Formation Of Calcium Sulfate Research Articles

Confinement Leads to Control over Calcium Sulfate Polymorph

AbstractAlthough many processes of great biological, technological, and environmental importance, such as the formation of biominerals, the templating of nanostructures, and salt weathering, occur in confinement rather than in bulk solution, little is known about the influence of confinement over the precipitation of inorganic crystals. The effects of confinement on the precipitation of calcium sulfate are investigated using a crossed‐cylinder apparatus which offers confinement that varies continuously from zero to tens of micrometers. While the thermodynamically stable form of calcium sulfate (gypsum) is always observed at large surface separations, a remarkable stabilization of the metastable phases amorphous calcium sulfate (ACS) and calcium sulfate hemihydrate (bassanite, plaster of Paris) is observed even at micrometer‐scale separations. For the first time, the approach is extended to study the combined effects of soluble additives and confinement, which are often present in natural or synthetic systems, and it is shown that this considerably extends the lifetimes of ACS and hemihydrate. These confinement effects are attributed to hindered aggregation of precursor particles at small surface separations, which limits polymorph conversion. While these results have immediate relevance to salt weathering and biomineralization processes, they are also important to the many crystallization and aggregation‐driven processes occurring in small volumes.

Modified pectin-based polymers as green antiscalants for calcium sulfate scale inhibition

In water processing industry, the precipitation of calcium and magnesium as carbonate, sulfate or phosphate and their adhesion to equipment surfaces is the main problem of mineral scaling. Many chemicals and antiscalants have been widely reported to mitigate the mineral scaling problems, but the efficiency is low on both economic and environmental grounds. In view of that, hydrolyzed pectin-based graft copolymers were synthesized and further functionalized by polymer analogous reactions for use as green antiscalant. The effect of functional graft copolymers on the crystal formation of calcium sulfate was investigated. From there it was observed that the ability to prevent the precipitation of calcium sulfate from homogeneous condition was almost independent of temperature and pH. The morphology and crystal structure of the precipitates were investigated using imaging and X-ray diffractometer (XRD). The result shows that the reported functional graft copolymers changed the morphology and shape of the crystal structures of CaSO4 precipitates from tubular to spherical.

High-resolution electron microscopy study of the early stages of calcium sulfate formation

High-resolution electron microscopy study of the early stages of calcium sulfate formation

Micro-scale deformation of gypsum during micro-indentation loading

The role of plastic deformation and fracture during small-scale, low-rate contact scenarios in gypsum is investigated. Results indicate that the hardness decreases with increasing load, on all axes, ranging from 216kg/mm2 at 0.25N to 91kg/mm2 at 4.91N on the (010) plane and from 122kg/mm2 at 0.98N to 75kg/mm2 at 4.91N on the (011) and (110) cleavage planes. Higher hardness values for lower loads are attributed to dislocation saturation and localized necking, which leads to densification and work-hardening. Transmitted light microscopy reveals that the decrease in hardness for increasing load is related to an increase in subsurface cracking along the (010) and (011) cleavage planes, enabling the indenter to penetrate deeper into the crystal. Coupled with an increase in subsurface cracking is an increase in surface smoothness as a result of increased plastic material flow beneath the indenter, with crack densities decreasing from 34% at a 0.25N load to 6% at a 4.91N loading. Raman spectroscopy reveals that the higher-hardness material comprises water-reduced forms of calcium sulphate: hemihydrate and α-CaSO4. Local temperatures required for the dehydration of gypsum range from 118 to 142°C, indicating such temperatures were reached during plastic flow, and related heat generation, of the material beneath the indenter under relatively quasi-static conditions. Evaporated water generates voids in the original crystal structure and enables the sulphate to deform more easily.

