Crystallization and Phase Stability of CaSO<Subscript>4</Subscript> and CaSO<Subscript>4</Subscript> – Based Salts

Summary.  Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum (CaSO₄·2H₂O), hemihydrate (CaSO₄·0.5H₂O) and anhydrite (CaSO₄). On the one hand the conversion of these phases into each other takes place in nature and on the other hand it represents the basis of gypsum-based building materials. The present paper reviews available phase diagram and crystallization kinetics information on the formation of calcium sulfate phases, including CaSO₄-based double salts and solid solutions. Uncertainties in the solubility diagram CaSO₄–H₂O due to slow crystallization kinetics particularly of anhydrite cause uncertainties in the stable branch of crystallization. Despite several attempts to fix the transition temperatures of gypsum–anhydrite and gypsum–hemihydrate by especially designed experiments or thermodynamic data analysis, they still vary within a range from 42–60°C and 80–110°C. Electrolyte solutions decrease the transition temperatures in dependence on water activity. Dry or wet dehydration of gypsum yields hemihydrates (α-, β-) with different thermal and re-hydration behaviour, the reason of which is still unclear. However, crystal morphology has a strong influence. Gypsum forms solid solutions by incorporating the ions HPO₄ ²⁻, HAsO₄ ²⁻, SeO₄ ²⁻, CrO₄ ²⁻, as well as ion combinations Na⁺(H₂PO₄)⁻ and Ln³⁺(PO₄)³⁻. The channel structure of calcium sulfate hemihydrate allows for more flexible ion substitutions. Its ion substituted phases and certain double salts of calcium sulfate seem to play an important role as intermediates in the conversion kinetics of gypsum into anhydrite or other anhydrous double salts in aqueous solutions. The same is true for the opposite process of anhydrite hydration to gypsum. Knowledge about stability ranges (temperature, composition) of double salts with alkaline and alkaline earth sulfates (esp. Na₂SO₄, K₂SO₄, MgSO₄, SrSO₄) under anhydrous and aqueous conditions is still very incomplete, despite some progress made for the systems Na₂SO₄–CaSO₄ and K₂SO₄–CaSO₄–H₂O. Corresponding author. E-mail: daniela.freyer@chemie.tu-freiberg.de Received December 17, 2002; accepted January 10, 2003 Published online April 3, 2003     

In situ synchrotron PXRD investigation of fresh and aged Equisetum ramosissimum Desf. (horsetail grass)

As one of the oldest species, horsetail grass (Equisetum ramosissimum Desf.) is known as a living fossil plant, dating back to the Mesozoic era. Horsetail grass is also considered one of the most important sources of bio-silica due to its ability to accumulate high amounts of silica in all parts of the plant; various minerals can also be isolated by heat treatment. Fresh and aged horsetail grass stored for 2 years under ambient conditions was investigated by synchrotron powder X-ray diffraction (PXRD). Clear crystallites were not observed in a fresh sample stored at room temperature; surprisingly, high amounts of gypsum (CaSO₄·2H₂O) and syngenite (K₂Ca[SO₄]₂·H₂O) were observed in the 2-day dried and 2-year aged samples, respectively. However, crystalline silica materials were not observed. In addition, in situ thermal treatment of up to 700°C was applied to investigate the crystals and phase transitions by focusing the X-ray beam onto a single stem. In situ synchrotron PXRD revealed that dehydration occurred in gypsum in the 2-day dried sample with an increase in temperature to hemihydrate (CaSO₄·xH₂O, 0.5 ≤ x ≤ 0.8) and anhydrite (CaSO₄). On the other hand, syngenite was transformed to calciolangbeinite (K₂Ca₂[SO₄]₃) at high temperatures in 2-year aged horsetail grass.     

