Sulfate Attack - Mechanism

Sulfate attack has occurred at various locations throughout the world. Some common sulfate environments are soils, groundwater, transport fluids, contained soils or fluids, and seawater. Many concrete structures are exposed to these environments and accumulation of sulfates at an exposed face increases the potential for deterioration.

The mechanism of distress for sulfate attack is an expansive pressure caused by the transformation of monosulfoaluminate to ettringite. This mechanism can be described as a sequence of processes. First, the external sulfate reacts with calcium hydroxide to saturate the pore solution and precipitate gypsum (CaSO₄ · 2H₂O). The increased concentration of SO₄ promotes the transformation of monosulfoaluminate to ettringite (3CaO· Al₂O_3· 3CaSO₄ · 32H₂O). This transformation causes an increase in solid volume, which results in the deterioration of concrete by inducing cracking, softening, and spalling. The expansion mechanism is caused by pressure from ettringite crystal growth or swelling due to absorption of water. Another measurable concrete property identified with the sulfate attack mechanism is a decline in compressive strength.

Different sulfates can effect the concrete differently. Magnesium sulfate is the most severe because of the presence of magnesium ions. These ions can cause additional corrosive reactions through the formation of Mg(OH)₂ and ettringite. This also decomposes the C-S-H.