Calcium Sulfate Phases Research Articles

Bio-Inspired Fluorescent Calcium Sulfate for the Conservation of Gypsum Plasterwork.

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO4·nH2O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO4·0.5H2O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO4·2H2O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance.

Solution-driven processing of calcium sulfate: The mechanism of the reversible transformation of gypsum to bassanite in brines

Here, we show that calcium sulfate dihydrate (gypsum) can be directly, rapidly and reversibly converted to calcium sulfate hemihydrate (bassanite) in high salinity solutions (brines). The optimum conditions for the efficient production of bassanite in a short time (<5 min) involve the use of brines with c(NaCl) > 4 M and maintaining a temperature, T > 80 °C. When the solution containing bassanite crystals is cooled down to around room temperature, eventually gypsum is formed. When the temperature is raised again to T > 80 °C, bassanite is rapidly re-precipitated. This contrasts with the better-known behaviour of the bassanite phase in low-salt environments. In low-salinity aqueous solutions, bassanite is considered to be metastable with respect to gypsum and anhydrite, and therefore gypsum-to-bassanite conversion does not occur in pure water. Interestingly, the high-salinity transformation of gypsum-to-bassanite has been reported by many authors and used in practice for several decades, although its very occurrence actually contradicts numerical thermodynamic predictions regarding solubility of calcium sulfate phases. By following the evolution of crystalline phases with in situ and time-resolved X-ray diffraction/scattering and Raman spectroscopy, we demonstrated that the phase stability in brines at elevated temperatures was inaccurately represented in the thermodynamic databases. Most notably for c(NaCl) > 4 M, and T > 80 °C gypsum becomes readily more soluble than bassanite, which induces the direct precipitation of the latter from gypsum. The fact that these transformations are controlled by the solution provides extensive opportunities for precise manipulation of crystal formation. Our experiments confirmed that bassanite remained the sole crystalline phase for many hours before reverting into gypsum. This property is extremely advantageous for practical processing and efficient crystal extraction in industrial scenarios.

Calcium Sulfate Nanoparticles in Surface Sediments of Lingding Bay of the Pearl River Estuary, China: Implications for the Nonclassical Crystallization Pathway of Gypsum in the Natural Estuary Environment

The mechanism of the nonclassical crystallization pathway of calcium sulfate dihydrate (gypsum) with calcium sulfate hemihydrate (bassanite) as a precursor has been considered in many studies. However, studies on the crystallization of gypsum in natural environments have rarely been reported, especially with regard to natural estuaries, which are one of the most important precipitation environments for calcium sulfate. Here, surface sediments (0–5 cm) of Lingding Bay of the Pearl River Estuary in China were sampled and analyzed. X-ray powder diffraction (XRD) analysis showed that calcium sulfate in the surface sediments mainly existed in the form of gypsum. In high-resolution transmission electron microscopy (HR-TEM) analysis, calcium sulfate nanoparticles were observed in the surface sediments. These particles mainly included spherical calcium sulfate nanoparticles (diameter ranging from 10–50 nm) and bassanite nanorod clusters (sizes ranging from 30 nm × 150 nm to 100 nm × 650 nm), and their main elements included O, S and Ca, with small amounts of N, Si, Na and Mg. The bassanite nanorods self-assembled into aggregates primarily co-oriented along the c axis (i.e., [001] direction). In epitaxial growth into larger bassanite nanorods (100 nm × 650 nm), the crystal form of gypsum could be observed. Based on the observations and analyses, we proposed that the crystallization of gypsum in surface sediments of the natural estuary environment could occur through the nonclassical crystallization pathway. In this pathway, bassanite nanoparticles and nanorods appear as precursors (nanoscale precursors), grow via self-assembly, and are finally transformed into gypsum. This work provided evidence supporting and enhancing the understanding of the crystallization pathway of calcium sulfate phases in the natural estuary environment. Furthermore, the interactions between calcium sulfate nanoparticles and the natural estuary environment were examined.