Formation of calcium sulfate whiskers from CaCO3-bearing desulfurization gypsum

Calcium sulfate whiskers can be used as the reinforcing agents in many composites, such as polymers, ceramics, cements, and papers, etc. This paper investigated the feasibility of preparing calcium sulfate whiskers using desulfurization gypsum as the raw material. The desulfurization gypsum composed mainly of CaSO₄·2H₂O (93.45 wt%) and CaCO₃ (1.76 wt%) were treated with dilute H₂SO₄ at room temperature to convert CaCO₃ to CaSO₄; the latter was then treated at 110?C150 °C to form CaSO₄·0.5H₂O whiskers. The removal of the CaCO₃ impurity from the desulfurization gypsum favored the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.     

Solubility study of sodium, potassium and calcium sulfates and chlorides, in ammonia

Solubility of sodium, potassium and calcium sulfates and chlorides in 28% ammonia solution was determined through monitoring conductivity measurements and kinetics of solids dissolution as a function of temperature and stirring time. The major findings of the present study show that Na₂SO₄ and K₂SO₄ solutions conductivity follow straight linear segments with different slopes. However, in case of NaCl, KC1, CaCl₂ and CaSO₄ · 2H₂O, conductivity curves were continuous, monotonous and reach constants maximum values. The hypothesis of complex formation or dissolution via intermediaries such as NaNH₄SO₄ and KNH₄SO₄ salts seams to be true, through X-ray diffraction study of resulting deposits. Furthermore, the dissolution rates at 25°C of potassium and calcium chlorides in ammonia solution are higher than that reported in the literature for water. In fact, ammonia significantly reduces the solubility of K₂SO₄; conversely, a slight increase in this parameter was observed for CaSO₄.     

Phase equilibria in the Na,K,Mg,Ca∥SO<Subscript>4</Subscript>,Cl–H<Subscript>2</Subscript>O system at 50°С in the polyhalite crystallization region

Phase equilibria of the Na,K,Mg,Ca||SO₄,Cl–H₂O system at 50°С in the polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) crystallization region were studied using the translation method. Polyhalite was found to be involved, as an equilibrium phase of the title system at 50°С, in 17 invariant points, 36 monovariant curves, and 24 divariant fields. A fragment of equilibrium phase diagram of the title system in the polyhalite crystallization region was constructed.     

Zur Darstellung und Charakterisierung von Calciumhydrogensulfaten

Preparation and Characterization of Calcium Hydrogen Sulfate CaSO₄ · H₂SO₄ was identified as calcium hydrogen sulfate whereas CaSO₄ · 3 H₂SO₄ is an adduct of CaSO₄ with H₂SO₄. Depending on the excessive amount of H₂SO₄ both compounds exist side by side up to a temperature of 343 K, whereas above this temperature only Ca(HSO₄)₂ is stable. The DTA curve of Ca(HSO₄)₂ shows two maxima at 488 K and 523 K, according to the separation of H₂O under formation of pyrosulfate and decomposition of this compound under elimination of SO₃. In comparison with other hydrogen sulfates Ca(HSO₄)₂ shows a considerable increased O? H distances. The d-values of Ca(HSO₄)₂ are calculated and represented.     

Thermal analysis of ternary gypsum-based binders stored in different environments

Although gypsum belongs to the low-energy environmentally friendly binders, its wider applications in building constructions are limited due to the negative effect of moisture on its mechanical properties. When calcined gypsum (CaSO₄·1/2H₂O) transforms into its hydrated form (CaSO₄·2H₂O), it is partially soluble in water and it has a relatively low strength. This problem can be resolved when gypsum is used as a part of binary or ternary binders. In this paper, a system consisting of calcined gypsum, lime, and silica fume is presented as a functional solution for a wider utilization of gypsum in wet environments. For this purpose, the newly designed materials were stored in different environments (laboratory conditions in air or water) up to 182 days. The effect of silica fume on the hydration process and the growth of the main products is evaluated by using differential scanning calorimetry and thermogravimetry in the temperature range from 25 to 1000 °C with a heating rate of 5 °C min^?1 in an argon atmosphere. The carbonation level of studied materials is also evaluated. Besides this, the information about the thermal stability of studied materials is provided. These results are supported by evolved gas analysis, X-ray diffraction, and scanning electron microscopy. The basic physical and mechanical properties are determined to provide more detailed information about the behavior of the designed materials under various conditions at selected days of hydration. The addition of silica fume to the gypsum–lime system activates the pozzolanic reaction of the analyzed pastes, which is proved by the presence of the CSH phase and by the consumption of portlandite in the mixtures. Wet environment speeds up the hydration processes and prevents samples from carbonation.     

