Temperature and humidity dependent formation of CaSO4·xH2O (x = 0...2) phases

https://doi.org/10.1016/j.gloplacha.2020.103132Get rights and content

Highlights

  • Phase formation kinetics study of CaSO4·xH2O (0≤x≤2) by in situ temperature and humidity dependent X-ray powder diffraction.

  • Experimental conditions based on climatic conditions in the Atacama Desert

  • Gypsum dehydration and bassanite rehydration proceed with pronounced particle size and temperature dependence.

  • β-anhydrite and bassanite formation is unlikely to occur during diurnal cycles at Atacama Desert climatic environments.

  • β-anhydrite formation by gypsum dehydration only conceivable over very extended time periods in the pure system CaSO4·xH2O.

Abstract

The temperature and humidity dependent formation of different crystalline phases CaSO4·xH2O with 0 ≤ x ≤ 2 was investigated by in situ X-ray powder diffraction using a Bruker D8 diffractometer equipped with an Anton Paar CHC+ cryo−/ humidity chamber. By isothermal measurements over time ranges up to 60 h at temperatures between 76 °C and 140 °C the dehydration of gypsum (CaSO4·2H2O) to bassanite (CaSO4·½H2O) and γ-anhydrite (γ-CaSO4) was studied under low relative humidity of <7% and under consideration of the dependence of the dehydration process on particle size. β-anhydrite (β-CaSO4, mineral anhydrite) was not found as dehydration product up to 140 °C, but only in small amounts as dehydration product of bassanite after 6 months at 80 °C. Rehydration of previously formed bassanite between 30° and 10 °C at relative humidity of 95% (without free H2O in the sample) was investigated analogously to the dehydration study. With quantitative phase analysis by the Rietveld method of the isothermal X-ray powder diffraction measurements fraction of conversion versus time curves were derived and the applicability of kinetic models is discussed. From the results it can be concluded that gypsum dehydration/rehydration under climatic conditions present in the Atacama Desert is unlikely to occur over diurnal cycles, however, very small fractions of gypsum dehydrated to bassanite and subsequently rehydrated to gypsum are conceivable over very extended time scales in the top layer of the soil surface. Faster dehydration and slower rehydration for small particle size (<20 μm) could favor a minor accumulation of gypsum dehydration products. However, the formation of large amounts of β-anhydrite (which is found, besides gypsum, as main sulfate constituent of ‘gypsum crust’ in the Atacama Desert) by gypsum/bassanite dehydration under climatic conditions in the Atacama Desert, is unlikely for the pure CaSO4·xH2O system without gypsum/bassanite nucleation-inhibiting additives.

Introduction

The Atacama Desert in Chile in its central hyperarid core is one of the driest areas on earth and, based on geological and mineralogical evidence it is likely, that hyperarid conditions have lasted for the past 3–4 million years (Hartley and Chong, 2002; Hartley et al., 2005; Dunai et al., 2005). In the hyperarid Atacama soils gypsum (CaSO4·2H2O) is the most dominant mineral (Ericksen, 1983; Chong Diaz, 1984; Ewing et al., 2006) and is believed to play, together with other compounds CaSO4 · xH2O (x = 0...2), an essential role concerning weathering of soil and rocks and soil dynamics (Ewing et al., 2006). In particular, a prominent feature in the Atacama Desert is the presence, formation (and disintegration) of so called ‘gypsum crusts’ (Ewing et al., 2006), which covers large areas with a firm surface. Constituents of ‘gypsum crust’ are, besides gypsum, bassanite (CaSO4 · ½H2O) and β-anhydrite (β-CaSO4) (Wierzchos et al., 2011; Flahaut et al., 2017), the two further calcium sulfate mineral phases. Our own X-ray diffraction phase analyses of surface crust samples corroborate the presence of the calcium sulfate minerals gypsum, bassanite and β-anhydrite, with metastable bassanite being a minor constituent, see Table 1.

