Admixtures |
Functions
|
Typical compounds |
Application |
Disadvantages |
Accelerating admixtures or accelerators |
More rapid gain of strength or higher early strength. More rapid setting. |
Calcium chloride Calcium formate Triethanolamine (TEA) Soluble inorganic salts Sodium nitrite Sodium sulphate Sodium aluminate Sodium silicate |
1. Normal rate of strength development at low temperature. 2. To counter retarding effects 3. Shorter stripping times. 4. Plugging of pressure leaks. 5. Sprayed concreting. |
1. Possible cracking due to heat evolution. 2. Possibility of corrosion of embedded effects reinforcement. |
Retarding admixtures or retarders |
Delayed setting |
Soluble carbohydrate derivatives: starch Hydroxylated carboxylic acids, Inorganic retarders Sugars |
1. Maintain workability at high temperatures. 2. Reduce rate of heat evolution. 3. Extend placing times, e.g., ready-mixed concrete. 4. Prevent cold joint formation. |
May promote bleeding. |
Water-reducing accelerators |
Increased workability with faster gain of strength. |
Mixtures of calcium chloride and lignosulfonate. |
Water reducer with faster strength development. |
Risk of corrosion. |
Water-reducing retarders |
Increased workability and delayed setting |
Mixtures of sugars or hydroxylated carboxylic acids and lignosulfonate. |
Water reducer, with slower loss of workability. |
|
Air-entraining agents |
Entrainment of air into concrete. |
Aluminum powders, |
Enhanced durability to frost without increasing cement content, improvement in workability, lowered permeability and cellular concrete. |
Careful control of air content, water-cement ratio, temperature, type and grading of aggregate and mixing time is necessary. |
Damp-proofing or water-proofing agents |
1. Water-repellent, i.e. prevention of water from entering cap 2. Reduced water permeability of concrete. |
Potash soaps, calcium-stearate, aluminium-stearate, butylstearate, petroleum |
1. Reduced permeability. 2. Enhanced durability. 3. Increased freeze-thaw resistance. 4. Reduced drying shrinkage. 5. Reduced surface staining. 6. Water tightness of structures without using every low water-cement ratio. |
1. Not efficient under high hydrostatic pressure. 2. Requires low water-cement ratio and full compaction. |
Plasticizers (water reducers)-8 to 15 percent water reduction |
Higher flowability |
Hydroxylated carboxylic acid derivatives Calcium and sodium lignosulfonates. |
1. Higher workability with strength unchanged. 2. Higher strength with workability unchanged. 3. Less cement for same strength and workability. |
Certain special types of cements like sulphate resistant cement (low C3A content) and expansive cement do not perform well. |
Superplasticizers (Super-water reducers) – 15 to 30 per cent water reduction |
Greatly enhanced workability. |
Sulfonated Melamine formaldehyde resin, sulfonated naphthalene-formaldehyde resin, Mixtures of saccharates and acid amides. |
1. Water reducer, but over a wider range. 2. Facilitate production of flowing or self-leveling concrete |
1. Tendency to segregate. 2. Flowability is not long lasting. 3. During hot weather the workability retention period decreases fast. |
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