Setting Time of Cement is one of the crucial indications required while handling cement or concrete on site. Setting of cement is a property of hydrated cement.
Setting of cement represents the transition of cement from fluid to rigid state. It can be evaluated either from cement paste or from mortar.
Setting of Cement
Stiffening of cement paste is termed as setting of cement.
The transition of cement from plastic state to solid state is represented as setting of cement.
Initial and Final Setting Time of Cement
Two of the setting times are considered as discussed below:
Initial Setting Time of Cement
The time from the moment water is added to the cement to the moment when cement starts losing plasticity is called initial setting time of cement.
In other words, IST is the time when the stiffening starts in cement paste. IST is the time until which the cement paste can be moulded into any desired shape without affecting its strength.
In actual construction practice, time is required for concreting operations like mixing, transporting, placing, finishing, compacting, etc. The cement should remain in plastic state until all these operations are completed.
There are certain points to be taken care of during these operations, you can read about these operations here- Steps in Concreting Process
For ordinary Portland cement, IST is 30 min. hence, all the concreting steps need to be completed within 30 minutes, else the concrete will start losing plasticity and the full strength of concrete would not be achieved.
Final Setting Time of Cement
The time when the water is added to cement until the time when the cement paste loses its plasticity completely is FST.
At FST, the cement attains sufficient firmness and it can resist definite pressure. FST is specified as well as measured so as to ensure normal hydration.
After the handling operations of cement are over, the cement should lose its plasticity at the earliest so that the vulnerability to the damage due to external factors is reduced.
FST should not be more than 10 hours. For OPC, the value of FST is 6 hours.
Significance of Initial and Final Setting Time:
- The properties of cement are affected if the concrete is not placed and finished within the initial setting time.
- The cement should not be disturbed till final setting time is over
- Moulds can be removed after FST as the cement gains enough strength
- FST marks the beginning of the development of mechanical strength of cement
- For removing the forms for various structural members, FST is considered
Relation between IST and FST
An approximate relationship between IST and FST is observed as follows:
FSTmin = 90 + 1.2 (ISTmin)
The above relation is not applicable for high alumina cement.
Test to Determine IST and FST
Vicat apparatus is used to determine the values of initial and final setting time of cement.
Initial and Final Setting Time of Cement Types
Type of Cement | Initial Setting Time | Final Setting Time |
---|---|---|
Ordinary Portland Cement | 30 | 600 |
Portland Pozzolana Cement | 30 | 600 |
Portland Slag Cement | 30 | 600 |
Rapid Hardening Cement | 30 | 600 |
Quick Setting Cement | 5 | 30 |
Low Heat Cement | 60 | 600 |
Sulphate Resisting Cement | 30 | 600 |
Super Sulphated Cement | 30 | 600 |
High Alumina Cement | 30 | 600 |
Hydrophobic Cement | 30 | 600 |
Masonry Cement | 90 | 1440 |
Why Setting of Cement Takes Place
As selective hydration of two components of cements, viz. C3A and C3S occurs setting of cement takes place.
With the setting of cement, rise in temperature is observed. During initial setting, rapid rise in temperature is observed while during final setting, peak temperature is observed.
Setting of Cement v/s Hardening of Cement
Setting of cement and hardening of cement are differentiated below:
Final Setting Time | Hardening of Cement |
---|---|
Stiffening of cement | Gain in strength of cement |
Cement loses plasticity upon setting | Hardening of cement occurs after setting of cement |
Factors Affecting Setting Time of Cement
Factors affecting setting time of cement are discussed below in detail:
1. Retarders– Adding retarders like gypsum decreases the setting of cement, and setting time is increased. Retarders are mixed with clinkers before final grinding or just after preliminary grinding of clinker.
2. Fineness of cement during grinding– The finer the cement particles are, the quicker will be its setting. Thus, setting time is reduced with increase in fineness of cement.
3. water/cement ratio– If water is increased by 1 % more than that required for normal consistency, then setting time is increased by 30 minutes or more. Thus, setting of cement is retarded with increase in water.
4. Composition of cement– Alkalis accelerates the setting of cement. Decreasing content of calcium oxide reduces setting time of cement. Aluminium oxide promotes setting of cement.
5. Admixtures– Some admixtures reduce the setting time of cement while some admixtures increase the setting time of cement. Set retarders are used to delay setting time while set accelerators increase hydration of cement, thus decreasing setting time of cement.
6. Temperature of mixing water– Temperature of mixing water also affects setting of cement. Cold water reduces cement setting while warm water accelerates the setting of cement.
7. Mixing time– As mixing time of concrete is increased, setting of cement is accelerated and the setting time decreases.
8. Degree of calcination– When gypsum is added before calcination, cement decomposes into lime and sulphur trioxide is liberated in kiln. Sulphur dioxide liberated affects setting of cement.
9. Aeration after grinding of clinker– Cement exposed to thoroughly saturated atmosphere sets much slowly in comparison to the cement exposed to dry atmosphere.
10. Atmosphere where cement is stored– Cement stored in warm room sets quickly as comparatively. Cement stored in atmosphere where considerable carbon dioxide is present also reduces the setting time of cement.
False Set of Cement
Sometimes cement stiffens in minutes when it is exposed to humidity during storage. This process is called false set of cement.
Very little or no heat is evolved during false set of cement.
Solution to False Set of Cement
- Remix the paste that has undergone false set vigorously without adding water
- Remixing vigorously restores the plasticity of the cement paste and allows it to set in normal way ensuring its specified strength.
- False set of cement is different from flash set and quick set of cement.
Temperature Effects on Setting Time of Cement
As noticed in false set, temperature affects the setting time of cement. Hence, temperature is recommended for mixing of cement and storage of cement.
As per BS EN 196- 3 : 1995, following temperatures and humidity are recommended:
Temperature (in °C) | Temperature (in °F) | Relative Humidity (in %) | |
Mixing of cement | 20 ± 2 °C | 68 ± 4 °F | 65 % (min) |
Storage of cement paste | 20 ± 1 °C | 68 ± 2 °F | 90 % (max) |
Key Take Away
IST = Time from the moment water is added to cement to the moment when cement starts losing plasticity
Cement can be moulded into any shape till initial setting starts. After this, stiffening of paste starts.
FST = Time from the moment water is added to cement to the moment when cement loses its plasticity
Cement starts achieving firmness and starts gaining strength as final setting time is reached.
For ordinary Portland cement, IST is 30 min and FST is 600 min.
Setting of cement represents stiffening of cement while Hardening of cement is the gain in strength of cement.
False set of cement is the stiffening of cement within minutes without much evolution of heat upon exposure to humidity.
FAQ
What is initial & final setting time of cement?
Initial setting time of cement is the time until which the cement paste starts loses its plasticity. Final setting time of cement is the time when the cement paste totally loses its plasticity.
What is the initial setting time of cement?
The time when water is added to cement to the time when the cement starts losing its plasticity is called initial setting time. Initial setting time is the time when water is added to cement until the moment when the square needle penetrates to a depth of 33-35 mm from the top of the mould.
Why is the initial setting of cement 30 minutes?
The initial setting time of cement should not be less than 30 minutes because the time is required for handling operations of cement like mixing, placing, finishing, etc. The cement operations cannot be carried out once initial setting of cement occurs as cement paste starts losing plasticity which will then affect the strength gaining of the paste.
What is cement setting?
Upon addition of water, the cement paste formed remains plastic for sometime.as the hydration of cement occurs, the cement paste gradually starts losing plasticity. This is called cement setting.
What causes setting of cement?
Selective hydration of two components of cements, viz. C3A and C3S causes cement is responsible for setting of cement.