Clays for Low-Carbon Cements: Overview, Progress, and Challenges

The replacement of Portland clinker with supplementary cementitious materials is a key approach to reducing the embodied carbon content of concretes. In this context, a widely studied family is the "limestone calcined clay cements, LC³." Within this eco-friendly family of materials, one composition is gaining popularity, LC³-50, a blend of ~50% of Portland clinker, 30% of activated clay, 15% of limestone and 5% of gypsum. This interest is due to a ~40% reduction of CO₂ emissions compared to Portland clinker, together with high compressive strengths after 7 days and very good durability against chloride and sulfate attacks. However, limitations still exist, such as low strengths at 1 day, workability loss during the first 2 h and reduced carbonation resistance. These drawbacks are being overcome with tailored admixtures and curing approaches. Here, after introducing low-carbon cements, pozzolans, and pozzolanic reactions, as well as phyllosilicate minerals, attention is given to recent progress in thermal and mechanical, aka mechanochemical, activations. Then, general correlations are established to assist in predicting compressive strength. This work concludes by highlighting the challenges that must be overcome for the widespread adoption of these classic rocks processed to yield advanced materials with the highest possible pozzolanic reactivity.