ASTM C150 SPECIFIED SET TIMES BY TEST METHOD:
How Setting Time of Cement is Affected:
Cement paste setting time is affected by a number of items including: cement fineness, water-cement ratio, chemical content (especially gypsum content) and admixtures.
Setting tests are used to characterize how a particular cement paste sets. For construction purposes, the initial set must not be too soon and the final set must not be too late.
Additionally, setting times can give some indication of whether or not cement is undergoing normal hydration (PCA, 1988). Normally, two setting times are defined:
Initial set. Occurs when the paste begins to stiffen considerably.
Final set. Occurs when the cement has hardened to the point at which it can sustain some load.