Papers by Keyword: Calcium Sulfate

Abstract: This study examined the effect of waste-derived calcium sulfate additions (2 to 8 weight percent) on the tribological properties of a sintered high-carbon steel material, commonly used for frictional applications. The calcium sulfate powder was obtained by crushing and subsequent calcination of waste plaster molds previously used for slip casting of ceramics. Samples were made using the powder metallurgy method. Powder mixtures were uniaxially die compacted and sintered at 1,100°C for 30 minutes in a reducing atmosphere. It was found that the density of the samples decreased with increased amounts of calcium sulfate. This was due to the lower theoretical density of the calcium sulfate as well as volumetric expansion of the samples. Ball-on-disc tribological testing was performed at room temperature. The addition of calcium sulfate resulted in an increase of the kinetic friction coefficient. The measured wear volume was slightly reduced with the addition of calcium sulfate less than 6 weight percent, beyond which the wear increased. The hardness of the samples was reduced with the addition of calcium sulfate. Increased calcium sulfate content decreased the sample deformation resistance while increasing the kinetic friction coefficient and the wear. This was likely due to the microstructural changes of the samples, which will be discussed.

Abstract: Autoclaved aerated concrete is an ecological building material with suitable properties and long-time tradition. The research of secondary raw materials usage and technology optimization is needed in order to increase the efficiency and sustainability of its production. It is important to focus on development of mineralogical composition while studying the influence of input materials and the parameters of hydrothermal synthesis on the properties of final product. This is especially important in case of tobermorite formation as the bearer of strength in the structure. Main focus of this paper is applicability of fly ash (silica component) and admixture of energo-gypsum (sulfate component) in production of lime-ash composite in hydrothermal conditions. The research was focused on autoclaving of samples at temperature of 170°C in three various isothermal sustains. Final samples were analyzed by XRD and SEM in order to proof the influence of input materials. Results show that 5% admixture of energo-gypsum had positive influence on tobermorite formation.

Abstract: This study examined the effect of calcium sulfate additions on the microstructure and frictional properties of a 95bronze-5graphite material, commonly used for frictional applications. The samples were prepared using a powder metallurgy process. The calcium sulfate powder was obtained from recycled plaster molds previously used for ceramic slip casting. The plaster molds were cleaned, crushed and ball milled to obtain powder, which was calcined at 400 °C for 4 hours and screened. 2 to 8 weight percent calcium sulfate powder was added to a premixed powder composed of copper and tin and mixed before the graphite was added. The powder mixture was compacted into a disc shape under a pressure of 500 MPa. The compacted samples were sintered at 750 °C for 30 minutes in a reducing atmosphere. It was found that the green density of the samples decreased with increased amounts of calcium sulfate. After sintering, the densities were found to have decreased due to sample expansion. A finer microstructure was observed in the samples containing calcium sulfate powder, possibly due to a grain boundary pinning effect. The addition of calcium sulfate resulted in the reduction of both the measured friction coefficient and wear of the bronze-graphite samples. However, the addition of calcium sulfate powder above 6 weight percent appeared to show decreasing effects.

Abstract: The effect of calcium sulfates varieties on the properties of calcium aluminate cement-based self-leveling mortar have been investigated, and the hydration kinetics, hydrated products and microstructures are characterized by isothermal calorimeter, X-ray diffraction and mercury intrusion porosimetry, respectively. The results show that the technological properties of mortars are significantly affected by calcium sulfate varieties and content. The setting times are shortened drastically with the addition of calcium sulfates. Mortars with hemihydrate show higher early strength and less drying shrinkage. In contrast, using anhydrite in mortars cause lower strength and higher drying shrinkage at early age but larger growth of strength in the late. The increasing calcium sulfates content may result in the delay of main hydration peak in the heat evolution curve. For formulation with hemihydrate, the appearance of main hydration peaks are advanced compared with formulation without addition of calcium sulfates. Moreover, mortar microstructures are optimized by addition of β-hemihydrate, the proportion of large pores are lower than that of mortars with anhydrite.

