Calcined Clays for Low-Carbon Construction: Effects of Production Technology on Carbon Footprint

Abstract

Calcined clays are increasingly recognized as strategic supplementary cementitious materials for reducing clinker consumption and the environmental impacts of construction. However, life cycle assessments typically represent metakaolin production using a single carbon footprint value, despite substantial differences in calcination technology, energy supply, and feedstock characteristics. This study develops a parameterized cradle-to-gate carbon inventory for metakaolin production and evaluates how this variability affects the environmental assessment of low-carbon construction materials. The methodology combines process-based theoretical modeling, mass and energy balances, literature-derived industrial data, and life cycle assessment. A full-factorial scenario analysis was performed for three calcination technologies: rotary kiln, fluidized bed, and flash calcination, while systematically varying fuel source, electricity mix, kaolinite purity, feedstock moisture, and transport distance. The resulting inventories were subsequently propagated into representative LC3 cement and metakaolin-based geopolymer formulations to quantify their influence at the construction-material level. Across 648 production scenarios, the carbon footprint of metakaolin ranged from 34 to 578 kg CO2e t−1, demonstrating that production conditions define the environmental performance. Fuel selection was identified as the principal emission driver, while moisture content and feedstock purity produced secondary effects. The variability propagated to downstream products, resulting in carbon footprints of 363–527 kg CO2e t−1 for LC3 cement and 167–357 kg CO2e m−3 for metakaolin-based geopolymers despite identical material compositions. For the building sector, the proposed framework enables designers, material producers, and LCA practitioners to select calcined clay production routes consistent with the carbon targets of concrete, mortar, masonry, precast elements, and other cement-based building applications. It therefore provides a practical basis for incorporating LC3 and geopolymer technologies into lower-carbon building projects while avoiding environmental benefits based on non-representative upstream assumptions.