zinc phosphate cement

zinc phosphate is oldest of luting cements

supply :- suppled as powder and liquid.

composition :- a.) powder contains ==> zinc oxide 90% and magnesium oxide 10%

b.) liquid contains ==> phosphoric acid, aluminium phosphate, water.

powder/liquid ration:-1.4 grms powder to 0.5 ml liquid

setting :- when powder and liquid are mixed, the phosphoric acid attacks the surface of the particles and releases zinc ions into liquid, aluminum which is already forms complex with phosphoric acid, reacts with zinc yields a 'zinc aluminophosphate' gel on the surface of remaining portion of particles

'set cement contains core of unreacted zinc oxide particles in a cohesive amorphous matrix of zinc alumino phosphate'

its an acid base reaction

addition of water accelerates the setting reaction

reaction is exothermic, cooling of glass slab retards the chemical reaction

setting time :- 2.5 -8 min

physical properties :- 1.)compressive strength 104 Mpa

2.)tensile strength:- 5.5 Mpa

3.)modulus of elasticity 13.7 Gpa

retention:- by mechanical interlocking

PH :- high pH at the time immediately after mix 2 after 2 minutes 4 after 1 hr