Actions during service 149
be accounted for when qualifying SCC mixtures for use in environments
conductive to TSA’. On the other hand, Irassar (2009) concluded that TSA
on SCC is mainly governed by transport properties, and by previous damage caused by expansive ettringite formation. As the surface area accessible
to sulfate ions increases substantially after expansive cracking due to ettringite formation, increasing dissolution of Ca(OH) 2 promotes instability of
ettringite and decomposition of C-S-H, favouring thaumasite formation in
the presence of carbonate and calcium ions. This latter view is supported
by Schmidt et al. (2009), who conclude from their investigation that ‘thaumasite formation was always preceded by expansion and cracking of the
samples due to ettringite formation and given the very slow kinetics of
thaumasite formation it was probably facilitated by the opening up of the
structure due to ettringite induced cracking’. This might lead to the conclusion that TSA can occur in real structures, but that most probably it is only
to be considered as a secondary damage mechanism, after previous damage
by a more classical form of sulfate attack or any other damage mechanism
leading to cracking.
5.3.2.1.1.5 SULFATE-ACID ATTACK
In some cases, e.g. exposure to sulfuric acid (H 2 SO 4 ) or ammonium sulfate
((NH 4 ) 2 SO 4 ), a combination of sulfate attack and acid attack is obtained.
Typical reaction products as in the case of sulfate attack will be formed,
e.g. gypsum, in combination with acid attack. For this latter degradation
mechanism, reference is made to Sections 5.3.3 and 5.3.4.
5.3.2.1.2 Influencing parameters
From the mechanism described in the previous section, important influencing parameters can be summarized as follows.
External chemical sulfate attack is promoted when sulfate ions can enter
the concrete more easily. The transport properties of concrete, as also
discussed in Chapter 1, thus significantly influence the process of sulfate
attack. In terms of concrete mix design, this is significantly linked to the
water/cement ratio.
Considering the chemical processes, the cement chemistry also plays
an important role, especially the aluminate phase. Some sulfate reaction
schemes are strongly linked to the presence of monosulfate and its conversion to ettringite.
A main influencing factor, of course, is the environment to which the
concrete is exposed. Of major importance are the sulfate concentration and
the type of sulfate solution. Furthermore, temperature and humidity also
influence the sulfate attack processes.
be accounted for when qualifying SCC mixtures for use in environments
conductive to TSA’. On the other hand, Irassar (2009) concluded that TSA
on SCC is mainly governed by transport properties, and by previous damage caused by expansive ettringite formation. As the surface area accessible
to sulfate ions increases substantially after expansive cracking due to ettringite formation, increasing dissolution of Ca(OH) 2 promotes instability of
ettringite and decomposition of C-S-H, favouring thaumasite formation in
the presence of carbonate and calcium ions. This latter view is supported
by Schmidt et al. (2009), who conclude from their investigation that ‘thaumasite formation was always preceded by expansion and cracking of the
samples due to ettringite formation and given the very slow kinetics of
thaumasite formation it was probably facilitated by the opening up of the
structure due to ettringite induced cracking’. This might lead to the conclusion that TSA can occur in real structures, but that most probably it is only
to be considered as a secondary damage mechanism, after previous damage
by a more classical form of sulfate attack or any other damage mechanism
leading to cracking.
5.3.2.1.1.5 SULFATE-ACID ATTACK
In some cases, e.g. exposure to sulfuric acid (H 2 SO 4 ) or ammonium sulfate
((NH 4 ) 2 SO 4 ), a combination of sulfate attack and acid attack is obtained.
Typical reaction products as in the case of sulfate attack will be formed,
e.g. gypsum, in combination with acid attack. For this latter degradation
mechanism, reference is made to Sections 5.3.3 and 5.3.4.
5.3.2.1.2 Influencing parameters
From the mechanism described in the previous section, important influencing parameters can be summarized as follows.
External chemical sulfate attack is promoted when sulfate ions can enter
the concrete more easily. The transport properties of concrete, as also
discussed in Chapter 1, thus significantly influence the process of sulfate
attack. In terms of concrete mix design, this is significantly linked to the
water/cement ratio.
Considering the chemical processes, the cement chemistry also plays
an important role, especially the aluminate phase. Some sulfate reaction
schemes are strongly linked to the presence of monosulfate and its conversion to ettringite.
A main influencing factor, of course, is the environment to which the
concrete is exposed. Of major importance are the sulfate concentration and
the type of sulfate solution. Furthermore, temperature and humidity also
influence the sulfate attack processes.