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 suitability of the rapid, relevant, and reliable (${\mathrm{R}}^3$ ) heat of hydration and bound water tests for measuring the reactivity of various supplementary cementitious materials (SCMs) and their blends with calcium hydroxide (CH) was investigated in this study. These tests have been recently standardized in current standards. The test was carried out on pastes of fly ash (FA), slag (S), kaolinitic calcined clay (K), limestone (LS), and their blends. A detailed analysis of the heat of hydration and bound water measurements specified according to the ${\mathrm{R}}^3$ test was carried out. A good correlation was found between the two types of test results and also between the reactivity test results versus strength activity index measured using cement mortar compressive strength. The formation of hydration products in the ${\mathrm{R}}^3$ test paste specimens was also studied using X-ray diffraction (XRD). The hydration products for SCMs formed in the test were similar to those reported in cement systems. It was seen that the interactions in the blended SCMs with CH were also similar to those reported in cement pastes. Further, the microstructure of the specimen was studied using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, and the overall hydration of various SCMs was studied. The influence of the proportions of the SCMs in the blended SCMs and that of fineness were also investigated. The applicability and limitations of this test method for SCMs and their blends were discussed.