Martin Dietzel

Understanding the hydration behaviour and distinct formation of MgO- and CaO-based components in aqueous media is crucial for the development of tailored construction and performance materials. The hydration reaction of Mg-/Ca-oxides often leads to heat release and volumetric expansion, promote strength development or can cause unwanted cracking in hardened products due to high crystallization pressures. Carboxylic compounds, like acetic and citric acid, are increasingly applied to control the MgO and CaO hydration reaction, but the underlying mechanisms are still not fully understood. Therefore, we performed a combined approach using thermodynamic modelling of potential MgO/CaO hydration paths coupled with pin-pointed validation experiments, which were evaluated by solution composition and solid phase characterization. The developed models successfully reproduce the observed evolution of the precipitate and the distribution of aqueous species in the reactive solution. Distinct pH-windows for the stability fields of individual Mg/Ca-organo phases, brucite and portlandite are clearly identified. Acetate significantly modulates the crystal morphology of the hydroxides, whereas citrate inhibits their formation by strong Mg/Ca aquo-complexation. The combined modelling and experimental approach help to understand the crystallization pathways in the MgO-CaO-H2O-organo system and provides a thermodynamic framework for exploring the system boundaries for e.g. concrete, refractory, and energy storage product development.

Aragonite abundantly crystallizes in marine and continental environments and occurs ubiquitously in biominerals, despite being metastable under ambient conditions. To resolve this apparent contradiction, we studied the crystal structure of aragonite that grew on calcite of biogenic (Berger Károly cave; Hungary) and inorganic (Sonnenberg; Italy and Erzberg; Austria) origin, in a range of natural settings and temperatures. Electron microscopy measurements show (001) stacking faults (SFs) in the first-grown aragonite. Structural modelling combined with theory-based optimization suggests that the crystal structure of this new type of SF can be explained by the presence of (000l) calcite-like layers within aragonite. Abundant (0001) calcite twins occur in the last-grown calcite, and hexarag, the hypothetical hexagonal polymorph of aragonite, was identified across the calcite twin interfaces. Oxygen isotope data indicate that both the Berger-Károly and Sonnenberg samples formed under non-equilibrium conditions, whereas the Erzberg aragonite formed near equilibrium, possibly explaining the defect-free structure. The presence of (001) SFs and (0001) calcite twins testifies to the formation of transitional calcite-aragonite structures, suggests the occurrence of a hexarag precursor phase and provides new insights into the long-debated mechanism of metastable aragonite formation in dynamically evolving (bio)geologic environments.

This study explores the isotopologues m/z 47 ( ¹³ C ¹⁸ O ¹⁶ O, denoted Δ ₄₇ ) and m/z 48 ( ¹² C ¹⁸ O ₂ , denoted Δ ₄₈ ) in CO ₂ derived from carbonate minerals, focusing on their temperature dependency, kinetic isotope effects, and distinct reaction pathways. By conducting experiments at four temperatures (5°C, 10°C, 15°C, and 25°C) using the enzyme carbonic anhydrase (CA) at pH 8.3, we approached isotopic equilibrium in dissolved inorganic carbon and measured Δ ₄₇ , Δ ₄₈ , and δ ¹⁸ O. Our results were compared with data from Devils Hole cave calcite and existing temperature calibrations, yielding regression equations correlating Δ ₄₇ and Δ ₄₈ with temperature: Δ _{47 I‐ CDES} = (2.43 ± 0.289) Δ _{48 CDES 90} − (0.006 ± 0.074); r ² = 0.96; Δ _{47 I‐CDES} = (0.038 ± 0.003) × 10 ⁶ T ⁻² + (0.161 ± 0.032); r ² = 0.99; Δ _{48 CDES 90} = (0.015 ± 0.002) × 10 ⁶ T ⁻² + (0.076 ± 0.025); r ² = 0.94. Further, calcite precipitated at varying temperatures and pHs of 8.3–11 had kinetic enrichments of Δ ₄₇ and depletions of Δ ₄₈ and δ ¹⁸ O at pH ≥ 9.5, with CA presence leading to distinct kinetic slopes and more efficient Δ ₄₈ catalysis. These findings are consistent with theoretical predictions for kinetic effects from CO ₂ hydration/hydroxylation. Additionally, methodological details for Δ ₄₈ measurements using the Nu Perspective mass spectrometer are provided, including shot noise calculations, baseline corrections, and how nonlinearity evolves over time. We show that these instruments do not have pressure baseline effects on m/z 48, which is a result of secondary electron suppression on the m/z 48 collector. Therefore, calibration data should be unbiased by these analytical effects.

We report the first calorimetric observations of glass transition temperatures and crystallization rates of anhydrous, amorphous calcium-magnesium carbonate using fast scanning differential scanning calorimetry. Hydrous amorphous Ca _0.95 Mg _0.05 CO ₃ · 0.5H ₂ O (ACMC) solid was precipitated from a MgCl ₂ –NaHCO ₃ buffered solution, separated from the supernatant, and freeze-dried. An aliquot of the freeze-dried samples was additionally dried at 250°C for up to 6 h in a furnace and in a high-purity N ₂ atmosphere to produce anhydrous ACMC. The glass transition temperature of the anhydrous Ca _0.95 Mg _0.05 CO ₃ was determined by applying different heating rates (1000–6000 K s ⁻¹ ) and correcting for thermal lag to be 376°C and the relaxational heat capacity was determined to be C p = 0.16 J/(g K). Additionally, the heating rate dependence of the temperature that is associated with the corrected crystallization peaks is used to determine the activation energy of crystallization to be 275 kJ mol ⁻¹ . A high-resolution transmission electron microscopy study on the hydrous and anhydrous samples provided further constraints on their compositional and structural states. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.