Portland cement that has been air-entrained is a unique type of cement that contains air bubbles within the cement or concrete. These air bubbles provide the necessary space for the expansion of tiny water droplets within the concrete when exposed to freezing and thawing. This expansion helps to protect the concrete from developing cracks and other forms of damage.
The process of manufacturing air-entrained Portland cement involves adding special air-entraining agents to the cement mixture during the production process. These agents are typically surfactants or foaming agents that help to create the air bubbles within the cement.
One of the key advantages of using air-entrained Portland cement is its ability to withstand the effects of freeze-thaw cycles. This makes it a popular choice for construction projects in areas with cold weather conditions. The air bubbles within the cement also help to improve the workability of the concrete, making it easier to pour and spread.
However, there are also some potential disadvantages to using air-entrained Portland cement. For example, it may have reduced strength and durability compared to non-air-entrained cement. Additionally, the air bubbles may also create a more porous concrete, which can make it more susceptible to water infiltration and damage from chemicals or other environmental factors.
Overall, air-entrained Portland cement is a specialized type of cement that offers unique properties and advantages for construction projects. It is important to carefully consider the specific needs of a project before deciding whether to use this type of cement.
Manufacture of Air-Entrained Cement
This paragraph describes the manufacturing process of a special type of concrete which is similar to normal Ordinary Portland Cement. The process starts with the formation of cement clinkers, which are then subjected to grinding. During the grinding process, air entraining materials are added to the cement clinkers. This addition of air entraining materials transforms the cement into air entrained cement, which is the special type of concrete being referred to.
The ways of incorporating air in concrete:
Gas forming materials such as aluminium powder, zinc powder, and hydrogen peroxide are commonly used in various industrial applications. These materials are capable of releasing gas upon reaction with other substances, making them useful for processes such as foam production and metal casting.
To aid in the efficiency of these processes, surface active agents are often added to the mixture. These agents help to reduce surface tension, allowing for better dispersion and mixing of the gas-forming materials. Natural wood resins and their soaps, animal or vegetable fats or oils, and alkali salts of sulfonated or sulphated organic compounds are some examples of surface active agents that are commonly used.
In addition to surface active agents, cement dispersing agents are also utilized in many industrial processes. These agents help to prevent clumping and settling of cement particles, allowing for a more uniform and consistent mixture. Calcium ligno-sulphate and calcium salts of glues are examples of cement dispersing agents that are commonly used in construction and other industrial applications.
Overall, the use of these materials and agents in various industrial processes helps to improve efficiency and ensure consistent results.
Fig 1: Process of expansion of water droplets in concrete.
Properties of Air-Entrained Cement
Air-entrained concrete is a type of concrete that has specific properties that make it unique compared to regular concrete. These properties are primarily due to the presence of small air bubbles within the concrete mixture. The air bubbles are created by the addition of a special admixture, which is typically a surfactant or a soap-like substance that causes the concrete to foam slightly.
One of the main benefits of air-entrained concrete is its improved durability. The air bubbles within the concrete act as tiny pressure relief valves, allowing the concrete to expand and contract without cracking or breaking. This makes air-entrained concrete particularly useful in regions with frequent freeze-thaw cycles, as it can resist the damaging effects of frost heave.
In addition to its durability, air-entrained concrete is also more workable than regular concrete. The small air bubbles create a smoother consistency, making it easier to mix, pour, and finish. It can also help reduce bleeding and segregation, which can improve the overall appearance of the finished product.
However, air-entrained concrete does have some drawbacks. It can be slightly weaker than regular concrete, and it may require more time to cure properly. It also tends to be more expensive due to the cost of the admixture used to create the air bubbles. Despite these drawbacks, air-entrained concrete remains a popular choice for many construction applications where durability and workability are key factors.
1. Workability
The high workability of this cement is attributed to the presence of air bubbles that act as a lubricant between the aggregates. This lubrication effect reduces the amount of water and sand required, which ultimately increases the strength of the concrete. Furthermore, the air bubbles help to prevent segregation and bleeding of plastic concrete. These properties are beneficial in the construction industry as they improve the quality and durability of concrete structures.
2. Strength
Abraham’s law is a well-known principle in civil engineering that states the strength and water-to-cement (W/C) ratio of concrete are inversely proportional to each other. This means that the lower the W/C ratio, the greater the strength of the concrete. To maintain a low W/C ratio while ensuring the strength parameter of the concrete is not compromised, air entrainers are used. These help to reduce the amount of water needed for the concrete mix, without sacrificing its strength. However, achieving high levels of strength in air-entrained cement can be challenging.
