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     

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.     

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.     

Thermodynamics of the silver-silver ion electrode and thermodynamic solubility product constants of silver halides and pseudo-halides in urea-water mixtures

The hydration kinetics of tricalcium aluminate, C₃A, with gypsum or anhydrite at 1:3 mole ratio were studied using hydration periods of 5 min up to 7 days. These studies were assessed with the aid of differential thermal analysis, thermogravimetric analysis and X-ray diffractometry as well as chemical analysis.The results revealed that hydration periods of up to 7 days of tricalcium aluminate with gypsum in paste form and in suspension, forms ettringite with unhydrated components of C₃A and gypsum. On the other hand, the only hydration product of the slurry of C₃A with anhydrite is ettringite after 7 days of hydration. The kinetics of hydration were studied by the quantitative determination of ettringite from the TG analysis. The results also illustrate that the rate of ettringite formation is slower in the presence of gypsum than in the presence of anhydrite and also that hydration of the slurry gives more ettringite than the hydration of the paste.     

The influence of Cd-impure gyrolite on the hydration of composite binder material based on α-C2S hydrate

In this study, we determined the parameters of a composite binder material (CBM) synthesis on α-C₂S hydrate basis as well as analyzed and explained the early stages of its hydration process. In addition, the utilization possibility of gyrolite impured with Cd²⁺ ions in the binder composite material was presented. The results have shown that α-C₂S hydrate was the dominant product of the hydrothermal synthesis at 175 °C after 16 h. The CBM was prepared by mixing synthetic α-C₂S hydrate with quartz sand and milling the mixture in a vibrating cup mill. The hydration study on both pure CBM and CBM with gyrolite (2.5, 5 or 7.5 % by mass) impured with Cd²⁺ ions (~97 mg Cd²⁺ g^?1) was conducted. The results showed that the additive of gyrolite impured with Cd²⁺ ions accelerates the initial hydration reaction (maximum heat flow of this stage increases from 0.006 W g^?1 for pure binder to 0.009 W g^?1 for the samples with 7.5 % gyrolite) while decreases both the rate of the main reaction (maximum heat flow of the pure binder estimated to be 0.0016 W g^?1, whereas it is 0.0009 W g^?1 in case of 7.5 % gyrolite additive) and total heat after 5 h of the hydration (approximately by 10 J g^?1).     

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.     

Specific heat and thermal conductivity of deuterated triglycine fluoroberyllate

The temperature dependence of the specific heat in deuterated triglycine fluoroberyllate (DTGFB) was measured over the range 294–375 K. The room temperature value was 0.317 cal g^?1 K^?1 and the jump in C_p between the ferroelectric and paraelectric phases is 0.048 cal g^?1 K^?1. Measurements of the thermal conductivity along the principal crystallographic axes [010], [100], [001] and [110] were temperature independent and had average values of 1.22, 1.61, 1.24 and 1.46 × 10^?3 cal s^?1 cm^?1 °C^?1, respectively. The effects of deuteration on these properties were also examined.     

Determination of a partial phase composition in calcined gypsum by calorimetric analysis of hydration kinetics

A calorimetric method for the determination of a partial phase composition in calcined gypsum is presented. The method is based on the separation of the reaction heat of the transformation of ??-anhydrite III to ??-hemihydrate from the main hydration reaction transforming ??-anhydrite III to dihydrate. Using the known hydration heats of ??-anhydrite III to dihydrate and ??-hemihydrate to dihydrate, the content of ??-calcium sulfate anhydrite III and ??-calcium sulfate hemihydrate as well as the amount of non-hydrating parts in gypsum can be determined. Practical application of the method is illustrated on three industrial products.     

Quantitative determination of specific heat by differential scanning calorimetry

The influence of some experimental parameters on the quantitative determination of specific heats by DSC is discussed. Conditions allowing measurement of the specific heat with a maximum relative error of 1.5 % are proposed. The specific heats of NaA zeolite,C _p=0.227±0.003 cal.°C^?1.g^?1, and AgA zeoliteC _p=0.205±0.003 cal.°C^?1.g^?1 have been determined.     

Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.     

Electrical conductivity of small ions in polyacrylonitrile in the glass–transition region

The electrical conductivity process in a new class of ion-containing polymers—highly concentrated solid solutions of hydrated perchlorate salts in polyacrylonitrile (PAN)—is described (σ_dc = 10^?7?10^?2Ω^?1cm^?1). A low-ac instrument (70 cps) is used to measure electrical conductivity. We present a cryogenic system in which the temperature dependence of the conductivity is studied (78–340°K). The ionic character of the conductivity process is established. The conductivity both above and below the glass-transition (T_g) point is thermally activated with an activation energy of 0.7–0.9 e V for the glassy state (?_g) and 0.12–0.6 eV for the rubber-like state (?_r). The systems described exhibit a compensation effect between the pre-exponential factor for the conductivity in the glassy state σ_0g and the difference in activation energy ?_g – ?_r      

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.     

A simplified method for simultaneous quantitation of alkylphenols and alkylphenol ethoxylates in meat and fish using high-performance liquid chromatography with fluorescence detection

Nonylphenol (NP), octylphenol (OP), nonylphenol monoethoxylate (NP₁EO) and nonylphenol diethoxylate (NP₂EO) are products of the biodegradation of alkylphenol polyethoxylates (AP _n EO) which are used worldwide as detergents and surfactants. NP and OP are categorized as definitely endocrine disruptors. 2,4-Tert-butylphenol (BP) is extensively used for anti-oxidant of rubber and plastics. This work proposed a simple and stable method for simultaneously determining the concentration of NP, OP, BP, n-NP₁EO and n-NP₂EO in meat and fish, without requiring the complex pretreatments of current methods. This study used liquid extraction with acetonitrile and hexane and solid extraction using Florisil, in that order to pretreat samples. The sample solutions were analyzed to identify NP, OP, BP, n-NP₁EO and n-NP₂EO by HPLC with fluorescence detection. The mean recoveries were 85.3?±?3.32% for OP, 87.5?±?6.01% for BP, 90.9?±?4.72% for NP, 86.4?±?4.81% for n-NP₂EO and 90.9?±?4.84% for n-NP₁EO. The average coefficients of variation were about 6%. The method's detection limits were 5.4?ng?g^?1 for OP, 5.2?ng?g^?1 for BP, 8.9?ng?g^?1 for NP, 8.7?ng?g^?1 for n-NP₂EO and 8.1?ng?g^?1 for n-NP₁EO. This work analyzed 5 kinds of usual foodstuffs of meat and fish that are frequently consumed by residents of Taiwan. All of these samples contained NP, but not detectable levels n-NP₁EO. Only salmon was contaminated with n-NP₂EO. The NP level was highest in cod (198.41?±?129.34?ng?g^?1, wet weight). The fried chicken had the highest BP level (48.0?±?41.3?ng?g^?1, wet weight), and the uncooked chicken had the highest OP level (66.6?±?53.0?ng?g^?1, wet weight).     

