Cement is a mixture of clay and limestone which is ground together and heated to a temperature of 1450 ° C, to obtain a powder with coherent properties which hardens in the presence of water on complex physico-chemical processes.
Cement = 80 percent limestone + 20 percent clay.
Pixabay
The stages of portland cement production:
1)Extraction and crushing:
The limestone is extracted from mines which are often near the cement plant, while the clay is extracted using mechanical machines and transported to the cement plant.
Before using the extracted raw material, the crushing process takes place on the extraction site itself to reduce the particle size of limestone and clay to 50 nm.
2)Preparation of vintage:
After crushing the raw material (Clay and limestone) and reducing its particle size, a mixture of 80% limestone and 20% clay is prepared by grinding them together to obtain a powder with a particle size less than 200 micrometers.
- vintage composition:
Calcium carbonate CaCO3: from 77 to 83 percent.
Clay:
Silica SiO2: 13 to 14 percent.
Alumina Al2O3: from 02 to 04 percent.
Ferric oxide: 1.5 to 05 percent.
3)Cooking:
The raw obtained from the previous process is placed in a rotary oven whose length varies between 70 to 120m and 05m in diameter. Inside the oven, there is an inclined tube (03 to 04 degrees from the horizontal) which rotates on itself from 02 to 03 rpm.
The cru advances and slides inside the internal walls of the oven which are covered by brick. The raw begins to agglomerate at a temperature of 1300 ° C, until the temperature reaches 1450 ° C, we get grains with a diameter of 0.5 to 04 cm called Clinker. The clinker cools quickly when it comes out of the oven to a temperature between 50 and 250 degrees Celsius to avoid strong crystallization.
- Thermogravimetric analysis:
at T = 100 ° C: evaporation of free water.
a T = 500 ° C: Departure of chemically bound water.
at T = 800 to 1000 ° C: Dissociation of limestone:
CaCO3 ====== CaO + CO2
a T = 1000 at 1450 ° C: Reaction between CaO and the clay elements (SiO2, Al2O3, Fe2O3).
Cement notions:
Calcium oxide CaO == C
Silicon oxide Si2O3 == S
Alumina oxide Al2O3 == A
Ferric oxide == F
- At the start of the fusion, the oxides combine to form:
Hydraulic silicates:
3CaO.SiO2 ==== C3S (50 to 70 percent of the clinker)
2CaO.SiO2 ==== C2S (10 to 30 percent of the clinker)
Hydraulic aluminates:
3CaO.Al2O3 == C3A (02 to 15 percent of the clinker)
4CaO.Al2O3.Fe2O3 == C4AF (5 to 15 percent of the clinker)
4)Grinding:
Since the diameter of the clinker grains is somewhat large when it comes out of the oven, it is ground to a size less than 80 picometers, after the addition of gypsum (CaSO4.2H2O) and other additions. This is called Portland cement.
Clinker + gypsum ===== grinding ==== portland cement.
Hydration, setting and hardening:
Hydration and setting:
- The hydration process begins when adding water to the clinker, the properties of the paste obtained change over time with an increase in viscosity and heat generation. But before that, that is to say when the time has not advanced much, the dough is soft, which allows us to form it to obtain the desired shape.
When the mixture hardens and the viscosity begins to increase, it is said to set. - Several factors affect this process:
The more advanced the grinding process, the shorter the setting time.
The setting stops at a temperature of 0 ° C, and the higher the temperature, the faster the setting:
at 02 ° C: 6 p.m.
at 2 ° C: 3h30min
at 35 ° C: 30min
In addition, the setting slows down if there is an excess of water or if the water contains organic matter.
Hardening:
- This stage occurs after setting, and it can last from 28 days to several months, during which the mechanical strength of the cement will continue to increase.
Use of additions in the manufacture of cement:
Cement production is a very expensive and polluting process that requires large amounts of water. Consequently, mineral additions are added to facilitate the production process and obtain a better quality of the product and also to reduce pollution and the cost of production.
1)Technical advantages:
The additions reduce the need for water, thus improving the resistance to chemical attack and the mechanical resistance of the product (Example: resistance to cracking).
2)Ecological advantages:
Large amounts of carbon dioxide CO2 are emitted during the cement production process (for 1 ton of cement === 1 ton of CO2 emitted), therefore, mineral additions are added to reduce these emissions.
3)Economic advantages:
Cement production consumes a lot of energy and therefore a high cost, and therefore additives are added to reduce energy needs and production costs.
Examples of some mineral additions:
Pouzzolan:
A silicon rock of volcanic origin, gives birth to new compounds which are stable, sparingly soluble in water, weakly agglomerated.Silica smoke:
A byproduct of silicon manufacturing,
Advantage: make a very reactive product.
Sources:
Ciment Portland — Wikipédia
Fabrication du ciment | lafarge.fr
Fabrication du ciment. Processus de fabrication du ciment
Clinker — Wikipédia
Béton à une faible teneur en clinker
Liant hydraulique — Wikipédia
CIMENT, Prise et durcissement du ciment - Encyclopædia
Etude physicochimique du ciment