In any construction project, the building itself is not a complete structure. It is an adjoined number of structural components that make our structures desirable. One of the many structural components is cement. It is a binder in concrete and mortar that holds the structure together. Understanding the setting time of cement is crucial to structural integrity. In generic terms, setting time is the duration that to transition from a fluid state to a solid state. This setting time can be divided into two types; they are the initial setting time and the final setting time. The difference between these two types is that in the initial setting time, the cement paste remains workable, whereas in the final setting time, the cement paste has completely lost its plasticity. This article provides proper knowledge of the above-mentioned setting times; failing to understand them would compromise the structural integrity.
What is the Final Setting Time?
The final setting time refers to the duration from the addition of water to the point of attainment of hardened cement. At this stage, the cement paste loses its plasticity. The hardened cement can bear loads and is stable for construction purposes. Observation of this process is important to determine the continuation of further processes such as formwork removal, and applying finishes. The parameter of the final setting time of the cement is influenced by the chemical composition of the cement, water-to-cement ratio, temperature, and the use of admixtures.
Procedure to Find Setting Time of Cement
The commonly used method or apparatus in the construction industry to determine the setting time of cement is the Vicat apparatus. This apparatus has a special rod with a needle at its end. Below are listed the procedures to determine the setting time of the cement.
- In both initial and final setting times, the mixture is prepared in paste form by mixing water and cement at standard consistency.
- To determine the initial setting time, the needle is lowered into the prepared cement paste that is filled in a mould. The process begins from here. The apparatus takes note of the time it requires for the needle to penetrate to 5mm from the bottom of the mould. The slow needle penetration signifies that the cement is in the initial setting phase.
- To determine the final setting time, after some time, the needle is again lowered. To signify that the cement is in the final setting time, the needle should not penetrate deep. The slight impression on the surface without much penetration implies that the cement is hardened and can support the load without significant deformation.
Final Setting Time of Various Cement Types
While almost all types of cements have the same initial and final setting time, some have differences in initial and final setting time. Below are listed the initial and final setting times of various cements.
- Ordinary Portland Cement (OPC) types 33, 43, and 53, along with Portland Pozzolana Cement, Portland Slag Cement, Rapid Hardening Cement, Sulphate Resisting Cement, and High Alumina Cement, all share a standard initial setting time of 30 minutes and a final setting time of 600 minutes.
- Masonry cement has an initial setting time of 90 minutes and a final setting time of 1140 minutes.
- Low Heat Cement has an initial setting time of 60 minutes and a final setting time of 600 minutes.
- Super Sulphated Cement has an initial setting time of 30 minutes and a final setting time of 600 minutes.
Factors Affecting Final Setting Time
Several factors can affect the initial and final setting time of the cement. They are,
- The setting time is greatly influenced by the composition of chemicals. The higher the percentage of tricalcium silicate, the lesser the time taken for setting. Similarly, the higher the dicalcium silicate slows the setting time of the cement.
- When a cement block is finely ground, it hydrates faster. This, in turn, reduces the setting time of the cement.
- The presence of excess water in the cement may prolong the setting time because the creation of more space for hydration.
- Even temperature plays a significant role. The high temperature decreases the setting time, whereas lower temperature increases the setting time.
- In admixtures, we have two types: retarders and accelerators. The addition of gypsum, which is a retarder, delays the setting time. The addition of calcium chloride, an accelerator, reduces the setting time.
Testing the Final Setting Time
As discussed above, the final setting time is determined by the Vicat apparatus. The procedure includes placement of prepared paste in the Vicat mould. The resistance of the mold is measured periodically. The final setting time is noted when the needle of the Vicat apparatus does not penetrate the prepared cement mold. This ensures the cement meets the required standard for construction needs.
Factors that can Accelerate or Delay Final Setting Time
The duration of the final setting can be modified with the addition of an accelerator or a retarder with the cement mixture.
- Chemicals like calcium chloride, which is an accelerator, are added to speed up the hydration in cold climates and are useful in cold temperatures.
- Chemicals like gypsum, which is a retarder, are added to delay the setting time. It is useful in hot temperatures.
- In some cases, environmental conditions such as high temperatures and low humidity accelerate the setting, whereas low temperatures and high humidity delay it.
Difference between Initial and Final Setting Time
| Initial setting Time | Final Setting Time |
| It is the duration of time taken by the cement to lose plasticity and begin hardening. | It is the duration of time taken by the cement to reach complete hardening. |
| The cement paste is workable at this stage. | This stage signifies that cement can bear loads and further construction can proceed. |
| The ideal duration for initial setting time is 30-40 minutes. | The ideal duration for the final setting time is 600 minutes. |
| This helps in the mixing and application process. | This helps in determining the continuation of further processess. |
Conclusion
The setting time of cement is one of the essential parameters in construction, influencing workability, strength, and durability. The final setting time indicates a stable state when the material can bear loads, and thus the construction can proceed safely and efficiently. Factors that affect the setting time, testing procedures, and the differences between initial and final setting times will be known by engineers, thereby optimizing construction processes and providing durable structures.