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Abstract
The reactivity of MgO obtained from calcination of magnesium carbonate at different temperatures has been investigated by means of hydration in a constant relative humidity environment at 40°C for periods up to 24 days. Natural magnesite and AR grade basic MgCO3 calcined in the range of 500–1000°C was characterised in terms of surface area, crystallite size, morphology, and hydration rate. It was found that the hydration rate is dependent on the surface area and crystallite size where temperature was the main variable affecting them. The most reactive MgO was produced at the lowest calcination temperature with the highest surface area and the smallest crystallite size. The basic MgO specimens showed higher degree of hydration compared to the natural MgO specimens due to the smaller surface area and larger crystallite size. The low MgO content of the starting natural magnesite is also attributable to the lower reactivity. This preliminary study serves as a mean to investigate potential utilisation of reactive MgO as a supplementary cementitious material in eco-friendly cements.
Thermal decomposition of SO4 2−-intercalated Mg–Al layered double hydroxide
Elimination behavior of sulfur oxides
) molar ratio (0.20 ≦ x ≦ 0.33) [ 1 , 2 ]. Mg–Al LDH consists of brucite-like octahedral layers that are positively charged due to the replacement of some Mg 2+ units by Al 3+ , while the interlayer anions help to maintain the charge balance. Water (H
Abstract
This paper reports an experimental study on the magnesium sulphate resistance of mortar specimens incorporating 0, 10 and 20% of metakaolin (MK). The evidence of the attack was evaluated through the content of calcium hydroxide (portlandite) and formation of magnesium hydroxide (brucite) by thermal analysis (thermogravimetric and derivative thermogravimetric analysis). The mechanical degradation of the mortar specimens was evaluated through splitting tensile tests after 200 days of exposition to the magnesium solution. The addition of metakaolin resulted in a reduction in the content of calcium hydroxide and in a smaller formation of brucite in comparison with reference mixture. A tensile strength loss of about 7% was observed for the metakaolin mortars submitted to the magnesium solution attack for 200 days.
Abstract
The mineral reevesite and the cobalt substituted reevesite have been synthesised and studied by thermal analysis and X-ray diffraction. The d(003) spacings of the minerals ranged from 7.54 to 7.95 Å. The maximum d(003) value occurred at around Ni:Co 0.4:0.6. This maximum in interlayer distance is proposed to be due to a greater number of carbonate anions and water molecules intercalated into the structure. This increase in carbonate anion content is attributed to an increase in surface charge on the brucite like layers. The maximum temperature of the reevesite decomposition occurs for the unsubstituted reevesite at around 220 °C. The effect of cobalt substitution results in a decrease in thermal stability of the reevesites. Four thermal decomposition steps are observed and are attributed to dehydration, dehydroxylation and decarbonation, decomposition of the formed carbonate and oxygen loss at ~807 °C. A mechanism for the thermal decomposition of the reevesite and the cobalt substituted reevesite is proposed.
Abstract
The reaction of magnesium minerals such as brucite with CO2 is important in the sequestration of CO2. The study of the thermal stability of hydromagnesite and diagenetically related compounds is of fundamental importance to this sequestration. The understanding of the thermal stability of magnesium carbonates and the relative metastability of hydrous carbonates including hydromagnesite, artinite, nesquehonite, barringtonite and lansfordite is extremely important to the sequestration process for the removal of atmospheric CO2. This work makes a comparison of the dynamic and controlled rate thermal analysis of hydromagnesite and nesquehonite. The dynamic thermal analysis of synthetic hydromagnesite proves that dehydration takes place in two steps at 135 and 184°C, dehydroxylation at 412°C and decarbonation at 474°C. Controlled rate thermal analysis shows the first dehydration step is isothermal and the second quasi-isothermal at 108 and 145°C, respectively. In the CRTA experiment both water and carbon dioxide are evolved in an isothermal decomposition at 376°C. CRTA technology offers better resolution and a more detailed interpretation of the decomposition processes of magnesium carbonates such as nesquehonite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal partial nesquehonite structure.
Abstract
Quaternary hydrotalcite-like precursors containing Mg(II)Pd(II)Cu(II)Al(III) with (Pd + Cu + Mg)/Al ≈ 3 and varying Pd/Cu molar ratios were prepared by coprecipitation of metal nitrates at constant pH. The precursors and their calcined products were characterized by elemental analysis, powder XRD, TG-DTA, FT-IR diffuse reflectance, TPR and N2 physisorption. Powder XRD indicated the formation of hydrotalcite of good crystallinity as the structure for all the precursors; the crystallinity decreased with increase of Pd2+ and Cu2+ in the Mg–Al brucite like layer. Thermal decomposition of HT-precursors at intermediate temperatures led to amorphous mixed oxides, PdCu/MgAl(O), which on reduction yielded a bi-functional catalyst, Pd0Cu0/MgAl(O). The resulting catalysts with acid, base and hydrogenating sites were highly effective and selective for the one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and hydrogen under mild conditions. Addition of copper with Pd inhibited the formation of hydrogenated by-products, propane-2-ol (IPA) and 4-methyl-pentane-2-ol (MIBA), which resulted an increase of MIBK selectivity and also the stability of the catalysts. A proper balance between acid-base and metallic sites was paramount for high catalytic performance.
Abstract
perpendicular to brucite-like layers. Only in recent years Xu and Zeng proposed different model [ 12 ]. According to their calculations, it is not “tilt-lying” arrangement (nitrates perpendicular to brucite-like layers) but “stick-lying” one (orientation of
Introduction Hydrotalcites are layered double hydroxides (LDHs) which show the brucite-like network characteristic of [Mg(OH) 2 ]. In brucite, Mg 2+ ions are octahedrally coordinated by hydroxyl anions, giving rise to the edge
consists of brucite-like layers with octahedrally centred Mg 2+ ions and some isomorphously substituted Al 3+ ions creating positive charge that is compensated with different anions present in the interlayer region together with water [ 7 – 9 ]. The
