Types of Admixtures

Concrete admixtures are used to improve the behavior of concrete under a variety of conditions and are of two main types: chemical and mineral

Chemical admixtures reduce the cost of construction, modify properties of hardened concrete, ensure quality of concrete during mixing/transporting/placing/curing, and overcome certain emergencies during concrete operations.

Chemical admixtures are used to improve the quality of concrete during mixing, transporting, placement and curing. According to concrete network, they fall into the following categories:

• air entrainers
• water reducers
• set retarders
• set accelerators
• superplasticizers
• specialty admixtures: which include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring.

Mineral admixtures

Mineral admixtures make mixtures more economical, reduce permeability, increase strength, and influence other concrete properties.

Mineral admixtures affect the nature of the hardened concrete through hydraulic or pozzolanic activity. Pozzolans are cementitious materials and include natural pozzolans (such as the volcanic ash used in Roman concrete), fly ash and silica fume.

They can be used with Portland cement, or blended cement either individually or in combinations.

Chemical Concrete Admixtures - ASTM Categories

ASTM C 494, “Standard Specification for Chemical Admixtures for Concrete,” classifies admixtures into seven types as follows:

• Type A: Water-reducing admixtures
• Type B: Retarding admixtures
• Type C: Accelerating admixtures
• Type D: Water-reducing and retarding admixtures
• Type E: Water-reducing and accelerating admixtures
• Type F: Water-reducing, high range admixtures
• Type G: Water-reducing, high range, and retarding admixtures

Note: Changes occur in the admixture industry faster than the ASTM consensus process. Shrinkage Reducing Admixtures (SRA) and Mid-Range Water Reducers (MRWD) are two areas for which no ASTM C494-98 specifications currently exist.

In discussing the commercially available water-reducing set-controlling admixtures, it is appropriate to consider five

classes of admixtures. Categorized by basic or primary ingredients, they are as follows:

• 1. Lignosulfonic acids and their salts;

• 2. Hydroxylated polymers;

• 3. Hydroxylated carboxylic acids and their salts;

• 4. Sulfonated melamine or naphthalene formaldehyde condensates; and

• 5. Polyether-polycarboxylates.