Cement and concrete Flashcards
The raw materials of Portland Cement are:
- Lime (C)- From Limestone
- Silica (S)- From china clay or Kaolin found in granite
- Alumina (A)- “
- Water (H)
- Iron Oxide (F)- Generally present as a flux
What is a flux?
A compound added to the cement composition to lower the temp. at which the melt forms
How do you produce Cement?
- Raw materials are quaried Rubble is crushed into fine particles
- Ground material blended and fed into a rotary kiln at 1500 degrees C
- Results in Cement clinker and this is pulverised into fine powder
- Gypsum is added to control setting rate
What is the final composition of Portland cement?
- Tricalcium Silicate (C3S) alite
- Dicalcium Silicate (C2S) belite
- Tricalcium Aluminate (C3A)
- Tetracalcium Alumino Ferrite (C4AF)
Discuss hydration of cement
- Cement hardens when it reacts with water, not by drying
- It’s important to keep cement wet till max hardness is reached
- Hydration is exothermic
- Cement goes v. alkaline during hydration. Do not handle setting cement
Pros and cons of the exothermic hydration reaction
- Pro In cold climates, it prevents the water in the cement freezing
- Con For v. large structures, water can evaporate before cement hardens. Need to use cooling pipes
What are the two stages of Hydration?
Setting- the dissolution of the components, forming a gel and interlocking of grains
Hardening- the formation of many more crystals and increased strength
What do the final components of Portland cement do during Hydration?
- Tricalcium Silicate (C3S)- Main strengthening agent, forms a gel during setting and crystalline fibrous grains during hardening. V. exothermic reactions
- Dicalcium Silicate (C2S)- Similar to above but slower and less exothermic
- Tricalcium Aluminate (C3A)- Setting and forms gels. Forms small needles when Gypsum is present. V. exothermic. Little influence on strength
- Tetracalcium Alumino Ferrite (C4AF)- Minor role in setting process, gives cement grey colour
What are the different types of cement?
- Low heat cement- Used in large buildings. More C2S and less C3S
- Sulphur Resistant (chemical resistant)
- Rapid Hardening- Roads
When does Hydration stop?
All components of the cement react for long periods of time over the hardening period. C2S is the longest, up to a year!
1st stage of Hydration
- Rapid hydration of C3S and C3A
- Gives out lots of heat
- pH goes up (alkaline)
- Silicate and aluminate gels form
2nd stage of Hydration
Needle-like crystals of Ettringite form, and slowly extend outwards in tubular gel structure thanks to the diffusion of ions (Ca2+ , Si4+, Al3+, OH- , etc).
3rd stage of Hydration
Ettringite crystals start locking the whole structure together. Precipitation of Ca(OH)2 crystals – known as Portlandite .
4th stage of Hydration
Ettringite crystals complete locking the structure together. Long fibrous crystalline C-S-H particles (Tobermorite) form which provide strength to the hydrated cement.
Concrete
A mixture of granular materials like sand, gravel and small rocks combined with cement and water
