Gypsum CaSO4 Research Articles

Stability under electron irradiation of some layered hydrated minerals

The structural damages caused to some layered hydrated minerals by 2.5 MeV electron irradiation using the SIRIUS platform were studied by powder X-Ray diffraction and, in some cases, by 1H MAS-NMR spectroscopy. It is clearly demonstrated that the radiation damages are distinguishable from the heating effects. It is shown that: i) in all cases electron irradiation leads to distortions of the unit cell and very limited volume expansion, compared to heating; ii) radiation damages increase with increasing the structural complexity of the mineral; iii) portlandite Ca(OH)2 and brucite Mg(OH)2 remain crystalline up to high doses (a few GGy), with appearance of stacking fault disorder especially in brucite; iv) brushite CaHPO4.2H2O and gypsum CaSO4.2H2O undergo a phase transformation of type amorphization for brushite involving the strongest intralayer H bond between the acidic proton and the phosphate tetrahedral, and decomposition for gypsum involving interlayer H bonds between water molecules.

Behavior of Calcium Compounds under Hydrothermal Conditions during Alkaline Leaching of Aluminosilicates with the Synthesis of Fillers for Composites

Calcium oxide plays an important role in alumina production by binding SiO2 from aluminosilicate raw materials (bauxite, nepheline, kaolinite, etc.) in aluminum-free compounds. The efficiency of the hydrochemical technology depends on the activities of calcium oxide or its compounds introduced into the alkaline aluminosilicate slurry. In this paper, we considered the effects of different calcium compounds (calcium carbonate CaCO3, gypsum CaSO4·H2O, calcium oxide CaO and calcium hydroxide Ca(OH)2), introduced during the hydrothermal stripping of aluminosilicates with alkaline solutions, on the degree of aluminum oxide extraction, with the subsequent production of fillers for composites. Ca(OH)2 was obtained by the CaO quenching method. Extraction of Al2O3 in an alkaline solution was only possible with Ca(OH)2, and the degree of extraction depended on the conditions used for CaO quenching. The effects of temperature and of the duration of CaO quenching on particle size were investigated. In potassium solution, the best results for Al2O3 extraction were obtained using CaSO4·H2O gypsum. The obtained solutions were processed using the crystallization method.

Cellular concrete waste: An efficient new way for H2S removal

The ability of cellular concrete waste (CC) to remove hydrogen sulfide (H2S) from a mimic biogas composed of nitrogen, oxygen and H2S was investigated using abiotic filtration systems for H2S concentration up to 1500 mg m−3 and Empty Bed Residence Time (EBRT) ranging from 30 to 180 s. The influence of the humidity of the material in H2S removal was studied. In wetted conditions (relative humidity > 97 %), 169 g of H2S were removed per kg of material. This value is probably lower than the maximum capacity of the cellular concrete since the experiment was ended before the end-of-life of the material. The H2S elimination capacity of cellular concrete waste could reach up to 32 g m-3 h−1. These performances are due to complex physico-chemical mechanisms operating simultaneously between H2S and the different components of cellular concrete (mainly calcium oxide CaO from the calcium silicate hydrate CaO SiO2 nH2O, and ferric oxide Fe2O3) leading to a modification of the mechanical structure of the material to calcium sulfate (gypsum CaSO4 2H2O) and the production of elemental sulfur (octasulfur S8). Empirical relationships based on H2S concentration and EBRT were established (i) to predict the Removal Efficiency (RE) of the filtration system (logistic equation); (ii) to assess the operating time of the material. Results obtained suggest that this simple, robust and cheap H2S removal technology would be a suitable and valuable technique to treat gas flowrates generated by decentralized domestic biogas digesters.

