Significant heaving has been observed over time at chromite ore processing residue (COPR) deposition sites in Maryland and New Jersey. Confined swell tests were employed in order to investigate the geochemical mechanisms that lead to the manifestation of heave in COPR. Ettringite, a known heave culprit in cement and soil-related literature, was identified in numerous samples across the sites and was therefore considered as the primary heaving mechanism in COPR. In addition, other possible mechanisms, such as brownmillerite hydration to hydrogarnets, carbonation reactions, calcium aluminum chromium oxide hydrate (CAC) formation, and change of hydration state were also investigated. The confined swell tests were conducted under wet/dry cycles. Sulfate, carbonate, and chromate solutions were introduced to the samples during wet cycles in order to validate the different heaving hypotheses associated with phase transformations. The test results showed that swell development occurred only in COPR upon 0.7 mole/I sulfate addition. Furthermore, x-ray diffraction analyses confirmed the formation of ettringite in the COPR sample following the addition of sulfate. Conversely, carbonation reactions led to no height change or even to consolidation, while the formation of hydrogarnets and CACs could not be established. It was, therefore, demonstrated that ettringite formation is a thermodynamically powerful reaction and also a viable expansion and failure mechanism in COPR.