The process of converting mixed cement from a fluid state to a solid state is referred to as the setting of cement. The time taken for this process to occur is called the setting time of cement, which is equivalent to the setting time of concrete. The setting of cement occurs in stages, and various factors influence the initial and final setting time of different types of cement.
When water is added to cement, a chemical reaction is initiated. The finely ground cement, in its pure form, is highly reactive to water. One of the three primary compounds found in cement, namely C3A, C3S, and C2S, reacts quickly with water to form a gel-like substance that begins to solidify. This process is known as the setting of cement.
To elaborate further, the setting time of cement can be affected by several factors, such as the amount of water used, the temperature, and the type of cement. Additionally, the setting of cement occurs in two stages: initial setting and final setting. The initial setting time is the time required for the cement to start solidifying after being mixed with water, while the final setting time is the duration needed for the cement to harden and attain its full strength.
The process of setting cement involves the formation of crystalline structures, and the speed at which these structures form is dependent on various factors. For instance, higher temperatures generally result in a faster setting time, while colder temperatures tend to slow down the process. The type of cement used also plays a role in determining the setting time.
In conclusion, the setting of cement is the process of converting mixed cement from a fluid state to a solid state, which occurs in stages and is influenced by several factors. The time taken for this process to occur is referred to as the setting time of cement, and it depends on factors such as water content, temperature, and the type of cement used.
Stages of Setting of Cement
When cement is mixed with water, it undergoes a complex chemical process that can be divided into three stages. These stages are crucial for the final setting of the cement and the resulting strength of the hardened material.
During the first stage, called the dormant period, there is no visible reaction between the cement and water. However, the cement particles start to absorb water, causing them to swell and begin to break down. This process leads to the formation of a gel-like substance that coats the cement particles, which is critical for the subsequent chemical reactions.
In the second stage, known as the induction period, the chemical reactions between the cement and water start to occur. The gel-like substance formed during the dormant period starts to react with the water, leading to the formation of calcium-silicate-hydrate (C-S-H) gel. This C-S-H gel is responsible for the strength and durability of the final cement product.
The final stage is the setting and hardening period. This is when the C-S-H gel starts to solidify and bind the cement particles together, resulting in a hardened material. The setting and hardening process is dependent on a range of factors, including temperature, humidity, and the chemical composition of the cement. However, once the cement has set, it is a strong and durable material that can withstand a range of environmental conditions.
1. Hydrolysis and Hydration Stage
The process of setting cement begins once water is added to it. During this process, the four compounds present in cement, namely C3S, C2S, 3CA1, and 4CAFe, undergo hydration. The hydration of the C3S compound of cement leads to the formation of complex hydro silicates. This chemical reaction is a crucial step in the setting of cement, as it allows the cement to harden and form a solid structure. Without this hydration process, the cement would not be able to bind with other materials and would remain in a powder form. Therefore, the hydration of the C3S compound is a vital process in the formation of concrete and other cement-based materials.
2. Colloidisation Stage
After the previous stage, the resulting products begin to separate and form a gel-like substance. This gel gradually thickens and acts as a type of glue around the aggregates, which helps initiate the setting of the cement. This process is important because it allows the cement to harden and become a solid structure.
As the gel thickens and hardens, the mortar – which is made up of a mixture of cement, water, and sand – becomes fully saturated. This means that the mortar has reached its maximum capacity for water absorption and can no longer take on any more water. This is an essential part of the process because it allows the mortar to solidify and form a strong bond with the aggregates. Overall, this stage plays a crucial role in the setting of the cement and the formation of a durable structure.
3. Crystallisation Stage
During the stage known as “crystallization,” the majority of components in a gel or colloidal state transform into a crystalline state. This process is particularly noticeable in compounds that are less stable, including tri-calcium hydro aluminate and calcium hydroxide. Calcium hydro silicate gel also hardens at a nearly simultaneous rate with the development of crystals. As a result, the combination of these processes leads to the creation of a strong and inter-grown mass of crystals and gels.
Setting Time of Cement
1. Initial Setting Time
The period between the addition of water to cement and the failure of a 1mm square needle to penetrate the test block to a depth of approximately 5mm from the bottom of the mold is known as Initial Setting Time. The ISI has set a minimum requirement of 30 minutes for the initial setting time of ordinary and rapid hardening Portland cements, while low heat cement must have an initial setting time of at least 60 minutes.
2. Final Setting Time
Final Setting Time is a crucial parameter for evaluating the quality and performance of Portland cement. It refers to the duration that passes from the moment when water is mixed with cement until the point when the needle of 1 mm square section with 5 mm diameter attachment creates an impression on the test block. The maximum time limit for the final set is 600 minutes for all types of Portland cements. This time interval is significant as it helps determine the optimal time for the placement and finishing of concrete.
To assess the strength characteristics of Portland cement, the Indian Standard IS: 269-1976 specifies the compression strengths on the standard mortar-cube. This test is performed by preparing a mortar mixture consisting of cement and standard sand in the ratio of 1:3 by weight. The mixture is then compacted in a cube mold and left to set for 24 hours. After that, the cubes are demolded and cured under standard conditions. The compression strength is measured at the age of 3, 7, and 28 days, respectively. This standard method provides a reliable and consistent means of evaluating the compressive strength of Portland cement, which is an essential property for its practical applications.
Fig 1: Vicat Apparatus for Cement Setting Time Test
Factors Affecting the Setting Time Of Cement
The setting time of cement after it is mixed with water can be influenced by several factors. One of the main factors is the composition of the cement itself. The amount of gypsum present in the cement also plays a role in setting time, as it can act as a retarder.
The fineness of the cement particles can also have an impact on setting time, with finer particles generally setting more quickly. Curing conditions, such as temperature and humidity, can also affect the setting time of cement.
The water-to-cement ratio is another crucial factor, as a higher ratio can lead to longer setting times. Additionally, the type of admixture used in the cement mixture can influence the setting time.
Finally, the storage of cement can also affect its setting time. Proper storage conditions, such as cool, dry, and dark environments, can help maintain the quality of the cement and prevent any changes that may affect its setting time.
Initial and Final Setting time of different type of cement.
Table 1: Initial and Final Setting time of different type of cement.
| Type/Name of Cement | Initial Setting Time in min | Final Setting Time in min |
| OPC(33) | 30 | 600 |
| OPC(43) | 30 | 600 |
| OPC(53) | 30 | 600 |
| Sulphate Resisting cement | 30 | 600 |
| Portland Pozzolana Cement | 30 | 600 |
| Rapid hardening cement | 30 | 600 |
| Portland Slag Cement | 30 | 600 |
| High alumina | 30 | 600 |
| Super sulphated | 30 | 600 |
| Low heat | 60 | 600 |
| Masonry cement | 90 | 1440 |