Фосфор өндірісінің қалдығы фосфогипсті тиімді пайдалану

Фосфор өндірісінің қалдығы болып табылатын фосфогипсті халық шаруашылығының түрлі салаларында, атап айтқанда, агроөнеркәсіп кешенінде қолдану үшін ұзақ мерзім бойы минералды тыңайтқыштың құрамына әсер ететін қолдану әдістері мен тәсілдеріне қысқаша шолу жасалды. Фосфогипстің физико-химиялық қасиеттерін зерттеу нәтижелері оның құрамындағы CaSO4, макро- және микроэлементтердің болуына байланысты қышқылданған топырақтарды кальцинациялауда мелиорант ретінде пайдалану мүмкіндігін негіздеу үшін ұсынылған. Фосфогипстің рентгендік фазалық талдауы бойынша ескі және жаңа үйінділердің құрамында: гипс CaSO4∙2H2O - 87,93% және 85,04%; кварц SiO2 - 9,10% және 10,58%; бассанит CaSO4 ∙ 0,67H2O - сәйкесінше 2,97% және 4,38%. «Қазфосфат» ЖШС Тараз филиалының ескі және жаңа үйінділерінен алынған фосфогипс үлгілерінің химиялық құрамы зерттелді. Көрсетілген үйінділердің фосфогипс үлгілеріне дифференциалды-термиялық және ИҚ талдаулары жүргізілді. Негізгі құрамдас бөліктердің құрамында ерекше айырмашылықтар жоқ екені анықталды. Жүргізілген зерттеулердің негізінде құрамында ұзақ әсер ететін өндірістік нысандардың қатты қалдықтарынан кешенді аралас тыңайтқыш қоспаларының жаңа түрлерін алудың мүмкіндігі зерттелді. Жоспардағы кешенді аралас тыңайтқыш қоспа құрамын фосфатты зат, ылғал сақтайтын компонент ретінде вермикулит, егіс алқаптарының қышқылдануын және тұздануын азайту үшін топырақ жамылғысын бейтараптандырғыш ретінде микроэлементтерді мазмұндайтын ішкі жыныстар және фосфогипс.

A Review of the Commercial Uses of Sulphate Minerals from the Titanium Dioxide Pigment Industry: The Case of Huelva (Spain)

This study was focused on the historical evolution of the waste management policy carried out by the Spanish industry devoted to titanium dioxide pigments manufacturing for minimising its environmental impact. This challenge was achieved by modifying the original chemical process and converting the originally dissolved sulphate and sulphuric acid present in the final streams of the factory into sulphate minerals (melanterite FeSO4·7H2O, szomolnokite FeSO4·H2O, and gypsum CaSO4·2H2O). These by-products were physicochemically, mineralogically and radiologically characterised in order to gain basic information for its subsequent commercial use. Some of the uses summarised in this study for both ferrous sulphates are as a supplier of iron to prevent chlorosis, animal food, manufacture of cement (to reduce Cr VI), primary flocculants for ferrous sulphates, magnetite nanoparticle and nano-Fe2O3 formation, production of magnetite concentrate, remediation of polluted soils with metals, and treatment of wastewaters. Red gypsum was analysed as a substitute for natural gypsum in the manufacture of cement, construction materials, inhibitor in soil erosion, and the immobilisation of heavy metals in agricultural soils and carbonation processes.

Emissivity and reflectance spectra at different temperatures of hydrated and anhydrous sulphates: A contribution to investigate the composition and dynamic of icy planetary bodies

Accurate analyses of existing spacecraft data and telescopic observations are of fundamental importance to describe in detail the surface composition of icy planetary bodies, such as the icy galilean moons. However, the spectral library data to compare the remote data with planetary observations are usually restricted to small spectral ranges and collected only at room temperature.In this study, selected hydrated Mg-sulphates were studied. Emissivity and reflectance spectra were collected in the 3–20 μm and 0.25–16 μm range, respectively, with emissivity collected in the temperature range 300–673 K, and reflectance from 300 to 193 K. All samples were recovered after the heating and cooling cycles and were characterized by means X-ray powder diffraction. Rietveld refinements of the collected data were performed to evaluate the mineralogical composition of the samples before and after the thermal treatment.Both reflectance and emissivity measurements gave us information about the vibrational modes and overtones of SO4 and H2O. Moreover, the careful analysis of the collected data allowed us to study the influence of the cation substitution in the sulphate's crystal structures on the wavelength position of the SO4 vibrational modes. In particular, in simple salts [kieserite MgSO4·(H2O); hexahydrite MgSO4·6(H2O); gypsum CaSO4·2(H2O); thenardite Na2SO4; arcanite K2SO4; anhydrite CaSO4; barite BaSO4], the increase of the cation's radius gives a shift of the ν3 overtone towards higher wavenumbers, varying in the range 1880–2300 cm−1. On the other hand, it was observed that, in hydrated sulphates, that the increase of the strength of the hydrogen bond gives a shift of the ν3(SO4) overtones towards lower wavenumbers.At high temperature, the depth of several absorption bands in the emissivity spectra increases; however, when total or partial de-hydration occurs, a discontinuity in the deepening is observed due to the endothermic character of the dehydration phenomena.Among the investigated samples, several hydrated and anhydrous sulphates undergo de-hydration and/or phase transition at specific temperature conditions. This leads to the stabilization of new crystal structures with higher density compared to the low temperature hydrated ones. The likely occurrence of minerals dehydration will strongly affect the availability of free water in planetary depths and, as a consequence, the thickness of the icy crust. Likewise, the density changes between the different polymorphs will affect the buoyancy. This means that the structural behavior of the “non-icy” components of the icy crust have a significant impact on the structure and dynamics of the planetary bodies and have to be considered in planetological models.

