Testing Framework for Analysis of Time-Dependent Behavior of Coal Combustion Products

Abstract

Coal combustion products (CCPs) are by-products created when coal is burned for energy production. These products include predominantly fly ash, bottom ash, and flue gas desulfurization (FGD) by-products. These materials can be a viable alternative to natural resources for the construction of engineered fills. Because of the variability in the chemical composition, CCPs require mineralogical, chemical, and mechanical characterization to ensure its applicability as a construction material. Furthermore, CCP characterization should also address the material’s changing properties with time. To date, only limited standards have been proposed to characterize these materials to determine the viability of their use as structural fill. The purpose of this study is to apply a minimal set of practical mechanical and chemical tests that will characterize and predict CCP time-based performance intended for large-volume civil engineering applications such as embankments and mine reclamation. Unconfined compressive strength tests, hydraulic conductivity measurements, and X-ray diffraction (XRD) analysis were performed at curing times from 1 to 180 days on three materials: fluidized bed combustion (FBC) ash, flue gas desulfurization (FGD) ash, and pulverized coal (PC) class F fly ash. Results show that the unconfined compressive strength varied between each of the materials and over time for both the FBC and PC fly ashes, whereas the strength of the FGD ash did not appear to change over time. The FBC ash experienced a significant strength gain with increased curing time. The XRD characterization shows that the strength gain in the FBC ash is likely caused by the formation of ettringite. The FBC ash also showed significant changes in hydraulic conductivity with curing time.