Impression plaster is:
IMPRESSION PLASTER
Impression plaster is a β-calcium sulfate hemihydrate used at a water/powder ratio of approximately 0.5 to 0.6. Its fluidity makes it suitable for making impressions of soft tissues in the uncompressed state, a characteristic of mucostatic impression material. 
Key concept: 
Impression plaster is less compressive than Alginate.
Ref: Phillips 12 ed page no 177
Less compressive than alginate
More compressive than alginate
As compressive as alginate
More compressive than hydrocolloids