One group of widely used cements is based on the vigorous reaction which occurs between certain basic oxides and phosphoric acid to form phosphate salt of low solubility.
Zinc phosphate cements
These materials are generally supplied as a powder and liquid which are mixed together by hand.
The major reactive component of the powder is zinc oxide.
Composition of zinc phosphate cements:
Approximately 90% as main active ingredient
Other metallic oxides
Approximately 10 % concentration
Aqueous solution of phosphoric acid
Up to 10% As buffers
The powder/liquid ratio depends on the application. For cavity lining a puttylike consistency having a powder/liquid ratio of about 3.5:1 is used.
The powder is the best incorporated into the liquid in small increments until the desired consistency is reached.
Mixing is easier if carried out on a cooled glass mixing slab.
Providing the materials are manipulated correctly. The phosphate cements have sufficient working time to allow placement of a cavity lining or cement lute before the viscosity has increased markedly. There is a minimum setting time of 2.5 minutes for luting cements and 2 minutes for lining cements. Phosphate cement are not adhesive to tooth substance or to restorative materials.
Silicophosphate cements are, essentially hybrids of zinc phosphate and silicate materials. They are supplied as a powder and liquid. The liquid is an aqueous solution of phosphoric acid whilst the powder is mixture of zinc oxide and black cupper oxide. The settings reaction is similar to that for zinc phosphate materials.
The two properties which distinguish these products from simple zinc phosphate materials are their black appearance and their bactericidal effects, produced by the presence of cupper.