Severe sulfate contamination in a crushed concrete base had resulted in considerable expansion of the base and total decomposition of the matrix, attributed to the formation of copious amounts of calcium sulfoaluminate hydrate and calcium silicosulfate hydrate minerals, ettringite and thaumasite, respectively. Incorporation of low volumes ranging from 5 to 13 percent (by mass) sulfate-resistant Type V cement was tested as a means of arresting the expansion and thus stabilizing the base. Class C fly ash was also introduced into the mixtures to dry the material to an acceptable moisture content for reworking and compaction. The presence of large amounts of calcium sulfoaluminate minerals promoted high levels of moisture retention within the base. These cementitious material-treated mixtures were subjected to different environmental conditions for 18 months, during which time their performance was evaluated. Their microstructural stability, evaluated by optical microscopy, scanning electron microscopy/energy dispersive X-ray analysis, X-ray diffraction, and differential thermal analysis/thermogravimetric analysis, is discussed.