Melt ceramics from coal ash: Constitutive product design using thermal and flow properties

Abstract

New US Environmental Protection Agency regulations for the disposal of coal combustion residues (CCR) incentivize bottom-up recycling efforts, to convert them into value-added applications. This study examines producing lightweight ceramic aggregates from CCR for concrete/geotechnical applications. More specifically, we argue that industrial residues such as coal bottom ash, despite their heterogeneity and diversity, are apt feedstock materials to constitutively design melt ceramics via high temperature recycling. A lot of knowledge on the feedstock (thermal and melt flow properties) is available, because of the historical interest in (molten) coal ash properties. It is shown how thermodynamics and empirically derived models and experimental observations on the viscosity, surface tension, heat capacity, enthalpy of fusion and thermal conductivity can be used to constitutively design melt ceramics. We created a custom model for the design of spherical porous reactive aggregates (SPoRA) from two different coal bottom ashes, using NaOH as an illustrative fluxing agent. To obtain the desired aggregate design, production should occur above the solidus temperature, yet viscous flow, caused by a low viscosity of the CCR melt, should be limited. The design method developed is able to discern the influence of various design parameters on the experimentally produced ceramic aggregates. A proper match between simulations and experimentally observed object shapes was obtained, allowing to define an operating window (temperature and residence time as function of fluxing agent addition) constitutively. This work shows how the available knowledge on coal ash assists the understanding and design of novel ceramic aggregate recycling processes.