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. 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.
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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.
ASTM Categories - Concrete Admixtures
ASTM C494 specifies the requirements for seven chemical admixture types. They are:
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.
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