Figure 8. SEM micrographs of cyclone fly ash (a) particles of irregular size and spherical morphology; (b) hollow sphere (plerosphere) containing smaller hollow and empty spheres (cenospheres)

From

Utilization of Water Treatment Plant Sludge and Coal Fly Ash in Brick Manufacturing

Denise Alves Fungaro, Mauro Valério da Silva

American Journal of Environmental Protection. 2014, 2(5), 83-88 doi:10.12691/env-2-5-2