American Mineralogist, Volume 97, pages 1060–1069, 2012 Minerals in cement chemistry: A single-crystal neutron diffraction and Raman spectroscopic study of thaumasite, Ca3Si(OH)6(CO3)(SO4)·12H2O G. DIEGO GATTA,1,2,* GARRY J. MCINTYRE,3 JULIA G. SWANSON,4 AND STEVEN D. JACOBSEN4 1Dipartimento di Scienze della Terra, Universita’ degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy 2CNR-Istituto per la Dinamica dei Processi Ambientali, Milano, Italy 3The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia 4Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois 60208, U.S.A. ABSTRACT Thaumasite, Ca3Si(OH)6(CO3)(SO4)⋅12H2O, is recognized as a secondary-alteration mineral and indicator of sulfate attack in Portland cement in contact with sulfate-rich groundwater, especially in cold regions. The hydrogen positions in thaumasite have been determined from single-crystal neutron diffraction structure refinements at 300 and 22 K. No phase transitions occur within the temperature range investigated. The structure of thaumasite is largely held together by hydrogen bonding. The major structural units [CO3 groups, SO4 tetrahedra, Si(OH)6 octahedra, and Ca(OH)4(H2O)4 polyhe- dra] are interconnected via 10 distinct hydrogen bonds. Analysis of the difference-Fourier maps of the nuclear density reveals the positions of all 10 hydrogen atoms in the structure, and the hydrogen bonding becomes shorter (stronger) upon decreasing temperature to 22 K. The SO4 tetrahedron ex- pands upon decreasing temperature (i.e., negative thermal expansion at the molecular level), driven 4+ by shortening of the hydrogen bonding between [Ca3Si(OH)6(H2O)12] columns. Polarized Raman spectra of thaumasite show that the ν1 symmetric stretching modes of Si(OH)6, SO4, and CO3 occur at –1 658, 983, and 1066 cm , respectively. In addition, the out-of-plane bending mode (ν2) and asymmetric –1 stretching mode (ν3) of the carbonate group are tentatively assigned to bands at 887 and 1400 cm , respectively. Bands at 418 and 455 cm–1 (and possibly at 477 cm–1) are attributed to the symmetric bending modes (ν2) of the sulfate group, and we observe a possible asymmetric stretching mode (ν3) –1 of SO4 at 1090–1100 cm . Splitting of some sulfate and carbonate vibrational modes may occur due –1 to hydrogen bonding on all the oxygen sites. At 1685–1710 cm we observe the H2O bending modes (H-O-H), and from 2900–3600 cm–1 there are 13 distinct bands associated with bending overtones and the O-H stretching vibrations corresponding to H-positions determined in the neutron diffraction study. The effect of the low-temperature stability of thaumasite on the pronounced “thaumasite sulfate attack” of Portland cements observed in cold regions is discussed. Keywords: Thaumasite, Portland cements, crystal chemistry, single-crystal neutron diffraction, Raman spectroscopy, hydrogen bonding INTRODUCTION provides most of the binding strength, thaumasite and ettringite formation leads to weakening and disintegration of the cement Thaumasite, Ca3Si(OH)6(CO3)(SO4)⋅12H2O [a ∼11.030, c matrix especially in concrete structures below ground (e.g., ∼10.396 Å, space group P63, Z = 2], occurs as a low-temperature secondary-alteration phase in mafic igneous and metamorphic Hartshorn et al. 1999; Bensted 1999; Hobbs and Taylor 2000; rocks. Usually it is associated with zeolites, apophyllite, anal- Santhanam et al. 2001; Zhang et al. 2009). Degradation of con- cime, calcite, gypsum, and pyrite. Thaumasite forms a solid crete by thaumasite sulfate attack (TSA) has been documented in various environments including an 80-year-old aqueduct in solution with ettringite, Ca6[Al(OH)6]2(SO4)3⋅26H2O (Moore and Taylor 1970; Barnett et al. 2000), and contains a highly unusual Manitoba, 33-year-old pavement in Ontario, test samples exposed Si site in six-coordination with hydroxyl, VISi-OH. to marine (tidal) conditions, and in laboratory samples exposed Thaumasite and ettringite also form in the porous matrix of to wet–dry cycles in sulfate solution (Thomas et al. 2003). The aging concrete, especially in contact with sulfate-rich soils or source of carbonate differs in each of these cases, deriving from fluids (Crammond 1985) or when limestone is used as a filling de-dolomitization of dolostone aggregates, carbonate ions in the material (Hartshorn et al. 1999). Cement hydration products, seawater, or atmospheric CO2 through the process of carbonation. calcium silicate hydrates—the so-called C-S-H and calcium Sulfate attack of Portland cement by thaumasite appears to be greatly accelerated at sub-zero temperatures, as observed from hydroxide—portlandite Ca(OH)2, are decomposed as a result of both sulfate attack and of carbonation. Because C-S-H in concrete rapid deterioration of reinforced concrete foundations in the Arctic within just two years of construction (Bickley 1999). * E-mail: [email protected] The crystal chemistry of thaumasite is unusual: it contains 0003-004X/12/0007–1060$05.00/DOI: http://dx.doi.org/10.2138/am.2012.4058 1060 GATTA ET AL.: HYDROGEN POSITIONS IN THAUMASITE 1061 about 42 wt% H2O, 7 wt% CO2, 13 wt% SO3, 28 wt% CaO, and 1997; Frost and Fei 1998). 