Studies of the stability of β-Ca₂SiO₄ doped by minor ions

The bulk of the cement industry's environmental burden is from the calcareous source. Calcium is mostly available naturally as limestone (CaCO₃), where almost half of the mass is eventually released as CO₂ during clinker manufacture. Iron (Fe) is the fourth most common element in the Earth's crust surpassed only by oxygen, silicon, and aluminium; therefore, potential raw materials for alternative cements can contain significant amounts of iron. This review paper discusses in detail the most abundantly available Fe-rich natural resources and industrial by-products and residues, establishing symbiotic supply chains from various sectors. The discussion then focusses on the impact of high iron content in clinker and on ferrite (thermo)chemistry, as well as the importance of iron speciation on its involvement in the reactions as supplementary cementitious material or alkali-activated materials, and the technical quality that can be achieved from sustainable Fe-rich cements.

Slags are an indispensable tool for the pyrometallurgical industry to extract and purify metals at competitive prices. Large volumes are produced annually, leading to important economical and ecological issues regarding their afterlife. To maximise the recycling potential, slag processing has become an integral part of the valorisation chain. However, processing is often directed solely towards the cooled slag. In this article, the authors present an overview of the scientific studies dedicated to the hot stage of slag processing, i.e. from the moment of slag/metal separation to complete cooling at the slag yard. Using in-depth case studies on C₂S driven slag disintegration and chromium leaching, it is shown that the functional properties of the cooled slag can be significantly enhanced by small or large scale additions to the high temperature slag and/or variations in the cooling path, even without interfering with the metallurgical process. The technology to implement such hot stage processing steps in an industrial environment is currently available. No innovative technological solutions are required. Rather, advances in hot stage slag processing seem to rely primarily on further unravelling the relationships between process, structure and properties. This knowledge is required to identify the critical process parameters for quality control. Moreover, it could even allow to consciously alter slag compositions and cooling paths to tailor the slag to a certain application.