Admixtures are chemicals that are added to concrete at the mixing stage to modify some of the properties of the mix. They play an important role in modern construction by enhancing the properties of concrete to meet specific project requirements.
They offer several uses and benefits, including:
Water Reduction:
Water-reducing admixtures (also known as plasticizers or superplasticisers) decrease the amount of water required to maintain the desired workability of concrete. This allows for higher strength and durability while maintaining proper consistency. For the same mix constituents (without reducing water), the addition of plasticisers can also improve the workability of a mix due to the dispersion effect that plasticisers have on cement conglomerates.
Increased Early Strength:
Admixtures such as accelerating admixtures speed up the hydration process, leading to faster strength development. On the other hand, retarding admixtures slow down the setting time, which is beneficial in hot weather or for large concrete placements.
Improved Workability:
Admixtures can enhance the workability and flowability of concrete, making it easier to place and consolidate. This is particularly useful in situations where concrete needs to be pumped or transported over long distances.
Decreased Permeability:
Admixtures such as water-reducing admixtures and pozzolanic materials reduce the permeability of concrete and increases its durability by making it less susceptible to water penetration, chemical attack, and carbonation. This is crucial for structures exposed to harsh environments or moisture-sensitive areas.
Shrinkage Control:
Shrinkage-reducing admixtures minimize the drying shrinkage of concrete, reducing the risk of cracking and improving long-term stability. This is particularly important for large slabs, pavements, and structures subjected to temperature variations.
Aesthetic Enhancements:
Other additives like pigments or colouring agents are used to enhance the appearance of concrete, providing a range of colour options. This is advantageous for architectural applications where visual appeal is crucial.
Sustainability:
Certain admixtures, such as supplementary cementitious materials (e.g., fly ash, slag, silica fume), can be used as partial replacements for cement, reducing the overall carbon footprint of concrete. Additionally, admixtures that enable the use of lower-grade aggregates or recycled materials help promote sustainable construction practices.
There are four commonly used cement admixtures in construction and understanding their functions, benefits, and applications will help you to optimize concrete performance for your project.
Accelerators are admixtures that speed up the initial set and early strength gain of concrete. They are particularly useful in cold weather conditions or when early strength development is critical. Commonly used accelerators include calcium chloride, calcium nitrate, and sodium nitrate. By increasing the rate of hydration, accelerators promote faster hardening and reduce the time required for formwork removal. However, excessive use of accelerators can lead to increased shrinkage and corrosion of reinforcement in the case of chloride accelerators.
In contrast to accelerators, retarders are admixtures that slow down the setting time of concrete. They are primarily used in hot weather conditions or when extended workability is necessary. Retarders such as sugar, lignosulfonates, and citric acid, delay the hydration process, allowing for better placement and finishing of concrete. This extended setting time reduces the risk of plastic shrinkage cracks. However, it is essential to consider the dosage and ambient temperature carefully to avoid excessive retardation, which may compromise setting times and the early strength gain.
Water-reducing admixtures, also known as plasticizers, are additives used to reduce the water content in concrete mixtures without affecting the workability. By dispersing cement particles and reducing the surface tension of water, they enhance the flowability and cohesiveness of the mixture. This results in higher strength, improved durability, and reduced permeability. Common types of water-reducing admixtures include lignosulfonates, polycarboxylate ethers (PCE’s), and melamine-based superplasticizers. It is crucial to select the appropriate admixture based on the project requirements, from a cost and overall performance point of view. Some water reducers may have specific limitations regarding dosage and compatibility with other admixtures.
Superplasticizers are a specialized type of water-reducing admixture that provides exceptional flowability to concrete. They are particularly effective in high-performance concrete and self-consolidating concrete (SCC). Superplasticizers disperse cement particles at a higher level, allowing for improved workability, reduced water content, and increased slump retention. This results in enhanced strength, reduced segregation, and an improved finish ability. Popular types of superplasticizers include polycarboxylates and sulfonated naphthalene formaldehyde condensates. Careful consideration must be given to dosage, mixing time, and temperature to ensure optimal performance.
It’s worth noting that the specific benefits and uses of admixtures can vary depending on the type and dosage of the admixture used, as well as the project requirements and environmental conditions. Consulting with a professional engineer or Sephaku technical representative is advisable to determine the most suitable admixture for a specific application.
For expert advice on the correct cement products to use to achieve the best possible results contact the Sephaku call centre on 0861 32 42 52 or speak to your technical representative.