Chemical Admixtures Flashcards
Name 4 types of chemical admixtures
Air entraining agents
Plasticisers
Accelerators
Retarders
Explain what air entraining admixtures do
Entrain air as microscopic uniformly dispersed discrete bubbles
0.02-1mm diameter
Closely spaced~0.4mm or less
Explain how air entraining agents work
Organic molecules with negatively charged hydrophilic group at one end, neutral hydrophobic group at other.
Molecules align radially at surface of air bubbles, hydrophilic in water, hydrophobic in air. Reduces surface tension & stabilises bubble. Residual negative charge on air bubbles mean they repel and are well dispersed.
How do air entraining agents improve workability
They act as lubricating ball bearings between air bubbles, cement particles and aggregates.
How do air entraining agents improve freeze/thaw resistance?
Negative air entraining molecules align radially at surface of air bubbles. The neutral hydrophobic groups prevent water molecules from filling the air bubbles with water. Under freezing conditions, ice is allowed to expand into the voids, preventing cracking.
What do plasticisers do?
Increase concrete workability, in plastic state, without increasing water content-higher strength for same cement content.
Why are super-plasticisers preferred over using more cement to gain high strength concrete
High workability, no increase in w/c
More cement increases likely hood of shrinkage, cost and heat evolutions
What are (super-) plasticisers? Give two classes and what mechanicms they operate by
Large polar organic molecules
Lignosulphonates/sulphonated naphthalene formaldehyde~electrostatic
Polycarboxylate ethers~steric
Explain how (super-)plasticisers that act through electrostatic modes work
Organic molecules physically absorbed onto surface of cement grains
Residual negative charge
Grains repel each other producing larger gaps
Break up cement flocs releasing trapped water, increasing fluidity of concrete, no increase in water content
Explain how steric (super-)plasticisers work
Steric plasticisers are big molecules
What do accelerators do?
Increase rate of hardening/strength development
What are accelerators used for?
Early form work removal
Overbuilding
Pre-casting
Cold weather
How are accelerators believed to work?
Increases dissolution of Ca from Alite + belite to form supersaturated solution that precipitates C-S-H
Why was calcium chloride banned as an accelerator and state an alternative
Chlorides cause corrosion of steel
Calcium formate-more expensive, not as effective
Explain why accelerators are used in cold weather conditions
Cold weather drastically reduces setting time of concrete, next layer of concrete may need to be placed but previous layer needs to develop enough strength to support. Accelerator speeds up setting time
