Concrete and Cement Flashcards
Portland cements are used to make…
Concrete + motar, buildings, infrastructures, roads
Special cements (such as low-co2 and rapid hardening) are used for…
Immobilisation of toxic/ radioactive waste
Medical cements are used for…
Tooth and bone placements
ASTM…
Consists of calcium and calcium sulfates
Basis of most concrete reactions is…
A hydraulic reaction whereby the cement reacts with water and hardens
Definition of a paste and its use =
Cement + water
Rarely used by itself and so is combined with aggregates
Definition of mortar and its use =
Paste + sand
Used to join bricks together or used as a coating
Definition of concrete and its use =
Paste + sand + coarse aggregate
which is gravel or crushed rock
Aggregates are…
Sand, gravel, rocks ( much cheaper than cements)
Aggregates are bound together by…
Cement as it acts as a glue
Best to use aggregates that do not react with cement such as..
Granite, quartz and basalt
or a slightly reactive rock such as limestone
What happens when using a reactive material with cement (such as glass aggregate or sulfide)
They expand and cause the concrete to fail
Fine vs coarse aggregate =
Fine = sand or crushed rocks Coarse = larger rocks
Why should we not use glass aggregates or sulfide?
Aggregate does not normally react with the cement so this does and causes concrete failure
Two reasons we use aggregates
- Strength; the concrete will be as strong as the aggregate
- Reduces thermal cracking (dilution of cement)
Shear-thinning is..
Concrete flows better if a force is applied to it otherwise it will set and harden
How to measure the flow of concrete:
-slump test; fill a cone with concrete then lift off and measure the distance in height
Role of water in cement:
Required for the hydration reaction and allows concrete to flow.
Too much water causes =
- Forms extra pores
- reduced durability and strength
- delay the setting process.
What is the normal water-cement ratio?
0.5 + - 0.2
What does adding polymeric molecules to concrete do? (superplasticisers)
Increase the plastic molecules making more brittle so more durable.
What happens when sulphate reacts with concrete?
The concrete reacts with the sulphate causing it to shrink
What are the four main minerals in portland cement clinker?
- Tricalcium silicate
- Dicalcium silicate
- Tetracalcium Aluminoferrite
- Tricalcium aluminate
What are the four main minerals in portland cement clinker in chemical symbols?
- C3s
- C2s
- C3A
- C4AF
Hydration reaction =
Mixing with water; dissolves minerals
Why is drying bad for fresh cement?
Stops the reaction process by evaporating water needed for reaction
Important hydrate phases “Calcium silicate hydrate”
- Formed by C3s and C2s
- Non-crystallin (amorphous)
- Main strength giving phase
Important hydrate phases “Calcium hydroxide”
- Formed by C3s and C2s
- large crystallites
- Protects from corrosion
Important hydrate phases “Ettringite”
- formed by c3a with water and gypsum
- Needle like crystals
- Initial setting of cement
Important hydrate phases “Aluminoterite or AFm”
m for monosulphate
-Important as sink for al but also cause durability
Importance of gypsum
- gypsum plays a role in the setting of cement; controls setting
- Without the cement would set straight away
Heat of hydration graph
-Steep gradient is an acceleration period
Three main types of cementitious materials:
- Pozzolans
- Ground granulated blast furnace slag
- Limestone
Pozzolanic reaction
Can blend portland cement with other forms of silicate
This is generally slower and can reduce early strength but contributes to long-term properties
Pozzolanic additives:
- Fly ash
- Natural pozzolans
- silica fume
- Calcined caly
- other waste silicate glass materials
How is fly ash produced?
- By product of coal combustion processes
- Helps fluidity of concrete
Calcines clay
- gives disordered reactive phase
- highly reactive
- Pozzolanic
- Takes up alot of water
Silica fume
- by product of simiconductor
- pozzolanic reaction
- high early strength development
- improves durability
Slag
- Reactives to form C-S-H or C-A-S-H
- Better than pozzolanic reaction
Limestone
Calcium carbonate
Good for reactivity
-Improved properties such as strength
5 key categories of portland cement:
- Portland cement
- Portland-composite cement
- Blastfurnace cement
- Pozzolanic cement
- Composite cement
Calcium aluminate cement =
High strength and rapid setting; strength loss under wrong conditions
Geopolymer
Much lower emissions than portland cement
Steel reinforcing=
Concrete is strong in compression but weaker in tension so increases tensile strength.
Steel is good in tension
chemistry of cement prevent steel from rusting
Main cause of concrete failure =
Steel failure as steel rusts so it expands and crack the concrete
How to prevent concrete failure =
Reduce permeability and water/cement ratio
Causes of degredation =
Corrosive substances
Freeze thaw or frost-salt cycling
poor workmanship
seawater has sulphate and chloride