Stable Bassanite Bulk Phase Formed in Aqueous Solution under the Control of Polymer-Mediated Water Activity

Bassanite, as the metastable phase in the family of calcium sulfate minerals, plays an important role in natural and industrial processes, but a stable bulk bassanite can rarely precipitate in nature or be fabricated under ambient conditions. Here, we report the first example of stable bulk bassanite precipitated in solution, which fills in the gap of calcium sulfate phase diagram. By precisely controlling water activity with polymer (polyvinyl pyrrolidone), bulk bassanite is formed, instead of transforming into thermodynamically favorable gypsum. A new thermodynamic model of the non-classical crystallization process is described in terms of water-driven colloidal assembly, and molecular dynamics simulation further demonstrates the role of water molecules in interacting with the polymer ligands to initiate the assembly and growth of bassanite into bulk. These findings not only contribute to a deeper understanding of the role of water activity in the formation of stable bulk bassanite in extreme natural environments but also allow for the fabrication of bassanite materials for industrial applications at room temperature.

Mapping mineralogical heterogeneities at the nm-scale by scanning electron microscopy in modern Sardinian stromatolites: Deciphering the origin of their laminations

Stromatolites are found throughout the geological record and have received strong attention because they provide precious information about paleoenvironments and microbial paleobiodiversity. However, while this information is interpreted based on our knowledge about modern analogs, the latter remains incomplete. Here, we investigated the environmental and biological information recorded by modern stromatolites in Mari Ermi, a coastal pond in Western Sardinia that experiences severe seasonal evaporation and large salinity variations. For this purpose, we combined a variety of analytical tools, allowing to characterize the mineralogical composition of these stromatolites from the bulk, centimeter-scale to the nanometer-scale and assess the spatial distribution of the mineral phases. In particular, we used quantified x-ray elemental maps provided by energy dispersive x-ray spectrometry analyses coupled with scanning electron microscopy (SEM-EDXS). These maps were processed using an innovative data treatment allowing (i) mineral phase recognition, (ii) assessment of the mineral formula of each phase and (iii) mapping of their spatial distribution with a spatial resolution down to a hundred nanometers. Overall, this proved as an efficient approach to unravel mineralogical heterogeneities and detect informative mineral phases overlooked by bulk analyses. Mari Ermi stromatolites were mostly composed of magnesian calcite with an average Mg/Ca of ~0.1–0.2 as indicated by bulk analyses. However, microscopy observations revealed variable Mg-contents of calcite with a specific distribution, the most Mg-enriched calcites (Mg/Ca up to ~0.5) being systematically distributed around microbial remnants. Moreover, Mari Ermi stromatolites comprised an alternation of Mg-richer and Mg-poorer laminae, which paralleled the varying enrichment in microbial remnants of alternating laminae. The combination of SEM-EDXS, focused ion beam milling and transmission electron microscopy allowed to detect several minor phases, hardly or not detected at all by bulk analyses, such as aragonite, a variety of clay minerals as well as a gypsum-like phase. An additional calcium sulfate phase, best interpreted as S-apatite (cesanite), was detected by SEM-EDXS. Overall, this mineral assemblage and its definite spatial distribution record only discrete stages of the evaporation of Mari Ermi lagoon. Moreover, microorganisms seem to have a major control on Mg substitution in calcite. As a result, the dynamics of microbial populations, influenced by the salinity variations induced by evaporation in the lagoons, generates the formation of laminae in Mari Ermi stromatolites.

Two-dimensional lamellar phosphogypsum/polyethylene glycol composite PCM: Fabrication and characterization

In this work, a nanocomposite phase change material (PCM) has been designed by combining two-dimensional lamellar anhydrous calcium sulfate with polyethylene glycol (PEG). We report a facile strategy to controllably fabricate two-dimensional lamellar anhydrous calcium sulfate (LAH) with the average thickness of 28.63 nm from phosphogypsum (PG) through ethylenediamine tetraacetic acid disodium (Na2EDTA) induction in glycerol and ethylene glycol solutions at 98 °C. The obtained 2D lamellar CaSO4 was a slit-type mesoporous material stacked by the nanosheet of calcium sulfate. It has a specific surface area of 70.02 m2/g, which is 10 times larger than phosphogypsum. Na2EDTA acts as a crystal habit-directing agent to regulate crystal morphology through nonclassical nanoparticle-mediated crystallization processes, resulting in the crystalline morphology tending to be lamellar. Lamellar anhydrous calcium sulfate phase change composites (LAHPCMs) were prepared with 2D lamellar anhydrous nano-CaSO4 and polyethylene glycol (PEG). The LAHPCMs had a high latent heat storage capacity (92.99 J/g). Lamellar anhydrous calcium sulfate phase change composites have good thermal stability and durability, structure stability, and good liquid leakage resistance. These results provide the possibility for phosphogypsum to be used for energy storage and thermal insulation.

