To develop a broader understanding of the engineering properties of fly ash, mixtures of five different soil types (ranging from ML to CH) and several different fly ash sources (including hydrated and conditioned fly ashes from six power plants in Iowa) were evaluated. The evaluation included strength, the influence of curing temperature on strength gain, and the effects of compaction delay time on density and strength gain. Strength and durability testing was conducted using the Iowa State University 2 x 2 apparatus, standard Proctor, and California bearing ratio (CBR) test methods.

The morphology of soil–fly ash mixtures and soil clay mineralogy were studied using x-ray diffraction and scanning electron microscopy techniques. To simulate harsh Iowa field conditions, several specimens were subjected to freeze/ thaw and wet/dry curing environments prior to testing. Some specimens were cured for up to 2.5 years.