Performance of the Halogen Technology for Determining the Moisture Content of Nanoparticulate Powders

Powder moisture content is a crucial parameter in many research and industry applications. Moisture content is required before conducting experiments, particularly when assessing nanomaterial risk using dustiness methods. Halogen moisture analyzers based on halogen technology are claimed to be more advantageous than other techniques for determining the moisture content of powders; they are rapid, accurate, reliable, cost effective and easy to use. However, no data in the open literature verify this claim, despite worldwide usage of these devices. In this technical note, we studied the performance of a representative halogen analyzer (HR83 manufactured by Mettler Toledo) for determining the water content of powder materials by comparing it with 3 reference thermogravimetric instruments. Our comparison covered 14 powders, of which 12 are nanoparticulate and representative of those most produced on an industrial scale. These test powders are of various chemical compositions (TiO2, SiO2, CuO and Gypsum CaSO4(H2O)1/2) and specific surface areas. Halogen technology was validated using 3 reference powder materials with known moisture contents. The biases on the reference values are within ±10% and the fairly small linearity in these biases confirmed the accuracy. The study gives good measurement repeatability using the halogen technology (5% in mean CV) and no significant differences in measured moisture content compared with reference devices were revealed. A relationship between the moisture contents and the powder specific surface areas (SSAs) was also highlighted. The powders characterized by high SSA values have the highest moisture contents. Overall, these results support the suitability of using the halogen technology to measure powder water content. More specifically, small companies and laboratories, which cannot invest in expensive sophisticated instruments, can effectively rely on halogen technology for this purpose.

Mechanisms of low-temperature vapor-gas streams formation from sulfide mine waste

This paper presents experimental data that revealed the potential for chemical element transport by low-temperature vapor-gas streams. The study was conducted on sulfide waste heap sites located in the Kemerovo region, Russia. Condensates of vapor-gas streams were collected and analyzed in the air above the waste heaps and during laboratory experiments using samplers specially designed for this purpose. The gas streams from a waste heaps are complex mixtures consisting of water vapor, sulfur- and selenium-containing compounds (sulfur dioxide SO2, dimethyl sulfide C2H6S, carbon disulfide CS2, dimethyl disulfide C2H6S2, dimethyl selenide C2H6Se, and dimethyl diselenide C2H6Se2), elemental sulfur (S6, S7, and S8) and various chemical elements, including rock-forming elements (Ca, Mg, Na, K, Si, Fe, Al, and Mn), metals (Cu, Zn, Pb, Ni, and Sn), and metalloids (As, Te, and Sb). The main sources of chemical elements in the gas streams are unstable secondary minerals associated with crystalline hydrates: gypsum CaSO4 × 0.5H2O, sideronatrite Na2Fe(SO4)2(OH) × 3H2O, serpierite CaCu3Zn(SO4)2(OH)6 × 3H2O, and copiapite (Mg,Zn,Fe2+Fe3+)4(SO4)6(OH)2 × 20H2O that formed during the oxidation of sulfide minerals. Some of the elements come from pore waters that are acidic, highly mineralized solutions. The mechanism of element migration from the pore waters is as follow: the water vapor phase transports elements in the form of aqueous ions, but complexed species (such as MeSO4(aq), MeCl(aq), Me(OH)+, etc.) remain in the salt residue. A significant contribution to the processes of transformation and transport of elements is made by biochemical methylation reactions, which occur in the presence of bacteria producers of methyl groups and are accompanied by the formation of volatile compounds of arsenic, selenium, sulfur, and tellurium.