Between these occurring mineral phases dehydration / rehydration processes can emerge depending on temperature, humidity (and pressure), which lead to phase transformations accompanied by substantial volume changes, but as well by solidification due to crystal growth processes. Here, the question arises concerning the requirements for these phase transformation processes to occur without the presence of liquid water (as it is given at hyperarid Atacama soil surfaces) but for typical temperatures and relative air humidity present in the Atacama Desert.

Because of the enormous technical importance of materials of the system CaSO4 - H2O this system has been investigated extensively. Disregarding high-pressure phases, at least eight duly structurally defined phases CaSO4 · xH2O, compiled in Table 2, are reported in literature:

Solely as technical products further CaSO4 · xH2O (x = 0…~0.5) phases are known (see, e.g., Gips, 2013). The subhydrates, i.e. phases with a water content per formula unit differing from 0, 0.5 or 2, were exclusively observed under laboratory conditions and are depicted as interim products that can occur during CaSO4 or CaSO4 · ½ H2O rehydration to gypsum (Bezou et al., 1995; Schmidt et al., 2011). The variety of subhydrate phases is regarded to result from the ability of the bassanite-type structure to accommodate various amounts of H2O molecules, dependent on the relative humidity of formation conditions (Schmidt et al., 2011).

Due to their technical and scientific significance processes of transformations (hydration and dehydration) between the above phases have been studied within a broad range of conditions, such as temperature (e.g., McAdie, 1964; Ball and Norwood, 1969; Christensen et al., 1985; Putnis et al., 1990; Badens et al., 1998; Chang et al., 1999; Prasad et al., 2005; Jordan and Astilleros, 2006; Lebron et al., 2009; Ballirano and Melis, 2009), atmospheric pressure (e.g., Molony and Ridge, 1968; McConnell et al., 1987; Carbone et al., 2008; Mirwald, 2008), further added chemical components (e.g., Hardie, 1967; Cody and Hull, 1980; Charola et al., 2007; Singh and Middendorf, 2007; Wilson and Bish, 2011; Ossorio et al., 2014) or partial pressure of H2O (e.g., McAdie, 1964; Ball and Norwood, 1969; Badens et al., 1998; Oetzel et al., 2000; Jordan and Astilleros, 2006; Lebron et al., 2009; Wilson and Bish, 2011; Robertson and Bish, 2013), applying a large number different experimental techniques (for a review see, e.g., Freyer and Voigt, 2003). However, despite of the large number of different studies, the results vary to a large extend, even for studies carried out under approximately similar experimental conditions, and in fact, there is still uncertainty concerning the mechanism of dehydration and hydration, the kinetics and number of steps as well as concerning the type of phases produced during dehydration and hydration. Consequently, emphasis needs to be placed on the parameters controlling hydration and dehydration processes of CaSO4 · xH2O phases, in particular, a simultaneous control of temperature, humidity and sample constitution (particularly grain size, sample thickness, porosity) by in situ measurements. The aim of this work is to investigate dehydration (and hydration) processes of CaSO4·xH2O (x = 0...2) phases without the presence of liquid H2O in the system, using in situ humidity- and temperature-controlled powder X-ray diffraction (powder-XRD). Applied temperature and humidity parameters are inspired by the extremes of climatic conditions present in the Atacama Desert, in order to constrain the stability of CaSO4 · xH2O phases under these conditions. In this study the number of variable parameters is limited to time, temperature, relative air humidity and sample particle size, keeping chemical composition fixed, in order to set a basis of data for the solid state phases of the pure system CaSO4 - H2O, before in the future extending the system towards natural sample compositions. Working with samples of chemical compositions like those found in the Atacama Desert will be subject to upcoming studies. The experimental data derived from the present study can give a contribution to answer the question, whether dehydration and hydration transformations of CaSO4·xH2O phases and their kinetics can occur over diurnal or seasonal cycles under hyperarid conditions of the Atacama Desert and therefore are either negligible or abundant phenomena, which can lead to formation or disintegration of ‘gypsum crusts’.