Structures and dynamic hydration of CaSO4 clusters in supersaturated solutions: A molecular dynamics simulation study

The characteristics of CaSO4 clusters before the formation of crystalline calcium sulfate phases in supersaturated solutions remain uncertain. We have used extensive molecular dynamics simulations to thoroughly investigate structures and dynamic hydration of [(CaSO4)n]0 clusters in supersaturated solutions. There are three typical CaSO4 clusters, dissolved ion associated species, stable aggregates and CaSO4 precursors. For the dissolved ion associated species [(CaSO4)n]0 (n ≤ 7), the aggrgegation of these clusters accompanies with dehydration and cooperative hydration. The neutral [(CaSO4)n]0 (7 < n < 70) clusters are stable aggregates, and the size is about 30– 50 Å. the aggregation of these clusters also accompanies with dehydration of Ca2+, but the cooperative hydration between Ca2+ and SO42− does not change obviously. There are two kinds of dynamic hydration around Ca2+ in [(CaSO4)n]0 structures. Water molecules shared by Ca2+ and SO42− can be retained during the aggregation processes. The further aggregation of stable aggregates accompanying with ion coordination recombination will result in the formation of hydrous or anhydrous CaSO4 precursors ([(CaSO4)n]0 (n ≥ 70)) under different conditions, such as the supersaturation degree and temperature

Force field for calcium sulfate minerals to predict structural, hydration, and interfacial properties

Calcium sulfates such as anhydrite, hemihydrate, and gypsum find widespread use in building materials, implants, and tissue healing. We introduce a simple and compatible atomistic force field for all calcium sulfate phases that reproduces a wide range of experimental data including lattice parameters, surface, hydration, mechanical, and thermal properties in 1% to 5% accuracy relative to experiments. The performance is several times better than prior force fields and DFT methods, which lead to errors in structures and energies up to 100%. We explain (hkl) cleavage energies, the dynamics of (hkl) water interfaces, and new insights into molecular origins of crystal-facet specific hydration and solubility. Impressive agreement of computed and experimentally measured hydration energies is shown. The models add to the Interface force field (IFF) and are compatible with multiple force fields (CHARMM, AMBER, GROMOS, CVFF, PCFF, OPLS-AA) for property predictions of sulfate-containing materials from atoms to the large nanometer scale.

Influence of cement with different calcium sulfate phases on cementitious tile adhesive mortars: Microstructure and performance aspects

Cement is generally used as a mineral binder in construction materials. The components formed as a result of the reaction between cement and water give physical strength to construction products. This reaction is influenced by the mineralogical composition of the cement. The clinker composition does not vary greatly for ordinary Portland cement. However, the calcium sulfate composition may vary from manufacturer to manufacturer, and even from batch to batch, depending on the effect of heat dissipation during the process of grinding the clinker with calcium sulfate. The type of calcium sulfate affects the hydration reaction and performance properties of cementitious tile adhesive (CTA). In this study, the hydration behavior and performance of CTA samples prepared from cement containing different calcium sulfate forms were investigated. The developed microstructures were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD), and calorimeter, while the performance was tested according to EN 12004 standards.

α or β?-hemihydrates transformed from dihydrate calcium sulfate in a salt-mediated glycerol–water solution

α-and β-hemihydrate calcium sulfate (HH), the most important calcium sulfate phases, are usually achieved by autoclave hydrothermal reaction and calcination in air, respectively. Herein, we report an investigation on preparation of HH from dihydrate calcium sulfate(DH) in a salt-mediated glycerol-water solution. We found that both α-HH and β-HH in forms of apparently single crystals could be obtained at 110 ℃ only by tuning the concentrations of glycerol, Na2EDTA, and NaCl. TG-DSC, XRD, SEM, and Raman spectroscopy were used to identify and characterize the product crystals. The results indicate that the high relative supersaturation of HH could be the key factor determining the generation of β-HH crystal in solution. Glycerol and NaCl generally decrease the water activity, enhance the crystallization kinetics, and therefore allow the presence of a metastable HH phase (especially for compact α-HH bulk crystal). But too high concentration of glycerol (i.e., ≥70%) and NaCl (i.e., ≥0.2 M) would be favorable for β-HH crystallization with obviously defective crystal morphology due to the dramatically increased local relative supersaturation. Na2EDTA could retard the recrystallization but well regulate the morphology of HH bulk crystals.

