This document discusses different types of chemical attacks on concrete, including sulfate attack, sea water attack, and acid attack. Sulfate attack occurs when sulfates react with hydrated lime and calcium aluminate in concrete, forming ettringite and gypsum that cause cracking. Sea water attack involves corrosion of steel reinforcement due to chlorides as well as chemical and physical deterioration. Acid attack, which can be caused by acid rain, sewage, or industrial sources, dissolves calcium compounds in concrete and weakens its structure over time. Proper concrete mix design and coatings can improve resistance to chemical deterioration.

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INTRODUCTION
 Most frequent form of chemical attack are due
to sulphates, sea water & natural acidic water.
 Concrete permeability is the single most
important factor that influences its durability.
 Chemical attack of concrete occurs by way of
decomposition of the products of hydration
and formation of new compound, which if
soluble, maybe leached out and if not soluble,
maybe disruptive in situ.
SULPHATE ATTACK

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INTRODUCTION
 Sulphate attack is one of the most important
causes of concrete deterioration.
 Soilds salts don’t attack concrete but when
present in solution, they can react with the
hydrated cement paste.
 Soluble sulfate (Na, Ca and Mg) is common in
mining operations, chemical and paper milling
industries.
 Sodium and calcium most common sulfate in
soils, water and industrial processes.
 Sulfates react chemically with cement paste’s
hydrated lime and hydrated calcium aluminate.
 When salts are in solution they can diffuse into
the pores of concrete and react with hardened
cement paste.
 The formation of gypsum and ettringite
expands, pressurizes and disrupts the paste
result in surface scaling, disintegration and
mass deterioration.
 Causing cracking and spalling of concrete due
to expansion.

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 Calcium sulphate reacts with the Calcium
aluminate hydrate in the concrete to form
calcium sulphoaluminate knowns as ettringite.
 Magnesium sulphate attacks calcium silicate
hydrate and Calcium aluminate hydrate.
 The critical consequence of the attack by
magnesium sulphate is the destruction of
C-S-H.
 Sodium sulphate attacks also the calcium
hydroxide in the concrete to give calcium
sulphate
 It is seen as progressive cracking and spalling
and a characteristic whitish appearance.
 Can be prevented by using sulphate resisting
cement or adding pozzolanic ash as partial
cement replacement material

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 The effect of sulphate attack:
a) disruptive expansion and cracking
b) loss of strength of concrete due to the loss
of cohesion in the hydrated cement paste
& loss of adhesion between it and aggregate
c) Concrete has a characteristic whitish
appearance
d)Damage usually start at edges and corner
and is followed by progressive cracking and
spalling which reduce the concrete to a
friable or even soft state.
Cracks formation because of sulphate attack

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Cracks occurrence because of sulphate attack
Deterioration of concrete floor due to sulphate attack

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Deterioration of concrete due to sulphate attack
Sulphate attacked fence post

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 Among the preventive measures that can be
taken:
a) Minimize C3A content in cement by using
Sulphate Resisting Portland Cement
b)Use of blended cement consisting pozzolanic
material to reduce the quantity of Ca(OH)2
c)Use of dense concrete with low permeability
concrete as possible
d)Application of high pressure steam curing
for concrete
 Resistance of concrete to sulfate attack can be
tested in the lab by storing specimens in a
solution of sodium or magnesium sulfate.
 Alternate wetting and drying accelerates the
damage due to the crystallization salts in the
pores of the concrete
 The effect of exposure can be estimated by
a) the loss in strength of the specimen
b) its expansion (normally mortar bar)
c) its loss of mass
d) can even be assessed visually

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SEA WATER ATTACK
INTRODUCTION
 Marine structures (dry dock and jetties,
breakwaters and tidal barriers, offshore
floating docks and drilling platforms, etc..)
 Sea water contains large amounts of chlorides
as well as sulphates which attack concrete
 The type and severity of attack on concrete in
a marine environment depend on the exposure
conditions.

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 Concrete exposed to sea water can be
subjected to various chemical and physical
action such as
a) chemical attack
b) chloride - induced corrosion of steel
reinforcement
c) freeze – thaw attack
d) salt weathering
 The presence and intensity of these various
forms of attack depend on the location of the
concrete with respect to the sea level.
Deterioration of concrete due to sea water attack

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Cracked and spalled marine bridge piling
Deterioration of concrete due to sea water attack

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Chloride-induced corrosion
Chloride induced spalling

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Chemical Composition of Sea Water
 The ion concentrations of Na and Cl are the
highest and the ion concentrations of Mg and SO
are sufficient to provide aggressive action on
cement hydration products.
 The dissolved CO2 in sea water forming
carbonic acid which will attack the cement
matrix, causing leaching of calcium from
hydrated cement paste.
 Crystallization of salts in the pores of concrete
can result in its disruption due to the pressure
exerted by the salt crystals.
3 different exposure zones
 Submerged zone
Is the part of the structure kept continuously
under water
 Splash zone
Is the part of the structure subjected to
repeated wetting and drying by sea water
 Atmospheric zone
Is the part of the structure above the splash
zone

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Deterioration of concrete in splash zone
 Concrete between the tide marks, subjected to
alternate wetting and drying, is severely
attacked, especially in tropical climates.
 Concrete above the water level is subject to
capillary action and can be in the splash zone.
 Permanently immersed concrete is attacked
least.
 Products of sea water, gypsum and calcium
sulphoaluminate are leached out by sea water
and this effectively inhibits expansion and
deterioration due to this form of attack.

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ACID ATTACK
INTRODUCTION
 Acids can come from sources external to the
concrete such as the earth surrounding a concrete
structure, groundwater, rainwater, and pollutants
in the air.
 Portland cement concrete being highly alkaline
is not resistant to attack by strong acids.
 Concrete can be attacked by liquids with a pH
value below 6.5, but the attack is severe at a pH
below 5.5; below 4.5 the attack is very severe.

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 The most vulnerable cement hydrate is
Ca(OH)2 but C-S-H can also be attacked.
 Acids can attack and leach away the calcium
compounds of cement paste formed in concrete
through the hydration process, as well as the
calcium in calcareous aggregate.
 The salts produced which then be leached
away would cause a loss of volume and
cohesion of the paste.
 Acid attack weakens the concrete structurally
and reduces its durability and service life.
 Acid rain that consists mainly of sulphuric acid
and nitric acid and has a pH value between 4 to
4.5, may cause surface weathering of concrete.
 Concrete in sewage and wastewater treatment
plants are among the structure that tend to
deteriorate because of sulfuric acid attack.
 If become severe, it would result in corrosion
of reinforcing steel.
 Sulphuric acid is produced in sewers by a
complex chain of microbiological and
gas/liquid solubility stages which occur as the
sewage travels through the sewerage system.

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 However, the severity of deterioration depends
on on the concentration of sulfuric acid, this in
turn being a function of both the specific
location within the plant and also the time over
which the concrete is exposed to elevated
concentrations of acid.
 Normally, the hardened cement paste is
gradually dissolved and progressive
deterioration of concrete take place.
Deterioration of sewerage concrete due to acid attack

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Deterioration of silo due to acid attack
Formation of gypsum in wastewater treatment plant

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Concrete bridge pylons corroded due to acid attack.
Corrosion of floodgate resulting from acid attack

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Corrosion of floodgate due to acid attack
Concrete structure affected due to acid attack

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 Among the methods that can be applied to
improve the concrete resistance to acids are:
a) choosing the right concrete composition to
make it as impermeable as possible
b) isolating it from the environment by using a
suitable coating
c) proper curing of concrete
d) use of blended cement which include
pozzolanic material