Thermoelastic properties of synthetic single crystal portlandite Ca(OH)₂ - Temperature-dependent thermal diffusivity with derived thermal conductivity and elastic constants at ambient conditions

Highlights

• Synthetic cm-sized portlandite single crystals for thermal diffusion experiments

• Practically identical elastic properties for natural and synthetic portlandites

• High temperature dependence of thermal diffusivity and thermal conductivity

• Strong anisotropy of elastic and thermal transport properties

• Dehydration affects thermal transport depending on sample size and heating rate.

Abstract

Synthetic portlandite single crystals were used to measure thermal diffusivity and elastic constants. The full tensor of elastic constants c_ijkl is derived by Brillouin spectroscopy at ambient conditions. The resultant aggregate bulk and shear moduli are K_{S, VRH}= 32.2(3) GPa and G_VRH= 21.2(2) GPa, respectively. The thermal diffusivity D was measured from −100 ^∘C to 700 ^∘C parallel [001] and perpendicular [100] to the crystallographic c-axis using laser flash method. The dehydration of the crystals influences the thermal diffusivity determination depending on sample size, orientation and heating rate. Thermal diffusivity and the derived thermal conductivity show a pronounced anisotropy with a maximum perpendicular to the c-axis, i.e. in the plane of the [CaO₆] octahedral layers. In the same direction the highest sound velocities (v_P and v_mean) and longest mean free path length of phonons are determined. The thermal diffusivity as well as the derived thermal conductivity show a distinct temperature dependence.

Introduction

Thermoelastic properties such as thermal diffusivity, thermal conductivity, elastic constants as well as dynamic elastic response (e.g. sound velocities) are a prerequisite to better understand and predict the behavior of composite materials [1]. The knowledge of thermoelastic properties of portlandite Ca(OH)₂ as one of the major phases (ca. 20 wt% [2,3]) in hydrated portland cement based composites is of fundamental importance, affecting the properties of buildings to a great extent [4]. Furthermore, Ca(OH)₂ is used as a reaction medium, especially in the context of numerous process variants for flue gas cleaning in combustion technology [5]. With regard to the further development of these technologies, many studies have addressed the reactivity of portlandite both with SO₂ (e.g. [6,7]) and with CO₂ (e.g. [8]). Reaction mechanisms and influencing factors are important in view of the variety of technical implementations, which also cover a wide range of temperature conditions [[9], [10], [11]]. It is common practice to assess the reaction kinetics and take into account mineralogical/chemical changes. However, thermal properties resulting from the crystal physics of the relevant compounds, e.g. Ca(OH)₂, have not yet been integrated into such considerations. Beyond that, in the topical field of thermochemical energy storage the reversible dehydration of portlandite is considered as a promising reaction and could play a leading role providing large storage capacities for intermittent renewable energy production in the future [12]. For ongoing numerical and experimental research a detailed knowledge about the thermal transport properties of the reactants is crucial and still lacking particularly for portlandite [13].

Portlandite Ca(OH)₂ is a trigonal hydroxide crystallizing in space group $P\overline{3}m1$ and is isostructural to brucite Mg(OH)₂. The structure is characterized by nearly close-packed oxides forming distorted edge-sharing [CaO₆] octahedral layers perpendicular to the 3-fold crystallographic c-axis [[14], [15], [16], [17]] (Fig. 1). H is pointing up- and downwards into the interlayer space, expanding the structure in [001] direction. H is displaced from the trigonal symmetry axis and disordered around the 3-fold rotation axis with maximum probability density along the c-axis [16,18]. Whereas ionic bonding is dominant within the [CaO₆] layers, hydrogen bonds connect the opposing octahedral layers [15,16]. As a results, portlandite shows a perfect cleavage along (001).

Apart from theoretical calculations by Laugesen [20] and Ulian and Valdrè [21], measured full sets of elastic constants c_ijkl of portlandite have been reported only by Holuj et al. [22] and Speziale et al. [4]. In contrast, thermal transport properties such as the thermal diffusivity and thermal conductivity seem to be still lacking especially as a function of temperature and in particular for single crystal portlandites. This is most likely due to its rare natural occurrence in general and in particular due to missing single crystals in a sufficient size to perform e.g. laser flash measurements.

The aim of this contribution is to enhance our profound knowledge about thermoelastic properties of portlandite with a focus on thermal transport properties. Therefore, large single crystal portlandites were grown by diffusion experiments. These are used to derive the full set of elastic stiffness constants c_ijkl by Brillouin scattering experiments at ambient conditions. The results on the elastic behavior of synthetic Ca(OH)₂ crystals are compared to published experimental data on portlandite [4,22]. Thermal transport properties are collected by means of laser flash measurements to derive the thermal diffusivity D in the temperature range from −100 ^∘C over the dehydration temperature of portlandite up to 700 ^∘C. For the temperature range of portlandite stability from −100 ^∘C to ~400 ^∘C, the full set of thermal diffusivity tensor components D_ij is obtained. Related thereto, the thermal conductivity tensor components κ_ij are derived using tabulated data for isobaric heat capacity and density. Additionally, averaged (Voigt, Reuss, Hill) elastic moduli and thermal diffusivity and thermal conductivity data are also given.