Errors during casting 49
As the aggressive substances are within the concrete from the beginning,
in some cases the degradation can go much faster than in cases where the
aggressive substances have to enter the concrete from the environment.
However, in many cases the degradation still depends on the availability
or transport of water and/or oxygen. Some potential aggressive substances
which have to be avoided in a concrete mix are listed hereafter, along with
explanations of the kind of problems to be expected.
3.4.1 sulfates
Sulfate attack in cementitious systems is a complicated issue and will be
explained in more detail in Chapter 5. However, in short, the problem of
sulfate attack can be summarized as follows. Portland cement consists
of different clinker minerals, among which is the aluminate phase C 3 A
(tricalciumaluminate). As a result of a normal hydration process, monosulfate is formed within the cementitious system (see also Section 1.4.1
in Chapter 1). When this monosulfate is later enriched by sulfates, it will
be transformed into ettringite (also called tri-sulfate). This conversion is
accompanied by a volume increase because the ettringite is much richer in
bound water. As a result, an internal pressure occurs within the hardened
concrete, possibly leading to severe cracking (see Chapter 5 for more details
on sulfate attack).
While the normal case of sulfate attack is caused by enrichment of the
cement stone by sulfates coming from the environment, it could also happen that sulfates are mixed within the concrete during production. As a
matter of fact, cement itself also contains sulfates. However, when cement
conforms to actual code provisions, the risk of sulfate attack by the sulfates
of the cement itself is practically excluded (except maybe the special situation of delayed ettringite formation in case of hydration at very high temperature, see further in Chapter 5).
Sulfates could also be added through additions such as ground granulated blast furnace slag or other fines like limestone powders. The sulfate
content of these additions should also be limited, as typically required in
relevant standards covering these materials.
A situation entailing more risk seems to be the application of recycled
aggregates, e.g. in the form of masonry rubble. These materials, which typically originate from the demolition of houses, can contain sulfate impurities coming from the interior plasterwork (gypsum). Due attention should
be given to avoiding these impurities because they will provide internal
sources of sulfate within the concrete, easily leading to damage due to sulfate attack. Also in the case of aggregates coming from the sea (sea sand
and sea gravel), due attention should be given to wash the aggregates with
clear water in order to avoid contamination with sulfates and other aggressive substances.
As the aggressive substances are within the concrete from the beginning,
in some cases the degradation can go much faster than in cases where the
aggressive substances have to enter the concrete from the environment.
However, in many cases the degradation still depends on the availability
or transport of water and/or oxygen. Some potential aggressive substances
which have to be avoided in a concrete mix are listed hereafter, along with
explanations of the kind of problems to be expected.
3.4.1 sulfates
Sulfate attack in cementitious systems is a complicated issue and will be
explained in more detail in Chapter 5. However, in short, the problem of
sulfate attack can be summarized as follows. Portland cement consists
of different clinker minerals, among which is the aluminate phase C 3 A
(tricalciumaluminate). As a result of a normal hydration process, monosulfate is formed within the cementitious system (see also Section 1.4.1
in Chapter 1). When this monosulfate is later enriched by sulfates, it will
be transformed into ettringite (also called tri-sulfate). This conversion is
accompanied by a volume increase because the ettringite is much richer in
bound water. As a result, an internal pressure occurs within the hardened
concrete, possibly leading to severe cracking (see Chapter 5 for more details
on sulfate attack).
While the normal case of sulfate attack is caused by enrichment of the
cement stone by sulfates coming from the environment, it could also happen that sulfates are mixed within the concrete during production. As a
matter of fact, cement itself also contains sulfates. However, when cement
conforms to actual code provisions, the risk of sulfate attack by the sulfates
of the cement itself is practically excluded (except maybe the special situation of delayed ettringite formation in case of hydration at very high temperature, see further in Chapter 5).
Sulfates could also be added through additions such as ground granulated blast furnace slag or other fines like limestone powders. The sulfate
content of these additions should also be limited, as typically required in
relevant standards covering these materials.
A situation entailing more risk seems to be the application of recycled
aggregates, e.g. in the form of masonry rubble. These materials, which typically originate from the demolition of houses, can contain sulfate impurities coming from the interior plasterwork (gypsum). Due attention should
be given to avoiding these impurities because they will provide internal
sources of sulfate within the concrete, easily leading to damage due to sulfate attack. Also in the case of aggregates coming from the sea (sea sand
and sea gravel), due attention should be given to wash the aggregates with
clear water in order to avoid contamination with sulfates and other aggressive substances.