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Introduction to Concrete Admixtures
Concrete admixtures are chemical or natural substances added to concrete during mixing to improve its workability and enhance its durability. In a general definition, they are materials other than Portland cement, aggregates, and water that are added to concrete in powder or liquid form as a constituent material. Admixtures are diverse, with each imparting specific properties to the concrete. Each of these admixtures has different applications, ultimately used to improve the performance of concrete.
Properties of These Admixtures
Can Properties Be Achieved in Concrete Without Using Admixtures?
In response to this question, the answer is yes. You can achieve certain properties without using admixtures by selecting appropriate materials, using optimal mix proportions, and considering economic factors. Therefore, it’s beneficial to understand what these properties are. The properties that can be achieved without the use of admixtures include:
History of Concrete Admixtures
Air-entraining admixtures are among the earliest types of concrete admixtures, emerging in the 1940s. These materials were primarily used in areas with frequent sub-zero temperatures where water in the concrete would freeze. The development of superplasticizers, starting with their naphthalene formaldehyde type in 1960 in Japan, followed by their melamine type later in Germany, could be considered a milestone in the concrete admixture industry.
In those years, concrete admixtures were mainly used to reduce water content and achieve consistent workability. Several years later, with the advent of higher-strength concretes, the role of these admixtures became more prominent. Today, admixtures are used to impart a variety of properties to concrete.
Types of Concrete Admixtures
Generally, admixtures are classified into two categories: chemical admixtures and mineral admixtures. A notable point regarding the types of concrete admixture materials is that different concrete grades and their applications may influence the dosage of these materials. In general, the types of concrete admixtures include:
These materials are further categorized into chemical and mineral admixtures as follows:
Chemical Concrete Admixtures
Chemical admixtures refer to materials produced industrially and chemically, and they encompass various classifications, which we will discuss further. The table below shows different types of single-purpose and multi-purpose chemical concrete admixtures.
| Single-Purpose Admixture Types | Multi-Purpose Admixture Types |
|---|---|
| Concrete retarder | Water reducer (plasticizer) |
| Concrete accelerator | Retarder, water reducer, plasticizer |
| Air-entraining agent | Accelerator, water reducer, plasticizer |
| Water retainer | Strong water reducer, strong plasticizer, or superplasticizer |
| Water absorption reducer | Strong water reducer, strong plasticizer or retarder, superplasticizer |
| Early strength improver or hardening time accelerator |
|
Single-Purpose Admixtures
If admixtures affect only one property of fresh or hardened concrete, they are called single-purpose admixtures.
Water-Reducing Admixtures (Including Plasticizers and Superplasticizers)
These materials are used to reduce the amount of water required for a given concrete workability or to increase workability. Other applications include:
Concrete Retarding Admixtures
These concrete admixtures are used for:
Concrete Accelerating Admixtures
Uses include:
Calcium chloride is one of the cheapest and most common materials used in accelerating admixtures.
Waterproofing or Damp-proofing Admixtures
These admixtures are available in liquid, powder, and paste forms. Their constituent materials include ammonium sulfate, zinc sulfate, ammonium chloride, calcium chloride, and sodium silicate. Chemically, these compounds act as active pore-filling agents.
Gas-Forming Admixtures
Uses include:
Aluminum powder, activated carbon, and hydrogen peroxide are known gas-forming admixtures. The amount of gas bubbles formed in concrete depends on factors such as the dosage of admixture used, cement chemical composition, temperature, fineness, etc.
Air-Entraining Admixture Antifoamers
Air-entraining antifoamers are used to remove excess air bubbles that have formed in concrete. The presence of certain aggregates can cause gas release in concrete, leading to an increase in entrained air beyond the permissible limit, which can be beneficial for the concrete. Materials used in these admixtures include tributyl phosphate, silicones, water-insoluble alcohols, etc.
Air-Entraining Admixtures
Adding these materials to concrete creates very fine, non-air-related bubbles with a diameter less than 0.05 mm. These materials improve the durability of concrete against moisture, freezing, repeated thawing, and de-icing chemicals. Ultimately, they enhance the workability of fresh concrete and the impermeability of hardened concrete.
Methods of Producing Bubbles in Air-Entraining Admixtures
Admixtures for Alkali-Aggregate (AAR) Expansion Inhibition
The reaction between alkaline cement and silica present in aggregates causes the expansion of alkali-aggregate in concrete. This reaction produces a gel-like substance that leads to volumetric expansion of the concrete, resulting in cracking and disintegration. These admixtures are used to prevent this phenomenon. In some cases, using air-entraining admixtures can also be effective. Admixtures used to reduce the risk of alkali-aggregate expansion generally contain aluminum powder and lithium salts.
Anti-Washout Admixtures
These admixtures are often used for concrete structures submerged in water. Anti-washout concrete admixtures are derived from natural and synthetic rubbers and cellulose-based thickeners. They are used for:
Corrosion-Inhibiting Admixtures
Since the corrosion of steel in reinforced concrete structures is very common, and this issue intensifies when the structure is exposed to saltwater, industrial fumes, and chlorides, these admixtures are used to prevent and reduce this corrosion in reinforced concrete.
Types of Corrosion-Inhibiting Admixtures
Bonding Admixtures
These admixtures are used to bond and connect previously hardened concrete surfaces with fresh concrete. If fresh concrete is poured onto a hardened concrete surface, severe stresses at the interface can cause fractures in the concrete. To prevent these fractures, these admixtures are used before pouring fresh concrete onto hardened concrete or in mortar. They are also used for repairs. Bonding admixtures are examples of aqueous emulsions formed from natural and synthetic rubbers such as polyvinyl acetate, polyvinyl acetate, etc.
Antifungal, Antibacterial, and Disinfectant Admixtures
These admixtures are used to prevent the growth of bacteria, microbes, and fungi in hardened concrete. Examples include polyhalogenated phenols, copper compounds, and Di-Aldrin emulsions.
Multi-Purpose Admixtures
If admixtures affect multiple properties of fresh or hardened concrete, they are called multi-purpose admixtures. Multi-purpose admixtures are examples of concrete admixtures.
Mineral Admixtures
These are very fine mineral particles that improve certain properties or provide specific properties in concrete. These admixtures can enhance the workability and cohesion of fresh concrete, as well as the strength of hardened concrete, and even alter the color of the concrete. Mineral admixtures are categorized into three types:
What is a Pozzolan?
Pozzolans are siliceous or aluminous materials that are not inherently adhesive. However, if these materials are finely ground, they can form fine particles similar to cement. In the presence of moisture and at ambient temperatures, they react with calcium hydroxide. Pozzolans gain adhesive properties if their structure is somewhat similar to the compounds produced by the hydration of Portland cement.
Pozzolanic Concrete Admixtures
This type of admixture is commonly used in dense concrete. Dense concrete is used in the construction of retaining structures such as dams, water tanks, etc. The advantages of using this admixture in concrete include:
Pozzolanic concrete admixtures exist in both natural and artificial types.
Natural Pozzolans
These include non-crystalline volcanic ashes such as shales, pumicites, diatomaceous earth, etc.
Artificial Pozzolans
Also known as industrial pozzolans, these include finely ground fly ash, pulverized coal ash, silica fume, etc.
In addition to the types of concrete admixtures discussed so far, there is another category in the form of fibers. Concrete fibers include polymer, metallic, mineral, natural, and synthetic fibers.
