Actions during service 147
As the attacking sulfate solution also provides the Ca 2+ ions, no decalcification of C-S-H and no associated strength loss occur in this case. Initially,
the concrete strength will even increase due to the formation of ettringite
filling the pores. However, upon further reaction and ettringite formation,
expansive stresses will occur, causing cracking of the cement matrix and
thus reducing the concrete strength at this stage.
5.3.2.1.1.3 MAGNESIUM SULFATE (MgSO 4 )
The main reaction process when a magnesium sulfate solution attacks a
Portland cement-based system is the formation of brucite (Mg(OH) 2 ) and
gypsum.
Mg 2+ + SO 4
2− + Ca(OH) 2 + 2H 2 O → Mg(OH) 2 + CaSO 4 ∙2H 2 O (5.7)
As Portlandite is consumed in this reaction, the C-S-H phase will gradually
decompose, forming an amorphous hydrous silica (SiO 2 ∙aq) and/or a magnesium silicate hydrate phase which is poorly crystalline (3MgO∙2SiO 2 ∙2H 2 O).
The decomposition of C-S-H proceeding simultaneously with the formation of brucite and gypsum leads to a quite fast degradation of the concrete,
which, in turn, leads to material softening and considerable strength reduction. This process also occurs in Portland cement-based systems with a
low C 3 A content, making magnesium sulfate more aggressive than sodium
sulfate.
When C 3 A is present, some ettringite formation can occur when exposed
to magnesium sulfate solutions. However, this ettringite formation will
only occur in more inward zones where the pH remains high enough. Since
the amount of ettringite in this case is rather low, the concrete will be disintegrated due to C-S-H decomposition before significant swelling pressure
can be formed.
Figure 5.29 provides an overview of the chemical reaction processes in
the case of exposure to magnesium sulfate solutions, as summarized by
Skalny et al. (2002).
5.3.2.1.1.4 THAUMASITE FORM OF SULFATE ATTACK (TSA)
In due presence of carbonate ions, thaumasite (3CaO∙SiO 2 ∙CO 3 ∙SO 3 ∙15H 2 O)
can be formed directly from C-S-H in Portland cement-based systems, provided that the temperature is low enough (below 15°C) and that the pH
levels are high enough (above 10.5) (Bassuoni and Nehdi 2009).
3Ca 2+ + SiO 3
2− + CO 3
2− + SO 4
2− + 15H 2 O →
3CaO∙SiO 2 ∙CO 2 ∙SO 3 ∙15H 2 O
(5.8)
As the attacking sulfate solution also provides the Ca 2+ ions, no decalcification of C-S-H and no associated strength loss occur in this case. Initially,
the concrete strength will even increase due to the formation of ettringite
filling the pores. However, upon further reaction and ettringite formation,
expansive stresses will occur, causing cracking of the cement matrix and
thus reducing the concrete strength at this stage.
5.3.2.1.1.3 MAGNESIUM SULFATE (MgSO 4 )
The main reaction process when a magnesium sulfate solution attacks a
Portland cement-based system is the formation of brucite (Mg(OH) 2 ) and
gypsum.
Mg 2+ + SO 4
2− + Ca(OH) 2 + 2H 2 O → Mg(OH) 2 + CaSO 4 ∙2H 2 O (5.7)
As Portlandite is consumed in this reaction, the C-S-H phase will gradually
decompose, forming an amorphous hydrous silica (SiO 2 ∙aq) and/or a magnesium silicate hydrate phase which is poorly crystalline (3MgO∙2SiO 2 ∙2H 2 O).
The decomposition of C-S-H proceeding simultaneously with the formation of brucite and gypsum leads to a quite fast degradation of the concrete,
which, in turn, leads to material softening and considerable strength reduction. This process also occurs in Portland cement-based systems with a
low C 3 A content, making magnesium sulfate more aggressive than sodium
sulfate.
When C 3 A is present, some ettringite formation can occur when exposed
to magnesium sulfate solutions. However, this ettringite formation will
only occur in more inward zones where the pH remains high enough. Since
the amount of ettringite in this case is rather low, the concrete will be disintegrated due to C-S-H decomposition before significant swelling pressure
can be formed.
Figure 5.29 provides an overview of the chemical reaction processes in
the case of exposure to magnesium sulfate solutions, as summarized by
Skalny et al. (2002).
5.3.2.1.1.4 THAUMASITE FORM OF SULFATE ATTACK (TSA)
In due presence of carbonate ions, thaumasite (3CaO∙SiO 2 ∙CO 3 ∙SO 3 ∙15H 2 O)
can be formed directly from C-S-H in Portland cement-based systems, provided that the temperature is low enough (below 15°C) and that the pH
levels are high enough (above 10.5) (Bassuoni and Nehdi 2009).
3Ca 2+ + SiO 3
2− + CO 3
2− + SO 4
2− + 15H 2 O →
3CaO∙SiO 2 ∙CO 2 ∙SO 3 ∙15H 2 O
(5.8)