What is Cement?
Cement is “a substance applied to the surface of solid bodies to make them
cohere firmly” or, more specifically, “a powdered substance which, made plastic
with water, is used in a soft and pasty state (which hardens on drying) to bind
together bricks, stones, etc in building”. Portland cement is a calcined
material comprising lime and silicates which is mixed with sand and stone and,
upon hydration, forms a plastic material which sets and hardens to a rock-like
material, concrete. Confusion between cement and concrete is endemic among the
uninitiated.
Portland cement is manufactured in a series of processes which may be
represented as shown:
History of Cement Manufacture
The ancient history of hydraulic mortars is extensive but becomes appreciable
with the widespread use of mixtures of natural pozzolans and burned lime by both
Greeks and Romans. The Pantheon in Rome is the only perfectly preserved building
from this period; it was constructed in 27BC and rebuilt 117-125AD and is of
pozzolan-lime concrete with an unsupported dome spanning 45M. Portland cement
was developed in the 19th century and is so called due to its resemblance in
colour and character to the naturally occurring stone of Portland Bill, off the
south coast of England.
Following are some of the more significant dates in the development of
Portland cement manufacture:
-
1824 Aspdin patented Portland cement
-
1845 Isaac Johnson recognised the significance of high temperature to produce
C3S. This was the first cement as we know it.
-
1880's Gypsum first added for set control.
-
1885 Ransome patented the rotary kiln.
-
1891 The continuously fed ball mill was patented.
-
1928 Introduction of the grate preheater kiln (Lepol) by Polysius provided the
first major improvement in thermal efficiency from the previous long, wet kilns.
-
1930's Roller mill first applied to cement manufacture; rapid development after
1960.
-
1930's Introduction of the roll press; rapid development after 1980.
-
1932 Patent of the cyclone preheater kiln with commercial development by KHD
dating from 1951.
-
1937 Introduction by Fuller of the grate cooler.
-
1950's Introduction of mechanical separators
-
1960 Introduction by KHD of the kiln bypass to allow use of raw materials with
high volatiles contents.
-
1966 Introduction of precalcination which was initially air-through
riser-firing.
-
1970's Introduction of high-efficiency separators.
-
1973 Introduction by IHI of the flash calciner with tertiary air duct.
The world consumption of Portland cement has grown:
|
|
Cement Demand |
World Population |
Per Capita |
|
1910 |
30 million t/Y |
1.5 billion |
20kg |
|
1925 |
150 million t/Y |
2.0 billion |
75kg |
|
1940 |
400 million t/Y |
2.2 billion |
180kg |
|
1955 |
600 million t/Y |
2.7 billion |
220kg |
|
1974 |
1,000 million t/Y |
4.0 billion |
250kg |
|
2000 |
1,500 million t/Y |
6.0 billion |
250kg |
This shows a long-term growth rate of 2-3% per year accelerating to a little
under 4% for the past decade.
Total international cement trade was 122 million tonnes in 2001 with 80 million
tonnes seaborn. As GDP per capita increases above $3000, cement consumption
increases substantially; above $15,000 consumption tends to reach a plateau.
World consumption is projected at 3,060 million tonnes per year by 2020.
Assuming an average selling price of $50/t, the cement industry world-wide has
revenues about one third of Walmart’s.
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