31-03-2014, 03:10 PM
LIGHTWEIGHT AGGREGATE CONCRETE Science, Technology, and Applications
Introduction
The use of lightweight aggregate concrete (LWAC) can be traced
to as early as 3000 BC, when the famous towns of Mohenjo-Daro and
Harappa were built during the Indus Valley civilization. In Europe, earlier
use of LWAC occurred about two thousand years ago when the Romans
built the Pantheon, the aqueducts, and the Colosseum in Rome. It is
interesting to note that pumice is still used today as an aggregate for
structural concrete in certain countries such as Germany, Italy, Iceland, and
Japan. In some places, like Malaysia, palm oil shells are used for making
lightweight aggregate concrete.
Earlier lightweight aggregates (LWAs) were of natural origin,
mostly volcanic: pumice, scoria, tuff, etc. These have been used both as fine
and coarse aggregates. They function as active pozzolanic materials when
used as fine aggregates. These interact with the calcium hydroxide gener-
ated from the binder during hydration and produce calcium silicate hydrate
which strengthens the structure and modifies the pore structure, enhancing
the durability properties.
LIGHTWEIGHT AGGREGATES (LWA)
Lightweight aggregates can originate from natural resources or they
can be man-made. The major natural resource is the volcanic material. Man-
made or synthetic, aggregates are produced by a thermal process in
factories.
Natural Aggregates
Volcanic Origin. When lava from a volcano cools down, it
produces a spongy well-sintered mass. Since there is an abrupt cooling of
the molten mass, the material freezes. With a sudden cooling of the molten
magma, there is no crystallization, and the material acquires a glassy
structure, a process similar to the production of the glass known as obsidian.
It can be called a supercooled liquid, which has no crystalline phase. It is
highly amorphous and has a glassy structure.
Lava is a boiling melt which may contain air and gases, and when
it cools down, it freezes to a spongy porous mass. In other words, it produces
lightweight material that is porous and reactive. This type of material is
known as volcanic aggregates, or pumice or scoria aggregates. The
aggregates are produced by mechanical handling of lava, i.e., crushing,
sieving, and grinding.
North America
Stephen John Hayde, of Kansas City, Missouri, was a contractor
and brickmaker. He observed abnormal bloating in the bricks when burnt.
This gave him the idea of making expanded clay pieces, e.g., aggregates,
from this clay. He did tests for producing the aggregates and in February,
1918, received a patent on his process for the production of expanded shale
aggregates in a rotary kiln.[5]
In 1920, the first commercial plant began operating in Kansas City,
producing “Haydite” expanded shale aggregates. By the year 1941, there
were seven plants in the USA and one plant in Canada. In 1952, the
Expanded Shale, Clay and Slate Institute (ESCSI) was founded.
Other Continents
In the USA and Canada, there are fourteen companies, some of
them owning five or more plants. The shipment in 1994 was about 4.6
million m3 of expanded clay, shale, and slate combined. The production
may increase. The sales volume is about 60–85% of the installed capacity.
In South America, there are two plants (Argentina and Venezuela).
The same holds for Africa (Algeria, Egypt). In Japan, there are two plants
with 1.2 million m3 capacity and 750,000 m3 shipment in 1994, mainly for
structural concrete, the bulk density for 5/15 mm size being around 800
kg/m3.
CONCLUDING REMARKS
Lightweight aggregate concrete was used even before the Christian
era. The concrete was made with natural volcanic aggregates of pumice and
scoria. With time, because of the advantages of lightweight aggregate
concrete, specifically its low density and thermal insulating properties, its
demand has increased. In recent years, it has become an important structural
material in off-shore construction. This has led to the development of
synthetic lightweight aggregates which are made from natural raw materi-
als like clay, slate, shale, etc., and from industrial by-products like fly ash,
slag ashes, etc.
There has been some research and development work on the
production of LWAC using organic natural aggregates such as palm oil and
shales, but it is still not in commercial production.
There are many types of lightweight aggregates of mineral origin,
ranging from weights below 50 kg/m3 up to heavy types of 1,000 kg/m3 or
even more. They enable the production of concrete and mortars in very wide
ranges with properties that will suit the requirements of different building
industries. But the term lightweight concrete is not well defined, hence, is
not used on a large scale.