Comparative Evaluation of the Efficacy of Calcium Sulfate Bone Grafts in Crystalline and Nano-Crystalline Forms in Fresh Extraction Socket Sites: A Radiographic and Histological Pilot Study

Aim: Extraction site reconstruction is essential for alveolar ridge preservation, keeping in mind future placement of implants. In an attempt to preserve the alveolar bone and to avoid ridge augmentation prior to implant placement, numerous biocompatibleregenerative materials have been used immediately following tooth extraction to fill the socket.Methodology: This article is a pilot trial presenting early trends of the comparative use of two forms of calcium sulfate, i.e. medical grade (Dentogen)® and nano-crystalline (Nanogen)® as bone graft materials in immediate extraction sockets of three cases, based on histological and radiological assessments.Results: At the end of 4 months following grafting, the extraction sites in all the three cases revealed no significant difference between the two materials. Radiographic assessment revealed an almost complete graft resorption and replacement with spongy bone with bone density comparable to the surrounding bone. Histopathological assessment revealed that 100% of the bone was vital with 52% trabecular bone and 48% bone marrow.Conclusion: Calcium sulfate both medical grade and nanocrystallineform hold a valid premise for use as regenerative materials in extraction sockets with no significant differences seen between them in this pilot trial.

Prevention of calcium sulfate formation in seawater desalination by ion exchange

In mechanical vapor compression (MVC) desalination, high operating temperatures (>120°C) allow for high heat transfer coefficients and reduced compressor size, both of which lower total capital costs. To prevent formation of calcium sulfate scale on high-temperature desalination heat exchangers, the inverse solubility of calcium sulfate requires high sulfate removal. To selectively remove sulfate from seawater, a weak-base anion-exchange resin (Relite MG 1/P) was used. In synthetic seawater, the resin shows high sulfate selectivity. Conveniently, the exhausted resin can be regenerated with concentrated acidified (pH 4) blowdown brine from the desalination heat exchanger. Using the same exhaustion and regeneration times, high desalination concentration factors increase chloride concentrations in the blowdown brine, which allows for slower regenerant flow rates thereby increasing regeneration efficiency and sulfate removal. Using a high concentration factor helps regeneration; however, this must be balanced...

Prevention of calcium sulfate formation in seawater desalination by ion exchange

Prevention of calcium sulfate formation in seawater desalination by ion exchange

Desalination of geothermal water by membrane distillation

Membrane distillation process was used for desalination of hot (333 K) geothermal water, which was applied in the plant producing heating water. The investigated water contained 120 g salts/dm3, mainly NaCl. The mineral composition was studied using an ion chromatography method. The obtained rejection of solutes was closed to 100%, but the small amounts of NH3 also diffused through the membrane together with water vapour. However, the composition of obtained distillate allowed to use it as a makeup water in the heating water system. The geothermal water under study was concentrated from 120 to 286 g NaCl/dm3. This increase in the solution concentration caused the permeate flux decline by a 10-20%. The geothermal water contained sulphates, which was subjected to two–fold concentration to achieve the concentration 2.4-2.6 g SO4 2−/dm3 and the sulphates then crystallized in the form of calcium sulphate. As a results, an intensive membranes scaling and the permeate flux decline was observed. The XRD analysis indicated that beside the gypsum also the NaCl crystallites were deposited on the membrane surfaces. The fresh geothermal water dissolved the mixed CaSO4 and NaCl deposit from the membrane surface. This property can be utilized for self-cleaning of MD modules. Using a batch feeding of MD installation, the concentration of geothermal water was carried out over 800 h, without significant performance losses.

Composite scale formation and assessment by the theoretical Scaling Potential Index (SPI) proposed previously for a single salt

Usually combination of various salts and other species co-exist in salty waters. However, traditionally the assessment of scaling propensity has been empirical and for a single salt. First the paper reviews the work of the author's research group in the composite scale formation and shows the application of the theoretical Scaling Potential Index (SPI) based on Gibbs Free Energy of reaction which was originally introduced [1] and later developed by the author [2, 3] to composite scale formation incorporating previously proposed [4] Pitzer model. Second, it discusses composite calcium carbonate and calcium sulfate formation in batch and RO dynamic system which has shown that co-existence of salts affects thermodynamics, kinetics and scale structure. Third it shows that the ratio of sulfate to carbonate significantly affects composite scale formation and flux decline. Fourth, it uses Sheikholeslami's theoretical SPI (taking into account ion interactive forces, concentration polarization, and osmotic pressure effects) to calculate the onset of scale formation on the membrane surface and compare with experimental results obtained in a reverse osmosis unit; comparison of flux decline and relevant analytical techniques showed that Sheikholeslami's theoretical SPI successfully predicted the onset of scale formation and crystal phases for calcium carbonate and calcium sulfate.