Copper and cobalt improve the acid resistance of alkali-activated cements

Experimental evidence of a new acid degradation mechanism in alkali-activated cements (AACs) micro-doped with copper (Cu) and cobalt (Co) is presented in this work. Cu and Co incorporation into binary metakaolin and basic oxygen furnace (BOF) slag-based AACs reduced bulk permeable porosity and acid penetration and retarded the formation of calcium sulfate phases upon exposure to acid. Analysis of microstructural evolution and elemental mobility using X-ray diffraction and electron microprobe analysis (EMPA) showed that Cu and Co doping was associated with major differences in AAC leaching patterns when exposed to sulfuric acid. Converging lines of evidence suggest that acid resistance is improved by the preferential mobilization of Cu and Co, along with other multivalent cations (i.e., magnesium), at the acid degradation front(s), stabilizing the AAC binder and inhibiting further deterioration.

ENSUBA–A new method for solving the gypsum problem in waste autoclaved aerated concrete

AbstractAccording to a previous study, the existing building stock in Germany contains up to 10% of gypsum building material, which is integrated in walls, ceilings, and floors. After the demolition of these buildings and their reuse in recycling building material (RC building materials), gypsum causes severe problems, such as sulfate attack and the corresponding concrete damages. Due to these problems the majority of the gypsum‐containing demolition waste is deposited in landfill sites. The other option – recycling the autoclaved aerated concrete (AAC) – is very difficult: according to the current technological standards and a previous study, almost all AAC formulations contain a certain amount of sulfate mineral phases in the form of gypsum or anhydrite. The use of the sulfate minerals is necessary for production reasons and to meet mechanical material requirements such as compressive strength and shrinkage. In this work a method known from the conservation and restoration field where ammonium carbonate compresses are used to remove gypsum from paintings and frescos is modified and applied to AAC recycling. It could be shown that the calcium sulfate phases can be removed to an amount, which allows to use AAC as an RC material or to make deposition much easier.

Sulfuric acid resistance of one-part alkali-activated mortars

One-part alkali-activated (geopolymer) mortars based on three different silica-rich starting materials and sodium aluminate, with and without ground granulated blast furnace slag (GGBFS) addition, were tested regarding sulfuric acid resistance according to DIN 19573:2016-03 (70 days at pH = 1). Corresponding pastes were characterized by XRD, SEM, chemical analysis, 29Si MAS NMR and 1H-29Si CPMAS NMR after water storage and after acid exposure. The mortars exhibited a high resistance against sulfuric acid attack, with the best ones conforming to the requirements of DIN 19573:2016-03. The analytical results showed that this was due to precipitation of silica gel at the acid-mortar interface, which formed a mechanically stable layer that protected the subjacent mortar and thus inhibited further degradation. The addition of GGBFS decreased the acid resistance via formation of expansive calcium sulfate phases.

Calcifying Response and Recovery Potential of the Brown Alga Padina pavonica under Ocean Acidification

Anthropogenic carbon dioxide (CO2) emissions are causing ocean acidification (OA), which affects calcifying organisms. Recent studies have shown that Padina pavonica investigated along a natural pCO2 gradient seems to acclimate to OA by reducing calcified structures and changing mineralogy from aragonite to calcium sulfate salts. The aim of the present study was to study the potential for acclimation of P. pavonica to OA along the same gradient and in aquaria under controlled conditions. P. pavonica was cross-transplanted for 1 week from a normal pH site (mean values of pHTS 8.1 and pCO2 = 391 μatm) to a low pH site (mean values of pHTS 7.4 and pCO2 = 1044 μatm) and vice versa. Results showed that this calcifying alga did survive under acute environmental pHTS changes but its calcification was significantly reduced. P. pavonica decalcified and changed mineralogy at pHTS 7.4, but once brought back at pHTS 8.1, it partially recovered the aragonite loss while preserving the calcium sulfate phases that formed...

