Portland cements (PCs) and cement blends are multiphase materials of different fineness, and quantitatively analysing their hydration pathways is very challenging. The dissolution (hydration) of the initial crystalline and amorphous phases must be determined, as well as the formation of labile (such as ettringite), reactive (such as portlandite) and amorphous (such as calcium silicate hydrate gel) components. The microstructural changes with hydration time must also be mapped out. To address this robustly and accurately, an innovative approach is being developed based on in situ measurements of pastes without any sample conditioning. Data are sequentially acquired by Mo K α ₁ laboratory X-ray powder diffraction (LXRPD) and microtomography (µCT), where the same volume is scanned with time to reduce variability. Wide capillaries (2 mm in diameter) are key to avoid artefacts, e.g. self-desiccation, and to have excellent particle averaging. This methodology is tested in three cement paste samples: (i) a commercial PC 52.5 R, (ii) a blend of 80 wt% of this PC and 20 wt% quartz, to simulate an addition of supplementary cementitious materials, and (iii) a blend of 80 wt% PC and 20 wt% limestone, to simulate a limestone Portland cement. LXRPD data are acquired at 3 h and 1, 3, 7 and 28 days, and µCT data are collected at 12 h and 1, 3, 7 and 28 days. Later age data can also be easily acquired. In this methodology, the amounts of the crystalline phases are directly obtained from Rietveld analysis and the amorphous phase contents are obtained from mass-balance calculations. From the µCT study, and within the attained spatial resolution, three components (porosity, hydrated products and unhydrated cement particles) are determined. The analyses quantitatively demonstrate the filler effect of quartz and limestone in the hydration of alite and the calcium aluminate phases. Further hydration details are discussed.

Understanding the factors that control the early age hydration kinetics of belite calcium sulfoaluminate (BCSA) cements is important for their use. Here, two industrial BCSA cements with different setting behaviour, BCSA-F and BCSA-S, were investigated to clarify the roles of soluble sulfates and citric acid during early-age hydration. Isothermal calorimetry, in-situ synchrotron X-ray powder diffraction, pore solution analysis and thermodynamic modelling with GEMS were used.,The two reference cements showed different hydration kinetics, mainly associated with the availability of soluble sulfates. In particular, the bassanite content affected the formation and subsequent dissolution of secondary gypsum. Citric acid strongly delayed ye’elimite dissolution, although the overall sequence of hydration products remained unchanged. Dissolved organic carbon measurements showed that citrate was rapidly removed from the pore solution. This behaviour is consistent with citrate adsorption on reactive surface sites rather than extensive complexation in the pore solution. Thermodynamic modelling reproduced the general phase evolution and indicated that the retardation was mainly kinetic, delaying the approach to equilibrium without changing the predicted hydration pathway.,The results indicate that the early hydration of these BCSA cements depends on both soluble sulfate availability and the interaction of citrate with reactive ye’elimite surfaces.

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.