Calcium and sulphur distribution in fired clay brick in the presence of a black reduction core using micro X-ray fluorescence mapping

A new finding of the observation of an elemental gradient or zoning of calcium and sulphur in fired brick bodies is described, that does not appear to have been reported in the literature before. Many raw clays used in brickmaking and pottery contain sulphur and calcium evenly distributed in low amounts in the unfired clay body. However, when the clay body is fired in the kiln at 1050 °C, the elements sulphur and calcium appear to combine to a compound, most probably calcium sulphate. The formation of calcium sulphate, visualised using energy dispersive imaging micro X-ray fluorescence ( μ-XRF), only occurs around a so-called reduction core, also known as ‘black core’, caused by reduced magnetite Fe 3O 4 in the centre of the brick body. The presence of a black reduction core appears to cause the formation of a calcium sulphate layer around the black reduction core. This research contributes to the understanding of phenomena like salt formation, efflorescence and durability in solid clay brick bodies. Large sums are spent on building conservation by national economies and improvement of bricks is of key interest to them.

Maximising RO recovery using a new antiscalant for high sulphate waters

Despite the current economic turmoil the need for good quality drinking water continues relentlessly. In the last eight years 11,825 reverse osmosis (RO) plants have come on line with a combined output of 25 million m3/d. Although large seawater plants take most of the ‘desalination news, awards and headlines’, a large number of small and medium sized plants (requiring less energy) are being commissioned that use other water sources. Recent data shows that some 8,500 of these systems are producing 62% of the extra water capacity installed since 2000; these plants use brackish, surface or waste water as a feed source. The increasing demand for the minerals that are found in arid and desert areas has increased the need for desalinated water in many mining areas. In these regions ground waters frequently contain high levels of calcium, magnesium, silica and sulphate; in such cases water desalination increasingly demands sophisticated speciality antiscalants and dispersants. This paper examines the use of new antiscalant compounds developed specifically to inhibit the formation of calcium sulphate. The chemistry and deposition mechanisms are presented along with details of formulating Genesys CAS a sulphate specific antisclant. Results from an operating plant are presented showing the improvement in plant performance and a simple model is developed to equate the improvement in plant operation associated with high recovery to actual cost savings. Membrane systems will continue to be built and operated with increasingly poorer quality feed waters. For this reason it is essential that new and improved speciality antiscalants are developed so that plants can continue to operate at their highest efficiency thereby saving water and energy.

Study on the Crystalline Morphology of Calcium Sulfate in H<sub>3</sub>PO<sub>4</sub> and HNO<sub>3</sub> Aqueous Medium

This study aims to investigate the morphologies and the existing phases of calcium sulfate crystal by the reaction of H2SO4 with Ca(NO3)2·4H2O in acid medium using microscopic method and XRD powder diffraction. A series of experiments were designed for studying the crystal morphology and the hydrated form under coexisitence of phosphoric acid and nitric acid at 338K. The micrograph results show that in the H3PO4-HNO3-H2O system, the crystal shapes are changed from thick-long rod to thin-short rod, then needle-like even tiny needle gradually with the increase of acid content in solution. The solubility of calcium sulfate increases gradually with the increase of phosphoric acid or nitric acid concentration in solution. The XRD results show that with the increasing of acid concentration, the form of calcium sulfate is transferred from dihydrate to hemihydrate, that is, at higher acid concentration condition calcium sulfate hemihydrate is more stable in solution with the particles to be tiny and needle-like; while at lower acid concentration condition the crystals are mainly calcium sulfate dihydrate with larger rod particles which indicate a good filtration properties.

