Chemical Admixture Flashcards
(35 cards)
What are admixtures?
The ingredients in concrete other than portland cement, water and aggregate that are added to the mix.
Why use admixtures?
- Modify the properties of hardened concrete: Increase early strength and reduce water content
- Reduce concrete construction cost: Saving time and increase workability
- Overcome emergencies during concrete operations: Cold weather casting and Narrow cross-sections
What are the different types of admixtures?
- Air Entrainment
- Shrinkage Reducers
- Corrosion Inhibitors
- ASR inhibitors
What is CSH?
The binder (glue) responsible for strength
How does the accelerator affect the mix?
- Increase in CSH
- Increase in Heat
- Increase in strength
How does a faster hydration reaction affect the mix?
- Higher strength
- Shorter setting time
When do you use accelerators?
Used in cold weather to accelerate construction:
- Reduces the Curing time
- Reduces formwork removal time
What are the different types of accelerators?
- Alkali Hydroxides
- Silicates
- Organic Compounds
- Sodium Chloride
- Calcium Nitrate
- Calcium Chloride
Which accelerator is the most used?
Calcium Chloride, because of availability, low cost and predictable performance.
What does Calcium Chloride do?
- Accelerates cement hydration
- Faster hardening and strength gain
What is the effect of accelerators in microstructure?
Change from fibrous structure of CSH to spherulite or lace-like structure is reported by researchers
What is the effect of accelerators on Fresh properties?
- Increases workability
- Reduces significantly both initial and final stting time
- Reduces both bleeding rate and bleeding capacity
What is the effect of accelerators on Mechanical Properties?
- Accelerates compressive strength gain at early stage
- Long-term strength is lower than that of a non-accelerated control mixture
What is the effect of accelerators on Durability?
Chlorides in CaCl2 increase corrosion of steel. CaCl2 usually banned in pre-stressed concrete.
What happens when the amount of CaCl2 is increased?
- Drying Shrinkage of concrete
- Alkali Aggregate expansion is increased
- Leads to lower durability under sulfate attack
- Leads to reduced frost resistance
What are retarding admixtures used for?
Delay the setting or hardening rate for:
- Hot weather concreting
- Need longer setting time: difficult placements or special finishing processes.
What can be used as a retarding admixture?
- Sugar
- Salt
What is the main use of Water reducer admixtures?
- Reduce the mixing water by around 5%
- Reduce the water-cement ratio
- Reduce cement content
What is the chemical composition of Water reducer?
- Lignosulfonates
- Hydrocarboxylic acid
- Carbohydrates
- other compounds
Lignosulfonate is the..?
- Most used raw material in the manufacture of water reducers
- Waste liquor in the production of paper making
- Contain about 30% reducing sugars
- They tend to entrain air in concrete
Hydrocarboxylic acid is…?
- Developed as water reducers in the 1950s
- Usually synthesized chemically
- Act also as retarding agents
Carbohydrates…?
- Include natural products such as glucose and sucrose
- Act also as retarding agents
What is the effect of water reducers on fresh concrete?
- Workability is improved, less cold joints and air pockets
- Decrease the bleeding rate and capacity
- Heat of Hydration: Delay the rapid heat evolution
What is the effect of water reducers on hardened concrete?
- Early strength can be decreased due to retarding effect, but 28 days strength is increased beyond what could be expected from reduction of w/c ratio.
- Better dispersion of cement grains leads to better particle packing and higher degree of hydration at later ages.