Polyhalite and its analogous triple salts

Polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) and analogue triple salts, where Mg²⁺ is substituted by Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, have been synthesized. The salts were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction. Diffraction patterns and Raman spectra resemble those of natural polyhalite, except K₂SO₄ · CuSO₄ · 2CaSO₄ · 2H₂O. The latter corresponds to the mineral leightonite, which is structurally different.     

Effects of the type of calorimeter and the use of plasticizers and hydrophobizers on the measured hydration heat development of FGD gypsum

Thermal phenomena at the hydration of calcium sulphate hemihydrate (CaSO₄·0.5H₂O) are investigated in the paper. Time development of hydration heat of β-calcium sulphate hemihydrate prepared from flue gas desulphurization (FGD) gypsum is determined using two different types of calorimeter, namely the differential calorimeter DIK 04 and the isothermal heat flow calorimeter KC 01, and the differences in measured data analyzed. Then, the effects of plasticizers and hydrophobizers on the hydration process of analyzed gypsum mixtures are studied.     

Enhancement on fireproof performance of construction coatings using calcium sulfate whiskers prepared from wastewater

This work is dealing with the fireproof performance of gypsum composite painting for fire passive protection in building construction. An efficient microwave-assisted method is adopted to fabricate high-crystalline CaSO₄·2H₂O whiskers from wastewater. The as-prepared CaSO₄·2H₂O whiskers display one-dimensional structure with a high aspect ratio of 40. The thermal resistive behavior of CaSO₄? containing paints is investigated using thermo-gravimetric analyzer, differential scanning calorimetry, and direct flaming test (at 150, 570, and 1100 °C). The addition of CaSO₄·2H₂O whiskers not only improves the anti-flammability but also reduces the ignition temperature of construction painting. This result can be attributed to the fact that the heat transfer through the CaSO₄-containing painting can be alleviated until the endothermic reaction steps [i.e., dehydration of gypsum (CaSO₄·2H₂O) and crystalline phase change of β-hemihydrated plaster] are totally completed. The burned fractions (including pyrogenation and carbonization) on CaSiO₃ substrate are decreasing functions of the content of CaSO₄·2H₂O whiskers, proving that gypsum works as an insulator against heat transfer and flame spreading. Accordingly, the CaSO₄·2H₂O whiskers can be considered as an effective fire retardant additive for improving the fireproof ability of construction coatings.     

Metastable phase equilibria for the ternary aqueous system of lithium sulfate and potassium sulfate at <Emphasis Type="Italic">T</Emphasis> = 308.15 K: Experimental data and prediction using Pitzer model

The metastable solubilities and the physicochemical properties including density, refractive index, pH and conductivity in the ternary system (Li₂SO₄ + K₂SO₄ + H₂O) at T = 308.15 K were determined experimentally using the isothermal evaporation method, and the metastable phase diagram and the physicochemical properties versus composition diagram were plotted. In the metastable phase diagram, there are two invariant points, three univariant curves and three crystallization regions corresponding to lithium sulfate monohydrate (Li₂SO₄ · H₂O), double salt (K₂SO₄ · Li₂SO₄) and arcanite (K₂SO₄). It was found that the double salt of K₂SO₄·Li₂SO₄ belongs to the incongruent double salt, and the hydrate of Li₂SO₄ · H₂O belongs to hydrate type I. On the basis of Pitzer model of the electrolyte solution theory, the mixing-ion parameter of θ_Li,K, \({\Psi _{Li,K,S{O_4}}}\) and the metastable equilibrium constants of the solid phases K₂SO₄, Li₂SO₄ · K2SO4 and Li₂-SO₄ · H₂O at 308.15 K were obtained for the first time. The calculated metastable solubility data for this ternary system at 308.15 K agree well with the experimental values, and this result indicates that the mixing-ion parameters and the metastable equilibrium constants obtained in this work are reliable.     

