Sulfate Attack - External and Internal Causes

The majority of studies done on sulfate attack have concentrated on its external effect, but the use of gypsum contaminated aggregates or by-product gypsum as concrete constituents demonstrates the importance of investigating its internal effect as well. Substantial differences have been found between the two types of sulfate attack. Both sulfate attacks result in the formation of ettringite, but the source of the sulfate ions and the time of their availability to reaction is where they differ.

In the case of the external attack, the sulfate ions must penetrate into the concrete to initiate the reaction. Any expansion caused by the penetration of these sulfates can cause cracking, which accelerates the penetration of more sulfate ions. The rate of the external attack is a function of the time of exposure. On the other hand, in the case of the internal attack, the sulfate ions exist inside the concrete from the time of mixing. The amount of sulfate ions decreases as the chemical reaction proceeds, and therefore the rate of internal attack tends to slow down over time.