Removal of CO2 already in the Earth's atmosphere through CO2 mineralisation with alkaline waste materials such as steel slag is one approach to mitigate the effects of anthropogenically-induced climate change. However, the microstructures produced during passive carbonation of slag are not well known. Here we use Scanning Electron Microscopy imaging and chemical mapping, X-Ray diffraction and stable isotopes (δ13C and δ18O) to show that ingassed and hydroxylated atmospheric CO2 reacts with Ca leached from slag to precipitate calcite directly on the slag surface. Precipitated calcite crystal morphologies vary, ranging from bladed and acicular crystals to layered deposits of micron-scale equant crystals. The variable morphology and extent of calcite precipitation documented is linked to a combination of internal (i.e. microstructural properties of the slag itself) and external (environmental conditions) factors. This work shows that atmospheric CO2 can be drawn down and mineralised passively by the slag at ambient conditions as part of the slag valorisation and reutilisation process.