Calorimetric studies of special cements

The kinetics and even the mechanism of cement reaction with water can be successfully investigated by use of microcalorimetry. In this study this method was applied to follow the hydration of the new family of portland cements containing C₁₂A₇ ^* and C₁₁A₇·CaF₂ addition as well as special cement with C₃A replacement by calcium sulphoaluminate. It has been found that C₁₁A₇·CaF₂ acted as hydration retarder. The heat evolution curves for C₁₂A₇ containing samples without CaF₂ are very similar to those for the reference portland cement samples. XRD and SEM studies confirm the results described above, relating to the retardation of alite hydration. The process is positively modified by the addition of anhydrite. In the presence of calcium sulphoaluminate (4CaO·3Al₂O₃·SO₃) the hydration at early stage occurs with the rapid formation of large amount of the ettringite phase. The calcium fluoride acts as a set retarder. The full compatibility of calorimetry with SEM and XRD results should be underlined. In cement chemistry the following notation is used:C=CaO,A=Al₂O₃,S=SiO₂,H=H₂O etc. for the main oxide constituents of portland cement clinker and hydrates.     

Heat of hydration as a method for determining the composition of calcined gypsum

The heat of hydration of a fresh, locally produced sample of some industrial gypsum (plaster of Paris, CaSO₄ · 0.5 H₂O) was determined. An adiabatic calorimeter was used for this purpose. The obtained heat of hydration was ?9 cal g^?1, which is higher than that for the hemihydrate (?5 cal g^?1). The calculated heat of hydration for calcium sulphate hemihydrate from the known heats of formation, and using ordinary thermochemical equations, is ?5 cal g^?1. In the same manner, however, the calculated heat of hydration for the anhydrite (CaSO₄) is ?29 cal g^?1. The higher heat of hydration (?9 cal g^?1) for the tested sample than that for the ordinary hemihydrate (?5 cal g^?1) was attributed to the presence of a certain percent of anhydrite. The composition of the tested sample was proposed by applying conventional chemical and rational analyses. The present work suggests the use of the heat of hydration as a tool for determining the composition of calcined gypsum.     

Influence of heavy metals on the early hydration of calcium sulfoaluminate

The hydration of calcium sulfoaluminate $ ( {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} ) $ in the presence of heavy metal is essential not only for applying the cement in solidification/stabilization (s/s) process, but also for preparing modern green cements from wastes containing heavy metals. In this study, the influence of gypsum, types, and concentrations of heavy metal nitrates (Pb(NO₃)₂, Cr(NO₃)₃·9H₂O, Cu(NO₃)₂·3H₂O, Zn(NO₃)₂·6H₂O) on the hydration of $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ during the first 24 h were investigated by isothermal conduction calorimetry, X-ray diffraction, and thermogravimetric analysis. The addition of 20 % of gypsum to $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ leads to a rapid formation of ettringite against monosulfate and acceleration of hydration. The effects of heavy metals on the hydration of $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ depend on the types of heavy metals and the addition of gypsum. Without any gypsum addition, heavy metal nitrates such as Cr, Cu, and Zn promote the hydration of $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ , whereas Pb presents a strong retardation effect at the early age of $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ hydration. When 20 % of gypsum is added to $ {\text{C}}_{4} {\text{A}}_{3} \overline{\text{S}} $ , heavy metals tend to accelerate the hydration of the blended pastes except Zn. However, heavy metal containing phases were not detected in this work, which needs to be supplemented by further investigations.     

