Setting Time of Cement: Hydraulic cementitious materials have a unique feature, which is they are moldable and castable materials in their early age once mixed, and then they transform into a solid/hard material with much useful engineering characteristics.
Hydration is the process that causes this structural transformation from plastic to hard material. Hydration refers to the chemical reactions between the cementitious material and water that leads to the consumption of free water and formation of calcium-silicate-hydrate that is responsible for the hardening behavior, which is defined as “setting.”
This process normally occurs in a matter of hours starting from the time of contact of cement with water. The hydration process is complex and involves a series of chemical reactions. Thus, any change in the content or properties of the cementitious materials results in changing the affect the time at which finishing and curing operations can be initiated.
Therefore, it is critical to have techniques for quantifying the rate of setting of the cementitious mixture.
What is Initial Setting Time of Cement?
The initial setting time of cement is defined as the time interval between the addition of water to the cement and the point at which the cement paste begins to lose its fluidity (plasticity).
In terms of testing, initial setting time is defined as the time interval between the point of water addition to the cement and the time at which Vicat’s needle reaches a depth of 5 to 7 mm from the bottom of Vicat’s mould.
What is Final Setting Time of Cement?
The final setting time of cement is defined as the time interval between the addition of water to the cement and the point at which the cement paste has totally lost its fluidity (plasticity).
Similarly, from the testing aspect. the final setting time of cement may be described as the time interval between the addition of water to the cement and the time at which Vicat’s needle produces a mark on the paste surface, but the annular collar does not cause any mark.
Basically, you will need Vicat’s apparatus as shown in the above photo. Also, a balance, stop watch, and trowel are required.
Steps for initial setting time
- Before starting mixing the paste required for testing the setting time of cement, a consistency test should be done as per the required code to determine the water required to produce a paste with normal consistency (P).
- Prepare a neat cement paste using 400 gm of cement and 0.85P of water by weight of cement.
- Start mixing the water with the cement for 3-5 minutes. At the same time, start the stopwatch. Record this time of starting the mixing process as (T1).
- Fill the Vicat mould with the paste. The glass plate should be placed below the mould. Completely fill the mould to the top and smooth off the surface of the paste making it level with the top of the mould. The prepared cement block is identified as the “test block”.
- Put the test block confined in the mould and resting on the glass plate under the needle.
- Gently lower the needle until it makes contact with the test block’s surface and quickly release, enabling it to penetrate the test block.
- Initially, the needle pierces the test block entirely. Repeat this process, rapidly releasing the needle after every two minutes, until the needle is unable to puncture the block for about 5 mm measured from the bottom of the mould. Take note of this time as (T2).
- Calculate the initial setting time as T2 – T1. It should not be less than 30 minutes for OPC.
Steps for final setting time
Final setting time is usually measured using the same test block as the initial time. In case it is required to measure only the final setting time, the procedure is as follows:
- Prepare a cement paste by adding water 0.85P% by weight of cement to 400 gm of cement. T1 is the time at the point of starting the mixing process.
- Mixing time should range between 3 and 5 minutes.
- Fill the mould to the top and level it off.
- Place the mould placed under the square needle that has an annular collar and bring the needle down to touch the surface of the paste.
- Suddenly release the needle to make on impression on the cement paste due to its own weight.
- Repeat the entire experiment with incremental time intervals until you reach a time where the square needle can make an impression on the surface whereas the annular collar fails to do so. This time is T2
- Calculate the final setting time as T2 – T1. It should not be more than 600 minutes for OPC.
Below are some important precautions that should be considered during the testing for setting time:
- Releasing the needle should be gentle.
- Stay away from any source of vibration or disturbance.
- Clean the needle from the cement paste all the times it is used.
- Slightly shift the mould after each penetration to prevent inserting the needle at the same position.
- Perform the test at the specified environmental conditions by governing codes.