3 10 wt% SiO2, giving a density of only ∼1.88 g/cm . The crystal The thermal behavior of thaumasite has been investigated structure of thaumasite was solved by Edge and Taylor (1971), by in situ single-crystal X-ray structure refinements at 130 and and later refined by Zemann and Zobetz (1981), Effenberger et al. 298 K by Jacobsen et al. (2003), and no phase transitions were (1983), and Jacobsen et al. (2003) by single-crystal X-ray diffrac- observed over this temperature range. The unit cell and CaO8 4+ tion. The thaumasite structure consists of [Ca3Si(OH)6(H2O)12] polyhedra showed significant positive thermal expansion over 2– columns running parallel to [001], connected by CO3 and the temperature range investigated (130–298 K), but the silicate 2– SO4 groups via hydrogen bonds with H2O molecules, as shown octahedron, sulfate tetrahedron, and the carbonate group show in Figures 1 and 2. Thaumasite is the only mineral containing zero or negative thermal expansion. Jacobsen et al. (2003) sug- silicon in six-coordination with hydroxyl (OH)− that is stable at gested that there is an important role played by the hydrogen low P-T conditions. High pressure favors six-coordination of Si bonds on this unusual thermal behavior, but clarification of this with oxygen, such as in stishovite (SiO2) and silicate perovskite role is obscured by the difficulties in defining the hydrogen-bond (MgSiO3), but the high-pressure synthetic phase D (MgSi2H2O6) geometry (i.e., O-H and H⋅⋅⋅O distances and O-H⋅⋅⋅O angle) is the only other phase known to contain VISi-OH (Yang et al. from X-ray data. FIGURE 1. The crystal structure of thaumasite viewed down [001] and [010] based on the neutron structure refinement at 22.5 K of this study. Thermal-ellipsoid probability factor: 50%. 1062 GATTA ET AL.: HYDROGEN POSITIONS IN THAUMASITE tions of all the proton sites and topological configuration of the hydrogen bonds, the anisotropic displacement parameters of the H sites, and discuss the role played by the H bonds on the low- temperature thermal behavior of thaumasite. Because we report the hydrogen-bonding environments in thaumasite with neutron diffraction techniques, new polarized Raman spectra, especially in the region of O-H stretching, are presented to complement the neutron study. EXPERIMENTAL METHODS Sample characterization A natural, transparent, and eudedral (hexagonal prism) centimetric crystal of thaumasite from Black Rock Mine, Kuruman, Kalahari, Northen Cape Province, South Africa, was obtained from the National Museum of Natural History, Smith- sonian Institution (NMNH no. 165279). The single crystal was cut into four sec- tions for microprobe, thermo-gravimetric, Raman spectroscopy, and single-crystal neutron diffraction experiments. FIGURE 2. Configuration of the [Ca3Si(OH)6(H2O)12] building-block Qualitative electron-probe X-ray microanalyses in the energy-dispersive mode units forming columns parallel to [001] (left side), and of the alternating (EDS-EPMA) and quantitative microanalyses in the wavelength-dispersive mode CO3 groups and SO4 tetrahedra running along [001] (right side), based (WDS-EPMA) were performed on a polished single crystal of thaumasite using a on the neutron structure refinement at 22.5 K of this study. Thermal- JEOL JXA-8200 electron microprobe. The system was operated using a defocused ellipsoid probability factor: 50%. electron beam to avoid dehydration and cation migration under the electron beam, following a protocol usually adopted for zeolite analysis (Gatta and Boffa Bal- The results of high-temperature synchrotron powder diffrac- laran 2004). The preliminary EDS-EPMA analysis showed that only the elements tion between 303 and 1098 K, and of TG/TDA up to 1073 K, were nominally expected in thaumasite (i.e., Ca, Si, S) occurred within the detection limit. A series of natural minerals (K-feldspar for Si, wollastonite for Ca, barite for reported by Martucci and Cruciani (2006). Rietveld refinements S) were used as standards for the WDS-EPMA analysis. The results were corrected showed no phase transition up to 417 K, whereupon thaumasite for matrix effects using a conventional ZAF routine in the JEOL suite of programs. decomposed. A continuous dehydration process was observed The average composition (30 point analysis) of thaumasite is: CaO 27.1(8) wt%, between 303–393 K, whereas at T > 393 the dehydration was SiO2 9.5(3) wt%, and SO3 12.7(6) wt%. The crystal was chemically homogeneous severe, leading to a collapse of the structure at T ≥ 417 K. The within analytical errors. Thermo-gravimetric analyses (TG) were performed with a NETZSCH STA449C thermoanalyzer under a 20 mL/min air flux with a heating dehydration-induced amorphization was confirmed by the TG/ rate of 5 K/min, using a powdered sample (crystallite diameter 2–5 µm) previously DTA analyses, which shows a slower weight loss at T < 353 dried at 310 K for 4 h.