Variable temperature PXRD investigation of the phase changes during the dehydration of potassium Tutton salts

The thermal dehydration of the potassium Tutton salts K₂M(SO₄)₂·6H₂O (M = Mg, Co, Ni, Cu, Zn) was investigated using thermal gravimetric analysis (TG), differential scanning calorimetry (DSC), FTIR, and variable temperature powder X-ray diffraction. While each Tutton salts lost all six waters of hydration when heated to 500 K, the decomposition pathway depended on the divalent metal cation. K₂Ni(SO₄)₂·6H₂O lost all six waters in a single step, and K₂Cu(SO₄)₂·6H₂O consistently lost water in two steps in capped and uncapped cells. In contrast, multiple decomposition pathways were observed for the magnesium, cobalt, and zinc Tutton salts when capped and uncapped TG cells were used. K₂Zn(SO₄)₂·6H₂O lost the waters of hydration in a single step in an uncapped cell and in two steps in a capped cell. Both K₂Mg(SO₄)₂·6H₂O and K₂Co(SO₄)₂·6H₂O decomposed in a series of steps where the stability of the intermediates depended on the cell configuration. A greater number of phases were often observed in DSC and capped-cells TG experiments. A quasi-equilibrium model is presented that could explain this observation. These results highlight that experimental conditions play a critical role in the observed thermal decomposition pathway of Tutton salts.     

Polyhalite and its analogous triple salts

Polyhalite (K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O) and analogue triple salts, where Mg²⁺ is substituted by Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, have been synthesized. The salts were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction. Diffraction patterns and Raman spectra resemble those of natural polyhalite, except K₂SO₄ · CuSO₄ · 2CaSO₄ · 2H₂O. The latter corresponds to the mineral leightonite, which is structurally different. For polyhalite analogues the cell parameters of the triclinic unit cell have been determined from the powder diffraction patterns. The length of the unit cell vectors varies regularly with the ionic radius of the substituted ion M ²⁺ and is explained by changes in the extension of the coordination octahedron of M ²⁺. Thereby increasing distances of the coordinated water molecules at M ²⁺ parallel with decreasing dehydration temperatures of the corresponding polyhalite. Correspondence: Daniela Freyer, Institut für Anorganische Chemie, TU Bergakademie Freiberg, 09599 Freiberg, Germany.     

Heat of solution of polyhalite and its analogues at T = 298.15 K

Calorimetric measurements have been performed to determine the heat of dissolution of polyhalite K₂SO₄ · MgSO₄ · 2CaSO₄ · 2H₂O and its analogues K₂SO₄ · MSO₄ · 2CaSO₄ · 2H₂O (M = Mn, Co, Ni, Cu, and Zn) at T = 298.15 K. The dissolution experiments were carried out in NaClO₄ solution with varying concentrations (0.5 to 2.0) mol kg^?1. All polyhalites dissolve exothermically. Exothermicity increases with concentration of NaClO₄. An extrapolation to infinite dilution was done using the SIT model.Within the limits of experimental uncertainty, the enthalpies of dissolution for the triple salts K₂MgCa₂(SO₄)₄ · 2H₂O with M = Mg, Mn, Ni, and Zn coincide. The value for the cobalt salt is noticeably less exothermic. Dissolution enthalpy of leightonite K₂CuCa₂(SO₄)₄ · 2H₂O, which does not crystallize in the polyhalite structure, deviates considerably within the series.     