3. Freezing and Thawing Durability
In cold environments, the low temperatures can cause water inside concrete to freeze and expand, which can result in the concrete rupturing. To prevent this from happening, a certain type of cement is used that provides the necessary space for the water to expand without damaging the concrete. Additionally, entrained air bubbles within the concrete act as reservoirs for the expanded water, which helps to relieve the expansion pressure and further prevents damage to the concrete. Overall, these measures help to ensure that the concrete remains intact and functional even in extremely cold conditions.
4. Sulphate Resistance
Air entrained cement has a high resistance ability against sulphate attack due to its ability to keep the water-to-cement ratio (W/c ratio) low, which is the main factor that initiates the sulphate attack. By introducing air bubbles into the cement mixture, the air entraining agents increase the workability of the cement while reducing the amount of water needed to achieve a desired consistency. This, in turn, reduces the W/c ratio and makes the cement less vulnerable to sulphate attack.
Sulphate attack is a chemical reaction that occurs when sulphates, such as those found in soil and groundwater, react with the compounds in cement. This reaction can cause significant damage to concrete structures over time, leading to cracking, weakening, and eventual failure. However, the air entrained cement’s ability to resist sulphate attack by keeping the W/c ratio low makes it a popular choice for use in construction projects where the concrete is likely to be exposed to sulphates.
5. Permeability
Air-entrained concrete is a type of concrete that contains tiny air bubbles throughout its structure. This feature has been found to significantly enhance the water tightness of the material, making it an ideal choice for projects that require a high degree of water resistance.
The reason for this improved water tightness lies in the fact that the air bubbles created during the mixing process of air-entrained concrete can disrupt the capillary channels that would otherwise allow water to penetrate through the material. This disruption of the capillary channels, in turn, reduces the permeability of the concrete, making it less susceptible to water penetration.
On the other hand, non air-entrained concrete does not contain these air bubbles, and as a result, it may not provide the same level of water tightness as air-entrained concrete. Therefore, it is advisable to use air-entrained concrete in construction projects where water tightness is a requirement.
6. De-Icing Resistance
Ice formation on buildings with sloped structures is a common issue, which is typically resolved through the use of de-icing chemicals. These chemicals are effective in removing snow and ice, but they can also cause a layer of scales to form on the concrete surface. To prevent this issue, it is recommended to use air entrained cement in all applications where the concrete comes into contact with de-icing chemicals. This type of cement is specifically designed to prevent scaling caused by these chemicals and is a suitable solution for buildings with sloped structures that are prone to ice formation.
Fig 2: Bubbles formation in concrete due to air entained cement.
Advantages of Air-Entrained Cement
Air entrained concrete has several advantages that make it a popular choice in construction projects. One of the main benefits of using air entrained concrete is that it enhances the workability of the concrete. This is because the tiny air bubbles in the concrete help to make it easier to place, compact and finish.
Another important advantage of air entrained concrete is that it is better able to withstand the effects of freezing and thawing. This is due to the air entraining agent that is added to the mix, which helps to reduce the potential damage caused by expansion and contraction due to temperature changes.
In addition, air entrained concrete can also help to reduce the occurrence of bleeding, segregation and laitance. These issues can be a concern in conventional concrete and can lead to weaker and less durable structures.
Furthermore, entrained air in concrete can also enhance its sulphate resisting capacity, making it more resistant to sulphate attack. This is particularly important in structures that are exposed to sulphate-rich environments, such as in coastal areas.
Lastly, the use of air entraining agents in concrete can also reduce the risk of shrinkage and cracking on the concrete surface. This is because the tiny air bubbles help to relieve the internal pressure that builds up as the concrete dries and hardens, which can lead to surface cracking.
Disadvantages of Air-Entrained Cement
Air entrained concrete comes with its fair share of disadvantages. One such drawback is the reduction in concrete strength that occurs as a result of using air entraining agents. Additionally, the use of such agents also leads to an increase in the porosity of concrete, which can in turn lower the unit weight of the concrete.
Another important point to note is that air entrained concrete must only be used in situations where the site control is of a high standard. This is because the amount of air entrained in concrete can vary greatly based on factors such as the sand grading, errors in proportioning and workability of the mix, and the temperature. Therefore, if site control is not optimal, it can lead to issues with the quality and consistency of the air entrained concrete.