The thermal properties of monochloro-para-xylylene

The three thermal properties that describe the heat transfer in a material were determined for a thin, tough, transparent, highly crystalline film of poly-monochloro-para-xylylene (PCPX). These three properties, viz. thermal conductivity (K), thermal diffusivity (α), and specific heat (C_p) were determined using a transient heating method.The experimental method used involved the heating of a sample of stacked polymer sheets by an ultrathin heating foil. The heating foil, located in the center plane of the stack provided a source of constant heat flux when a current of known amperage was passed through it. By careful consideration of sample dimensions, the sample simulated an infinite solid. The thermal properties were calculated using standard solutions of the heat transfer equations of an infinite solid over a temperature range of ?192 to 130°C. The experimental method was repeated to check the reproducibility of the results and compared with differential scanning calorimeter results.A data acquisition system was developed to facilitate data handling for the transient experiments. The system included hardware capable of punching data on paper tape and a software package to analyze these data.The conclusions drawn include: (1) the reproducibility of the experiments was well within the experimental errors; (2) the data acquisition system greatly facilitates acquisition of thermal data; (3) an incremental change occurs in C_p of PCPX in the vicinity of the γ relaxation reported by dynamical relaxation measurements and its occurrence indicates that this relaxation involves a cooperative motion of molecules; (4) owing to the significant magnitude of the C_p jump and the appreciable degree of crystallinity of PCPX, these internal motions occurring at the γ transition probably involve both amorphous and crystalline regions; (5) a direct relationship between thermal expansion and specific heat was indicated in PCPX as well as for polystyrene (PS) at relatively low temperatures (?200 to ?20°C); (6) the overall low values of thermal conductivity (1.0 to 2.5 × 10^?4 cal sec^?1 cm^?1) and thermal diffusivity (9.5 to 5.3 × 10^?4 cm² sec^?1) of PCPX indicate that it is ideally suited for insulation applications.     

Nature of influence exerted by Na<Subscript>2</Subscript>SiF<Subscript>6</Subscript> on REE recovery from orthophosphoric acid solution in the course of CaSO<Subscript>4</Subscript>·0.5H<Subscript>2</Subscript>O crystallization

Chemical and X-ray phase analyses were used to study the influence exerted by Na₂SiF₆ on the isomorphic inclusion of cerium into the structure of CaSO₄?0.5H₂O precipitates formed from solutions of phosphoric acid hemihydrate (38 wt % P₂O₅). The poorly soluble suspensions of CaSO₄?0.5H₂O precipitates can serve as adsorbents for cerium compounds, with CaSO₄?0.5H₂O–NaCe(SO₄)₂?H₂O and CaSO₄?0.5H₂O–CePO₄?0.5H₂O solid solutions formed. The introduction of Na₂SiF₆ makes the sorption properties CaSO4?0.5H2O several times better because the Na₂SiF₆ phase is a source of sodium cations and creates the necessary Na: Ce ratio of 1: 1 for extracting cerium from the liquid phase into a precipitate in the form of a CaSO₄?0.5H₂O–[NaCe(SO₄)₂?H₂O + CePO₄?0.5H₂O] solid solution. Under the industrial conditions in which extraction phosphoric acid is manufactured, a similar isomorphous capture of rare-earth elements of the cerium group (La–Sm) may occur in joint precipitation of CaSO₄?0.5H₂O and Na₂SiF₆.     

Kinetics and thermodynamics of anionic polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphosphorinane

Kinetic activation parameters and thermodynamic functions describing the reversible anionic polymerization of 2-methoxy-2-oxo-1,3,2-dioxaphosphorinane (1,3-propylene methyl phosphate) were determined. Enthalpy and entropy of the anionic propagation ? depropagation equilibrium were found to be close to those found previously by the present authors for the cationic polymerization, while the activation parameters of propagation and depropagation differ substantially for both processes and reflect the differences in the involved mechanisms. Thus, data for anionic polymerization (and cationic polymerization in parentheses) are: ΔH_1s° = ?0.7 kcal/mole (?1.1); ΔS_1s° = ?2.8 cal/mole-deg (?5.4); ΔH_p^? = 26.7 kcal/mole, and ΔS_p^? = ?6.1 cal/mole-deg. The polymers obtained have low degrees of polymerization (DP _n ≤ 10) because of the extensive chain transfer, leaving cyclic end groups in macromolecules. The presence, structure and concentration of the end groups have been determined by ¹H-, ³¹P-, and ¹³C-NMR spectra.     

