3. Chemical Admixtures
Chemical admixtures are the most commonly used type of admixture in modern concrete industries. These materials work by altering the properties of fresh and/or hardened concrete through chemical reactions or specific physical effects. Below, we will discuss several major types of chemical admixtures based on their primary functions.
3.1. Water Reducing Admixtures
Water-reducing admixtures are chemicals that reduce the amount of water needed in the mix without affecting the workability of the concrete. With less water, concrete achieves higher compressive strength due to a lower water-cement ratio.
There are three main types of water reducers:
- Normal Water Reducer (Type A – ASTM C494): Used for general applications, it reduces water demand by 5-10%.
- Mid-range Water Reducer: Developed to meet the needs between normal and high-range reducers, improving workability without segregation.
- High-range Water Reducer (Superplasticizer – Types F and G): Reduces water demand by 12-30%, enabling the production of high-strength concrete with high workability or self-compacting concrete.
Examples of materials include lignosulfonates, naphthalene sulfonates, and polycarboxylate ethers (PCE). Common applications include reinforced concrete structures, precast concrete, and concrete with high reinforcement density.
3.2. Retarders (Set Retarders)
Retarders are admixtures that slow down the initial setting time of cement, preventing the concrete from hardening too quickly. This is particularly useful in hot weather, large volume pours, or when concrete needs to be pumped and transported over long distances.
Examples of retarders include lignosulfonates, hydroxycarboxylic acids, or glucose mixtures. Retarders help prevent cold joints in staged pours and allow more time for finishing work.
3.3. Accelerators
The opposite of retarders, accelerators are admixtures that speed up the initial setting time and the development of early strength in concrete. These are beneficial for cold weather concreting, rapid repairs, or structures that require early formwork removal.
Examples include:
- Calcium chloride (CaCl₂): Effective but can increase the risk of reinforcement corrosion, so its use is limited.
- Non-chloride accelerators (e.g., sodium thiocyanate, calcium nitrate): Safer for reinforced concrete.
3.4. Air Entraining Agents
Air entraining agents are admixtures that create microscopic air bubbles in the concrete, which are evenly distributed. These air bubbles increase the concrete’s resistance to freeze-thaw cycles by providing space for ice expansion without damaging the cement paste. They also help improve workability in concrete with coarse aggregates.
Examples include natural resins, wood sulfate salts, and fatty acids. These are mainly used in structures exposed to cold climates or freeze-thaw conditions.
3.5. Specialized Admixtures
In addition to the primary admixtures mentioned above, there are also specialized admixtures designed for specific needs:
- Corrosion-inhibiting admixtures: Protect reinforcement from corrosion, suitable for marine structures or bridges.
- Waterproofing admixtures: Increase concrete’s resistance to water penetration. These can be integral (mixed into the concrete) or applied as a coating.
- Shrinkage-reducing admixtures: Minimize plastic shrinkage and drying shrinkage, reducing the risk of cracking.
The use of chemical admixtures should be carefully controlled, following the manufacturer’s recommended dosage and ensuring compatibility with the type of cement used to avoid negative reactions in the concrete.
Next, we will discuss Mineral Admixtures in the following section.