Hydration Of Periclase Research Articles

The hydration of periclase: Atomistic insights from quantum-chemical look

A first principles density functional theory study has been carried out to investigate the hydration kinetics of periclase (MgO) through the interaction between water and periclase (0 0 1) and (1 1 1) surfaces. A single water molecule associatively adsorbs on both surfaces. Dissociative adsorption of water molecule only occurs by the co-adsorption of water on the (0 0 1) surface. Excepting this co-adsorption on the MgO(0 0 1) surface, the adsorption of two or monolayer water molecules on both surfaces are associative in nature. The interaction of water monolayer with the MgO(1 1 1) surface leads to the formation of Mg(OH)2. The calculated energy barrier for the formation of Mg(OH)2 from MgO is 93.6 kJ/mol, which is higher than the experimental values (66–84 kJ/mol). The absence of defect in the model surface sets the upper limit to the energy barrier for the hydration reaction. The effect of surface defects is demonstrated through the adsorption of a single water molecule. A single water molecule adsorbs both associatively and dissociatively on the (1 1 1) defect surface. The energy barrier for the conversion of associative to dissociative adsorption is 36.7 kJ/mol. The important new insights from this study are: (1) only MgO(1 1 1) surface is involved in the hydration of periclase and (2) surface defects play very significant role in the experimental measurement of energy barrier for hydration reaction.

Modeling Porosity Evolution Throughout Reaction‐Induced Fracturing in Rocks With Implications for Serpentinization

AbstractNumerical modeling based on the discrete element method was used to explore the kinetics and mechanics of fractures induced by mineral volumetric expansion during rock hydration. Two systems were considered: the hydration of periclase into brucite and the hydration of peridotite into serpentine. We modeled the coupling between mineral transformation, stress, volume increase, and deformation by simulating the volumetric growth of discrete elements based upon an Avrami‐type kinetics equation. The model was implemented to consider the effects of stress and temperature on reaction kinetics as well as the alteration of material properties during hydration reactions. We were able to reproduce experimental evidences observed during the transformation of periclase into brucite, including volume growth, fracturing of periclase, formation of a porosity pulse, as well as the slow‐down effect of effective stress on the kinetics of the transformation. The model was also applied to study the serpentinization process. We estimated a bell‐shaped relationship between temperature and reaction rate of peridotite transformation following observations made during serpentinization experiments. For both periclase and peridotite systems, we characterized a relationship between the formation of a porosity pulse and the rate of fracture development within the medium. During serpentinization, the amplitude of the porosity pulse and the duration of this pulse depend on the reaction rate and, therefore, on the temperature. Our investigations provide geomechanical explanations on how native dihydrogen formed during serpentinization can be expelled and initiates its migration process.

Mixed‐Mode Strain Localization Generated by Hydration Reaction at Crustal Conditions

AbstractHydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X‐ray synchrotron microtomography experiments to investigate reaction‐induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190 °C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time‐resolved three‐dimensional imaging, we quantify the spatial and temporal distribution of the reaction‐induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed‐mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction‐induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency‐magnitude domain, indicative of long‐range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization.

Phase composition and burning history of high-fired medieval gypsum mortars studied by Raman microspectroscopy

The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Not hydrated clusters of firing products preserved in the binder matrix are a typical feature of such mortars. A novel Raman microspectroscopic approach, providing access to the burning history of individual anhydrite grains, was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for tracing and visualising pyrometamorphic reactions in natural impurities of the kiln run. In the discussed examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 to periclase MgO and lime CaO yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Hydration of periclase in the mixed gypsum paste containing sulphate ions also resulted in magnesium sulphate hydrates, here identified in the form of hexahydrite MgSO4·6H2O. Lower burning temperatures left the accessory minerals in their pristine form, but can be traced by measuring the spectra of individual anhydrite crystals in grains of firing products and evaluating Raman band widths. Throughout the present study, calcination temperatures ranging from approx. 600°C to 900°C were determined.

Effects of Confinement on Reaction‐Induced Fracturing During Hydration of Periclase

AbstractHydration of a nominally dry rock can cause expansion of the solid volume, resulting in reaction‐induced fracturing and an associated increase in the porosity and permeability of the rock. We study the effect of confinement on the coupling between stress generation, reaction‐induced fracturing, and porosity evolution during the hydration of periclase (MgO) into brucite (Mg(OH)2). Samples of a microporous MgO ceramic were hydrated at 170–210 °C, 5–80 MPa confining pressure, 6–95 MPa differential stress, and 5–75 MPa pore fluid pressure in a purpose‐designed triaxial load cell. Hydration‐induced changes were recorded in situ by X‐ray microtomographic imaging at 5‐min intervals. Below 30 MPa effective mean stress, the fraction of periclase replaced by brucite is a sigmoidal function of time. After a slow start, the replacement rate picks up with concomitant intense fracturing. The porosity increase resulting from the reaction‐induced fractures is transient (pulse‐like). Following the porosity pulse the rate of replacement declines until the replacement is almost complete. Above 30 MPa, the reaction rate is slow, porosity decreases monotonically without any observable fracturing during the time of the experiment. At these stress conditions, the lack of fracturing cannot be limited by the thermodynamic affinity of the reaction. A possible interpretation is that the stress generated by the reaction may overcome the disjoining pressure at the grain‐grain interface, expelling the water film trapped there and thereby dramatically reducing the reaction rate.

Cement Growth Failure Mechanism in Porcelain Suspension Insulators

The paper describes a study undertaken to elucidate the mechanisms of cement grout expansion which leads to radial cracks in the shells of porcelain suspension insulators. The delayed hydration of periclase (MgO) is isolated as the dominant mechanism responsible for cement growth failure although sulphate related expansion may be a secondary influence. The expansion required to cause failure is quantified through extensive long term water expansion tests on cement pastes and autoclave tests on insulators assembled with both industrial and laboratory produced cements. From this study, a maximum ASTM C 151 autoclave expansion limit of about 0.12 percent is recommended for suspension insulator grout.

Brucite in limestone near Wilkinson, Ontario

Brucite is present as granules, blades and needles disseminated in magnesian limestone at Wilkinson, Ontario. It is similar in character and occurrence to brucite described from limestones in other localities. In several of these occurrences brucite has been considered to have been formed by hydration of periclase resulting from decomposition of dolomite. The brucite from Wilkinson contains no remnants of periclase and is considered to be the result of a single process in which H 2 O, probably in the vapor phase, replaced CO 2 of dolomite.