156 Damage to concrete structures
accelerated by a higher temperature in hot climates. Due attention should be
paid to the local exposure conditions when dealing with marine structures.
5.3.3 acid attack
As concrete is a calcium-rich alkaline environment, it will react with
nearly all possible acids. The solubility of the reaction products will be
of major importance for the resulting degradation level of the concrete,
especially the solubility of the calcium salt, which is typically the first salt
to be formed in case of an acid attack. This also explains why the rate of
acid attack quite often more significantly depends on the level of water
movement near the concrete surface, rather than on the type of cement or
aggregate. Nevertheless, the latter parameters surely have an influence (De
Belie 1997). It is often considered that cement types with a lower calcium
content will be more resistant to acid attack. Although this often seems
the case, it cannot be fully generalized to all cases. In some cases, a low
Ca(OH) 2 content in the matrix can lead to a lower buffer capacity and a
faster degradation of the C-S-H phase. A similar reasoning can be followed
concerning the aggregates.
The main effect of the acid is the dissolution of the cement matrix, leading to a degradation of the concrete. In contrast with typical cases of sulfate
attack, the degradation process in the case of acid attack is not linked to
any significant expansion reaction because ettringite and thaumasite are
not stable in the acid environment. The level of degradation largely depends
on the type of acid to which the concrete is exposed. Furthermore, it is
important to mention that in real exposure cases wastewaters can contain
a multitude of different acids, which makes the degradation process very
complex. The resulting degradation cannot be predicted by simply considering a superposition of the degradation caused by each acid separately.
Larreur-Cayol et al. (2011) studied the effects of three different organic
acids typically present in agro-industrial wastewaters, namely oxalic, citric,
and tartaric acids, and made a comparison with the effect of acetic acid.
The specimens immersed in citric acid showed large amounts of white salts
on their surfaces, which, however, could be removed very easily. A significant and rapid erosion of the outer part was also noticed. In the case of acetic acid, there was no clear salt precipitation, which is in accordance with
the good solubility of calcium acetate. Some cracking was observed, probably caused by shrinkage due to the loss of calcium. Samples immersed in
tartaric acid did not show visible degradation during the first month; but,
after one month, a layer of very loose yellowish salt precipitated on the surface. After two months of exposure, the outer layer of the samples gradually dissolved. Very little attack was noticed on the specimens immersed in
oxalic acid. This comparative study clearly showed that the most important
parameters leading toward aggressiveness of acids is the solubility of their
accelerated by a higher temperature in hot climates. Due attention should be
paid to the local exposure conditions when dealing with marine structures.
5.3.3 acid attack
As concrete is a calcium-rich alkaline environment, it will react with
nearly all possible acids. The solubility of the reaction products will be
of major importance for the resulting degradation level of the concrete,
especially the solubility of the calcium salt, which is typically the first salt
to be formed in case of an acid attack. This also explains why the rate of
acid attack quite often more significantly depends on the level of water
movement near the concrete surface, rather than on the type of cement or
aggregate. Nevertheless, the latter parameters surely have an influence (De
Belie 1997). It is often considered that cement types with a lower calcium
content will be more resistant to acid attack. Although this often seems
the case, it cannot be fully generalized to all cases. In some cases, a low
Ca(OH) 2 content in the matrix can lead to a lower buffer capacity and a
faster degradation of the C-S-H phase. A similar reasoning can be followed
concerning the aggregates.
The main effect of the acid is the dissolution of the cement matrix, leading to a degradation of the concrete. In contrast with typical cases of sulfate
attack, the degradation process in the case of acid attack is not linked to
any significant expansion reaction because ettringite and thaumasite are
not stable in the acid environment. The level of degradation largely depends
on the type of acid to which the concrete is exposed. Furthermore, it is
important to mention that in real exposure cases wastewaters can contain
a multitude of different acids, which makes the degradation process very
complex. The resulting degradation cannot be predicted by simply considering a superposition of the degradation caused by each acid separately.
Larreur-Cayol et al. (2011) studied the effects of three different organic
acids typically present in agro-industrial wastewaters, namely oxalic, citric,
and tartaric acids, and made a comparison with the effect of acetic acid.
The specimens immersed in citric acid showed large amounts of white salts
on their surfaces, which, however, could be removed very easily. A significant and rapid erosion of the outer part was also noticed. In the case of acetic acid, there was no clear salt precipitation, which is in accordance with
the good solubility of calcium acetate. Some cracking was observed, probably caused by shrinkage due to the loss of calcium. Samples immersed in
tartaric acid did not show visible degradation during the first month; but,
after one month, a layer of very loose yellowish salt precipitated on the surface. After two months of exposure, the outer layer of the samples gradually dissolved. Very little attack was noticed on the specimens immersed in
oxalic acid. This comparative study clearly showed that the most important
parameters leading toward aggressiveness of acids is the solubility of their