Elemental Analysis of Tarhana by Microwave Induced Plasma Atomic Emission Spectrometry

Elemental analysis of tarhana, a traditional Turkish cereal soup, has been conducted. A new method was developed for the determination of calcium, iron, magnesium, manganese, potassium, and sodium, in tarhana by microwave induced plasma atomic emission spectrometry. A sample of 0.1?g were mineralized by microwave digestion in 10?mL of 65% HNO₃. A wheat flour standard reference material (GBW 08503) was used for validation. Linear calibration using standards prepared in acid was conducted for all determinations. The limits of detection were 1.21?µg?g^?1 for Ca at 393.366?nm, 0.43?µg?g^?1 for Fe at 259.940?nm, 11.5?µg?g^?1 for K at 766.491?nm, 0.12?µg?g^?1 for Mg at 285.213?nm, 0.04?µg?g^?1 for Mn at 403.076?nm, and 0.04?µg?g^?1 for Na at 588.995?nm. Ca, K, Fe, Mg, Mn, and Na were determined in tarhana with values from 0.73 to 1.61, 0.016 to 0.061, 2.02 to 4.09, 0.473 to 1.414, 0.019 to 0.043, and 0.26 to 1.83?mg?g^?1, respectively.     

Thermal properties of some cyclic disulfides: naphthalene disulfide and diphenylene disulfide

The specific heat, the melting heat and entropy, the vaporization heat of naphtalene disulfide (C₁₀H₆S₂) and of diphenylene disulfide (C₁₂H₈S₂) have been determined by differential scanning calorimetry (DSC).Over the temperature range examined the specific heat may be represented as follows:

where T is the temperature in degrees Kelvin, while melting heat, vaporization heat, melting entropy are for naphtalene disulfide: 3.10 kcal mol^?1, 6.42 kcal mol^?1, 7.87 cal deg^? mol^?1 and for diphenylene disulfide: 4.62 kcal mol^?1, 6.90 kcal mol^?1 and 11.87 cal deg^?1 mol^?1.     

A hierarchical porous MnO2-based electrode for electrochemical capacitor

A hierarchical porous MnO₂-based electrode was prepared and its electrochemical performance for electrochemical capacitors was investigated. In this work, porous MnO₂ film with pore size of 2?C3?nm in diameter was deposited on a three-dimensional porous current collector by cathodic electrodeposition associated with subsequent controlled heat treatment at 200°C for 2?h. Transmission electron microscopy and X-ray photoelectron spectroscopy showed that the heat treatment has a great effect on the formation of the porous structure of MnO₂ layer, and the disordered porous structure was caused by dehydration during the heat treatment. Cyclic voltammetry and galvanostatic charge?Cdischarge tests showed that both energy and power densities are enhanced due to the unique hierarchical porous structure. The electrode delivers a high specific capacitance of 385?F?g^?1 at a high current density of 5?A?g^?1 within a potential window of ?0.05????0.85?V, and also exhibits excellent rate capability and electrochemical stability.     

Hydrogen and Methane Production, Energy Recovery, and Organic Matter Removal from Effluents in a Two-Stage Fermentative Process

This study evaluates the potential for using different effluents for simultaneous H₂ and CH₄ production in a two-stage batch fermentation process with mixed microflora. An appreciable amount of H₂ was produced from parboiled rice wastewater (23.9?mL g^?1 chemical oxygen demand [COD]) and vinasse (20.8?mL g^?1 COD), while other effluents supported CH₄ generation. The amount of CH₄ produced was minimum for sewage (46.3?mL g^?1 COD), followed by parboiled rice wastewater (115.5?mL g^?1 COD) and glycerol (180.1?mL g^?1 COD). The maximum amount of CH₄ was observed for vinasse (255.4?mL g^?1 COD). The total energy recovery from vinasse (10.4?kJ g^?1 COD) corresponded to the maximum COD reduction (74.7?%), followed by glycerol (70.38?%, 7.20?kJ g^?1 COD), parboiled rice wastewater (63.91?%, 4.92?kJ g^?1 COD), and sewage (51.11?%, 1.85?kJ g^?1 COD). The relatively high performance of vinasse in such comparisons could be attributed to the elevated concentrations of macronutrients contained in raw vinasse. The observations are based on kinetic parameters of H₂ and CH₄ production and global energy recovery of the process. These observations collectively suggest that organic-rich effluents can be deployed for energy recovery with sequential generation of H₂ and CH₄.