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 The
use of Cement in making concrete for construction
activity is age old, since last more than
two centuries. However, greater awareness
and emphasis on durability of concrete structures
is seen since last about a decade, as concrete
is constantly subjected to forces of nature, humidity,
heat, cold and rains etc. Concrete cancer
is the disintegration of concrete due to corrosion
in the steel reinforcement, caused by the ingress
of corrosive agents present in the atmosphere.
The durability means the capacity of the structure
to effectively withstand the attack of the deleterious
agents, both external and internal, throughout
the life of the concrete structures for
which it is designed. The common causes of distress
and deterioration in the concrete structure are
– Heat of hydration, sulphate attack, chloride
attack and Alkali aggregate reaction (ASR).
More than hundred years of experience with blast
furnace slag cement has unambiguously proved that,
structures made with Portland Slag Cement can
be durable even under most harsh marine conditions.
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Eastern Scheldt Barrier , built
with blast furnace slag cement
with a design service life of 200 years.
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King
Fahad Causeway (Saudi Arabia-Bahrain
Causeway) |
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| SUPERIOR PROPERTIES OF SLAG CEMENT |
Heat of Hydration :
The measured heat of hydration is less
in Bagalkot shakti cement compared to other
cements of ‘OPC grades’. Slag
Cement is more suitable alternative to the low
heat Portland Cement. With controlled
chemical reaction in the portland Slag Cement,
amount of heat liberated is less by 30% compared
to that of Ordinary Portland Cement. This
low heat of hydration helps in reduction
of structural cracks due to thermal stress.
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Resistance to Sulphate attack :
Sulphates are present in Sea water, certain soils / ground water, sewage and industrial effluents. The presence of sulphates is a major contributing factor to the eventual disintegration of concrete built in such areas.
The
disintegration is caused by the reaction between
soluble sulphates in water/soil and -Tricalcium
Aluminate (C3A) - present in the cement.
The product of this reaction is called "Ettringite"
and is volumetrically grater than the original
constituents.
This
can lead to excessive expansion, cracking and
loss of strength in concrete. The sulphate
attack can be controlled either by restricting
C3A content in the Cement like sulphate resistant
cement (C3A <6%) or by using Portland Slag
Cement.
Sulphate resistant Cement (SRC) provides protection
against only sulphates attack but is not suitable
for protecting reinforced steel and concrete
where there is also danger of chloride attack.
If both chlorides and sulphates are present
in soil / water, slag cement is the ideal choice.
The study conducted by leading research institutes
reveals that expansion value in Slag Cement
is very much less compared to that of
ordinary Portland Cement.
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Report on slag cement by Hottan, , Emery
& Osbore (Published - National seminar
, Delhi)
Study reported by “ Hootan ”
and “Emery (1990) on exposure of
10 years of concrete made with portland
blast furnace slag cement. The replacement
of slag up to 45% by mass of portland
cement with higher C3A value (12.3%) showed
equivalent or better resistant to sulphate
expansion compared to sulphate resistant
cement. The concrete from 45-70% slag
replacement showed no deterioration to
exposure over “10” years in
solution of 3000mg/litre of Na2So4 or
MgSo4. In another study by “ Osborne”
(1991) the sulphate resistance of 70%
replaced slag is almost similar to the
low “C3A” cement.
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Resistance to Chloride Attack :
Water containing chloride is dangerous as it causes leaching of free lime making the concrete more porous, resulting in corrosion of reinforcement. It is established that pore structure of cementous matrix changes through the reaction of granulated slag with calcium hydroxide and alkalies released during hydration of Cement. Pores in concrete, normally containing calcium hydroxide, are then, in part filled with calcium silicate hydrate resulting in reduction in permeability in concrete.
The reduction in permeability increases durability of concrete since chlorides and other chemicals cannot penetrate the concrete surface. This will increase resistance to corrosion of reinforcement. Bagalkot shakti cement having lower values of chloride content (less than 0.03%) is ideally suited for protecting concrete structure against chloride attack. In case the water and soil contain both sulphate and chloride only Portland Slag Cement has to be used, because the effect of chloride on sulphate resistant Cement is much higher.
From the table, it is very clear that coefficient for diffusion of chloride ion into cement paste in case of slag cement is low at 0.41 compared to 4.47 in OPC and 10.0 in sulphate resistant Cement. This reduction in diffusivity has been confirmed by many investigators and would appear to be due to two mechanisms i.e 1) The use of slag reduces permeability of the concrete 2) The hardened paste of slag cement bind greater amount of chloride than that of other cement resulting in much lower proportion of free chloride in the pore solution. Hence Slag cement is better choice for construction works where water / soil contains both sulphate and Chloride.
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Chloride
Attack
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Resistance to Alkali Aggregate Reaction :
The use of Bagalkot Shakti cement is recommended in the case of aggregates, which are susceptible with respect to alkali silica reaction leading to swelling of concrete. The allowable limit of alkali content in cement increases with the percentage slag used.
In case of OPC alkali content equivalent to Na2O should not exceed 0.6% where as higher Alkali content is allowed in case of slag cement. In Netherlands alkali content up to 2% is allowed if the slag content in cement is above 65% and alkali content up to 1.1% is allowed if the slag content in the cement is above 50%.
With the Alkali content about 0.9% and slag
content up to 50% in BAGALKOT SHAKTI makes cement
having similar properties to that of low Alkali
Cement. From the graph this can be seen
that expansion values of Portland Slag Cement
are just 12.5% of that concrete made out by
using Ordinary Portland cement.
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Alkali
Aggregate Reaction
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