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Clay calcination is an industrial process used to produce a reactive supplementary cementitious material (SCMs), calcined clay – which is a partial clinker replacement in cement. Despite the well-documented reactivity of calcined clays, existing test methods such as R3 test and Strength Activity Index (SAI) are time-intensive and, thus, impractical for rapid process control in industrial settings. This prohibits plant operation to take immediate action to ensure the minimum targeted reactivity is reached at all times, resulting in: a) increased OpEx and CO2 emissions since the potential partial clay substitution in cement ought to be reduced or, in the worst case, rejected and production lost; and/or b) larger storage facilities at high CapEx due to the longer response time to the ensure the material quality is confirmed. To address this challenge, a 5-minute Ultra-Rapid Reactivity (UR2) test was recently proposed. To assess the UR2 feasibility as an at-line control tool, a natural clay was calcined in a pilot-scale flash calciner across temperatures of 825 to 925°C under oxidizing and reducing conditions. Also, the clay was calcined at laboratory scale, extending the temperature range down to 550°C. All samples were assessed by UR2 and SAI; with a subset evaluated using R3. The UR2 showed a strong correlation with R3 and SAI across all calcination temperatures. This trend was sustained when data from laboratory and pilot scale are evaluated together – including samples with maximum particle size ranging from 45 to 90 µm – showcasing the robustness of the UR2 to capture clay reactivity. Overall, our findings support the application of the 5-minute UR2 as a rapid screening method for assessing calcined clay reactivity and its integration into industrial settings – upon controlled sample preparation – for at-line monitoring and evaluation of clay suitability, prior to full-scale calcination trials.