Gypsum Mold Research Articles

Effect of Different Starting Hemihydrate Powders on the Performance of Gypsum Moulds

In order to determine the effect of different starling hemihydrate, (Plaster of Paris) on the life of gypsum moulds, microstructure of two different Plaster of Paris and gypsum moulds made from these hemihydrates have been characterised using x-ray diffraction (XRD), themiogravimetry and a scanning electron microscope (SEM) attached with an energy dispersive x-ray spectrometer (EDX). XRD results have shown that one of the Plaster of paris contains anhydrite phase which retains even after moulds are made. Both the gypsum moulds are immersed in a solution containing sodium silicate (Na 2 SiO 3 ) deflocculant to determine the possible interactions between the moulds and the Na 2 SiO 3 containing slip casting slurry. Experimental results have shown that a layer formed on the mould surface as a result of a reaction between the deflocculant and the gypsum mould The reaction layer was thicker for the mould containing anhydrite phase.

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda ‘white earth’ (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from −3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from −6·34‰ to −0·39‰ PDB (mean=−3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=−0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3−/CO32− groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.

Cake growth control in slip casting

A control technique of cake growth in slip casting was studied. The influence of geometrical features of the gypsum mold upon the cake growth pattern was examined in experimental slip casting using T-shaped gypsum molds. The T-shaped molds were of two types; one had walls inside and the other did not. These walls were formed from silicone rubber and did not transmit water. A water flow in the gypsum mold was controlled by silicone rubber walls, so that cake growth was controlled. Cake growth during slip casting using T-shaped mold was predicted by computer simulation.

Research on Diageneses of Cambrian Shoal Facies Carbonate Rocks in the Xiadong Area, Hubei Province

AbstractWith continuous outcrops, developed shoal facies rocks, complete types of diagenesis and changeable diagenetic environments, Cambrian strata are well developed in the Xiadong area, Yichang, Hubei Province. Under the combined influence of numerous diageneses, secondary pores can be formed, which result in better reservoir properties of the rock strata.The Cambrian rocks in this area consist of mainly carbonate rocks and secondarily detrital rocks. The carbonate rocks are dominated by grainstones including wormkalk, calcirudite‐calcarenite, oolitic limestone and oncolitic limestone. Graded bedding and cross bedding are well developed in the strata, which indicates that the formation environment is of a high‐energy shoal facies.In this area, there has developed a sequence of stable Cambrian platform carbonate deposition. The evolution trend is as follows: open sea shelf faciesŕintertidal low‐energy restricted sea faciesŕintertidal high‐energy shoal faciesŕcoastal shoal faciesŕevaporite tidal‐flat facies.The diageneses that the strata have experienced mainly include dolomitization, dedolomitization and dissolution, which are constructive diageneses for the formation of secondary pores, such as intercrystal pores, intercrystal solution pores, gypsum mold pores and caverns of dolomite.The diagenetic facies intervals can be divided into the unitary and the compound ones, totalling 22 in the area.In the early atmospheric fresh‐water diagenetic environment and the late epidiagenetic environment, Cambrian rocks, especially dolomite of the Middle and Upper Cambrian, experienced extensive and profound fresh water corrosion, forming pore intervals with a porosity ranging from 5% to 15%.

Translucent Alumina Produced by Slip-Casting Using a Gypsum Mold.

Translucent Al2O3 ceramics could be successfully produced by slip casting using a gypsum mold, provided that CaSO4 impurities, which had penetrated into the green compacts from the gypsum mold, were removed by the wash of calcined compacts with HCl aqueous solution. Al2O3 ceramic samples were sintered at 1350°C for 2h under vacuum. The relative densities of the sintered Al2O3 ceramic compacts with the HCl treatment were slightly higher than those of the untreated samples. Grains in the HCl-treated samples grew homogeneously with about 1μm in diameter. The sintered Al2O3 ceramics with the HCl treatment were translucent. Transmittance values were measured by spectroscopy. Transmittance values in the HCl-treated samples increased from 0 to 12% as the wavelength increased from 300 to 900nm.