Section snippets

Materials

Calcium sulfate dihydrate from Emsure Merck (CAS 10101–41–4, ≤0.01% fractions insoluble in HCl, precipitated for analysis), was treated with demineralized water at 45 °C for one day to rehydrate any residual traces of other calcium sulfates to gypsum. The recrystallized material was ground in an agate mortar and sieved into fractions of <50 μm for particle size independent experiments and into fractions of <20 μm, 20–25 μm, 25–32 μm and 32–50 μm for particle size dependent experiments. As

Results and discussion

In powder X-ray diffraction the differentiation between the three mineral phases gypsum, bassanite and β-anhydrite is straightforward, as Fig. 1 demonstrates, due to the rather different crystal structures and powder patterns. While gypsum crystallizes with a monoclinic (space group I2/a) layer structure of alternating Ca + [SO4]2− and H2O layers (e.g., Atoji and Rundle, 1958) the monoclinic (space group I2), pseudo-trigonal structure of bassanite consists of six-membered interlinked distorted

Conclusion

The results of the present study of temperature and humidity dependent formation of CaSO4·xH2O (x = 0...2) phases by dehydration and rehydration processes show, that gypsum dehydration to bassanite (and further on to β-anhydrite during long time range) can proceed already at rather low temperatures of 80 °C and even lower, however, the conversion rate decreases rapidly with lower temperature, leading to very long time for gypsum dehydration to take place. Rehydration of bassanite to gypsum

Declaration of Competing Interest

None.

Acknowledgment

The authors thank Dr. D. Hoffmeister, University of Cologne, SFB 1211, project Z02, for the supply of data concerning soil surface temperatures at the meteorological stations in the Atacama Desert. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 268236062 – SFB 1211 (project C04).

References (73)

  • Bruker

    Diffrac Suite 5.0 (containing Diffrac Eva 3.0 and Diffrac Topas 5.0)

    (2014)
  • PDF-2

    Powder Diffraction FileTM

    (2019)
  • W. Abriel

    Calcium Sulfat Subhydrat, CaSO4.0,8H2O

    Acta Cryst. C

    (1983)
  • W. Abriel et al.

    Bestimmung der Kristallstruktur von CaSO4(H2O)0,5 mit Röntgenbeugungsmethoden und mit Potentialprofil-Rechnungen

    Z. Kristallogr.

    (1993)
  • S.M. Antao

    Crystal-structure analysis of four mineral samples of anhydrite, CaSO4, using synchrotron high-resolution powder X-ray diffraction data

    Powder Diffract.

    (2011)
  • M. Atoji et al.

    Neutron diffraction study of gypsum, CaSO42H2O

    J. Chem. Phys.

    (1958)
  • M. Avrami

    Kinetics of phase change. II Transformation-time relations for random distribution of nuclei

    J. Chem. Phys.

    (1940)
  • M.C. Ball et al.

    Studies of the system calcium sulphate-water. part i kinetics of dehydration of calcium sulphate dihydrate

    J. Chem. Soc. A

    (1969)
  • P. Ballirano et al.

    Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction

    Phys. Chem. Miner.

    (2009)
  • P. Ballirano et al.

    The monoclinic I2 structure of bassanite, calcium sulphate hemihydrate (CaSO4 · 0.5H2O)

    Eur. J. Mineral.

    (2001)
  • C. Bezou et al.

    Compt. Rend. Acad. Sci. Paris, Ser. II

    (1991)
  • J.C.A. Boeyens et al.

    Redetermination of the crystal structure of calcium sulphate dihydrate, CaSO4·2(H2O)

    Z. Kristallogr. NCS

    (2002)
  • N.N. Bushuev

    Water of crystallization in the CaSO4.0.67H2O and CaSO4.0.5H2O structures

    Russ. J. Inorg. Chem.

    (1982)
  • N.N. Bushuev et al.

    X-Ray diffraction investigation of CaSO4.0.67H2O

    Russ. J. Inorg. Chem.

    (1982)
  • N.N. Bushuev et al.

    Phase transformations in the dehydration of CaSO4.2H2O

    Russ. J. Inorg. Chem.

