Alginate impression material is:
Alginate
The freshly spatulated material has low viscosity, although this can be varied to some extent by alterations in the amount of inert ﬁller incorporated by the manufacturer. The low viscosity, coupled with a degree of pseudoplasticity, classiﬁes alginates as mucostatic impression materials. They are able to record soft tissues in the uncompressed state. For some applications low viscosity may be a disadvantage, for example, when trying to record the depth of the lingual sulcus. A higher viscosity is required to displace the lingual soft tissues in order that the full depth can be recorded. 
Materials which are initially very ﬂuid are often classiﬁed as mucostatic impression materials because they are less likely to compress soft tissues, whilst materials which are initially more viscous are classiﬁed as mucocompressive.
KEY CONCEPT:
Alginate impression material is less mucostatic than ZOE.
Reference: Applied Dental Materials Ninth Edition  McCabe and Walls; page  no :159, 136
Non-elastic impression material
Thermoplastic impression material
More mucostatic than ZOE
Less mucostatic than ZOE