Structural behaviour of an Australian silty clay (Coode Island silt) stabilised by treatment with slag lime

Abstract A microstructural study of Coode Island Silt (CIS), a soft silty clay from the Melbourne area of Australia, stabilised with slag lime is reported. Slag lime is a blend of 80–85 wt% slag, 15 wt% hydrated lime, Ca(OH)2 and 3–8 wt% gypsum CaSO4.2H2O, and is typically used for soil stabilisation in roading applications. The morphologies of several homogeneous mixtures of slag lime and CIS were studied by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD), which indicated the formation of the cementitious phases calcium carbonate, calcium silicate hydrate, calcium aluminium silicate hydrate, and calcium aluminium silicate carbonate. The progress of the CIS-slag lime reactions was also investigated by pH measurements. Slag lime was found to very actively promote pozzolanic reactions with CIS, shown by SEM to form crystalline reticular calcium silicate hydrate and other cementitious products. The most effective pozzolanic reactions occurred in a composite containing 12.5 wt% slag lime with CIS and contained the cement mineral ettringite, contributing to its high strength. These composites constitute a new class of materials with excellent potential for construction applications.

Cryogenic Minerals in Hawaiian Lava Tubes: A Geochemical and Microbiological Exploration

ABSTRACTThe Mauna Loa volcano, on the Island of Hawaii, has numerous young lava tubes. Among them, two at high altitudes are known to contain ice year-round: Mauna Loa Icecave (MLIC) and the Arsia Cave. These unusual caves harbor cold, humid, dark, and biologically restricted environments. Secondary minerals and ice were sampled from both caves to explore their geochemical and microbiological characteristics. The minerals sampled from the deep parts of the caves, where near freezing temperatures prevail, are all multi-phase and consist mainly of secondary amorphous silica SiO2, cryptocrystalline calcite CaCO3, and gypsum CaSO4·2H2O. Based on carbon and oxygen stable isotope ratios, all sampled calcite is cryogenic. The isotopic composition of falls on the global meteoric line, indicating that little evaporation has occurred. The microbial diversity of a silica and calcite deposit in the MLIC and from ice pond water in the Arsia Cave was explored by analysis of ∼50,000 small subunit ribosomal RNA gene fragments via amplicon sequencing. Analyses reveal that the Hawaiian ice caves harbor unique microbial diversity distinct from other environments, including cave environments, in Hawaii and worldwide. Actinobacteria and Proteobacteria were the most abundant microbial phyla detected, which is largely consistent with studies of other oligotrophic cave environments. The cold, isolated, oligotrophic basaltic lava cave environment in Hawaii provides a unique opportunity to understand microbial biogeography not only on Earth but also on other planets.

Effect of Microwave Heating on Moulding Sand Properties with Gypsum Binder

AbstractThe paper presents results of initial research on the possibility of applying microwave radiation in an innovative process of making casting moulds from silica sand, where gypsum CaSO4∙2H2O was acting as a binding material. In the research were compared strengths and technological properties of moulding mixture subjected to: natural bonding process at ambient temperature or natural curing with additional microwave drying or heating with the use of microwaves immediately after samples were formed. Used in the research moulding sands, in which dry constituents i.e. sand matrix and gypsum were mixed in the ratio: 89/11. On the basis of the results of strength tests which were obtained by various curing methods, beneficial effect of using microwaves at 2.45 GHz for drying up was observed after 1, 2 and 5 hours since moisture sandmix was formed. Applying the microwaves for hardening just after forming the samples guarantees satisfactory results in the obtained mechanical parameters. In addition, it has been noted that, from a technological and economic point of view, drying the silica sand with gypsum binder in microwave field can be an alternative to traditional molding sand technologies.

Solvothermal recrystallization of α-calcium sulfate hemihydrate: Batch reactor experiments and kinetic modelling

Under appropriate temperature conditions, natural gypsum CaSO4·2H2O, dispersed in an aqueous solution, turns into calcium hemihydrate CaSO4·½H2O. This transformation is performed in a 2L stirred baffled reactor, where the temperature increase is measured and controlled on line. The water content of the suspension and its size distribution are measured on samples during the transformation. Experiments are achieved at nominal temperature of 140°C, with three initial solid mass fractions 0.5, 0.33 and 0.25. The transformation takes place through a dissolution followed by re-crystallization. A model is proposed which takes into account the size distribution of the particles of gypsum, their dissolution rate, primary and secondary nucleation and growth rates of calcium hemihydrate. The set of equations is solved with a MATLAB software, which allows to test the assumptions on the kinetics of the transformation and fit their parameters. A satisfying representation of the variations of the extent of transformation and of volume and surface mean diameters of the suspension is obtained.