In Vitro Elution of Daptomycin by a Synthetic Crystallic Semihydrate Form of Calcium Sulfate, Stimulan

A synthetic crystallic semihydrate form of calcium sulfate, Stimulan, was evaluated as a biodegradable carrier for the daily in vitro elution of daptomycin. Daptomycin and Stimulan were admixed at a ratio of 95:5. Elution lasted for 28 days. Eluted concentrations peaked on days 1 and 11, when the mean values were 1,320.1 and 949.2 microg/ml, respectively. The lowest eluted concentration was detected on day 28. These results support the application of the system described in experimental models of osteomyelitis.

Treatment of experimental osteomyelitis caused by methicillin-resistant Staphylococcus aureus with a synthetic carrier of calcium sulphate (Stimulan ®) releasing moxifloxacin

The objectives of this study were to assess the efficacy of a synthetic semihydrate form of calcium sulphate (Stimulan ®) in experimental bone infection caused by methicillin-resistant Staphylococcus aureus (MRSA). Osteomyelitis was induced after inoculation of the test pathogen in the left tibia of 72 New Zealand rabbits assigned to the following groups: 18 control rabbits (Group A); 18 rabbits with Stimulan ® implanted (Group B); and 36 rabbits with moxifloxacin-impregnated Stimulan ® implanted (Group C). Rabbits were sacrificed at weekly intervals and cancellous bone was harvested for histopathology and for estimation of bacterial growth and concentrations of moxifloxacin. Bacterial growth from cancellous bone of Group C was significantly lower than the respective growth of Groups A and B on all days of sacrifice. The main histological finding of animals in all three groups was a moderate to intense inflammatory reaction accompanied by fibrosis. The degree of fibrosis was higher in Group C compared with both other groups. Infiltration by giant cells was also observed, which was greater in Group C on Day 42. Antibiotic levels in bone were higher for bone samples closer to the site of implantation. In conclusion, Stimulan ® admixed with 10% moxifloxacin was very effective in achieving complete eradication of the causative pathogen in experimental osteomyelitis caused by MRSA.

In vitro elution of moxifloxacin and fusidic acid by a synthetic crystallic semihydrate form of calcium sulphate (Stimulan™)

Stimulan™ was evaluated in vitro as a biodegradable carrier for local delivery of moxifloxacin and fusidic acid. Moxifloxacin or fusidic acid was mixed with calcium sulphate at a ratio of 95:5 to prepare five replicas per antibiotic. In vitro elution was estimated daily using a high-performance liquid chromatography (HPLC) system. Elution of moxifloxacin lasted for 31 days. Eluted concentrations reached their peak on Day 13 (mean level 745 μg/mL); the lowest eluted concentration was detected on Day 30 (mean level 367 μg/mL). Elution of fusidic acid lasted for 14 days. Eluted concentrations reached their peak on Day 6 (mean value 249.5 μg/mL); the lowest eluted concentration was detected on Day 13 (mean value 10.9 μg/mL). The presented results revealed that Stimulan™ may allow adequate in vitro elution of moxifloxacin and fusidic acid. The latter results support the application of this system in experimental models of osteomyelitis.

Characteristics of salt inclusions in polar ice from Dome Fuji, East Antarctica

We studied the depth profile of the chemical properties, number concentration and average diameter of micro‐inclusions in Antarctic ice from Dome Fuji. Analysis of the data indicated that the distribution of inclusions was not significantly modified during ice sheet flow, although a main constituent Na2SO4·10H2O changed into another solid phase in the deep ice. In the chemical form, sodium sulfate and magnesium sulfate were dominant in the interglacial and early‐glacial ice, but were not detected in the glacial‐maximum ice. Instead, the sulfate in the glacial‐maximum ice was mainly in the form of calcium sulfate. This is strong support for the hypothesis that the neutralization regime of acid aerosols in the atmosphere and snowpack dramatically changed from chemical reaction with sea salt to that with dust during glacial maxima.

