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Sulfates occur in ground water, soils, sea water, fertilizers,
and industrial pollutants and also get formed during oxidation of sulfides.
When present in significant amounts they have long been a recognized cause of
durability failure of Portland cement concrete. More recently, sulfates have
been associated with internal causes of durability problems such as delayed
ettringite formation and thaumasite attack . Search for a solution to the sulfate
problem (particularly in Portland Cement concrete structures in France)
led to development of high alumina cement.
As its name suggests, it contains a high
content of Alumina (about 40%). Lime content is also about 40%, followed by
about 8% of silica.
High alumina cement (calcium aluminate cement, ciment fondu) is made in kiln by the
fusion of bauxite and limestone with some compounds of iron, which act as flux.
The principal minerals formed in the kiln are CA (CaO.Al2O3)
and C12A7 (12CaO.7Al2O3).
The material formed in the kiln is ground
to a fine powder which when mixed with water forms a paste which hardens and gains strength rapidly
with considerable evolution of heat. Strength may reach 80 per cent
of the ultimate strength within 24 hours.
This
is a special type of cement that has high rate of strength development,
refractoriness and possesses high chemical resistance. About 80% of its
ultimate strength is achieved at the age of 24hrs. Even at 6-8
hrs the concrete is strong enough for side formwork to be struck.
This
high rate of strength development is due to rapid hydration which implies a high
rate of heat development (about 9cal/g/h
compared to 3.5cal/g/h for rapid hardening Portland cement). However,
the total heat of hydration is of the same order for both types of cement.
Some of the applications of HAC are as under:-
Rapid hardening
Compared
to high-early-strength or ordinary Portland cement, High alumina cement hardens
rapidly. It strengthens enough even at low temperatures. When mixed with
Portland cement or other additives, they cause quick setting according to their
formulation. Therefore, they are used for urgent construction of heavy
traffic roads, railways, bridges, etc.
Refractoriness
High Alumina Cement is often used as a bonding
material for refractory castables because it forms ceramic bonds at high
temperature and maintains its strength even after cooling. With the
accurate selection of aggregates, it is possible to produce refractory
castables capable of resisting a maximum temperature of 1600°C.
Resistance to chemical attack
It
resists sulfuric acid corrosion better than Portland cement, and demonstrates high resistance to chemical
corrosion. Therefore, it is useful in construction of ground water pipes,
water pipes, sewage pipes, factory drains, coastal establishments, and factory
chimneys.
Also, High Alumina Cement has been found to be very useful
both in improving existing materials and in developing new materials by
combination with other cements/aditives.
A phenomenon known as "Conversion” occurs
in HAC.
Cement is a substance
with binding property, which sets and hardens independently, and can bind
other materials together.
Cement can also be
described as a material with adhesive and cohesive properties which makes it
capable of bonding mineral fragments into a compact whole (Neville) .
Interesting facts:
The word cement is derived
from the Latin word “caementum” which means rough
stone.
The Early Romans used a
masonry (which resembled modern day concrete) made from crushed rock
with burnt lime as binder. They used the term "opus caementitium"
to describe this masonry.
The name concrete is
derived from the Latin word “concretus; concretus is the past participle of the word concrescere meaning “to
grow together”, “to harden”.
Cements used in
construction are characterized as Hydraulic or Non-Hydraulic.
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