New Structure Type among Octahydrated Rare‐Earth Sulfates, β‐Ce2(SO4)3·8H2O,and a new Ce2(SO4)3·4H2O Polymorph

Syntheses, crystal structures and thermal behavior of two new hydrated cerium(III) sulfates are reported, Ce₂(SO₄)₃·4H₂O ( I ) and β‐Ce₂(SO₄)₃·8H₂O ( II ), both forming three‐dimensional networks. Compound I crystallizes in the space group P2₁/n. There are two non‐equivalent cerium atoms in the structure of I , one nine‐ and one ten‐fold coordinated to oxygen atoms. The cerium polyhedra are edge sharing, forming helically propagating chains, held together by sulfate groups. The structure is compact, all the sulfate groups are edge‐sharing with cerium polyhedra and one third of the oxygen atoms, belonging to sulfate groups, are in the S–Oμ₃–Ce₂ bonding mode. Compound II constitutes a new structure type among the octahydrated rare‐earth sulfates which belongs to the space group Pn. Each cerium atom is in contact with nine oxygen atoms, these belong to four water molecules, three corner sharing and one edge sharing sulfate groups. The crystal structure is built up by layers of [Ce(H₂O)₄(SO₄)]_nⁿ⁺ held together by doubly edge sharing sulfate groups. The dehydration of II is a three step process, forming Ce₂(SO₄)₃·5H₂O, Ce₂(SO₄)₃·4H₂O and Ce₂(SO₄)₃, respectively. During the oxidative decomposition of the anhydrous form, Ce₂(SO₄)₃, into the final product CeO₂, small amount of CeO(SO₄) as an intermediate species was detected.     

Comparative studies on fire-rated and standard gypsum wallboard

The long-term goal of this research is to improve the fire resistance of gypsum wallboard (GWB). GWB consists mainly of gypsum, i.e., calcium sulfate dihydrate, CaSO₄·2H₂O. In buildings, the chemical, mechanical, and thermal properties of GWB play an important role in delaying the spread of fire. To build a fire resistant GWB, it is very important to study the thermal, mechanical, physical, and chemical properties of regular GWB and various types of fire resistant wallboards available commercially in the market. Various fire resistant GWBs have been compared and contrasted with reference to a standard wallboard in this study. Regardless of the type of wallboard, the main component is gypsum. The fire resistance property is mainly attributed to the absorption of energy related with the loss of hydrate water going from the dihydrate (CaSO₄·2H₂O) form to the hemihydrate (CaSO₄·½H₂O) and from the hemihydrate to the anhydrous form (CaSO₄) in a second decomposition. The present paper is a comparative study of commercially available standard, fire-rated Type X, and fire-rated Type C GWBs. Type X wallboards are typically reinforced with non-combustible fibers so as to protect the integrity of the wallboard during thermal shrinkage, while the Type C wallboards are incorporated with more glass fibers and an additive, usually a form of vermiculite. These Type C wallboards have a shrinkage adjusting element that expands when exposed to elevated temperature. Differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and powder X-ray diffraction were used to characterize and compare the materials. Various properties, such as the heat flow, mass loss, dimensional changes, morphology, and crystalline structures of the GWBs were studied using these techniques.     

Conversion of phosphogypsum to potassium sulfate

Summary Solubility of calcium sulfate in concentrated aqueous chloride solutions is of particular significance in chloride hydrometallurgy and various crystallization processes, such as the production of potassium sulfate from phosphogypsum and potassium chloride. This paper examines an example of the second type of application in which gypsum and potassium chloride are reacted to form K₂SO₄. The solubility of phosphogypsum in aqueous solutions of KCl, HCl, and mixtures of both has first been measured at various temperatures and concentrations. The parameters investigated are HCl concentration up to 6M, KCl concentration up to 180 g L^-1 and temperature from 25 to 80°C. In addition, the influence of co-existing chloride salts, such as (HCl+KCl), on the solubility of calcium sulfate is estimated from 25 to 80°C. The solubility increases obviously with the temperature increment as it does initially with acid concentration, reaching a maximum of about 3M HCl, 130 g L^-1 KCl and then drops. At the same time, the solubility of CaSO₄·2H₂O decreases with increasing KCl concentration.