Abstract: Calcium sulfate/hydroxyl apatite whiskers composite is possible to be used as bone graft substitute, for its biocompatibility, controllable degradation, and suitable mechanical properties. In this study, calcium sulfate/hydroxyl apatite whisker composites were fabricated and characterized. The characteristics of the composites were assessed by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and immersing testing techniques. The composites prepared in this article have been confirmed to be ideally used as biodegradable implants.

Abstract: Phosphogypsum is a kind of by-product of the phosphate fertilizer production process, mostly in the form of CaSO₄·2H₂O, the analytical determination of the content of the main component is the basis for the comprehensive utilization of phosphogypsum. This paper introduces three analytic procedure to determinate the content of crystal water, SO₄^2-, Ca²⁺ to calculate the CaSO₄ content of phosphogypsum, through a comparison between the results of three methods, we can conclude that granularity of phosphogypsum has the smallest influence on the crystal water determination method, but only applicable to determination of gypsum containing crystal water in single phase. However, the measure of SO₄^2-, Ca²⁺ is not restricted by the phosphogypsum phase, but influenced by the particle size. SO₄^2- determination operation is relatively simple and easy to control, reversely, Ca²⁺ determination operation is complex and hard to control.

Abstract: The formation of ettringite and the expansion associated with it were studied on paste made from Portland cement, two Al-bearing compounds such as calcium aluminate cement and calcium sulphoaluminate cement, and calcium sulfate with different reactivities (anhydrite, α-hemihydrate and gypsum). The hydration process and formation of ettringite in the binders with different content of calcium sulfate was investigated by free expansion testing, isothermal conduction calorimetry, X-ray diffraction (XRD) and environmental scanning electronic microscope (ESEM). Results showed that the reactivity and amount of the calcium sulfate determined the balance between the hydration products of ettringite and monosulfate, and also the early hydration kinetics including the formation content, size and location of ettringite. It was also found that all the ternary systems with higher addition of calcium sulfate expand significantly. When a higher content of α-hemihydrate was added, besides ettringite much secondary gypsum also forms in voids between cement granules, which exerted adverse effects on the properties of ternary systems. However, the formation content of ettringite appeared to have no connection with the expansion value.

Abstract: Calcium sulfate is an important inorganic chemical products which is widely used in construction, pharmaceutical and food industries. Especially, calcium sulfate whisker with fibrous structure developed in recent years is further used in rubber, coating, plastic, adhesives, textile, papermaking, and many other industries due to its superior performance and characteristics of environmental protection and nontoxic.The paper reviewed the preparation technology of calcium sulfate according to the different raw materials, especially for industrial waste (by-product).

Abstract: The effect of calcium sulfate whisker (CaSO₄) on mechanical property, melting, crystallization and rheological properties of polyamide 1012 (PA1012) was studied by differential scanning calorimeter (DSC), rheometer and material testing machine. It was shown that CaSO₄ had no influence on melting temperate of PA1012, but can effectively enhanced nucleation ability of PA1012, which indicated that CaSO₄ was good nucleation agent for PA1012. The viscosity and storage modulus of PA1012 firstly increased and then decreased as CaSO₄ content increased. The results showed that CaSO₄ had little effect on tensile strength, but impact largely flexural strength.

Abstract: Calcium sulfate hemihydrate has been used for many years as a biomaterial. However, too fast a degradation rate and lack of bioactivity have limited its application in orthopaedic field. Herein, α-Calcium sulfate hemihydrate is used as the raw material to prepare spherical particles calcium sulfate with stir method. Microstructures, degradation, and bioactivity of the materials were characterized by XRD, FT-IR and SEM. The effects of solutions of sodium silicate (Na₂SiO₃) on the structure and properties of the materials were analyzed. The results indicate that both the interface structure and the surface crystal binding state of the spherical particles calcium sulfate are changed by the solutions. The bioactivity was significantly improved by the solutions of Na₂SiO₃. With increasing the concentration of solutions in the materials, the degradation rate of the materials is decreased. Si-O network membrane which contains a large amount of Si-OH was formed on the surface of calcium sulfate. Therefore, the degradation rate was decreased and the bioactivity was significantly improved.