Cement is a fine powder which sets after a few hours when mixed with
water, and then hardens in a few days into a solid, strong material.
Cement is mainly used to bind fine sand and coarse aggregates together
in concrete. Cement is a hydraulic binder, i.e. it hardens when water
is added.
There are 27 types of common cement which can be grouped into 5
general categories and 3 strength classes: ordinary, high and very
high. In addition, some special cements exist like sulphate resisting
cement, low heat cement and calcium aluminate cement.
The quarry is the starting point
Cement plants are usually located closely either to hot spots in the
market or to areas with sufficient quantities of raw materials. The aim
is to keep transportation costs low. Basic constituents for cement
(limestone and clay) are taken from quarries in these areas.
A two-step process
Basically, cement is produced in two steps: first, clinker is
produced from raw materials. In the second step cement is produced from
cement clinker. The first step can be a dry, wet, semi-dry or semi-wet
process according to the state of the raw material.
Making clinker
The raw materials are delivered in bulk, crushed and homogenised into
a mixture which is fed into a rotary kiln. This is an enormous rotating
pipe of 60 to 90 m long and up to 6 m in diameter. This huge kiln is
heated by a 2000°C flame inside of it. The kiln is slightly inclined to
allow for the materials to slowly reach the other end, where it is
quickly cooled to 100-200°C.
Four basic oxides in the correct proportions make cement clinker:
calcium oxide (65%), silicon oxide (20%), alumina oxide (10%) and iron
oxide (5%). These elements mixed homogeneously (called “raw meal” or
slurry) will combine when heated by the flame at a temperature of
approximately 1450°C. New compounds are formed: silicates, aluminates
and ferrites of calcium. Hydraulic hardening of cement is due to the
hydration of these compounds.
The final product of this phase is called “clinker”. These solid grains are then stored in huge silos. End of phase one.
From clinker to cement
The second phase is handled in a cement grinding mill, which may be
located in a different place to the clinker plant. Gypsum (calcium
sulphates) and possibly additional cementitious (such as blastfurnace
slag, coal fly ash, natural pozzolanas, etc.) or inert materials
(limestone) are added to the clinker. All constituents are ground
leading to a fine and homogenous powder. End of phase two. The cement is
then stored in silos before being dispatched either in bulk or bagged.
What is concrete?
Concrete is a solid material made of cement, water, aggregates and
often with admixtures. When fresh, it has a certain workability and
takes the form of the mould into which it is put. When set and hardened,
it is as strong as natural stone and resists time, water, frost,
mechanical constraints and fire. Typically, concrete is the essential
material used in all types of construction [residential (housing),
non-residential (offices) and civil engineering (roads, bridges, etc.)].
