ZnPO4 setting time is 4-7 min. All of the following methods are employed to prolong its setting time except:
Addition of drop of distilled water
Prolong spatulation time
Small increment of addition of powder to liquid
Mixing on cooled glass slab
Control of setting time​:
​Manufacturing process​

Sintering temperature : The higher the ​temperature, the more slowly the cement sets.​
Particle size : Finer particles react more quickly as a greater surface area is exposed to ​the liquid.​
Water content of liquid : Presence of excess water accelerates, whereas insufficient water ​retards the reaction.​
Buffering agents : When added, slow down the reaction.​

​Factors under control of operator​

Temperature : Higher temperature accelerates the reaction. Cooling the mixing slab is an effective way of slowing the reaction and prolonging the working time.​
Powder-liquid ratio : More the liquid, slower the reaction. ​
Rate of addition of powder to liquid : The reaction is slower, if the powder is incorporated slowly.​
Mixing time : The longer the mixing time (within practical limits), the slower is the rate of reaction.

Ref: Manappalil P: 90​