In this study, spent foundry sand (SFS) was treated and used as a source to synthesis sodium silicate. Initially, SFS was subjected to calcination (C-SFS) followed by acid bleaching (A-SFS) to yield quartz silicate in high purity. After each treatment, the samples were studied using field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) to analyse their properties. The hydrothermal process was performed using A-SFS with alkali hydroxides to obtain the corresponding alkali silicates. The optimisation studies conducted at different temperatures and time periods revealed that the maximum yield of 97.6% was obtained at 225 °C for 3 h. A plausible depolymerisation mechanism is also proposed based on the observations from 29Si NMR and XRD results of residues. Thus, present study facilitates the synthesis of sodium silicate effectively by utilising SFS, which in turn minimizes the waste disposal.