Introduction
The freshly-mixed and hardened properties of concrete may be changed by adding liquid (chemical) or mineral admixtures to the concrete, during batching. Admixtures are commonly used to (1) intentionally entrain air, (2) reduce water demand, (3) increase workability, (4) adjust setting time or hardening, and (5) adjust other concrete properties, such as strength.
Common chemical admixtures include water reducers, set retarders and accelerators, air entraining, and super-plasticizers. Common finely-divided mineral admixtures are usually classified as either cementitious materials (i.e. ground-granulated blast furnace slag and lime), or pozzolans (i.e. fly ash and silica fume).
Admixtures that affect the workability of a concrete mixture will also affect the slump, which is a test of the consistency of the mix. The slump of a mix with the same aggregate, cement and water can vary significantly by adding an admixture such as a high-range water-reducing admixture (super-plasticizer), which will increase the strength and provide greatly increased workability with the addition of excessive amounts of water. The admixture does not, however, reduce the quality of the material.
Air bubbles are entrained in concrete using an air-entraining admixture. The tiny microscopic bubbles provide free space within the paste to relieve hydraulic pressure when concrete freezes. Without the bubbles, the paste may crack when it freezes because water expands 9% in volume when it turns to ice. With entrained air, there is free space within the concrete to relieve pressure in the paste during freezing.
Common Chemical Admixtures
Chemical admixtures are usually materials that meet ASTM C494 or C1017 criteria. Other chemicals can be used, however, and the following table should provide guidance as to their use and effect.
Table 1. Chemical Admixtures and Their Effects
| Type of chemical admixture |
Desired effect |
Material |
| Accelerators (ASTM C494, Type C) |
Accelerate setting and early-strength development |
Calcium chloride (ASTM D98)
Triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite, calcium nitrate |
| Air-entraining admixtures |
Improve durability in environments of freeze-thaw, deicing chemicals, sulfate, and alkali reactivity
Improve workability |
Salts of wood resins (vinsol resin)
Some synthetic detergents
Salts of sulfonated lignin
Salts of petroleum acids
Salts of proteinaceous material
Fatty and resinous acids and their salts
Alkylbenzene sulfonates
Salts of sulfonated hydrocarbons |
| Alkali-reactivity reducers |
Reduce alkali-reactivity expansion |
Pozzolans (fly ash, silica fume), blast-furnace slag, salts of lithium and barium, air-entraining agents |
| Corrosion inhibitors |
Reduce steel corrosion activity in a chloride environment |
Calcium nitrite, sodium nitrite, sodium benzoate, certain phosphates or fluosilicates, fluoaluminates |
| Permeability reducers |
Decrease permeability |
Silica fume
Fly ash (ASTM C618)
Ground granulated blast-furnace slag (ASTM C989)
Natural pozzolans
Water reducers
Latex |
| Retarders (ASTM C494, Type B) |
Retard setting time |
Lignin
Borax
Sugars
Tartaric acid and salts |
| Super-plasticizers (ASTM C1017, Type 1) |
Flowing concrete
Reduce water-cement ratio
Reduce water demand (minimum 12%) |
Sulfonated melamine formaldehyde condensates
Sulfonated naphthalene formaldehyde condensates
Lignosulfonates |
| Water reducer (ASTM C494, Type A) |
Reduce water demand at least 5% |
Lignosulfates
Hydroxylated carboxylic acids
Carbohydrates
(Also tend to retard set so accelerator is often added) |
| Workability agents |
Improve workability |
Air-entraining admixtures
Finely-divided mineral admixtures, except silica fume
Water reducers |
Common Mineral Admixtures
Finely-divided mineral admixtures are powdered or pulverized materials added to concrete before or during mixing to improve or change some of the plastic or hardened properties of portland cement concrete. These admixtures are generally natural or byproduct materials. Also referred to as Supplementary Cementing Materials.
Table 2. Mineral Admixtures and Their Effects
| Type of mineral admixture |
Desired effect |
Material |
| Cementitious |
Hydraulic properties
Partial cement replacement |
Ground granulated blast-furnace slag (ASTM C989)
Natural cement
Hydraulic hydrated lime (ASTM C141) |
| Pozzolans* |
Pozzolanic activity
Improve workability, plasticity, sulfate resistance
Reduce alkali reactivity, permeability, heat of hydration
Partial cement replacement
Filler |
Diatomaceous earth, opaline cherts, clays, shales, volcanic tuffs, pumicites (ASTM C618, Class N)
Fly ash (ASTM C618, Class F and C)
Silica fume |
| Pozzolanic and cementitious |
Same as cementitious and pozzolan categories |
High calcium fly ash (ASTM C618, Class C)
Ground granulated blast-furnace slag (ASTM C989) |
| Nominally inert |
Improve workability
Filler |
Marble, dolomite, quartz, granite |
*A siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
For more information, consult Portland Cement Association's Design and Control of Concrete Mixtures (EB001)
