Silicate Solubility in Various Aqueous Media

Sammanfattning

Calcium silicate (CaSiO₃) is a potential source of calcium and silicon for both agricultural and cosmetic applications, but its limited solubility in water poses a challenge for practical use. This study investigated dissolution behavior of calcium silicate in different aqueous environments: pure water, NPK fertilizer solution, and citric acid-containing systems. Solubility was assessed through measurements of total and free calcium concentrations using ICP-OES and ion-selective electrode (ISE), along with pH, TGA, SEM-EDS, and XRD analyses to characterize solid residues. Results confirmed very low solubility in water, with a sharp increase in pH and rapid saturation of free Ca²⁺ ions. In NPK solutions, calcium silicate partially dissolved but showed signs of secondary precipitation, likely due to interactions with phosphate. The addition of citric acid significantly improved calcium ion availability, particularly at a concentration of 0.035%, which maintained higher levels of free Ca²⁺ without excessive complexation. Solid-state analysis showed that crystalline residues remained at higher dosages, while low concentrations favored amorphous or partially reacted phases. Overall, the findings suggest that citric acid could be an effective additive to improve calcium silicate solubility and bioavailability in nutrient formulations but needs further investigation and also comparison with other co-solvents. This work provides insight into optimizing calcium silicate use in controlled-release systems.