Cement

What is cement and how is it made?
Cement is a fine, soft, powdery-type substance. It is made from a mixture of elements that are found in natural materials such as limestone, clay, sand and/or shale. When cement is mixed with water, it can bind sand and gravel into a hard, solid mass called concrete.

An example of how cement can be made

1.) Limestone is taken from a quarry. It is the major ingredient needed for making cement. Smaller quantities of sand and clay are also needed. Limestone, sand and clay contain the four essential elements required to make cement. The four essential elements are calcium, silicon, aluminum and iron.

2.) Boulder-size limestone rocks are transported from the quarry to the cement plant and fed into a crusher which crushes the boulders into marble-size pieces.

3.) The limestone pieces then go through a blender where they are added to the other raw materials in the right proportion.

4.) The raw materials are ground to a powder. This is sometimes done with rollers that crush the materials against a rotating platform.

5.) Everything then goes into a huge, extremely hot, rotating furnace to undergo a process called "sintering". Sintering means: to cause to become a coherent mass by heating without melting. In other words, the raw materials become sort of partially molten. The raw materials reach about 2700° F (1480°C) inside the furnace. This causes chemical and physical changes to the raw materials and they come out of the furnace as large, glassy, red-hot cinders called "clinker".

6.) The clinker is cooled and ground into a fine gray powder. A small amount of gypsum is also added during the final grinding. It is now the finished product - Portland cement. The cement is then stored in silos (large holding tanks) where it awaits distribution. The cement is usually shipped in bulk in purpose-made trucks, by rail or even by barge or ship. Some is bagged for those who want small quantities

Comparing Reactions1

Hydration Reactions

Hydration reactions are semi stoichoimetric2 – for example two moles of di calcium silicate
(a monomer) take on water releasing calcium (which then combines with hydroxides from
water) to form a mole of di calcium silicate hydrate (a dimer)3
2Ca2SiO4 + 5H2O ==> Ca3Si2.O7.4H2O + Ca(OH)2
Using “atom and stick” nomenclature

The pozzolanic reaction is similar to the hydration reaction for di and tri calcium silicate;
the difference is that the components all come together to form compounds that are
hydrated silicates with less calcium and bound water.
1.1Ca(OH)2 + SiO2 + .67H2O ==> Ca1.1SiO3.1.2.1H2O
Atoms don’t come in fractions so the formula suggest that larger molecules are formed that
are probably oligopolymers – and with the pozzolanic reaction they are.

Silicification Reactions

Hydrolysis and Recombination

As portlandite is formed in reactions like the one depicted above the pH of the mix rises to
around 12.5 at equilibrium4.
At high pH the surfaces of silica and alumina containing compounds tend to hydrolyze.
Strength giving reactions occur when the hydrolyzed surfaces then bind back together
losing water as depicted in the “atom and stick” diagram below.

In tec-cement concrete formulations many wastes containing silica and alumina such as fly
ash and ground granulated blast furnace slag5 can be added in high proportion and tend to
recombine in the above manner as well as becoming involved in the pozzolanic reaction.

Comparing Bonds with Aggregates

In PC concretes there is no chemical bond with aggregates – they are physically held in
place. Portlandite also tends to form around larger aggregates weakening the bond to
them. A further source of weakness is the reaction between alkali and silica
Geopolymers are different in that the geopolymeric paste tends to bond with silica in the
micro aggregates and aggregates and this together with cross linking are reasons for high
strength, .