Sodium silicate glass as an inorganic binder in foundry industry

Mixtures of sand, sodium silicate, some additives such as iron oxide, pitch or sugar and an ester can form a temporary mould. Sodium silicate is used in foundries to bind sand grains by means of an appropriate acid, either directly such as CO2, or indirectly with an organic ester, which hydrolyzes and subsequently gels the silicate sand mass. A gel network is prepared by acidifying concentrated sodium silicate (3.3 ratio Of SiO2:Na2O) solution. Sodium silicate solution, under acidic conditions is polymerized to silica and acts as an inorganic binder. In other word, lowering the pH of an alkali silicate solution below about 10.9 causes the polymerization to occur, thus leading to high molecular weight aggregates of hydrated silica. The produced hydrated silica namely silica gel is responsible for giving the necessary strength to the mould. Concentration of the sodium silicate solution is adjusted to 65%(w/w) in water. The viscosity of this solution is adjusted on 500 cp by addition of sugar in an amount of 1.0-2.0% based on weight of sodium silicate. The sugar or pitch is used as a thickening agent, to concentrate the binder solution at a desirable limit. Sodium silicate solution as an inorganic binder is of particular importance mainly because, the necessary time for curing the silicate is low, and it is environmentally extremely beneficial.