Setting and Hardening of Cement
The term ‘Setting‘ is used to describe the stiffening of the cement paste. Setting of cement refers to changes of cement paste from a fluid to rigid state. Setting differs from Hardening of cement.
The term ‘Hardening’ refers to the gain of strength of a set cement paste, although during setting the cement paste acquires some strength.
The setting characteristics of Portland cement paste are defined by initial set and final set. Initial set indicates the approximate time at which the paste begins to stiffen considerably. Final set roughly indicates the time at which the cement paste has hardened and can support some load.
Initial setting time indicates the beginning of the setting process when the cement paste starts losing its plasticity. Final setting time is the time elapsed between the moment water is added to the cement and the time when the cement completely lost its plasticity and can resist certain definite pressure.
These times of set are tested according to standardized procedures and have no special relationship to concrete setting behavior. Setting types are affected by minor constituents in the cement such as alkalis and Sulfates, by fineness, water-cement ratio, ambient temperature and inclusion of mineral and chemical admixtures. Concrete generally sets more slowly than cement paste because of the higher water-cement ratios.
A table is shown below for approximate setting time at different temperature.
|Temperature||Approximate Setting Time (hours)|
|20oF (-7oC)||Set will not occur|
There are 2 types of abnormal setting behavior that should be mentioned.
- False Set: This refers to the rapid setting that occurs without the liberation of much heat. Plasticity can be regained by further mixing without the need to add more water, and thus is not a problem where concrete is mixed for long periods (ready-mixed concrete). Increasing mixing time when possible will help to reduce a false set problem.
- Flash Set (or quick set): This behavior is accompanied by the liberation of considerable heat. The plasticity of the mixture cannot be regained with additional mixing or water.