The Purpose of Concrete Admixtures
The Purpose of Concrete Admixtures
Concrete admixtures are an important part of the concrete production process. They help improve the quality and durability of the concrete, as well as help it meet specific application needs. There are various types of admixtures, each with its own set of benefits. Understanding the purpose of each type is essential to ensuring that you select the right admixture for your project.Admixtures: What To Use and When
Admixtures are added to concrete for a variety of reasons. Some admixtures adjust the properties of fresh concrete to make it more workable, while others modify the hardened concrete to improve durability or control shrinkage cracking. Admixtures can also be used to accelerate or retard the set time of concrete, and to reduce water demand, shrinkage and bleeding. When selecting an admixture, it’s important to consider the desired outcome and how that relates to the specific site conditions.There are two main categories of admixtures: chemical and mineral. Chemical admixtures are further classified into five groups, according to their function in concrete:
- Air-entraining agents
- Water reducers
- Plasticizers
- Superplasticizers
- Set retarders
Mineral admixtures include fly ash, silica fume, ground granulated blast furnace slag, and natural pozzolans. White Cap carries all of the admixtures that you need to achieve your desired result.
Chemical Admixtures
Chemical admixtures are materials added to concrete, either before or during mixing, in order to modify one or more of its properties. Commonly used admixtures include air-entraining agents, water reducers, plasticizers, superplasticizers, and others that accelerate curing, retard setting time, increase compressive strength, or improve workability.
Air-entraining agents are used to entrain microscopic air bubbles in concrete, which improves workability and durability. This is especially important in concrete that will be exposed to freeze-thaw cycles or deicing salts.
Water reducers are either normal or high range and are used to increase workability while reducing water content. Too much water weakens concrete by reducing the amount of cement paste available for bonding purposes.
Plasticizers also increase workability but do not reduce water content like water reducers do.
Superplasticizers provide the highest level of workability without affecting the water-cement ratio. Set retarders are used to delay the initial set of concrete, which can be important in hot weather or when placing large slabs.
Water reducers are either normal or high range and are used to increase workability while reducing water content. Too much water weakens concrete by reducing the amount of cement paste available for bonding purposes.
Plasticizers also increase workability but do not reduce water content like water reducers do.
Superplasticizers provide the highest level of workability without affecting the water-cement ratio. Set retarders are used to delay the initial set of concrete, which can be important in hot weather or when placing large slabs.
Mineral Admixtures
Mineral admixtures can also improve the properties of concrete, such as workability, durability, and strength. Mineral admixtures can also be used to change the color of concrete or to make it more resistant to weathering and other environmental factors. Common mineral admixtures as mentioned previously include fly ash, silica fume, ground granulated blast furnace slag, and natural pozzolans. Each of these materials has unique properties that can be used to improve the quality of concrete.Fly ash is the most commonly used mineral admixture and is a by-product of coal combustion. It improves workability, freeze-thaw durability, and shrinkage resistance.
Silica fume is another by-product of industrial processes and provides similar benefits as fly ash but at a much higher level.
Ground granulated blast furnace slag is a by-product of steel production, and like fly ash and silica fume, it improves durability and shrinkage resistance.
Natural pozzolans are volcanic materials that can be used in place of Portland cement to make concrete. Pozzolans improve workability and durability and can be used to increase the number of recycled materials in concrete.
Supplementary Cementitious Materials
Supplementary cementitious materials are materials that, when used in conjunction with cement, can enhance the performance of concrete. SCMs can be used to improve the strength, durability, and/or aesthetic properties of concrete.There are a variety of SCMs available on the market, each with its own unique set of properties. Common SCMs include fly ash, slag cement, silica fume, and rice husk ash.
When selecting an SCM for a particular project, it is important to consider the specific needs of the project. Some factors that should be considered include:
- The desired properties of the final concrete product
- The compatibility of the SCM with other
- Materials in the mix
- The cost of the SCM
- The availability of the SCM
Fly ash is a byproduct of coal combustion and is one of the most commonly used SCMs. It is typically used to improve the strength and durability of concrete.
Slag cement is a byproduct of the steelmaking process. It can be used to improve the workability, finish, and/or strength of concrete.
Silica fume is a byproduct of silicon and ferrosilicon production. It is used to improve the strength and durability of concrete.
Rice husk ash is a byproduct of rice milling. It can be used as an admixture or replacement for Portland cement.
Mixing Water
When mixing concrete, the water used should be clean and free from any impurities that could compromise the finished product's quality.However, if clean, fresh water is not available, other types of water can be used as long as they do not contain any impurities that could harm the quality of the finished product.
For example, when building a pier or another structure in seawater, it is important to use fresh water for the concrete mix. Seawater contains high levels of salt, which can corrode metal reinforcement rods and cause them to rust. In addition, seawater can also contain algae and other organisms that can grow in damp concrete and cause problems. Finally, sugar can also be present in seawater, which can cause the concrete to set too quickly and not cure properly. For all these reasons, it is important to use fresh water when making concrete mix for structures in seawater.
There are another couple of things to keep in mind when choosing the type of water to use for mixing concrete:
- The setting time of the concrete can be affected by the type of water used. In general, using clean, fresh water will result in a shorter setting time than using other types of water.
- The long-term properties of the concrete can also be affected by the type of water that is used. Again, using clean, fresh water will result in concrete with better long-term properties.