Composition of Cement clinker and Hydration of Cement

 Composition of Cement clinker and Hydration of Cement 

Chemical composition of Portland Cement:

1. a) Tricalcium Silicate (50%)
2. b) Dicalcium Silicate  (25%)
3. c) Tricalcium Aluminate (10%)
4. d) Tetracalcium Aluminoferrite (10%)
5. e) Gypsum (5%)

Composition of Cement clinker

The various constituents combine in burning and form cement clinker. The compounds formedin the burning process have the properties of setting and hardening in the presence ofwater.They are known as Bogue compounds after the name of Bogue who identified them. These compounds are as follows: Alite (Tricalcium silicate or C₃S), Belite (Dicalcium silicate or C₂S), Celite (Tricalciumalluminate or C₃A) andFelite (Tetracalciumalumino ferrite or C₄AF). 

Tricalcium silicate

It is supposed to be the best cementing material and is well burnt cement.It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind,increases resistance to freezing and thawing, hydrates rapidly generating high heat and developsan early hardness and strength. However, raising of C₃S content beyond the specified limitsincreases the heat of hydration and solubility of cement in water. The hydrolysis of C₃S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C₃S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.

Dicalcium silicate

It constitutes about 25-40% (normally about 32 per cent) of cement. It hydrates andhardens slowly and takes long time to add to the strength (after a year or more). It impartsresistance to chemical attack. Rising of C₂S content renders clinker harder to grind, reducesearly strength, decreases resistance to freezing and thawing at early ages and decreases heat ofhydration. The hydrolysis of C₂S proceeds slowly. At early ages, less than a month, C₂S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C₃S. The heat of hydration is 260 J/g.

Tricalciumalluminate

It is about 5-11% (normally about 10.5 per cent) of cement. It rapidlyreacts with water and is responsible for flash set of finely grounded clinker. The rapidity ofaction is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalciumaluminate is responsible for the initial set, high heat of hydration and has greater tendency tovolume changes causing cracking. Raising the C3A content reduces the setting time, weakens resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contractionduring air hardening. The heat of hydration of 865 J/g.

Tetracalciumalumino ferrite

It constitutes about 8–14% (normally about 9 per cent) of cement. It isresponsible for flash set but generates less heat. It has poorest cementing value. Raising theC4AF content reduces the strength slightly. The heat of hydration is 420 J/g.

Function Of Cement

1. to bind the sand and coarse aggregate together
2.  to fill voids in between sand and coarse  aggregate particle
3.  to form a compact mass

Types of Cement

2 types of cement normally used in building industry are as follows:

1. a) Hydraulic Cement
2. b) Nonhydraulic Cement

Hydraulic Cement

Hydraulic Cement sets and  hardens by action of water. Such as Portland Cement

In other words it means that hydraulic  cement are:

“ Any cements that turns into a solid product  in the presence of water (as well as air) resulting in a material that does not disintegrate in water.”

Non-hydraulic Cement

Any cement that does not require water to transform it into a solid product.

Manufacturing Of Portland Cement

The 3 primary constituents of the raw  materials used in the manufacture of  Portland Cement are:

1. a) Lime
   b) Silica
   c) Alumina
   
   
   

Lime is derived from limestone or chalk

Silica & Alumina from clay, shale or bauxite

 There are 2 chief aspects  of the manufacturing process:

 First
To produce a finely divided mixture of raw materials – chalk / limestone and clay / shale

Second

To heat this mixture to produce chemical composition

There 2 main process that can be used in manufacturing of Portland Cement that is

1. i) wet process        ii) dry process

Rotary Kiln

Raw materials are homogenized by crushing, grinding and blending so that approximately 80% of the raw material pass a No.200 sieve. The mix will be turned into form of slurry by adding 30 – 40% of water. It is then  heated to about 2750ºF (1510ºC) in horizontal revolving kilns (76-153m length and 3.6-4.8m in diameter. Natural gas, petroluem or coal are used for burning. High fuel requirement may make it uneconomical compared to dry process.

DRY PROCESS

Raw materials are homogenized by crushing,  grinding and blending so that approximately 80% of the raw material pass a No.200 sieve. Mixture is fed into kiln & burned in a dry state. This process provides considerable savings in fuel consumption and water usage but the process is dustier compared to wet process that is more efficient than grinding.

