Non-isovalent substitution in a Zintl phase with the TiNiSi type structure, CaMg1–xAgxGe [x = 0.13 (3)]

Single crystals of the title Ag-substituted calcium magnesium germanide, CaMg1–xAgxGe [x = 0.13 (3)] were obtained from the reaction of the corresponding elements at high temperature. The compound crystallizes with the TiNiSi structure type (Pearson code oP12) and represents an Ag-substituted derivative of the Zintl phase CaMgGe in which a small fraction of the divalent Mg atoms have been replaced by monovalent Ag atoms. All three atoms in the asymmetric unit (Ca, Mg/Ag, Ge) occupy special positions with the same site symmetry (.m.). Although the end member CaAgGe has been reported in an isomorphic superstructure of the same TiNiSi type, higher Ag content in solid solutions could not be achieved due to competitive formation of other, perhaps more stable, phases

Calcium platinum aluminium, CaPtAl

A preliminary X-ray study of CaPtAl has been reported previously by Hulliger [J. Alloys Compd (1993), 196, 225–228] based on X-ray powder diffraction data without structure refinement. With the present single-crystal X-ray study, we confirm the assignment of the TiNiSi type for CaPtAl, in a fully ordered inverse structure. All three atoms of the asymmetric unit have .m. site symmetry. The structure features a ∞ 3[AlPt] open framework with a fourfold coordination of Pt by Al atoms and vice versa. The Ca atoms are located in the large channels of the structure

Fixed-Bed Adsorption of an Azo Dye (Methyl Orange) onto Chemically and Thermally Regenerated Activated Carbons

In this study, the effectiveness of the recovery method for spent granular activated carbon (SGAC) for application in dye removal was evaluated. A comparative study of the textural (porosity), compositional, surface functionality, and adsorption performance of chemically (CAR400) and thermally regenerated activated carbons (CAR700 and CAR900) was conducted for the elimination of methyl orange (MO) dye by using a fixed-bed system. The results were compared with those of commercial activated carbon (CA). The influence of parameters such as the initial dye concentration, the flow rate, the internal diameter of the column, and the bed height was evaluated. Adsorption data were modelled by using the Thomas, Adams–Bohart, and Yoon–Nelson equations. The CAR400 activated carbon had a microporosity (1045 mg/g) comparable to that of the reference commercial (CA) activated carbon (1052 mg/g) but exhibited the least adsorption capacity. The breakthrough curves were best described by the Thomas model more than the Bohart–Adams and Yoon–Nelson’s models. Thomas’s model depicted that an increase in column diameter resulted in a decrease in the maximum adsorption capacity (qo). The CAR900 material exhibited the highest adsorption capacity (15.72 mg/g) comparable to that of commercial activated carbon, CA (16.90 mg/g). These results show that the physical/thermal regeneration of spent granular activated carbons (SGAC) is more suitable for the valorization of these waste materials for water purification applications

Role of ɣ-Al2O3 on the mechanical and microstructural properties of metakaolin-based geopolymer cements

The main target of this work is to investigate the influence of ɣ-Al2O3 on the properties of metakaolin-based geopolymer cements. The kaolin used as starting material for producing geopolymer cements contains approximately 28 and 64% of gibbsite and kaolinite, respectively. This kaolin was transformed to metakaolins by calcination at 500, 550, 600, 650, and 700 °C for 1 h. Gibbsite contained in kaolin was transformed to γ-Al2O3 during the calcination process. The hardener was obtained by mixing commercial sodium silicate and sodium hydroxide solution (10 M) with a mass ratio sodium silicate/sodium hydroxide equal to 1.6:1. Geopolymer cements, GMK-500, GMK-550, GMK-600, GMK-650, and GMK-700, were obtained using the prepared hardener with a mass ratio hardener/metakaolin equal to 0.87:1. It could be seen that the specific surface area of metakaolins decreases with increasing the calcination temperature of kaolin owing to the formation of the particles of γ-Al2O3. The compressive strengths 18.21/29.14/36.61/36.51 increase in the course GMK-550/GMK-600/GMK-650/GMK-700. The X-ray patterns and micrograph images of geopolymer cements, GMK-600, GMK-650, and GMK-700, indicate the presence of γ-Al2O3 in their structure. It was typically found that γ-Al2O3 remains largely unaffected during the geopolymerisation, and therefore could act as an inert filler and reinforce the structure of geopolymer cements. [Figure not available: see fulltext.]

Investigation of the relationship between the condensed structure and the chemically bonded water content in the poly(sialate-siloxo) network

The main objective of this work was to investigate the relationship between the condensed structure and the chemically bonded water content in the metakaolin-based geopolymer network. The kaolin used in this work as an aluminosilicate source was transformed to metakaolin by calcination at 700 °C. The powder of the waste glass and the silica fume were used as silica sources for the synthesis of the hardeners, dissolving with caustic soda solution. The obtained hardeners were characterized by infrared spectroscopy and MAS-NMR 29Si. The metakaolin and the hardeners were used for producing geopolymers cements. The synthesized products were characterized by X-ray diffractometry, infrared spectroscopy, mercury intrusion porosimetry, scanning electron microscopy, MAS-NMR 29Si and 27Al, thermal analyses (TG and DSC) and compressive strength. The results show that the compressive strength of geopolymer cements using hardener from silica fume and the one from waste glass are 61.93 and 25.51 MPa, respectively. The microstructure (SEM observations) of geopolymer cements obtained using hardener from silica fume is homogeneous, compact and dense with an average pore diameter around 13 nm. Whereas, the one obtained using hardener from waste glass are heterogeneous and contains larger pores (with average pore diameter around 168 nm). MAS-NMR 29Si and 27Al results show that the specimen obtained using hardener from the silica fume contains more aluminum in four-fold coordination in its network than the ones using hardener from waste glass. This indicates a higher degree of crosslinking of poly(sialate-siloxo) chains which could lead to a smaller pore sizes and a higher water uptake in the structure of the sample. The amount of chemically bonded water contained in the network of geopolymer cements using hardeners from waste glass and silica fume were 6.82 and 11.23%, respectively, as determined from weight loss in the range 100–300 °C. All these results indicate that the higher content of chemically bonded water in the network of geopolymer cement obtained using hardener from silica fume is related to the much smaller average pore size diameter and the hydrophilic character of aluminum, which reveals obviously better mechanical and microstructural properties of the specimen. This could indicate here a higher degree of condensation using silica fume based hardener for geopolymerization