Continuous Determination of the Early-Age Hydration Kinetics of Limestone Calcined Clay Cements: The Influence of a C-S-H Seeding Admixture

Limestone calcined clay cements offer a promising route to reducing CO2 emissions in cement production, but their relatively low reactivity and strength at very early ages may limit their use in construction applications requiring rapid demolding or early load-bearing capacities. Understanding hydration during the first 1–2 days is therefore important for practical implementation. Conventional Cu-Kα1 reflection X-ray powder diffraction is prone to bleeding, water condensation, and preferred orientation effects. Here, the “Mix and Measure III” methodology, using Mo-Kα1 radiation in transmission geometry, was extended to continuously investigate the first 48 h of hydration. Three LC3-50 binders with different metakaolin contents were studied, together with the influence of C-S-H nucleation seeding. The results show that gypsum depletion controls the transition from AFt to Hc formation, whereas both the onset and extent of the pozzolanic reaction are primarily governed by the metakaolin content. At 48 h of hydration, the metakaolin degree of reaction reached approximately 10% and 6–7% for the high- and medium-metakaolin LC3 binders, respectively. C-S-H nucleation seeding accelerated gypsum dissolution kinetics and C3A and C4AF hydration without significantly modifying C3S hydration or the kinetics of the pozzolanic reaction. The results provide a robust laboratory-based framework for quantifying phase development during the early hydration of low-carbon cements and for understanding their behavior in construction applications requiring rapid early strength development.