Enhanced adhesive performance of silicate binder via phosphate modification for permanent casting coatings

Environment-friendly silicate binders are commonly applied in permanent mold coatings; however, their adhesive performance requires further improvement. Sodium hexametaphosphate and sodium tripolyphosphate are typical additives for water-based casting coatings, but their effects on the structural evolution behaviors, adhesive performance, and inherent modification mechanisms of silicate binders remain unclear. In this work, systematic comparative studies on the adhesive performance of different contents of sodium hexametaphosphate and sodium tripolyphosphate modified silicate binders were presented, and their structural evolution behaviors were thoroughly explored. The results show that the silicate binder modified with 0.5wt.% sodium hexametaphosphate exhibits the optimal modification effect, with the compressive strength of silicate-bonded samples increasing from 46.58 MPa to 54.30 MPa, the bonding strength of the corresponding coating rising from 2.43 MPa to 3.50 MPa compared with the unmodified counterpart, and better service performance under simulated practical casting conditions. This indicates that sodium hexametaphosphate, by regulating the degree of silicon-oxygen polymerization, enhances interfacial bonding and alleviates internal stress within the coating while promoting the formation of a denser silica-oxygen network, thereby improving the adhesive performance of the binder. By contrast, sodium tripolyphosphate tends to inhibit the dehydration-condensation reaction, plausibly via competitive complexation with Na + , which correlates with its adverse modification effect. This work provides theoretical support for rational use of phosphates to enhance the adhesive performance of silicate binders in permanent mold coatings.