Recycling of hydrated Portland cement paste into new clinker

The recycling of concrete is essential because it reduces the environmental impact of concrete construction and demolition waste. For recycling, concrete waste is crushed and the coarse fraction of crushed material can be recycled as a coarse aggregate. Concrete waste fines, which are a mix of fine aggregates, coarse aggregate debris, and the hydrated cement paste, are currently not a part of the recycling process. Since hydrated cement paste has all the necessary chemical elements for clinker production but without heavy carbon dioxide emissions associated with traditional clinker raw material, it would be beneficial to recycle concrete fines for the production of clinker. However, the data available in the literature about the transformations of hydrated cement paste upon heating are contradictory. It is not clear whether hydrated cement paste can be converted back to clinker. In this research, the phase transformations in hydrated cement paste upon heating in the temperature range from 600 to 1450 °C were investigated using thermal analysis and X-ray diffractometry. The results show that hydrated Portland cement paste can be completely recovered into a new clinker using a thermal treatment. The main phase transformations during heating are described and compared to the literature data. The results indicate a great potential for the complete recycling of concrete fines for the production of cement with a low carbon footprint.