Removal of Lead lons Using Porous Hydroxyapatite Monoliths Synthesized from Gypsum Waste.

Porous hydroxyapatite (HAp) monoliths synthesized from gypsum mold waste were used for removing lead ions from aqueous solution. The gypsum waste was crushed into small blocks and converted directly to porous HAp pieces in a 0.5mol/dm3 diammonium hydrogen phosphate solution at 55 and 80°C. Porous structures were effected by reaction temperatures. Twenty and 40mg of lead ions were removed from 100cm3 aqueous solution by stirring with 1.0g porous HAp monoliths at pH 3 for 5 to 30min. HAp was dissolved in the pH 3 acidic solution, but Pb-Cl apatite, which was formed from Pb2+, PO43- and Cl-, was stable in the pH 3 acidic solution.

Slip Casting of Alumina Powder Mixtures with Bimodal Size Distribution. Influence of Particle Size Difference between Fine and Coarse Powders on Packing and Consolidation Process.

Slips prepared from bimodal size distributed alumina powder mixtures, which consisted of 30% fine powder and 70% coarse powder, were cast. The two average particle sizes of the bimodal distribution were varied. When the difference between the average particle sizes was small, green compacts had a low packing density and uniform packing structure. In this case, the fine and coarse particles were deposited by filtration of gypsum mold, due to the high viscosity of the slips, an insufficient dispersion of the fine particles and the small size of the coarse particles. On the other hand, when the difference between the average particle size was large, green compacts had a high packing density and nonuniform packing structure. In this latter case, the coarse particles were first deposited by sedimentation to form the skeletal structure; then, the fine particles percolated through the voids of the skeletal structure by filtration and were deposited in the voids. On approaching the bottom part of the green compacts, the skeletal structure showed increased density. Since the water absorption rate was slow, the fine particles did not fill the voids sufficiently. As a result, the maximum packing density of 84% was obtained in the bottom part of the green compact. The packing density decreased with increasing distance from the bottom part of the specimen.

Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada

The Steep Rock Group of northwest Ontario's Wabigoon subprovince is one of the world's thickest Archean carbonate platform successions. It was deposited unconformably over a 3001-2928 Ma gneissic terrane, and contains a remarkable group of biogenic and oolitic limestones, dolostones, micrites, and karst breccias capped by a thick paleosol developed between and over karst towers. The presence of aragonite fans, herringbone calcite, and rare gypsum molds suggests that the carbonate platform experienced at least local anaerobic and hypersaline depositional conditions. This sequence shows that a combination of chemical and biological processes was able to build a carbonate platform 500 m thick by 3 billion years ago. The carbonate platform is structurally overlain by a mixture of complexly deformed rocks of the Dismal Ashrock forming a mélange with blocks of ultramafic volcaniclastic rocks, mafic volcanics, carbonate, tonalite, lenses of Fe-ore rock, and metasedimentary rocks, in a shaly, serpentinitic, and fragmental ultramafic volcaniclastic matrix. The mélange shows evidence of polyphase deformation, with early high-strain fabrics formed at amphibolite facies, and later superimposed brittle fabrics related to the final emplacement of the mélange over the carbonate platform. An amphibolite- through greenschist-grade shear zone marks the upper contact of the mélange with overlying mafic volcanic and tuffaceous rocks of the ca. 2932 Ma Witch Bay allochthon, interpreted as a primitive island arc sequence. We suggest an evolutionary model for the area that begins with rifting of an arc sequence (Marmion Complex of the Wabigoon arc) that initiated subsidence and sedimentation on the Steep Rock platform and its correlatives that extend for a restored strike length exceeding 1000 km. Shallow water carbonate sedimentation continued until the platform was uplifted on the flanks of a flexural bulge related to the approach of the Witch Bay allochthon, representing collision of the rifted arc margin of the Wabigoon subprovince with the Witch Bay arc. Mélange of the Dismal Ashrock was formed as off-axis volcanic rocks were accreted to the base of the Witch Bay allochthon prior to its collision with the Steep Rock platform.

