SCAS-monolith: II. Microstructure and mineralogical composition

We present a microscopic investigation using scanning electron microscopy and energy-dispersive X-ray microanalysis (SEM/EDX) of SCAS monolith manufactured from fly ash (a large-tonnage waste of combustion of coal dust at power plants), lime, and sand in a three-step process comprising filtration combustion under superadiabatic heating, fine milling, and pressing. The microstructure and mineral composition of the monolith are described after seven years of exposure in natural (laboratory) settings. The major crystalline phases are melilite minerals (including gehlenite) and dicalcium silicate. Inclusions are represented by dioctahedral calcium mica and xonotlite, as well as chromferide Fe3Cr0.4, magnesioferrite MgFe23+O4, and greigite-violarite minerals (Ni, Fe2+, Cu)Fe23+S4. Three types of glasses have been identified: cristobalite, calcium aluminosilicoferrite, and calcium aluminosilicate. The binder is a calcium hydrosilicate gel of variable composition (1.20–1.31)CaO · SiO2 · (0.68–2.04) · H2O. Sulfur is found in small amounts in the sulfate form. No direct evidence of formation of portlandite (Ca(OH)2) or carbonization products in the form of CaCO3 polymorphs is obtained.