Engineering and Construction

There has been considerable research, testing, and chemical analysis of sulfate attack. Two different mechanisms of sulfate attack have been discovered: chemical reactions and the physical growth of crystals.

Chemical Reactions. The chemical reactions involving sulfate attack of concrete are complex and studies have discovered two main chemical reactions (Lea, 1971; Mehta, 1976). The first is a chemical reaction of sulfate and calcium hydroxide (which was generated during the hydration of the cement) to form calcium sulfate, commonly known as gypsum. The second is a chemical reaction of gypsum and hydrated calcium aluminate to form calcium sulfoaluminate, commonly called ettringite (ACI, 1990).

As with many chemical reactions, the final product of ettringite causes an increase in volume of the concrete. Hurst (1968) indicates that the chemical reactions produce a compound of twice the volume of the original tricalcium aluminate compound. Concrete has a low tensile strength and thus the increase in volume fractures the concrete, allowing for more sulfates to penetrate the concrete, resulting in accelerated deterioration.

Physical Growth of Crystals. It has been shown that there can be crystallization of the sulfate salts in the pores of the concrete. The growth of crystals exerts expansive forces within the concrete, causing flaking and spalling of the outer concrete surface. Besides sulfate, the concrete, if porous enough, can be disintegrated by the expansive force exerted by the crystallization of almost any salt in its pores (Tuthill, 1966; Reading, 1975).

Damage due to crystallization of salt is commonly observed in areas where water is migrating through the concrete and then evaporating at the concrete surface. Examples include the surfaces of concrete dams, basement and retaining walls that lack proper waterproofing, and concrete structures that are partially immersed in salt-bearing water (such as seawater) or soils. Figures 12.3 and 12.4 show two examples of concrete patio deterioration due to the crystallization of salts.