Actions during service 145
• Decomposition of previously formed hydration products
• Formation of gypsum
• Formation of ettringite
• Formation of thaumasite
• Formation of brucite and magnesium silicate hydrate
• Formation of hydrous silica (silica gel)
• Penetration into concrete of sulfate anions and subsequent formation
and repeated recrystallisation of sulfate salts
Most of these processes will be explained in the following sections.
However, it is not the intention of this general textbook to provide advanced
chemical details of concrete damage. Further reference will be made to
literature.
It should be mentioned that in some cases of sulfate attack (e.g. ettringite formation), expansion of the cement paste causes concrete damage; however, in other cases, it concerns a decomposition of hydration
phases. In some cases, it is debated whether a reaction causes expansion,
e.g. in the case of gypsum formation. In cementitious systems, most of the
chemical reactions involving sulfate ions show some chemical shrinkage
(Skalny et al. 2002), meaning that the sum of the absolute volumes of the
reacting products is larger than the absolute volume of the reaction products. Nevertheless, in the case of a reaction showing chemical shrinkage,
an expansion of the cement paste can occur. Several theories have been
developed to explain the resulting expansion. A comprehensive overview
is reported by Skalny et al. (2002). Further details on volumetric relations
and expansions in case of sulfate attack can also be found in Clifton and
Ponnersheim (1994).
5.3.2.1 Chemical sulfate attack
5.3.2.1.1 Mechanism
The mechanism of a chemical sulfate attack depends on the type of ions
that are combined with the sulfate ions. In a natural environment, sulfate
ions are most commonly combined with alkali or calcium ions, although
magnesium ions may also be significantly present in some cases. An overview of the main chemical reactions is given below.
5.3.2.1.1.1 SODIUM SULFATE (Na 2 SO 4 )
When a Portland cement-based system is attacked by a sodium sulfate
solution, a set of chemical reactions takes place as summarized by Skalny
et al. (2002) in Figure 5.28. In a simplified way, it can be summarized that
SO 4
2− ions enter the pore solution and react with Ca 2+ ions provided by
• Decomposition of previously formed hydration products
• Formation of gypsum
• Formation of ettringite
• Formation of thaumasite
• Formation of brucite and magnesium silicate hydrate
• Formation of hydrous silica (silica gel)
• Penetration into concrete of sulfate anions and subsequent formation
and repeated recrystallisation of sulfate salts
Most of these processes will be explained in the following sections.
However, it is not the intention of this general textbook to provide advanced
chemical details of concrete damage. Further reference will be made to
literature.
It should be mentioned that in some cases of sulfate attack (e.g. ettringite formation), expansion of the cement paste causes concrete damage; however, in other cases, it concerns a decomposition of hydration
phases. In some cases, it is debated whether a reaction causes expansion,
e.g. in the case of gypsum formation. In cementitious systems, most of the
chemical reactions involving sulfate ions show some chemical shrinkage
(Skalny et al. 2002), meaning that the sum of the absolute volumes of the
reacting products is larger than the absolute volume of the reaction products. Nevertheless, in the case of a reaction showing chemical shrinkage,
an expansion of the cement paste can occur. Several theories have been
developed to explain the resulting expansion. A comprehensive overview
is reported by Skalny et al. (2002). Further details on volumetric relations
and expansions in case of sulfate attack can also be found in Clifton and
Ponnersheim (1994).
5.3.2.1 Chemical sulfate attack
5.3.2.1.1 Mechanism
The mechanism of a chemical sulfate attack depends on the type of ions
that are combined with the sulfate ions. In a natural environment, sulfate
ions are most commonly combined with alkali or calcium ions, although
magnesium ions may also be significantly present in some cases. An overview of the main chemical reactions is given below.
5.3.2.1.1.1 SODIUM SULFATE (Na 2 SO 4 )
When a Portland cement-based system is attacked by a sodium sulfate
solution, a set of chemical reactions takes place as summarized by Skalny
et al. (2002) in Figure 5.28. In a simplified way, it can be summarized that
SO 4
2− ions enter the pore solution and react with Ca 2+ ions provided by