Decomposition Of Brucite Research Articles

Mechanical and microstructural changes in reactive magnesium oxide cement-based concrete mixes subjected to high temperatures

This study investigated the mechanical, microstructural, and chemical changes in reactive MgO cement-based concrete cured under ambient and accelerated carbonation conditions, followed by exposure to high temperatures. The compressive strength of the ambient-cured samples increased from 10 to 30 MPa when subjected to up to 200 °C, induced by increased hydration of the remaining MgO. The accelerated formation of brucite at 50 °C also enhanced the compressive strength of the carbonated samples (58 vs. 65 MPa). A relatively stable performance (~56 MPa) was observed at temperatures ranging between 100 and 300 °C for the carbonated samples, associated with additional formation of brucite and transition of nesquehonite and hydromagnesite to artinite. The hydrated magnesium carbonates (HMCs) forming around brucite acted as barriers and delayed its dehydroxylation. The decomposition of brucite and HMCs at 400 °C caused a porous microstructure and a low residual strength (5–8 MPa) in both the ambient-cured and carbonated samples.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy E a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent E a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E a versus α plots.

Decomposition of brucite up to 20 GPa: evidence for high MgO-solubility in the liquid phase

We performed in-situ X-ray diffraction and ex-situ textural observations and thermodynamic calculations in order to confirm phase relations in the system MgO-H 2 O at high pressure and high temperature conditions. In-situ and ex-situ observations reveal that the brucite decomposition boundary has a negative slope from 11 GPa to 20 GPa. Phases obtained at 2.1 GPa and 1503 K were periclase and a fluid with a small MgO content while those at 12 GPa and 1473 K were periclase and an MgO-rich liquid. Quench textures suggest that brucite dehydrates at least up to 4 GPa but melts incongruently at higher pressure. The incongruent melting temperature of brucite, that is wet solidus temperature of periclase, decreases with increasing pressure up to 20 GPa. The experimental results are supported by thermodynamic calculations.

Nanocrystalline, porous periclase aggregates as product of brucite dehydration

Transmission electron microscopy (TEM) techniques were employed to in situ study the electron-beam induced dehydration of brucite Mg(OH)2. Under the electron beam, the hexagonal platelets of brucite immediately decompose and show a morphological shrinkage of 5% and 10–20% in the a and c directions, respectively. The volume contraction occurs first in the rim and then affects the center of grains. Electron energy low-loss spectra reveal a simultaneous change in the local mass thickness of 50–55%. Combining these data, it follows that the porosity in the dehydrated material is 37.5–50%. The decomposition product is composed of numerous, tiny MgO crystallites and voids. Electron diffraction reveals a topotactic relationship between brucite and MgO with [0001]Bru // [111]MgO and [1120]Bru // [110]MgO. Since the porosity of the dehydrated material is slightly smaller than the maximum theoretical porosity (54%), only a small fraction of the voids is transported out of aggregates. Information on the local environment of the oxygen atoms was derived from extended energy-loss fine (EXELFS) and energy-loss near-edge structures (ELNES). In the time course of dehydration the coordination number of oxygen shows the expected increase from 3 for brucite to 6 for MgO. In a transient state the Debye-Waller factor reaches a maximum, indicating a highly disordered intermediate state. These data allow us to model the water loss and to examine reaction kinetics applying the Avrami equation. The decomposition of brucite is interpreted as a complex three-stage process: ( i ) It proceeds first via an interface-controlled process, starting at the rim of brucite; water escapes through the basal plane. ( ii ) The dehydrated lattice collapses then at the rim, whereas the core region is still hydrated. To further dehydrate the grain, the voids have to interconnect and rearrange in the form of a network slowing down the decomposition. At this stage, the process is diffusion-controlled. ( iii ) Finally, the pores are interconnected and reach the surface. The dehydration accelerates and is again an interface-controlled process. Dedicated to Prof. Dr.Wolfgang F.Muller on the occasion of his 60th birthday

Observations of the Thermal Decomposition of Brucite

Natural Brucite (Mg(OH)2) decomposes on heating to form magnesium oxide (MgO) having its cubic ﹛110﹜ and ﹛111﹜ planes respectively parallel to the prism and basal planes of the hexagonal brucite lattice. Although the crystal-lographic relation between the parent brucite crystal and the resulting mag-nesium oxide crystallites is well known, the exact mechanism by which the reaction proceeds is still a matter of controversy. Goodman described the decomposition as an initial shrinkage in the brucite basal plane allowing magnesium ions to shift their original sites to the required magnesium oxide positions followed by a collapse of the planes along the original <0001> direction of the brucite crystal. He noted that the (110) diffraction spots of brucite immediately shifted to the positions required for the (220) reflections of magnesium oxide. Gordon observed separate diffraction spots for the (110) brucite and (220) magnesium oxide planes. The positions of the (110) and (100) brucite never changed but only diminished in intensity while the (220) planes of magnesium shifted from a value larger than the listed ASTM d spacing to the predicted value as the decomposition progressed.

Mechanism of decomposition of brucite

The inhomogeneous mechanism for the crystallographic transformation of brucite (Mg(OH)2) to MgO postulated by Ball and Taylor 1 is re-examined. This postulate was based upon the observation of X-ray reflections attributable to a spinel structure in decomposed brucite crystals. In the present work a relatively pure Mg(OH)2 powder and Mg(OH)2 powders containing Fe and Mn were studied. It is shown that the spinel reflections arise from the presence of these impurities and that no spinel phase could be detected (i.e., it is <0.3 %) on decomposition of pure hydroxide. All the observations of Ball and Taylor are satisfactorily accounted for and the experimental basis for the inhomogeneous mechanism shown to be invalid.

Thermal Decomposition of Brucite: I, Electron and Optical Microscope Studies

The decomposition of brucite was studied directly in electron and optical microscopes. A small amount of decomposition caused marked changes in the physical state of the brucite crystal. The decomposition was interpreted as a normal nucleation and growth process in which magnesium oxide nuclei formed coherently with the brucite matrix. This process accounts for: (a) the structural relations between parent and product phases, (b) the small crystallite size of the product, (c) the cracking process occurring in the initial stages of decomposition, and (d) the range of d spacings observed for the magnesium oxide formed during the decomposition.