    (1983)
  • M. Carbone et al.

    Kinetics of gypsum dehydration at reduced pressure: an energy dispersive X-ray diffration study

    Eur. J. Mineral.

    (2008)
  • A.E. Charola et al.

    Gypsum: a review of its role in the deterioration of building materials

    Environ. Geol.

    (2007)
  • G.C.H. Cheng et al.

    The crystal structure of anhydrite (CaSO4)

    Acta Cryst

    (1963)
  • G. Chong Diaz

    Die Salare in Nordchile

    Geotecton. Res.

    (1984)
  • A.N. Christensen et al.

    A time-resolved neutron powder diffraction investigation of the hydration of CaSO4.½D2O and of the dehydration of CaSO4.2D2O

    J. Appl. Crystallogr.

    (1985)
  • A.N. Christensen et al.

    Formation and transformation of five different phases in the CaSO4-H2O system: Crystal structure of the subhydrate β-CaSO4·0.5H2O and soluble anhydrite CaSO4

    Chem. Mater.

    (2008)
  • A.N. Christensen et al.

    A new calcium sulfate hemi-hydrate

    Dalton Trans.

    (2010)
  • R.D. Cody et al.

    Experimental growth of primary anhydrite at low temperatures and water salinities

    Geology

    (1980)
  • W.F. Cole et al.

    A refinement of the crystal structure of gypsum CaSO4.2H2O

    Acta Cryst. B

    (1974)
  • A.G. De la Torre et al.

    Structure and microstructure of gypsum and its relevance to Rietveld quantitative phase analyses

    Powder Diffract.

    (2004)
  • T.J. Dunai et al.

    Oligocene-miocene age of aridity in the Atacama desert revealed by exposure dating of erosion-sensitive landforms

    Geology

    (2005)
  • Cited by (13)

    • Long-term environmental monitoring for preventive conservation of external historical plasterworks

      2022, Journal of Building Engineering
      Citation Excerpt :

      In this regard, Winkler & Wilhelm (1970) indicated the relationship that must exist between ambient temperature and relative humidity to transform gypsum (CaSO4·2H2O) into bassanite (CaSO4·0.5H2O) [31], In this case, the material is likely to lose stability if exposure to these unfavorable environmental conditions is prolonged over time [32] (Fig. 1-A, 1-B). However, this transformation is part of a reversible process in plasterwork, which means that a rehydration would allow the recovery of the initial properties as long as the dehydration had not been prolonged in time until it remained constant, due to a change in climatic conditions, causing the mechanical weakening of the plasterwork [32]. Additionally, gypsum has low hygroscopicity [33,34] and medium-low solubility [35] that do not represent a direct risk in the preservation of plasterwork [36].

    • Established soil science methods can benefit the construction industry when determining gypsum content

      2021, Cleaner Engineering and Technology
      Citation Excerpt :

      The transformation of bassanite in to gypsum has been studied at nanoscale by Saha et al. (2012). Ritterbach and Becker (2020) stress that the kinetics of gypsum dehydration and bassanite rehydration are dependent of particle size, and have shown that bassanite can re-hydrate to gypsum with high relative air humidity of 95% without liquid water. False records of anhydrite and/or bassanite can be obtained when observed under a microscope.

    • Whitepaper: Earth – Evolution at the dry limit

      2020, Global and Planetary Change
      Citation Excerpt :

      To quantitatively interpret topographic and hydrologic patterns concerning long-term and transient regional differences in runoff and erosion (Bartz et al., in 2020; Binnie et al., 2020, May et al., 2020; Medialdea et al., 2020; Mohren et al., 2020; Ritter et al., 2018b; Walk et al., 2020); To investigate the formation of calcium sulphates, chlorides and nitrates to better understand their potential for the reconstruction of humidity changes and role for landscape modulation (Ritterbach and Becker, 2020; Voigt et al., 2020). Some methodological challenges and tasks have to be met by the CRC 1211.

    View all citing articles on Scopus
    View full text