Cement has a variety of applications in construction, including the construction of roads, buildings, and other structures. A number of factors can affect the initial setting time, like the type of cement used and the ratio between cement and water. In this article, we will talk about the significance of the initial setting time of cement, as well as the elements that influence it.
Initial setting time of cement: Meaning
Cement hydrates and transforms into cement paste when it is combined with water in a certain ratio. Because of the fluidity of this paste, it may be shaped into almost any form that one could wish.
During this period of time, the cement continues to react with the water, and gradually, the cement begins to lose its plasticity and solidify into a more permanent state.
The whole duration of this process is referred to as the setting time of cement. The period of time up to which cement may be shaped into any desired form without suffering a loss in its strength is referred to as the initial setting time of cement.
Initial setting time of cement: Significance
Cement should not harden too quickly or too slowly. The rapid setting would leave insufficient time for concrete to be transported and poured. If it’s delayed too long, it may delay the task and reduce productivity at the optimal moment.
The initial setting time of cement is very important for the transportation, placement, and compaction processes. It slows down the hydration and the hardening of the material.
Initial setting time of cement: Factors influencing it
The following is a detailed discussion of the factors that impact the initial setting time of cement:
The rate at which cement sets is slowed down and the amount of time it takes to set is lengthened by the use of retarders like gypsum. Before the clinkers are put through their final grinding process, or soon after the preliminary grinding process, retarders are added to the mix.
Cement fineness during grinding
If the particles are smaller, the cement will set more quickly. As a result, the finer the cement, the faster it sets.
If the amount of water is raised by one per cent above what is necessary for the regular consistency, then the time it takes to set will increase by at least thirty minutes. As a result, an increase in the amount of water slows down the process by which cement sets.
The cement hardens more quickly when alkalis are present. The amount of calcium oxide in cement may be decreased to shorten the amount of time it takes to set. The cement hardens more quickly when aluminium oxide is present.
Mixing water temperature
The temperature of the water used in mixing the cement also has an effect on how quickly the cement sets. The setting time of cement is sped up by using warm water, whereas using cold water slows down the process.
The amount of time it takes for cement to set may be altered by adding certain admixtures, some of which speed up the process while others slow it down. Set retarders are added to cement in order to lengthen the time it takes for the cement to set, whilst set accelerators speed up the setting process by increasing the hydration of the cement.
When you mix concrete for a longer period of time, the cement will set faster, which will result in less time being needed for the setting process.
When gypsum is introduced to cement prior to calcination, the cement is broken down into lime, and the kiln produces sulphur trioxide as a byproduct of this process. The amount of sulphur dioxide released has an effect on the cement’s ability to set.
Aeration after clinker grinding
When compared, the setting time for cement that has been exposed to an environment that is completely saturated with water is much longer than that of cement that has been exposed to an atmosphere that is dry.
The environment in which cement is kept
Comparatively, cement placed in a warm environment sets in a short amount of time. The setting period of cement is shortened if it is kept in an environment with high levels of carbon dioxide.
Initial setting time test: Vicat Apparatus Test
The Vicat apparatus is used to find the initial setting time of cement. The devise has a 1 mm size square needle that is 50 mm long and a mould with a height of 40 mm and diameter of 80mm.
Mentioned below is the test procedure.
- Make a paste of the cement by mixing water that is 0.85P% by weight of the cement. Note that the time to mix should be more than 3 minutes and less than 5 minutes.
- Once mixed, fill the paste in the vicat mould with a properly levelled top surface.
- Next, the mould is places under square needles which has to be brought down so that the surface of the paste touches it.
- Next, the square needle is let off to sink in the paste of cement by its own weight. Note the depth till which the needle has penetrated inside the cement paste.
- Repeat this in intervals till square needle penetrates till 33 to 35 mm from the mould’s top.
- Initial setting time of cement should be calculated as the time between when water is added to the cement to when the needle penetrates a depth of 33 to 35 mm from the top mould.
- For ordinary Portland cement, the initial Setting Time should be more than 30 minutes.
Why should the initial setting of cement be necessarily more than 30 minutes?
The reason why the initial setting time of cement should not be less than 30 min is that the initial setting time is the exact time when the concrete starts to harden and can be used for construction purposes. This time starts as soon as water is mixed with cement to make the mixture. The initial setting time has to be more than 30 minutes so that the concrete is proper and there is no quality issues faced during construction. The final setting time is the time the concrete has hardened so much that the square needle is no longer able to penetrate through it.
Initial setting time: Different types of cement
|Type of Cement||Initial Setting Time (minutes)|
|Ordinary Portland Cement||30|
|Portland Pozzolana Cement||30|
|Portland Slag Cement||30|
|Rapid Hardening Cement||30|
|Quick Setting Cement||5|
|Low Heat Cement||60|
|Sulphate Resisting Cement||30|
|Super Sulphated Cement||30|
|High Alumina Cement||30|
What causes the initial setting time of cement to increase?
Cement composition, the ratio of water to cementitious material (measured in w/cm), temperature, and the use of admixtures are the primary elements that influence the amount of time it takes for the mixture to set. When cement hydrates more quickly, the amount of time needed for setting is cut down significantly. When the width per centimetre (w/cm) is raised, the amount of time needed to set increases as well.
Does gypsum prolong the initial setting time?
If the gypsum content is below 15 wt.%, the setting time will rise dramatically. When increasing from 7 wt.% gypsum to 15 wt.%, the first and last setting times go up from 16 minutes and 22 minutes, respectively, to 37 minutes and 55 minutes.
Does temperature affect cement setting time?
The pace at which cement hydrates is heavily influenced by a number of factors, including the temperature of the air and ground, as well as the circumstances of the weather. The amount of time it takes for concrete to set increases as the temperature rises, but if the temperature reaches 30 degrees Celsius or more, the opposite effect may be seen.
Why is the default value thirty minutes?
The primary reason why the initial setting time of cement shouldn’t be any less than thirty minutes is that the initial setting time of cement represents the precise instant when the concrete begins to harden. In principle, the countdown to this time begins when the liquid is put into the cement.