Formation of an anthropogenic rock: mechanisms and microstructures of lithification of a legacy deposit of cementitious material

Billions of tonnes of anthropogenic geomaterials, such as slags, ashes and cementitious materials, are deposited on the Earth's surface globally every year. These materials have the potential to become rock but little is known about how they may become lithified. In this study, we document lithification of a legacy deposit of cementitious material at a former cement works in Scotland, and uncover the mechanisms and drivers for its conversion from a loose granular sediment to an anthropogenic rock. Optical and electron microscopy and chemical mapping, coupled with X-ray diffraction, indicate the presence of particles of waste cementitious material interpreted to be calcium–silicate–hydrate (C-S-H) formed through hydration of cement clinker; these particles are rimmed by a secondary calcite mineral cement which binds the C-S-H particles together. Partial dissolution of C-S-H followed by diffusion of Ca and OH− into porewaters promoted ingassing and hydroxylation of atmospheric CO2 (fingerprinted by carbon isotope analysis). The CO2 reacted with the dissolved Ca to precipitate the calcite cement rimming the C-S-H grains. This calcite mineral cement has therefore bonded the grains to create an anthropogenic rock. This process can potentially create benefits in terms of ground stabilization and atmospheric CO2 sequestration.