Sulfate attack in concrete and mortar can be 'external' or 'internal'. External: due to penetration of sulfates in solution, in groundwater for example, into the concrete from outside. Internal: due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate, for example.
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external attack
This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. . Sulfate ions that penetrate the concrete react with CH to form gypsum ŜH+CH→CSH2 (Cement chemist notation)
C3A + 3CŜH2 + 26H→C3A.3CŜ.H32
tricalcium aluminate + gypsum → ettringite
when concentration of sulfate ions decrease the ettringite breaks down into monosulfates
2C3A + C3A.3CŜ.H32 → 3C3A.3CŜ.H12
internal
This form occurs when the source of sulfates is excess gypsum . The gypsum present in the concrete it reacts with monosulfates to form ettringite
C3A.3CŜ.H12 + 2CSH2 + 16H → C3A.3CŜ.H32
A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include:
The effect of these changes is an overall loss of concrete strength.
The above effects are typical of attack by solutions of sodium sulfate or potassium sulfate. Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.