Introduction

Glass ionomer cement as the name suggests it has silicate glass particles in the powder and polyacrylic acid as liquid. Introduced in 1970, has less pulp irritating capacity compared to silicate cements. It is considered superior to many cements because of its adhesive property and translucency. The carboxy terminal in the polyacrylic acid makes it adhere to the tooth structure. Newer variations of GIC are available incorporating water soluble polymers, resin monomers in liquid, particles of ceramic in powder for better mechanical properties etc. Newer formulations are chemically cured, light cured or both. The GIC is originally classified as

Type I: Luting crowns, bridges, and orthodontic brackets 

Type IIa: Esthetic restorative cements 

Type IIb: Reinforced restorative cements 

Type III: Lining cements, base

Composition 

Powder:

It consists of silica, alumina, aluminium fluoride, calcium fluoride, sodium fluoride, aluminium phosphate, barium and strontium salts to increase radiopacity. Silica glass is melted between 1100° and 1500°C. The glass is ground into a powder with particles ranging from less than 15 µm to about 50 µm.

Liquid:

The copolymers of itaconic, maleic, or tricarboxylic acids are used replacing polyacrylic acid as it is highly viscous and has low shelf life. 

Tartaric acid is a rate-controlling additive in the GIC liquid that improves handling properties, decreases viscosity, lengthens shelf life before gelling of the liquid occurs, increases working time, and shortens the setting time.

How does GIC set?

When powder and liquid are mixed together, the glass in the powder is dissolved by the acid in the liquid releasing the calcium, aluminium, sodium and fluoride ions. Here, water acts as a reaction medium.

The polyacrylic acid crosslinks with the calcium ions and aluminium ions(after 24 hours) forming the matrix phase. 

Sodium and fluoride are dispersed in the matrix phase. Sodium sometimes replaces the hydrogen ions in the carboxylic group of matrix. As the matrix matures, it is hydrated. The unreacted powder particles are coated by silica gel formed during cross linking.

So basically set cement consist of unreacted powder particles coated by silica gel embedded in the amorphous matrix of hydrated calcium and aluminium polysalts containing fluoride.

How does it adhere to the tooth?

The carboxylic group in the polyacrylic acid chelates with the calcium in the hydroxy apatite crystals of enamel and dentin forming a chemical bond. 

The mixture should be glossy during insertion. The glossiness means the carboxylic group are free and it will bond to the tooth surface.