Here's brief summary of how chemical admixtures affect properties of both plastic and hardened concrete:
- AIR-ENTRAINING ADMIXTURES: Entrained air greatly improves concrete resistance to damage caused by freezing and thawing and deicing agents. Any concrete that will be exposed to freezing and thawing when wet should contain entrained air, usually 4 percent to 8 percent by volume of the concrete. Entrained air also reduces bleeding, aids cohesiveness, and improves pumpability.
- WATER REDUCERS AND RETARDERS: Water-reducing admixtures reduce the amount of water needed to make concrete placeable. The main mechanism is cement dispersal that frees water trapped within clusters of cement grains. Water reducers can be used to increase slump with no change in water-cement ratio, to lower the water cement-ratio with no change in slump or cement content, or to lower the cement content with no change in the slump or water-cement ratio.
- SUPERPLASTICIZERS: Superplasticizers are potent water reducers that permit water reductions of 12 percent to 30 percent. They can also be used to produce flowing concrete - a high-slump mix that doesn't segregate readily or bleed excessively.
- ACCELERATORS: Accelerators speed cement hydration, making concrete set and gain strength faster. Accelerators are used mainly to offset the effects of low temperatures on cement hydration. Calcium chloride is the most commonly used accelerator and is added in amounts up to 2 percent by weight of cement.