The use of calcined clays as supplementary cementitious materials provides the opportunity to significantly reduce the cement industry’s carbon burden; however, use at a global scale requires a deep understanding of the extraction and processing of the clays to be used, which will uncover routes to optimise their reactivity. This will enable increased usage of calcined clays as cement replacements, further improving the sustainability of concretes produced with them. Existing technologies can be adopted to produce calcined clays at an industrial scale in many regions around the world. This paper, produced by RILEM TC 282-CCL on calcined clays as supplementary cementitious materials (working group 2), focuses on the production of calcined clays, presents an overview of clay mining, and assesses the current state of the art in clay calcination technology, covering the most relevant aspects from the clay deposit to the factory gate. The energetics and associated carbon footprint of the calcination process are also discussed, and an outlook on clay calcination is presented, discussing the technological advancements required to fulfil future global demand for this material in sustainable infrastructure development.

The RILEM technical committee 282-CCL: Calcined Clays as Supplementary Cementitious Materials, investigates all the aspects related to calcined clays, from clay exploration and characterization to calcination process, hydration reactions and concrete properties. This white paper focuses on the hydration mechanisms of calcined clay-blended Portland cements, covering both 1:1 and 2:1 calcined clays. The pozzolanic reaction of calcined clay is detailed, and the main reaction products are described. The differences observed depending on the clay type are also discussed, as well as the potential influence of the secondary phases present in calcined clay. The factors controlling and limiting the reaction of calcined clay are investigated, evidencing the role of porosity saturation and refinement of the microstructure. The complete characterisation of the hydration of calcined clay cements is made possible by the determination of the reaction degree of calcined clay. Several methods are compared to estimate the extent of calcined clay reaction. The influence of clinker and limestone mineralogy are also discussed. Finally, guidelines for optimising the mixture design of calcined clay blended cements are provided, with special attention to sulphate adjustment and clinker factor.

The calcination of calcium carbonate (CaCO₃) is a major contributor to carbon dioxide (CO₂) emissions that are changing our climate. Moreover, the calcination process requires high temperatures (∼900 °C). A novel low-temperature process for the decarbonisation of CaCO₃ is tested whereby the CO₂ is directly sequestered/mineralised in sodium carbonate. CaCO₃ is reacted with an aqueous sodium hydroxide solution by mixing under atmospheric temperatures and pressures. The reaction products are calcium hydroxide (hydrated lime; Ca(OH)₂) and sodium carbonate (soda ash; Na₂CO₃). For the first time, the extent of this reaction at ambient conditions is studied along with the NaOH requirements. Conceptual process designs, which include procedures to separate and recover material, as well as energy calculations, are also presented to demonstrate the technical/industrial feasibility of the process. The technology is also successfully tested on industrially sourced limestone chalk, and the silica impurity remains inert throughout the process. This technology will enable industrial symbiosis by combining the high-temperature lime and sodium carbonate manufacturing processes into a single low-temperature process and greatly reduce the chemical (raw material) CO₂ emissions associated with the production of cement and lime.