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 a cement is undergoing normal hydration (PCA, 1988). Normally, two setting times are defined (Mindess and Young, 1981):
- 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.
These particular times are just arbitrary points used to characterize cement, they do not have any fundamental chemical significance. They describe the setting of the cement and are not tied to the setting time of concrete.
Both common setting time tests, the Vicat needle and the Gillmore needle, define initial set and final set based on the time at which a needle of particular size and weight either penetrates a cement paste sample to a given depth or fails to penetrate a cement paste sample. The Vicat needle test is more common and tends to give shorter times than the Gillmore needle test. Table 1 shows ASTM C 150 specified set times.
|Test Method||Set Type||Time Specification|
|Vicat||Initial||≥ 45 minutes|
|Final||≤ 375 minutes|
|Gillmore||Initial||≥ 60 minutes|
|Final||≤ 600 minutes|
Standard Test Methods
- AASHTO T 131 and ASTM C 191: Time of Setting of Hydraulic Cement by Vicat Needle
- AASHTO T 154: Time of Setting of Hydraulic Cement by Gillmore Needles
- ASTM C 266: Time of Setting of Hydraulic-Cement Paste by Gillmore Needles