Sulfate attack occurs when sulfates react with compounds in the cement paste such as monosulfate, portlandite, and C-S-H gel. The reaction products may include ettringite, gypsum, and thaumasite. Destabilization of the C-S-H gel may be another consequence of the reaction. External sulfate attack occurs when the sulfates enter the concrete from the surrounding environment. Internal sulfate attack occurs when there is an excess of sulfate in the original mixture. Delayed ettringite formation, or DEF, is a form of internal sulfate attack that can happen with steam-cured and mass concrete when internal concrete temperatures exceed ~ 70oC (158oF). DRP has the expertise and experience to differentiate between different sulfate attack mechanisms. We are able to objectively assess the impact of sulfate attack on the performance of the concrete and help engineers develop rational solutions to remediate problems.
Reflected light photomicrograph of deposits of thenardite near the outer end of a core extracted from a foundation subjected to chemical sulfate attack.
Reflected light photomicrograph of deposits of gypsum (red arrows) in voids and microcracks in concrete subjected to marine water attack. The green arrows indicate empty microcracks.
Secondary electron micrograph of deposits of gypsum observed on the surface of a foundation undergoing sulfate attack.
Cross-polarized transmitted light photomicrograph of microcracks filled with thaumasite (red arrows) and gypsum (green arrows).
Reflected light photomicrograph of polished surface where SEM/EDS analysis of the gelatinous material indicated a composition consistent with ettringite, rather than ASR gel. This is a hallmark of Delayed Ettringite Formation.
Cross-polarized transmitted light photomicrograph of thin section where red and yellow bars indicate zones thenardite and gypsum mineralization, respectively. The green arrow indicates a void filled with gypsum and the yellow arrows indicate gypsum in the ITZ of an aggregate particle. The blue arrow indicates the outer surface of the core.