144 Damage to concrete structures
Ground water and sea water typically contain sulfates. High sulfate
concentrations may also be found in industrial wastewater. Sulfate ions
will react with the hydration products present in cementitious materials,
causing chemical sulfate attack. In partly submerged elements, sulfate
salts can crystallise on the drying surface. The crystallisation pressure can
damage the concrete surface, causing so-called physical sulfate attack.
In cases of accelerated curing conditions involving high temperature, a
special case of chemical sulfate attack can occur at later age. This is called
delayed ettringite formation. These degradation mechanisms, all linked
to the presence of sulfate, will be explained in some more detail in the
following sections.
It should be mentioned, however, that sulfate attack on concrete is
a very complex and sometimes even confusing item (Neville 2004).
The classical distinction between chemical and physical sulfate attack
(which will also be adopted in this book for reasons of clarity) is not
entirely justified scientifically because ‘physical’ sulfate attack clearly
also involves ‘chemical’ processes. Furthermore, processes of sulfate
attack quite often run parallel with other degradation processes. When
concrete is exposed to NaSO 4 , it can be degraded by both sulfate attack
and alkali silica reaction (ASR, see Section 5.3.1). Sulfuric acid will
lead to a combination of acid attack and sulfate attack. When microorganisms are involved, as in sewer pipes, it will become an even more
complicated degradation mechanism; this is called biogenic sulfuric acid
attack (see Section 5.3.4).
In literature, a distinction is often made between internal and external
sulfate attacks. Internal sulfate attack occurs when the source of sulfate is
within the concrete, e.g. caused by cement with a too high sulfate content
or by sulfate-containing aggregates. This has already been mentioned in
Chapter 3. Delayed ettringite formation is a special case of internal sulfate
attack, as will be explained further on. External sulfate attack refers to the
situation where the sulfate comes from an external source, e.g. from sea
water, wastewater, or ground water.
Sulfate attack can lead to a variety of damage patterns, such as spalling,
delamination, cracking, and loss of cohesion. Skalny et al. (2002) give a
comprehensive list of complex physico-chemical processes which may be
involved in a typical sulfate attack:
• Dissolution or removal from the cement paste of calcium hydroxide
• Complex and continuous changes in the ionic composition of the pore
liquid phase
• Adsorption or chemisorption of ionic components present in the
pore liquid phase on the surface of the hydrated solids present in the
cementing system
• Decomposition of still unhydrated clinker components
Ground water and sea water typically contain sulfates. High sulfate
concentrations may also be found in industrial wastewater. Sulfate ions
will react with the hydration products present in cementitious materials,
causing chemical sulfate attack. In partly submerged elements, sulfate
salts can crystallise on the drying surface. The crystallisation pressure can
damage the concrete surface, causing so-called physical sulfate attack.
In cases of accelerated curing conditions involving high temperature, a
special case of chemical sulfate attack can occur at later age. This is called
delayed ettringite formation. These degradation mechanisms, all linked
to the presence of sulfate, will be explained in some more detail in the
following sections.
It should be mentioned, however, that sulfate attack on concrete is
a very complex and sometimes even confusing item (Neville 2004).
The classical distinction between chemical and physical sulfate attack
(which will also be adopted in this book for reasons of clarity) is not
entirely justified scientifically because ‘physical’ sulfate attack clearly
also involves ‘chemical’ processes. Furthermore, processes of sulfate
attack quite often run parallel with other degradation processes. When
concrete is exposed to NaSO 4 , it can be degraded by both sulfate attack
and alkali silica reaction (ASR, see Section 5.3.1). Sulfuric acid will
lead to a combination of acid attack and sulfate attack. When microorganisms are involved, as in sewer pipes, it will become an even more
complicated degradation mechanism; this is called biogenic sulfuric acid
attack (see Section 5.3.4).
In literature, a distinction is often made between internal and external
sulfate attacks. Internal sulfate attack occurs when the source of sulfate is
within the concrete, e.g. caused by cement with a too high sulfate content
or by sulfate-containing aggregates. This has already been mentioned in
Chapter 3. Delayed ettringite formation is a special case of internal sulfate
attack, as will be explained further on. External sulfate attack refers to the
situation where the sulfate comes from an external source, e.g. from sea
water, wastewater, or ground water.
Sulfate attack can lead to a variety of damage patterns, such as spalling,
delamination, cracking, and loss of cohesion. Skalny et al. (2002) give a
comprehensive list of complex physico-chemical processes which may be
involved in a typical sulfate attack:
• Dissolution or removal from the cement paste of calcium hydroxide
• Complex and continuous changes in the ionic composition of the pore
liquid phase
• Adsorption or chemisorption of ionic components present in the
pore liquid phase on the surface of the hydrated solids present in the
cementing system
• Decomposition of still unhydrated clinker components