Calcium Sulphate Ceramics for Orthopaedic Applications

The use of calcium sulphate externally on the human body in the form of Plaster of Paris (POP) has been well established, but its possible use inside the human body as bone void filler is a recent idea. It has been realized that the quality of the calcium sulphate and its physical characteristics are the key factors for its reproducible performance inside human body, and most commercial powders may not be suitable. Controlling the size and shape of the calcium sulphate hemihydrate/bihydrate crystals is necessary to obtain a product, which will get resorbed in human body at a rate consistent with the new bone growth.It has been realized that the α-hemihydrate form of calcium sulphate is a more suitable candidate for such applications. Hence, α-hemihydrate of calcium sulphate was prepared in the laboratory and ceramic shapes with requisite porosity and strength were fabricated. The powders and pellets were characterized for their morphology and microstructure respectively. These shapes after radiation sterilization have been used in certain test cases for orthopaedic surgery patients with bone defects. These trials have met with considerable success. Results obtained during the actual trials at L.T.M.G. Hospital, Mumbai are described.

Development of acrylonitrile copolymers for scale control in cooling water systems

The formation of calcium sulphate and carbonate scale has been prevented by using acrylonitrile-acrylic acid (AN-AA) and acrylonitrile-methacrylic acid (AN-MAA) copolymers over a temperature range of 50–80°C at different pHs from 7.0 to 8.5. The efficiency of the antiscalants was assessed by various techniques such as a chemical screening test, constant potential electrolysis and impedance measurements. The change in crystal habits by the addition of antiscalant was seen using XRD spectrum and SEM photographs. The effectiveness of polymers on microorganisms and forming gelation with calcium ion and iron-dispersing ability were also studied. The amount of scale deposition increased with increase in temperature and pH. The polymers acted as good antiscalants at low temperature and pH ranges. They also acted as good flocculants under some special conditions. For example, at a higher pH of 8.5 and temperature 80°C with 20 ppm dosage level of the AN-AA polymer, 100% was achieved. Further, they had good iron dispersibility and can gelate with Ca 2+ ions.

An injectable calcium sulfate-based bone graft putty using hydroxypropylmethylcellulose as the plasticizer.

The addition of a plasticizer to synthetic bone graft substitutes can improve handling characteristics, injectability, and the ability to uniformly fill defects. Restoration of large medullary bone defects using an injectable calcium sulfate-based putty using hydroxypropylmethylcellose as the plasticizer was compared to conventional calcium sulfate paste in a canine model. Beginning 2 weeks following implantation, serial clinical and specimen radiographs demonstrated a similar progressive resorption of the implanted materials and replacement with new bone for both the putty and paste forms of calcium sulfate. The area fraction of new bone and remaining implant material in bone defects treated with the putty were not significantly different from defects treated with conventional calcium sulfate paste after 13 and 26 weeks. In addition to its handling characteristics, the putty was biocompatible and as effective as conventional calcium sulfate paste in achieving substantial bony restoration of a large, critical-size bone defect.

Activation of fly ash/cement systems using calcium sulfate anhydrite (CaSO 4)

A number of studies had been conducted on the activation of fly ash using gypsum and sodium sulfate. Anhydrite, another form of calcium sulfate, has not been used for this purpose. This paper presents an exploratory study on the effectiveness of anhydrite in activating fly ash cement systems. Anhydrite (10%) was added into cement mortars with up to 55% fly ash replacement. The prepared mortars were allowed to cure in steam at 65°C for 6 h before normal room temperature water curing. Significant strength increases (up to 70%) compared to the control mortars were observed as early as after 3 days curing. Improvements in the pore size distribution of the mortars were also observed due to the activation. The results of scanning electron microscopy (SEM) examination and quantitative X-ray diffraction (XRD) analysis show that, with accelerated curing, a large quantity of ettringite (AFt) was formed during the early stage of hydration of the anhydrite-activated fly ash cement pastes. This might be the main cause of the high early strength of the activated fly ash cement systems. A comparison was made using anhydrite and gypsum as activators. For an equivalent SO 3 content, anhydrite is more effective in increasing the early-age strength of the cement/fly ash mortars, but less effective in increasing the later-age strength than gypsum.