Minerals of the Wooltana Cave, Flinders Ranges, South Australia

Analysis of guano specimens from Wooltana cave, formerly occupied by ghost bats (Macroderma gigas), shows the principal mineral is whitlockite with seven other crystalline phases. These include dittmarite NH4MgPO4 ·H2O, biphosphammite (NH4K)H2PO4, syngenite K2Ca(SO4)2 ·H2O, görgeyite K2Ca5(SO4)6 ·H2O, brushite CaHPO4 ·2H2O, gypsum CaSO4 ·2H2O and whewellite CaC2O4 ·H2O. The mineralogy of the guano samples was determined by powder X-ray diffraction and field emission SEM and the phase analysis was quantified by Reitveld methods. Dittmarite is a very rare mineral and only reported from a small number of localities. It cannot be formed directly in caves as it is normally crystallised from boiling water. We propose that it is formed by decomposition of struvite, NH4MgPO4 ·6H2O, in the presence of ammonia arising from the fresh bat guano. The minerals dittmarite, struvite and newberyite, MgHPO4 ·3H2O, can transform between themselves in caves and in kidney stones (in vitro) according to the conditions. Two minerals, görgeyite, and whewellite, have not previously been reported from guano deposits. Gypsum and brushite are present in some samples and show up to 18% PO4 for SO4 in gypsum substitution and up to 18% SO4 for PO4 in brushite. Some pure end-member crystals were also identified. The whitlockite is also substituted by sulfate by up to 30% and averaging 10%. The cave walls are dolomite and a speleothem present shows calcite, magnesium calcite, gypsum and leonite, K2Mg(SO4)2 ·4H2O an unusual sulfate mineral. The extent and presence of the sulfates and of whewellite confirms that substantial ground water has entered the cave consistent with the known wetter past climate.

Monitoring of Calcium Sulphate Phase Transformations Using High-Temperature X-Ray Diffraction

Within the task the so called high-temperature X-ray diffraction analysis (HT-XRD) was used in the field of sulphated binders. Gypsum or calcium sulphate CaSO4.2H2O is the basic raw material for the sulphated binder production. As it is known by gradual warming of gypsum its different phase transformations can be obtained which significantly differ in its properties. Using this analysis identifications of temperatures of particular phase transformations in the CaSO4·xH2O system can be performed.

Elucidating the Mechanism by Which <i>Gypsum fibrosum</i>, a Traditional Chinese Medicine, Maintains Cutaneous Water Content

Aquaporin-3 (AQP3) plays an important role in maintaining the normal water content of the skin. Previously, we revealed that the expression of cutaneous AQP3 increased following oral administration of Gypsum fibrosum (main component: CaSO₄) to mice. The purpose of this study is to elucidate the mechanism by which Gypsum fibrosum increases the expression of cutaneous AQP3 in a keratinocyte cell line. Gypsum fibrosum or CaSO₄ was added to keratinocytes, and the expression level of AQP3, the Ca concentration, the activity of protein kinase C (PKC), and the degrees of phosphorylation of both extracellular signal-regulated kinase (ERK) and cAMP response element binding protein (CREB) were measured. The mRNA and protein expression levels of AQP3 increased significantly 6 h-post addition of Gypsum fibrosum. In keratinocytes treated with Gypsum fibrosum, increases in the concentration of intracellular Ca, PKC activity, and the phosphorylation of ERK and CREB were observed. Pre-treatment with GF109203X, a PKC inhibitor, suppressed the mRNA expression levels of AQP3. Similarly to treatment with Gypsum fibrosum, the addition of CaSO₄ led to the same observations in keratinocytes. It is hypothesized that Gypsum fibrosum causes an increase in the intracellular Ca concentration, PKC activity, and the phosphorylation levels of ERK and CREB, resulting in increased AQP3 expression in keratinocytes. In addition, it is possible that the effect of Gypsum fibrosum is attributable to CaSO₄, based on the results demonstrating that the mechanisms of action of Gypsum fibrosum and CaSO₄ were nearly identical.

Radium Uptake by Recrystallized Gypsum: An Incorporation Study

Batch experiments of 226Ra uptake by recrystallized gypsum CaSO4.2H2O(s), monitored over 200 days, have shown that the published value for the distribution coefficient of 226Ra between gypsum and an aqueous phase, 0.03 (Gnanapragasam and Lewis, 1995), is an upper limit. This suggests that this value needs to be confirmed. A solid solution between gypsum and radium sulfate (Ca,Ra)SO4.2H2O(s) cannot be considered per se, which is not surprising considering gypsum's high solubility product (lg Ks = -4.58) and the ionic radius of VIIIRa (1.48Å), when compared to that of VIIICa (1.12Å).