Heat-resistant saggers produced by slip casting

Heat-resistant saggers containing granular silicon carbide filler with particles up to 130 µm in size were obtained by slip casting in gypsum molds. The silicon carbide content varied from 30 to 55 wt.%. Prototype batches of saggers of different shapes were tested in gas-heated and mazut-heated kilns and exhibited good service properties, which made it possible to decrease the amount of reject articles in firing.

Effect of Slurry Characters of Titanium Nitride on Forming and Sintering Behaviors.

Titanium nitride (TiN) slurry was prepared for slip casting with water, and the effects of slurry characters on forming and sintering behaviors were studied. 46.4vol% TiN powder was dispersed in water with triethylamine (TEA) and polycalboxyl ammonium (PCA), and formed in a gypsum mold. Formed compacts were sintered at about 0.7MPa N2 pressure, at 1750°C for 3h. The slurries with TEA content above 0.02mass% showed low viscosity and non-thixotropy. The slurries with PCA content in the range 0.06-0.08mass% showed the minimum viscosity and minimum thixotropy. High-density specimens (density of 3.8-3.9×103kg·m-3) could be sintered from the low viscosity slurry, prepared by the addition of TEA with pH higher than 10 and optimum addition of PCA.

Fabrication of 3Y-TZP/SUS304 functionally graded materials by slip casting; application of porous alumina molds

Three mole percent Y 2O 3 doped tetragonal zirconia polycrystals (TZP) and stainless steel 304 (SUS) functionally graded materials (FGM) were fabricated by slip casting. To overcome the problems of the gypsum mold (GM) in the slip casting process, an alumina mold (AM) was prepared and its properties investigated after controlling the microstructure and the sintering conditions. The optimal conditions for the dispersion of monophase and binary-phase slurries were investigated. According to these results, eleven layer TZP/SUS-FGM were fabricated with a GM and an AM. The specimens, using an AM, showed no contamination on the surface, easier thickness control of each layer and resulted in higher productivity. Especially, no symptoms of degradation were observed in the SUS made by AM. Therefore, it is more desirable to use a porous AM than a GM in the fabrication of TZP/SUS-FGM by the slip casting process.

The effect of Al and Ca impurities on the properties of quartz ceramics

The effect of aluminum and calcium impurities on quartz ceramic properties is studied and its causes are elucidated. A possible mechanism of migration of these impurities from gypsum molds into ceramic crock in the course of casting is considered. The necessity of the presence of Al2O3, if quartz ceramic products are used at temperatures above 1450°C, is stated.

Sinterability of SiC powder coated uniformly with Al ions

SiC particles coated uniformly with Al ions (0.25 mass% Al2O3) in an aluminum nitrate solution were consolidated by filtration through a gypsum mold. Hot-pressing in vacuum gave dense SiC (above 99% relative density) in the temperature range of 1900–1950 °C under a pressure of 39 MPa. The microstructures of dense SiC consisted of 2–5 μm grains of low aspect ratios (below 2). The fracture toughness and flexural strength of SiC increased gradually as the hot-pressing temperature became higher and were 4.3 Mpa m0.5 and 350 MPa, respectively, with hot-pressing at 1950 °C. Crack propagation in SiC shifted from intergrain to intragrain with increasing hot-pressing temperature.

Fabrication of Net Shape High Strength TiB<sub>2</sub>/SiC Composite by Slip Cast Molding

The strength and the size accuracy of TiB2/SiC composite sintered bodies, which were shaped by the slip cast molding using gypsum molds and hot isostatic pressing (HIP), were investigated. Near net shaped bodies with no cracks were obtained after optimizing the dispersibility of the slurry by adjusting the amount of a dispersant and the pH value. The increase in the solid concentration of the slurry improved the dispersibility of TiB2 particles that acted as fracture origins in the HIPed body. The four-point flexural strength of the sintered body by HIP was 710MPa, which was same level of that made by hot-pressing. Combustors were fabricated by using this slurry. The standard deviation of each parts, 110mm in diameter was 0.1mm. The coefficient of variation in the size among eleven parts was 0.9%. The flexural strength of the specimen cut from the combustor was also equivalent to that of the hot-pressed body.

