Chapter 07 · Materials & footprintsCircular Economy & Materials
Calcined clay
Definition
Clay that has been heated (calcined) at roughly 700–850 °C to activate its pozzolanic reactivity, enabling it to replace a substantial portion of clinker in cement. Its flagship application is Limestone Calcined Clay Cement (LC3), a blend of calcined clay, limestone and reduced clinker that can cut production-related CO₂ emissions by up to 40% relative to ordinary Portland cement.
References
LC3 composition (limestone + calcined clay); up to 40% CO₂ reduction; abundance of low-grade clays; cost-effectiveness without major plant modification; SDC funding and mainstreaming goal.
Overview
What it means
Clinker production is cement's emissions core (process CO₂ from limestone calcination plus fuel), and scalable substitutes have been the industry's bottleneck: slag and fly ash supplies are limited and shrinking. Calcined clays are abundant worldwide — including low-grade clays unsuitable for other uses — and can be produced with existing plant types without capital-intensive modification, keeping costs competitive.
The LC3 project, led by EPFL with partners and Swiss Agency for Development and Cooperation funding, drove research, standard-setting and pilot production across the Global South.
How it is used
Cement producers in India, Latin America and Africa are commercialising LC3; standards bodies have added calcined-clay cements to national norms; decarbonisation roadmaps rank calcined clay among the most scalable clinker-substitution routes.
Why it matters
Cement is ~7–8% of global CO₂; calcined clay is currently the clinker substitute that can scale to the volumes and geographies where construction demand is growing fastest.