Crystallographic Control in the Replacement of Calcite by Calcium Sulfates

The transformation of calcite into calcium sulfate phases in the presence of an aqueous phase is relevant to many technological processes, including remediation of acid mine drainage, exploitation of subsurface reservoirs, geological CO2 sequestration, and the conservation of building stone. At acidic pH, the interaction of sulfate-bearing aqueous solutions with calcite cleavage fragments results in the replacement of gypsum, bassanite, and/or anhydrite depending on the reaction time and temperature. Here we show that a fraction of the calcium sulfate crystals (gypsum, bassanite, or anhydrite) grows in all cases oriented onto the calcite surfaces, forming a product layer that reproduces the initial morphology of the calcite crystals. Calcium sulfates form on calcite surfaces as thick three-dimensional crystals, suggesting a Volmer–Weber mechanism of epitactic growth. The epitaxial relationships found by two-dimensional X-ray diffraction are (010)Gp//(104)Cal; (010)Bs//(104)Cal; and (200)Anh//(104)Cal and ...

Tissue reaction and material biodegradation of a calcium sulfate/apatite biphasic bone substitute in rat muscle

SummaryBackground/ObjectiveA biphasic ceramic bone substitute consisting of calcium sulfate and hydroxyapatite has been reported to give good clinical outcome regarding bone regeneration and may serve as a carrier for antibiotics in the treatment of bone infections. Often, the overlying muscle is in direct contact with the synthetic graft. The dissolving bone substitute induces inflammation, which may be harmful to the surrounding soft and muscle tissue. The aim of the present study was to evaluate the surrounding soft tissue reaction and the biodegradation of the biphasic bone substitute.MethodsRods (3 mm × 6 mm) were cast and implanted in the rat abdominal rectus muscle. The rods were either soaked or not soaked in autologous bone marrow before insertion to induce bone formation. Thirty-two rats underwent bilateral operation. After 6 weeks and 12 weeks, the bone substitute material and the surrounding muscle were harvested. The right rod was evaluated by histology to study tissue reaction and the left rod was analysed with micro-computed tomography and scanning electron microscopy to study bone substitute degradation.ResultsThe muscle tissue around the material was similar at 6 weeks and 12 weeks, with or without prior treatment with bone marrow. The remaining material showed close contact with the muscle, and blood vessels penetrated the material in both groups. Wide bundles of collagen were embedded around the apatite particles, more at the 12-week time point. No bone formation was found, either at 6 weeks or 12 weeks, and scanning electron microscopy showed that the calcium sulfate phase was resorbed after 6 weeks with the calcium phosphate phase remaining intact. Micro-computed tomography showed significantly more hydroxyapatite at 6 weeks than after 12 weeks.ConclusionCalcium sulfate hydroxyapatite bone substitute can be used as a carrier for antibiotics or other drugs, without adverse reaction due to the fast resorption of the calcium sulfate. No bone formation was seen despite treating the bone substitute with autologous bone marrow.

Effect of biomass-sulfur interaction on ash composition and agglomeration for the co-combustion of high-sulfur lignite coals and olive cake in a circulating fluidized bed combustor

This study aimed to investigate the effect of biomass-sulfur interaction on ash composition and agglomeration for the co-combustion of high-sulfur lignite coals and olive cake in a circulating fluidized bed combustor. The tests included co-combustion of 50–50% by wt. mixtures of Bursa-Orhaneli lignite+olive cake and Denizli-Kale lignite+olive cake, with and without limestone addition. Ash samples were subjected to XRF, XRD and SEM/EDS analyses. While MgO was high in the bottom ash for Bursa-Orhaneli lignite and olive cake mixture, Al2O3 was high for Denizli-Kale lignite and olive cake mixture. Due to high Al2O3 content, Muscovite was the dominant phase in the bottom ash of Denizli Kale. CaO in the bottom ash has increased for both fuel mixtures due to limestone addition. K was in Arcanite phase in the co-combustion test of Bursa/Orhaneli lignite and olive cake, however, it mostly appeared in Potassium Calcium Sulfate phase with limestone addition.

Carboxylic acids: effective inhibitors for calcium sulfate precipitation?

