What Is Sulfate Attack in Concrete?
Sulfate attack is the deterioration of concrete by sulfate ions in soil or groundwater reacting with the cement paste to form expansive compounds — causing cracking, spalling, and strength loss, typically in foundations, piers, and below-grade walls. It is the reason geotechnical reports include sulfate testing and why specifications call for Type V cement or SCMs in sulfate soils. Like ASR, it is prevented at mix design, not repaired afterward.
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Where sulfates come from
Sulfate-bearing soils and groundwater are common in the western US (California, Texas, the Southwest), in marine environments, and near industrial sites. Seawater, wastewater, and some agricultural runoff carry sulfates. The geotechnical report's chemical testing quantifies sulfate exposure class (S0 through S3) — that classification directly sets the concrete requirements.
How the damage progresses
Sulfates penetrate the concrete and react with calcium aluminate hydrates to form ettringite, which expands and cracks the paste from within. The damage starts at the surface in contact with sulfate soil or water and works inward — scaling, cracking parallel to the surface, and eventual disintegration. It is slow (years to decades) but relentless once started, and it accelerates with wetting-drying cycles.
Prevention in specifications
ACI 318 Table 19.3.1.1 sets maximum water-cement ratios and minimum strengths by sulfate exposure class, and requires Type V cement or equivalent SCM blends (slag, Class F fly ash) for severe exposures. Below-grade waterproofing that keeps sulfate water off the concrete is complementary protection. The structural drawings should note the exposure class and required cement type — this is a specification item plan checkers verify.
The bottom line
Sulfate attack expands and destroys concrete in sulfate soils — common in the West. Test soils, specify Type V cement or SCMs per ACI 318 exposure class, and waterproof below-grade concrete.
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