The Application of Spray Dryer Absorber Ash in Green Cementitious Systems

Abstract

Spray dryer absorber (SDA) ash represents a small but important source of underutilized coal combustion products (CCPs) suitable for commercial concrete applications. SDA ash is a by-product of the flue gas scrubbing used in the coal combustion process to reduce sulfur oxide (SOx) emissions. Currently, this by-product material is not specified for applications in concrete and is mostly landfilled due to elevated sulfite/sulfate levels that pose concerns for potential sulfate attack in concrete applications. The reported research efforts have focused on novel ternary cementitious binder systems combining portland cement, SDA product, and a pozzolan (i.e., Class F or harvested fly ash). The premise for the addition of a pozzolan is to reduce porewater pH through the binding with portlandite resulting in an “additional” calcium silicate hydrate. The process can inhibit the formation of ettringite responsible for loss of durability. Test results demonstrated promise for improving durability, but further research is warranted regarding the solubility and interaction effects between the sulfite and sulfate. The percentage of sulfite in SDA ash is typically much higher, and it has a significantly lower solubility than observed for systems with sulfate. Accordingly, it is hypothesized that the presence of a sulfite in the cement system poses a lower concern for ettringite/thaumasite formation unless it is exposed to mechanisms that would promote the oxidation of sulfite to more soluble sulfate. Ettringite formation is considered a precursor to thaumasite formation that more readily occurs at lower temperatures and higher humidities. Research presented herein further evaluates the mechanisms of carbonation capable of promoting the oxidation of sulfite to sulfate and the formation of ettringite/thaumasite that could lead to a loss of durability over time. Models are developed that can help to explain the behavior of cement systems incorporating SDA ash concerning hydration, chemical stability, and carbonation.