Results are reported here of an investigation into the effects of three carboxylic acid additives (tartaric, maleic and citric acids) on the precipitation of calcium sulfate phases. Precipitation reactions were followed at pH 7 in the pure CaSO4system and in experiments with 0–20 ppm carboxylic acids added using in situ UV-VIS spectrophotometry (turbidity). The solid products were characterized in terms of their mineralogical composition, using X-ray diffraction, during and at the end of each reaction, and in terms of their morphological features, by scanning electron microscopy. All additives increased the time needed for turbidity to develop (induction time, start of precipitation) and the comparison between additive and additive-free experiments showed that, at equivalent concentrations, citric acid performed far better than the other two carboxylic acids. In all cases bassanite precipitated first and with time it transformed to gypsum. The addition of citrate stabilized bassanite and changed the final gypsum habit from typical needle-like crystals in the pure CaSO4system to plates in the citrate-additive experiments.

Quantitative X-Ray Powder Diffraction Analysis of Portland Cements: Proficiency Testing for Laboratory Assessment

Abstract Quantitative X-ray powder diffraction analysis (QXRD) is being used within the cement industry for phase characterization of hydraulic cement. The current ASTM standard test method for powder diffraction analysis of cements provides guidance, but not an explicit method, for quantifying phase concentrations. The standard utilizes qualification criteria, where an analysis of a set of certified reference materials must fall within stated precision and bias limits. Validation of X-ray powder diffraction analyses by the Rietveld method is particularly important because the normalization inherent in the mass fraction calculations can obscure accuracy problems. Currently, the only certified reference materials for phase abundance are a set of NIST SRM clinkers, which lack the calcium sulfate and carbonate phases found in portland cements. A set of portland cements was distributed to 29 laboratories for analysis according to each lab’s individual protocols. The objective was to provide each lab with quantitative feedback on its precision and accuracy performance. The results from all the labs are presented graphically with Youden plots that incorporate ranking to illustrate relative lab precision and accuracy based upon a consensus mean for each phase and ASTM C1365 performance qualification criteria. Labs that fall outside of the compliance limits are provided with information via the Youden plots to assess their systematic and random error. Proficiency testing of this sort provides participating laboratories with a quantitative assessment of their performance relative to peers using a wider range of materials encompassing the broad spectrum of modern hydraulic cement production. These newer materials may include, for example, the calcium sulfate phases and the limestone additions that have become commonplace in today’s cements. Such a quantitative assessment could be used to qualify laboratories and may be stipulated in a specification.

Phase Stability and Inhibition of Calcium Sulfate in the System NaCl/Monoethylene Glycol/H2O

Summary Calcium sulfate is one of the major mineral scales in oil and gas production. Hemihydrate (CaSO4·0.5H2O) and anhydrite (CaSO4) are the predominant sulfate scales formed at high temperature, while gypsum (CaSO4·2H2O) scale may form at low temperatures (&amp;lt;~45°C). However, it has been shown in this study that anhydrite can form at low temperature in the presence of excess amounts of monoethylene glycol (MEG), and this may occur during offshore production with long tie-backs. The prediction and prevention of calcium sulfate scales requires knowledge of the phase behavior of the three major phases of calcium sulfate. The phase behavior of different calcium sulfate phases is related to the supersaturation state, temperature, and fugacity of water. In this study, the effect of a common hydrate inhibitor, MEG, on calcium sulfate solubility and phase behavior was investigated. This study was run with NaCl/CaSO4/MEG/H2O solutions at 0–6 molality (M) NaCl and 0–95 wt% MEG at 4–70°C. Three approaches were taken to determine the kinetics of calcium sulfate phase transition at various temperatures, ionic strengths, and MEG concentrations: (1) dissolution of gypsum, (2) dissolution of anhydrite, and (3) nucleation and precipitation of calcium sulfate by mixing calcium- and sulfate-containing solutions. The effect of scale inhibitors on phase transition was also evaluated. Phase transition of gypsum to anhydrite was observed in the presence of high concentrations of NaCl and MEG, regardless of the experimental approach. The transition boundary of temperature and concentrations of NaCl and MEG can be estimated from solubility of calcium sulfate and the fugacity of water. The inhibition mechanism of hexamethylene diamine tetra (methylene phosphonic acid) (HDTMP), one of the most effective inhibitors for calcium sulfate scale, was also tested by investigating the kinetics of precipitation and inhibition of calcium sulfate.