Anisotropic thermal expansion and hydrogen bonding behavior of portlandite: A high-temperature neutron diffraction study
Graphical abstract
With increasing temperature, the Ca(OD)2 structure expands ∼4.5 times larger between the [CaO6] octahedral layers than within the layers. Correspondingly, the D-mediated interatomic interactions become significantly weakened, and the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at elevated temperatures.
Introduction
Portlandite [Ca(OH)2], a member of the layered hydroxide family (space group P3¯m1), is composed of edge-sharing [CaO6] octahedral layers parallel to (001) with hydrogen bonding between the layers (Fig. 1) [1]. Within the interlayer, each H interacts with three OH bonds from the neighboring [CaO6] layer through H⋯O attraction and H⋯H repulsion. These unique structural features make Ca(OH)2, together with other M(OH)2 layered hydroxides, a model system for studying hydrogen-mediated interactions, which are among the most common interatomic interactions in physical and biological systems. M(OH)2 compounds are also interesting from a geological viewpoint, since they are present as component units in the structures of complex hydrous minerals, which are potential hosts for water in the Earth's mantle [2], [3], [4]. Thus studying their structures and stability will provide insights into the mechanisms of water storage in the deep Earth. Moreover, portlandite is of importance in the cement industry because it is one of the major phases in cement paste, a hydration product of Portland cement. Thus the proportion of portlandite in cement paste and its response to changing temperature and/or other conditions during the curing process will ultimately affect the strength and durability of concrete. To better utilize cement/concrete for various applications, an accurate determination of the stability and properties of portlandite such as its thermal expansion coefficients is necessary.
Most structural studies of portlandite have treated hydrogen as occupying a single site, , similar to oxygen, thereby forming OH bonds along the c-axis. In this model, H exhibits very large anisotropic displacement parameters in the (001) plane, which Busing and Levy [1] described as a “riding” motion of H relative to O. Desgranges et al. [5] proposed an alternative, three-site split-atom model to describe the hydrogen positions based on their results from single-crystal neutron diffraction. Specifically, H is displaced from the 3-fold symmetry axis and disorders over three positions with occupancy of about the 3-fold rotation (Fig. 1). More recently, using ab initio molecular simulation, Raugei et al. [6] demonstrated that portlandite adopts the three-site split-hydrogen model at high pressure and, further, the distance between the three sites increases with increasing pressure. This model has also been used to interpret neutron diffraction results for other hydroxides such as brucite, Mg(OH)2 [7], [8], [9], β-Co(OH)2 [10] and β-Ni(OD)2 [11]. However, as most of the structural studies on portlandite used X-ray diffraction, where H is a weak scatterer, they simply treated hydrogen as occupying the single site. Furthermore, despite a number of diffraction studies of portlandite at high temperature and pressure, measurements of its coefficients of thermal expansion (CTEs) are surprisingly scarce [12], [13], [14], [15], and the detailed mechanisms of thermal expansion remain largely unclear. Since changes in cell dimensions with temperature are likely to be coupled with changes in the hydrogen bonding behavior and since neutron scattering is sensitive to the position of hydrogen (and its isotopes), high-temperature neutron diffraction studies of portlandite are particularly useful to unravel its thermal expansion mechanisms.
In this study, we carried out in situ neutron diffraction of portlandite using a pulsed neutron source at temperatures up to 643 K (the sample started to decompose into CaO plus water vapor at 613 K). To avoid the large incoherent scattering of neutrons by hydrogen, we synthesized deuterated portlandite, Ca(OD)2, via hydration of CaO with D2O. Rietveld analysis of the time-of-flight neutron data allowed determination of structural parameters as a function of temperature. In particular, the atomic positions and displacement parameters of D at high temperatures have been obtained, and implications for D motion and D-mediated interactions as well as their effects on thermal expansion are discussed.
Section snippets
Sample synthesis
The portlandite sample used in this study was prepared via hydration of CaO powders with D2O. First, CaO powders were obtained by heat-treating CaCO3 powders (Alfa Aesar, 99.95%) at 1233 K for ∼5 h with the emitted CO2 gas constantly pumped out. Second, the CaO powders were slowly added to boiling D2O in a custom-built, sealed reaction line to minimize possible reaction of CaO with CO2 from air. Third, the resulted sample was dried at ∼373 K under vacuum and stored in a desiccator. The product, a
Results and discussion
Our high-temperature neutron diffraction patterns indicate that the Ca(OD)2 sample was stable from 308 to 583 K. However, it started to decompose into CaO and D2O(g) when the temperature reached 613 K (the molar ratio CaO:Ca(OD)2 obtained from the Rietveld analysis is 1.2:98.8). Thus the onset temperature of the dehydroxylation (Td) lies between 583 and 613 K. As demonstrated in previous studies, the dehydroxylation temperature of portlandite increases with increasing water vapor pressure (Pwater
Conclusions
We have studied the high-temperature structural behavior of Ca(OD)2 using neutron diffraction in conjunction with Rietveld analysis up to its dehydroxylation point. With increasing temperature, both the a and c dimensions expand, but the latter at a rate ∼4.5 times larger. The larger expansion along c is due to the weak D-mediated interatomic interactions within the interlayer and thus the ease of changing its thickness. At a given temperature, the amplitude of thermal vibration of D is much
Acknowledgments
We are grateful to J. William Carey and the anonymous reviewers for helpful comments and Patrick Woodward for handling this paper. This work has benefited from the use of the Lujan Neutron Scattering Center at LANSCE, which is funded by the Department of Energy's Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, under DOE Contract DE-AC52-06NA25396.
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