Nathaniel Findling

Mechanism of wollastonite carbonation deduced from micro- to nanometer length scale observations

Abstract The microstructural evolution of CaSiO3 wollastonite subjected to carbonation reactions at T = 90 °C and pCO2 = 25 MPa was studied at three different starting conditions: (1) pure water; (2) aqueous alkaline solution (0.44 M NaOH); and (3) supercritical CO2. Scanning and transmission electron microscopy on reacted grains prepared in cross-section always revealed unaltered wollastonite cores surrounded by micrometer-thick pseudomorphic silica rims that were amorphous, highly porous, and fractured. The fractures were occasionally filled with nanometer-sized crystals of calcite and Ca-phyllosilicates. Nanoscale chemical profiles measured across the wollastonite-silica interfacial region always revealed sharp, step-like decreases in Ca concentration. Comparison of the Ca profiles with diffusion modeling suggests that the silica rims were not formed by preferential cation leaching (leached layer), but rather by interfacial dissolution-precipitation. Extents of carbonation as a function of time were determined by quantitative Rietveld refinement of X-ray diffractograms performed on the reacted powders. Comparing the measured extents of carbonation in water (condition 1) with kinetic modeling suggests that carbonation was rate-controlled by chemical reactions at the wollastonite interface, and not by transport limitations within the silica layers. However, at conditions 2 and 3, calcite crystals occurred as a uniform surface coating covering the silica layers, and also within pores and cracks, thereby blocking the connectivity of the originally open nanoscale porosity. These crystals ultimately may have been responsible for controlling transport of solutes through the silica layers. Therefore, this study suggests that pure silica layers were intrinsically non-passivating, whereas silica layers became partially passivating due to the presence of calcite crystallites

Fast slip with inhibited temperature rise due to mineral dehydration: Evidence from experiments on gypsum

Anomalously low heat flow around active faults has been a recurrent subject of debate over past decades. We present a series of high-velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large-scale endothermic dehydration reaction. The tests were performed at 1 MPa normal stress and a velocity of 1.3 m s −1 , while measuring the temperature close to the sliding surface and the relative humidity around the sample. The temperature close to the sliding surface is remarkably stable at ∼100 °C during the dehydration reaction of gypsum. Microstructural and X-ray diffraction investigations show that dehydration occurs at the very beginning of the test, and progresses into the bulk as slip increases. In the hottest parts of the sample, anhydrite crystal growth is observed. The half-thickness of the dehydrated layer ranges from 160 μm at 2 m slip to 5 mm at 68 m slip. Thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10% to 50% of the total mechanical work input. The temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam. We suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake. This is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature.

Raman mapping and numerical simulation of calcium carbonates distribution in experimentally carbonated Portland-cement cores

The spatial distribution of CaCO 3 polymorphs formed during the experimental carbonation of water saturated Portland-cement cores (30 mm in diameter), with supercritical CO 2 at 90 °C and 30 MPa, has been investigated using Raman microspectrometry on polished sample sections and X-ray microdiffraction. The three calcium carbonate polymorphs (calcite, aragonite and vaterite) were clearly distinguished using both techniques and their distribution along the main CO 2 diffusion direction could be mapped at the millimetre scale using a dynamic line-scanning Raman mapping tool. The calcium carbonate 2D distribution clearly shows that vaterite, the least stable of the three CaCO 3 polymorphs, is mostly located in a 500 μm wide ring ahead of the carbonation zone. This feature indicates that vaterite is the first CaCO 3 polymorph to crystallize within the cement sample in the course of the carbonation process. The presence of a vaterite front indicates that local mineral-solution equilibration can be slower than species transport, even above ambient conditions, and that kinetics cannot be ignored in the cement carbonation process. By using calcite and vaterite precipitation kinetic data from the literature and assuming water-mineral kinetics based on the Transition State Theory, the vaterite front inferred from Raman mapping is reproduced with a purely diffusive 1D transport code.

Nucleation and Growth of Chrysotile Nanotubes in H2SiO3/MgCl2/NaOH Medium at 90 to 300 °C

Herein, we report new insights into the nucleation and growth processes of chrysotile nanotubes by using batch and semi-continuous experiments. For the synthesis of this highly carcinogenic material, the influences of temperature (90, 200, and 300 °C), Si/Mg molar ratio, and reaction time were investigated. From the semi-continuous experiments (i.e., sampling of the reacting suspension over time) and solid-state characterization of the collected samples by XRPD, TGA, FTIR spectroscopy, and FESEM, three main reaction steps were identified for chrysotile nucleation and growth at 300 °C: 1) formation of the proto-serpentine precursor within the first 2 h of the reaction, accompanied by the formation of brucite and residual silica gel; 2) spontaneous nucleation and growth of chrysotile between about 3 and 8 h reaction time, through a progressive dissolution of the proto-serpentine, brucite, and residual silica gel; and 3) Ostwald ripening growth of chrysotile from 8 to 30 h reaction time, as attested to by BET and FESEM measurements. Complementary results from batch experiments confirmed a significant influence of the reaction temperature on the kinetics of chrysotile formation. However, FESEM observations revealed some formation of chrysotile nanotubes at low temperatures (90 °C) after 14 days of reaction. Finally, doubling the Si/Mg molar ratio promoted the precipitation of pure smectite (stevensite-type) under the same P (8.2 MPa)/T (300 °C)/pH (13.5) conditions.

Formation of porous calcite mesocrystals from CO2–H2O–Ca(OH)2 slurry in the presence of common domestic drinks

This study reports a simple, innovative and fast method to synthesize porous calcite mesocrystals with high specific surface area from Ca(OH)2–water–CO2 slurry in the presence of common domestic drinks (soluble coffee, orange juice, carrot juice, white wine, sugar–water and milk). As already reported in previous studies, calcite nanoparticles (<100 nm) can be obtained at low temperature (≤30 °C) in the absence of additives. We demonstrate herein that the use of common domestic drinks as additives can induce the formation of calcite mesocrystals with a peanut-like morphology, i.e. the formation of a nanostructured material in which the constituent calcite nanoparticles (10 < size < 50 nm) are aligned and/or oriented, forming regular micrometric (<3 μm) 3D porous aggregates. We note that the additives used in the system did not induce polymorphism because only calcite was measured/observed in the solid products using XRD, FESEM and TEM or in collected-time suspensions using Raman spectroscopy. This innovative method for synthesizing porous calcite mesocrystals has significant relevance because only a few hours (6 h < time < 24 h) were required and synthesis was possible using a dispersed triphasic gas–liquid–solid system under high non-constant CO2 pressure (anisobaric conditions), contrary to available methods requiring days or weeks, in which reactant diffusion is typically imposed since these systems were initially designed to mimic biomineralization processes. Moreover, this new synthesis method could easily be scaled to industrial processes to produce calcite mesocrystals with high specific surface area (up to 30 m2 g−1). The nanostructured state, the mesoporosity and the high specific surface area for these synthesized calcite mesocrystals could improve the typical industrial and medical uses for synthetic calcite.