Cement Flashcards
Please explan the preparation of Portland Clinker (OPC)
- The raw materials Limestone (calcium oxide) and clay (silicone dioxide) are mixed together to a fine powder.
- Heat up: The raw powder is fed into a long rotating kiln heated to extremely high temperatures (1450°C). Inside the kiln the raw material react and forms the clinker balls.
- After the reaction the four phases the portland cement is then produced in a fine pulverized portland clinker with 5% gypsum mixed into it.
What does gypsum fo to the portland cement?
Gypsum helps control the setting time of the final cement
Please explain the heating process in more details
The heating goes up to 1450 °C, but at 1000°C the formation of calcium silicates (bicalcium silicate and tricalcium silicate) occur.
At 1250°C the formation of iron oxides occur (tricalcium aluminate and iron).
The silicates are good for the mechanical properties
The two other are good for the formation of the liquid phase.
What is the hydration of Portland cement?
This is the process when the Portland cement is mixed with water. The w/c ratio needs to be 0.25.
The different constituents of the cement will react differently:
* Aluminates will hydrate faster than the silicates. Since this is very fast , you can add gypsum to slow it down.
* The silicates will give the mechanical properties due to the formation of calcium silictae hydration products.
Bicalcium silicate C2S provides long-term strength due to higher C-S-H content
Tricalcium silicate C3S is crucial for early strength development in cement.
What are two important considerations from the hydration and production?
A high w/c → lower contact points among fibers → higher porosity → lower mechanical properties
Thermal conductivity problem: The dater the heat of the hydration is obtained, the core of the material will remain hot while the surface of the material is colder - thermal gradient between core and the surface and will lead to cracks.
* Solve this by using less tricalcium aluminate
Name the different types of cement
Portland cement
Portland cement blends: with additives
Pozzolan cement: Blend of Portland cement (clinker + gypsum) with pozzolan (up to 55% wt, volcanic ash, fly ash, burned clays etc). A siliceous rich material that reacts with portlandite
Describe pozzolan cement
When Portland cement hydrates, it produces calcium silicate hydrate (C-S-H) and portlandite (Ca(OH)2).
Pozzolanic materials (such as fly ash, silica fume, volcanic ash) react with the portlandite produced during the hydration of Portland cement in a secondary reaction called the pozzolanic reaction.
What is portlandite?
Portlandite = Calcium hydroxide
Portlandite increases the pH of the pore solution in concrete, typically to around 12.5-13.5. This high alkalinity forms a passivating layer on the steel rebars, protecting them from corrosion.
Pozzolanic materials react with portlandite and reduces the amount of calcium hydroxide in the cement.
* not as good for reinforced concrete
The use of pozzolanic materials enhances the overall durability of concrete. The denser microstructure and reduced permeability mean that the concrete is less likely to suffer from cracking and degradation over time.
Describe the weathering mechanisms for cement
Presence of free MgO and CaO in the hardened cement:
Can cause volume expansion due to delayed hydration of these two, which will cause micro cracks
Temperature changes:
Cement is a bad heat conductor → faster shrinkage on the surface in comparison to the core → stresses and microcracks.
Fire: evaporation of free water inside of pores + elimination of crystallization = empty pores and lower mechanical properties.
Freezing thaw cycles:
Gelivity problems due to pores - same as stones
Solve by decreasing the water content and the w/c.
Effects of fluent water:
Water reacts with the cement during hydration adn forms calcium hydroxide (portlandite)
Calcium hydroxide is soluble in water, leading to gradual dissolution and movement through the pore water.
For further reactions the calcium hydroxide can react with CO2 to form calcium carbonate CaCO3 which weakenes and increases the porosity even more.
Sulphate - containing water:
Reacts with the hydrated calcium carbonate in the hardened cement leading to delayed ettrignite.
What are the consequences of pores in the hydrated cement?
The higher w/c the more capillary pores.
The higher the degree of hydration, the lower the volume of pores.
The lower the porosity, the higher the compressive strength
The higher the effective porosity, the higher the permeability
Some differences between ordinary portland cement and pozzolan cement?
The pozzolanic material (fly ash being a common example) present in PPC reacts with the Ca(OH)2 released during OPC hydration. This reaction is slower than the initial hydration but occurs over an extended period.
- Ordinary portland cement hardens quicker
- Pozzolan cement gets higher mechanical properties
- Ordinary portland cement is preferred for applications where early strength development is crucial, such as precast concrete elements or repairs.
- Pozzolan cement has No problems with sulfate-containing water or fluent water.
- Pozzolan is not ideal for reinforced concrete since Portlandite helps with the alkaline environments protecting rebars from corrosion
What is blast furnace cement?
Slaggcement. A byproduct obtained during the production of iron in blast furnaces.
Lower heat hydration than Portland cement
High ultimate strength
Cheaper than portland cement
High resistance to attack of fluent water and sulfate rich water
The hydration rate is very slow and if slag content is increased then early strength is reduced in comparison to Portland cement. The trend of development of mechanical resistance is analogous to that observed for pozzolan cement.
What is calcium aluminate cement?
Non-portland cement, consists of hydraulic calcium almuniates
Quick hardening
Slow setting
Can only be used in cold climates