Chemical, morphological and chromatic behavior of mural paintings under Er:YAG laser irradiation

Several pigments (malachite CuCO3⋅Cu(OH)2, azurite 2CuCO3⋅Cu(OH)2, yellow ochre (goethite α-FeOOH, gypsum CaSO4⋅2H2O), St. John’s white CaCO3 formed from slaked lime) and respective mural paintings specimens were subjected to the free-running Er:YAG laser radiation in order to study their damage thresholds, in a broad range of laser fluences, both in dry and wet conditions. The specimens’ damage thresholds were evaluated by spectroscopic methods, colorimetric measurements and microscopic observation. The pigments containing –OH groups were found to be more sensitive than St. John’s white; hence the most sensitive paint layers in dry conditions are those containing malachite, azurite (both 1.3 J/cm2) and yellow ochre (2.5 J/cm2) as compared to the ones containing St. John’s white (15.2 J/cm2). The presence of wetting agents (w.a.) attenuated the pigments chemical alteration. The damage thresholds of all the paint layers, in presence of w.a., were found to be around 2.5 J/cm2. The alteration was caused by thermo-mechanical damage and by binding medium ablation of a fresco and a secco prepared specimens, respectively.

Adsorption inhibition of the growth of CaSO4 · 2H2O crystals in aqueous solutions

The growth of gypsum CaSO4 · 2H2O crystals in aqueous solutions has been investigated, both on individual crystals and on crystals in a homogeneous suspension in the supersaturation range 1–20 and the temperature range 291–303 K. The dependence of the crystal growth rates on supersaturation is described by an S-shaped curve, which is due (i) to the specific features of CaSO4 self-adsorption on the surface of growing gypsum crystals and (ii) to the multistage character of dehydration of Ca2+ and SO42− ions at the transition from the solution to the solid. At solution supersaturations of ∼20, at which the adsorption layer is close to saturation, the growth rate weakly depends on supersaturation, approaching the limit value Gjp. A model of the crystal hydrate growth in the kinetic mode is constructed which reveals the conditions at which the growth rate reaches the limit value Gjp and does not change with a further increase in supersaturation.

X-ray diffraction analysis of urinary calculi: need for heat treatment

Although X-ray diffraction (XRD) is the most reliable method for analysis of urinary stones, it has its specific limitations. It fails to detect amorphous phases, cannot distinguish between chemically different phases having identical lattice geometry (e.g., brushite CaHPO4.2H2O and gypsum CaSO4.2H2O) and may miss some phases (e.g., apatite and calcium urates) due to peak overlaps. XRD of urinary stones was performed using a DRON 2.0 diffractometer with CuKalpha radiation and repeated after calcining the sample, preferably with weighing. XRD of the calcined samples enabled detection of amorphous magnesium phosphates, poor crystallized apatite mixed with struvite, weddellite and/or organic matter, hidden organic calcium salts mixed with uric acid; unambiguously discriminated between brushite and gypsum, struvite and its potassium analogue; confirmed presence of quartz in one stone. Statistical study of 341 samples from Rostov region has shown that three-phase mixtures are most frequent (32.3%). Redoing XRD phase analysis after heat treatment, preferably at 500 and/or 900 degrees C, considerably enhances capabilities of the method due to (i) avoiding peak overlaps; (ii) crystallization of amorphous phases; (iii) concentrating minority inorganic components in organic stones; (iv) different decomposition products from indistinguishable phases; (v) semi-quantitative information from the weight loss data.

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

At least five crystalline phases can be found in the CaSO4−H2O system, which are gypsum CaSO4·2H2O, the subhydrates α- and β-CaSO4·0.5H2O, and the soluble and insoluble anhydrite CaSO4. The formation of these five phases in the CaSO4−H2O system and their transformations were investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) in this study. Furthermore, revised structural models for β-CaSO4·0.5H2O and soluble anhydrite CaSO4 are presented. The hydration of α-CaSO4·0.5H2O was studied at 25 °C and showed that the reaction with H2O started immediately after mixing the two reactants and that the formation of CaSO4·2H2O was coupled to the depletion of α-CaSO4·0.5H2O. The thermal decomposition of CaSO4·2H2O was investigated in the temperature range of 25−500 °C and showed the formation of α-CaSO4·0.5H2O followed by the formation of soluble anhydrite AIII-CaSO4, which was gradually converted to insoluble anhydrite AII-CaSO4. The thermal decomposition of α-CaSO4·0.5D2O wa...