This article is focused on air-entrained Portland cement, a unique type of cement that contains tiny air bubbles. These bubbles allow for the expansion of water droplets during freezing and thawing, which can help protect the concrete from damage and cracks. We will explore the production process of this cement, the air-entraining agents used, as well as its properties, advantages, and disadvantages.
Manufacture of Air-Entrained Cement
The process of producing this particular type of concrete is similar to that of regular ordinary Portland cement. The first step involves the formation of cement clinkers. Once these clinkers are formed, they undergo a grinding process. During this process, certain air entraining materials are added to the cement clinkers. These materials are responsible for converting the cement into air entrained cement.
The ways of incorporating air in concrete:
Gas forming materials such as aluminium powder, zinc powder, and hydrogen peroxide are commonly used in various industries. They are known for their ability to generate gas when mixed with other substances, making them useful for a range of applications.
Surface active agents are another group of substances that are frequently used in industrial processes. These agents are designed to reduce surface tension, which can help to improve the mixing and dispersal of other materials. Examples of surface active agents include natural wood resins and their soaps, animal or vegetable fats or oils, and alkali salts of sulfonated or sulphated organic compounds.
In addition to gas forming materials and surface active agents, cement dispersing agents are also commonly used in various industries. These agents are designed to help break up clumps and aggregates of cement, allowing it to mix more easily with other materials. Calcium ligno-sulphate and calcium salts of glues are two examples of cement dispersing agents that are commonly used in industrial processes.
Fig 1: Process of expansion of water droplets in concrete.
Properties of Air-Entrained Cement
The following are properties of air entrained concrete:
1. Workability
The high workability of this type of cement is attributed to the presence of air bubbles within the mixture. These bubbles serve as a lubricant between the aggregates, leading to a reduction in the amount of water and sand needed. Consequently, the strength of the resulting concrete is increased. Additionally, the presence of air bubbles helps to prevent segregation and bleeding of the plastic concrete, which can be a common issue in the mixing process. Overall, the inclusion of air bubbles in this cement contributes to its excellent workability properties and makes it a preferred choice for concrete applications.
2. Strength
Abraham’s law is a well-known concept in civil engineering, which states that the strength and the water-to-cement (W/C) ratios of concrete are inversely proportional. This means that the lower the W/C ratio, the higher the strength of the concrete. To maintain a low W/C ratio, engineers often use air-entraining agents, which help prevent the formation of voids in the concrete and increase its durability. However, achieving high-strength concrete while using air-entrained cement can be a challenging task. This is because the presence of air bubbles in the concrete can interfere with the formation of strong bonds between the cement particles, thereby compromising the strength of the final product.
3. Freezing and Thawing Durability
In cold environments, concrete is susceptible to damage due to the expansion of water inside it as a result of low temperatures. This expansion can cause the concrete to rupture if appropriate measures are not taken to prevent it. To avoid such damage, a specific type of cement can be used which creates space for the water to expand.
Furthermore, entrained air bubbles within the concrete serve as reservoirs for the expanded water. This helps to relieve the pressure of the expansion and prevent damage to the concrete. These air bubbles provide a safe space for the water to expand into, thereby reducing the risk of the concrete rupturing or cracking. Overall, the use of this type of cement and entrained air bubbles are effective strategies for protecting concrete from damage caused by low temperatures.
4. Sulphate Resistance
Air entrainment is a beneficial characteristic for cement as it increases the resistance to sulphate attack. This is because the addition of air allows for a lower water to cement ratio (W/c ratio), which can help prevent the initiation of sulphate attack. A low W/c ratio is important because it reduces the amount of water available for sulphate ions to react with. Therefore, the use of air entrained cement can help to mitigate the harmful effects of sulphate attack.
5. Permeability
When it comes to water tightness in concrete, the permeability of air-entrained concrete is superior to that of non-air-entrained concrete. This is because the presence of air bubbles in the former disrupts the capillary channels and prevents water from seeping through. As a result, it is advisable to opt for air-entrained concrete in situations where water tightness is a crucial factor. By doing so, you can ensure that the concrete structure is better equipped to withstand any potential water damage or seepage.
6. De-Icing Resistance
When a building has a sloped structure, it is common for ice to form on its surface. To remove this ice, de-icing chemicals are typically used to melt the snow and ice. However, the use of these chemicals can lead to the formation of a layer of scales on the concrete surface of the building. This scaling can cause significant damage over time and may even compromise the integrity of the structure.
One solution to prevent scaling caused by de-icing chemicals is the use of air entrained cement. This type of cement is recommended for all applications where the concrete will come into contact with de-icing chemicals. The addition of air to the cement helps to create tiny bubbles within the concrete, which can accommodate the expansion and contraction caused by freeze-thaw cycles. This prevents the formation of scales and helps to maintain the durability of the concrete over time.
Overall, the use of de-icing chemicals to remove ice from sloped structures can lead to significant damage to the building’s concrete surfaces. To prevent this damage, it is important to use air entrained cement, which can accommodate the expansion and contraction caused by freeze-thaw cycles and maintain the durability of the concrete. By taking these precautions, buildings can remain safe and structurally sound over time, even in the harshest of winter conditions.
Fig 2: Bubbles formation in concrete due to air entained cement.
Advantages of Air-Entrained Cement
Air entrained concrete offers several advantages that make it a popular choice for construction projects. One of the primary benefits of using air entraining agents in concrete is that it increases the workability of the material. This can help improve the overall quality of the concrete and make it easier to work with during the construction process.
Another advantage of air entrained concrete is that it can reduce the negative effects of freezing and thawing. When water freezes, it expands, which can cause significant damage to concrete structures. However, the air bubbles in air entrained concrete help to relieve the pressure that occurs when water freezes, reducing the likelihood of cracks and other damage.
In addition, air entraining agents can also help to reduce problems such as bleeding, segregation, and laitance in the concrete. These issues can affect the strength and durability of the concrete, so reducing them can help to ensure that the finished product is of the highest quality.
Entrained air also improves the sulphate resisting capacity of concrete, making it more resistant to damage from exposure to sulphate compounds. This can be particularly beneficial in environments where concrete is exposed to high levels of sulphates, such as in coastal areas.
Finally, air entrained concrete can reduce the possibility of shrinkage and crack formation on the surface of the concrete. This is because the air bubbles help to relieve the internal pressure that can cause shrinkage and cracking, resulting in a smoother and more uniform surface. Overall, these advantages make air entrained concrete an attractive option for a wide range of construction applications.
Disadvantages of Air-Entrained Cement
Air entrained concrete has some drawbacks that need to be considered. One of the main concerns is that the strength of the concrete can decrease as a result of air entrainment. Additionally, the use of air entraining agents can lead to an increase in the porosity of the concrete, which in turn can reduce the unit weight.
It is important to note that air-entrainment in concrete should only be carried out if there is good site control. This is because the amount of air entrained in the concrete can vary depending on factors such as sand grading, proportioning errors, workability of the mix, and temperature changes. Therefore, proper supervision and monitoring of these variables is essential to ensure that the desired level of air entrainment is achieved.
In summary, while air entrainment in concrete can be beneficial in certain situations, it is not without its drawbacks. Proper site control and monitoring are essential to ensure that the desired level of air entrainment is achieved without compromising the strength or weight of the concrete.