DRY PROCESS & WET PROCESS

In the kiln, water from the raw material is driven off and  limestone is decomposed into lime and Carbon Dioxide.

limestone  =        lime + Carbon Dioxide

In the burning zone, portion of the kiln, silica  and alumina from the clay undergo a solid state chemical reaction with lime to produce calcium aluminate.

silica & alumina + lime    =  calcium aluminate

The rotation and shape of kiln allow the blend to flow down the kiln, submitting it to gradually increasing temperature. As the material moves through hotter regions in the kiln, calcium silicates are formed. These products, that are black or greenish black in color are in the form of small pellets, called cement clinkers. Cement clinkers are hard, irregular and ball shaped particles about 18mm in diameter. The cement clinkers are cooled to about 150ºF (51ºC) and stored in clinker silos.

When needed, clinker are mixed with 2-5%  gypsum to retard the setting time of cement when it is mixed with water. Then, it is grounded to a fine powder and then the cement is stored in storage bins or  cement silos or bagged. Cement bags should be stored on pallets in a dry place.

 

Hydration of Cement

In the anhydrous state, four main types of minerals are normally present: alite, belite, celiteand felite. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and gypsum, which was added when the clinker was ground up to produce the familiar grey powder.

When water is added, the reactions which occur are mostly exothermic, that is, the reactions generate heat. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry.Almost immediately on adding water some of the clinker sulphates and gypsum dissolve producing an alkaline, sulfate-rich, solution.Soon after mixing, the (C₃A) phase (the most reactive of the four main clinker minerals) reacts with the water to form an aluminaterich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C₃A) reaction is with water is strongly exothermic but does not last long, typically only a few minutes, and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II).The first part of the dormant period, up to perhaps half-way through, corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable.At the end of the dormant period, the alite and belite in the cement start to react, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of hydration (Stage III), during which time concrete strengths increase. The individual grains react from the surface inwards, and the anhydrous particles become smaller. (C₃A) hydration also continues, as fresh crystals become accessible to water.The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing OPC only, most of the strength gain has occurred within about a month. Where OPC has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year.Ferrite reaction also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.

 

Products of Hydration

During Hydration process several hydrated compounds are formed most important of which are, Calcium silicate hydrate, calcium hydroxide and calcium aluminium hydrates which is important for strength gain. 

Calcium silicate hydrate:

This is not only the most abundant reaction product, occupying about 50% of the paste volume, but it is also responsible for most of the engineering properties of cement paste. It is often abbreviated, using cement chemists' notation, to "C-S-H," the dashes indicating that no strict ratio of SiO₂ to CaO is inferred. C-S-H forms a continuous layer that binds together the original cement particles into a cohesive whole which results in its strong bonding capacity. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50 in hydrated Portland cement but up to perhaps about 0.6 if slag or fly ash or microsilica is present, depending on the proportions.

Calcium hydroxide:

The other products of hydration of C3S and C2S are calcium hydroxide. In contrast to theCS-H, the calcium hydroxide is a compound with distinctive hexagonal prism morphology. It constitutes 20 to 25 per cent of the volume of solids in the hydrated paste. The lack ofdurability of concrete is on account of the presence of calcium hydroxide. The calcium hydroxide also reacts with sulphates present in soils or water to form calcium sulphate which further reacts with C3A and cause deterioration of concrete. This is known as sulphate attack. To reduce the quantity of Ca (OH)₂ in concrete and to overcome its bad effects by converting it into cementitious product is an advancement in concrete technology.The use of blendingmaterials such as fly ash, silica fume and such other pozzolanic materials are the steps toovercome bad effect of Ca(OH)2 in concrete. However, Ca(OH)₂ is alkaline in nature due to which it resists corrosion in steel. 

Calcium aluminium hydrates:

These are formed due to hydration of C₃A compounds. The hydrated aluminates do notcontribute anything to the strengthof concrete. On the other hand, theirpresence is harmful to the durabilityof concrete particularly where theconcrete is likely to be attacked bysulphates. As it hydrates very fast itmay contribute a little to the earlystrength.

Read More:

What is Cement? What are the types of cement and their uses?

Composition of Cement clinker and Hydration of Cement 

Types of Tests on Cement to Check the Quality

PPC or OPC And Their Difference

All Types of Cement in Construction

Must-Know Points to Check on a Cement Bag Before Buying It