Porous hydroxyapatite monoliths from gypsum waste

Porous hydroxyapatite (HAp) monoliths were synthesized from gypsum mold waste with diammonium hydrogen phosphate solution by hydrothermal treatment at 50-190°C and their properties studied. Gypsum waste samples could be completely converted to HAp with 0.5 mol dm –3 (NH 4 ) 2 HPO 4 at 50°C in 15 days and 100°C in 2 days. This paper describes the effect of hydrothermal reaction conditions such as reaction temperature, time and concentration of (NH 4 ) 2 HPO 4 on the formation of new HAp monoliths and their porous properties.

Colloidal processing and mechanical properties of silicon carbide with alumina

Submicrometer-sized SiC coated with SiO2 of 0.4–1.8 wt.% and α–Al2O3 powder of median size 0.2 μm were mixed in aqueous solutions in the pH range 3.0–10.0. The SiC/Al2O3 (4.3–6.9 wt. %) powders were consolidated by filtration through gypsum molds and hot-pressed at 1600°–2040 °C under a pressure of 39 MPa. These compacts were densified to near the theoretical density at 1700°–1800 °C. The sintering mechanisms are discussed based on the analysis of shrinkage curves of SiC/Al2O3 compacts during hot-pressing. The equiaxed SiC grains grew with low aspect ratios below 1800 °C and changed to plate-like grains at 1900 °C. The fracture toughness of SiC as a function of average grain size reached a maximum of 5 Mpa · m0.5 at 2.5 μm grains of low aspect ratios of 1–2. The flexural strengths at room temperature were 230–430 MPa in the SiC above 98% of the theoretical density and showed a similar grain size dependence.

Microwave pressure chamber processing of a denture repair polymer.

Most denture base repairs are made using a chemically activated acrylic resin polymer for reasons of cost and expediency. Heat-activated polymers are not typically used because they require a custom split cast gypsum mold, longer polymerization time, and higher laboratory fees. Repairs made using a chemically activated polymer compared with a heat-activated polymer generally have reduced transverse strength, poorer color stability, increased water sorption, and weak bond strength to existing acrylic resin. A method of utilizing a microwave polymer in a commercially available microwave flask is described. This technique allows denture base repairs to be made with a heat-activated polymer in a manner similar to that of chemically activated repairs.

Sinterability of monodisperse TiO2 powders

Sintering of monodisperse titania powders with a relatively large grain size (0.84±0.24 µm) is investigated. It is established that the use of nonagglomerated monodisperse powders makes it possible to fabricate ceramics with a density of 99.5% of the theoretical value and a narrow distribution of pore sizes without preliminary milling and by relatively simple technological means, namely, cold pressing and casting into gypsum molds followed by sintering at a relatively low temperature. In combination with a high density such a structure makes the developed ceramic material applicable for the production of porous membranes for microbiology and medicine.

Electrophoretic deposition of ceramic slips under a pulse-periodic current

Experimental data on dectmphmetic precipitation of ceramic slips (for porcelain, faience, and household ceramics) onto an aluminum electrode under apulse-periodic current with different parameters (frequency. voltage, time) me presented, Advantages ofdoctrophnrmic technology with dbe use ofupulse-periodic current with u low-frequency spectrum are described. Tbc body grows 25 times faster than in gypsum molds, the density of the precipitate and the strength of the dried and fired specimens increase. and the total consumption of power is reduced. The intensification of the technological process is explained by electromagnetic activation of the working medium and the shaped mixture by the longwave spectrum.

Porous alumina ceramics for slip casting molds

Although a slip cast process using a gypsum mold enables large and complicated dense green body to be made, the green body is contaminated with calcium and sulfur impurities from the gypsum mold. In the case of a resin mold, external pressure must be applied due to its poor water wettability. Porous alumina ceramics are used as a mold material for the first time to overcome these problems. The porous alumina ceramics show higher water wettability than gypsum or resin. The porous alumina molds have submicron pores, and provide larger penetration pressure and higher casting rate than gypsum molds.