Strength of zinc phosphate is affected by
COMPRESSIVE STRENGTH
The fully set zinc phosphate cement has a relatively high compressive strength ranging from 104 to 119 MPa. The set cement gains approximately 70% of its maximum strength in the first 30 minutes. The strength continues to rise with time and maximum strength is attained at the end of 24 hours.
The strength of zinc phosphate cement is sufficient when used as a base or luting agent. However, when it is exposed to the oral environment, e.g. temporary restorations, its brittleness and low strength causes it to fracture and disintegrate. Also, the prolonged contact with the oral fluids or water gradually reduces its strength. This may be due to the slow dissolution of the cement.
TENSILE STRENGTH
The set cement is weaker in tension (5.5 MPa), thus making it brittle.
Key concept​​:
Factors affecting strength​:

Powder-liquid ratio: More the powder, greater the strength.​
Water content of the liquid: Both loss or gain, reduces the strength.

Ref: Phillips Ed 12th P: 318
Temperature of glass slab
P:L ratio
Consistency of mix
Alteration of mixing time