Accelerators and retarders are used with gypsum products mainly to control:
MODIFIERS FOR CONTROLLING SETTING TIME
Chemical modifiers have been used extensively to increase or decrease the setting time of gypsum products; they are called retarders and accelerators, respectively. The manufacturers use both types of compounds to formulate gypsum products with specific setting times.
The chemical that increases the rate of hemihydrate dissolution or precipitation of dihydrate accelerates the setting reaction. The most commonly used accelerator is potassium sulfate, which is particularly effective in concentrations greater than 2%. Slurry water flowing out from a model trimmer contains numerous fine gypsum particles that act as nuclei of crystallization and that can serve as an effective accelerator.
Key Concept: 
Chemical modifiers have been used extensively to increase or decrease the setting time of gypsum products; they are called retarders and accelerators, respectively.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 186
Setting time
Setting expansion
Hardness of the set product
None of the above