Western Research Institute, in cooperation with the US Department of Energy Federal Energy Technology Center, initiated a multi-year program to examine the relationship between CFBC ash chemistry and geotechnical properties as they relate to ash disposal and utilization. Four CFBC facilities supplied ash from their units for the study representing high-sulfur (4%) and medium-sulfur (1.8%) bituminous coal. Sub-bituminous coal (0.9% sulfur) and petroleum coke (5--6% sulfur) fired ashes were also included in the study. The ashes were composed principally of large quantities of anhydrite (CaSO{sub 4}) and lime (CaO) and minor amounts of calcite (CaCO{sub 3}). The ash curing study addressed the impact of curing conditions (sealed and saturated curing and 23 C and 5 C curing temperature) on the geochemical and geotechnical properties of the ash. The strength development and expansion varied with the type and characteristics of the ashes. The expansion appeared to be inversely related to strength development. As the strength decreased under saturated curing, the expansion increased significantly. The application of 5 C saturated curing resulted in further strength loss and increased expansion. The hydration reaction products appeared to be principally the hydration of lime (CaO) to portlandite (Ca[OH]{sub 2}), the hydration of anhydrite (CaSO{sub 4}) to gypsum (CaSO{sub 4} {center{underscore}dot} 2H{sub 2}O), and the precipitation of ettringite (Ca{sub 6}Al{sub 2}[SO{sub 4}]{sub 3}[OH]{sub 12} {center{underscore}dot} 26H{sub 2}O) from the soluble calcium, sulfates and alumina. No thaumasite was noted in the specimens. The ashes appeared to follow one of several hydration reaction trends: (1) ettringite-only development, (2) ettringite and/or gypsum early followed by later gypsum formation, or (3) gypsum-only formation. Testing confirmed that the hydration reaction chemistry was related to geotechnical properties of the ashes. Strength development and expansion appeared to be related to ettringite and/or gypsum formation. The expansion increased with saturated curing and appeared to be predominantly gypsum based. A pore filling model was found to be consistent with the observed relationships between hydration phases (ettringite and gypsum) and strength development and expansion, as well as SEM observations and void reduction observations.