Article pubs.acs.org/crystal
Direct and Facile Room-Temperature Synthesis of Nanocrystalline Calcium Sulfate Dihydrate (Gypsum) Kapil Gupta,*,†,§ Shubra Singh,‡ and M. S. Ramachandra Rao*,† †
Department of Physics, Nano Functional Materials Technology Centre and Materials Science Research Centre, Indian Institute of Technology Madras, Chennai 600 036, India ‡ Crystal Growth Centre, Anna University, Chennai 600 025, India S Supporting Information *
ABSTRACT: In this report, we have evolved a new method to synthesize nanocrystalline calcium sulfate dihydrate (gypsum) via electrochemical oxidation. This method provides a facile, direct way of synthesizing nanocrystalline gypsum at room temperature. Field-emission scanning electron microscopy micrographs present an interesting phenomenon of formation of microcracks in the nano-CaFeO2.5 pellet during electrochemical oxidation leading to nucleation of gypsum within the cracks. A known theoretical model has been invoked to explain the gypsum growth within the cracks. Transmission electron microscopy studies indicate electron beam-induced phase transformation of single-crystalline calcium sulfate dihydrate to polycrystalline CaO nanoparticles. The hardness was found to be improved (63% higher) by the addition of a mere 5 wt % of nano-gypsum to commercial gypsum, which is almost 200% higher than nano-gypsum synthesized earlier via an indirect route of flame synthesis and hydration. Moreover, the compressive strength of the gypsum mixture (with 5 wt % of nano-gypsum) was found to be improved by a factor of 1.7 (92.05 MPa), in comparison to commercial gypsum (54.23 MPa). The improved hardness and compressive strength by reduction in particle size of gypsum are noteworthy.
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INTRODUCTION Gypsum, also known as calcium sulfate dihydrate (CaSO4· 2H2O), is one of the most abundant evaporite minerals found in sedimentary deposits.1 Gypsum is widely used in many fields of industrial technology, such as, a setting retarder for Portland cement, in soil treatment in the agriculture industry (e.g., as chemical amendment for sodic soil reclamation, as conditioner, and as fertilizer), and in dental industry in plaster form.2 The importance of gypsum in a wide range of industrial technologies has motivated research on the improvement in its mechanical properties.3−6 It has been found that the mechanical properties (e.g., hardness, modulus of elasticity) are affected significantly by intergrowth and interlocking of gypsum crystals.7 Therefore, reduction in particle size could give rise to better mechanical properties due to better interlocking between the crystals. This motivated us to investigate the changes in the physical properties of gypsum when obtained in nanocrystalline form. So far, there has been a single report on nano-gypsum synthesis by an indirect and complicated route of hydration of CaSO4 nanoparticles which were prepared by flame synthesis.5 Nano-gypsum prepared by this technique exhibited 2−3 times higher Vickers hardness compared to micron-sized gypsum, and this increment in hardness was attributed to the stronger entanglement of gypsum crystallites due to the formation of calcium sulfate nanoneedles.5 In this report, we have evolved a new method to synthesize nanostructured gypsum using electrochemical oxidation of Brownmillerite nano-CaFeO2.5 at room temperature. © XXXX American Chemical Society
RESULTS AND DISCUSSION The experimental setup for electrochemical oxidation of nanoCaFeO2.5 pellet (Figure 1a) indicates that the oxidation starts on the surface of the nano-CaFeO2.5 pellet within ∼4 h duration (Figure 1b) and initiates the process of conversion of CaFeO2.5 to gypsum (CaSO4·2H2O). After ∼12 h of the reaction process, loose particles of gypsum are found to have formed (as can be seen from Figure 1c), and these particles
Figure 1. Electrochemical oxidation of Brownmillerite nano-CaFeO2.5. (a) Brownmillerite nano-CaFeO2.5 pellet. (b, c) Gypsum formed on the surface of CaFeO2.5 pellet, during intermediate steps of electrochemical oxidation. (d) Starting from the surface, the entire CaFeO2.5 pellet converts into gypsum. Loose particles of gypsum fall off and settle down at the bottom of the cell holder as shown in the right side picture (true color images). Received: February 10, 2016 Revised: April 19, 2016
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DOI: 10.1021/acs.cgd.6b00216 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. Phase purity of as-obtained nano-gypsum using XRD pattern, FT-IR, Raman spectrum, and simultaneous TG/DSC analysis. (a) Rietveld refinement was performed on X-ray powder diffraction pattern using FullProf suite showing monoclinic structure with space group C2/c. The reliability factors for Rietveld refinement of as-obtained gypsum are χ2 = 4.30, RBragg = 7.33%, Rf = 5.40%, and GOF index = 2.1. Red dots correspond to observed patterns, the black lines are the calculated patterns, the blue lines are the difference between the calculated and observed patterns, and the green verticals correspond to the expected Bragg positions. (b) FT-IR spectrum of as-obtained gypsum in the spectral range of 450−4000 cm−1. Inset of (b) shows the antisymmetric stretch mode (ν3) of SO4 tetrahedron as a doublet at 1123 and 1142 cm−1. (c) Raman spectrum of as-obtained gypsum. (d) TG-DSC curve of nano-gypsum in nitrogen atmosphere. Inset of (d) shows two-step dehydration of gypsum at temperatures 115 and 127 °C using the first derivative of weight loss in TG measurement.
anion (Figure 2b). A free sulfate-ion has the symmetry of a regular tetrahedron, and it belongs to the Td symmetry group; however, in gypsum molecule, there is a deviation in SO4 geometry from the ideal tetrahedral configuration to a lower molecular symmetry C2.9−12 Therefore, a weak band at 1005 cm−1 is observed in the FTIR spectrum of gypsum, which corresponds to ν1 (S−O symmetric stretch) of SO4 tetrahedron in gypsum. This symmetric stretching mode is observed due to the lower symmetry of sulfate-ion in gypsum and is otherwise IR inactive in the free sulfate-ion. The absorptions at 604 and 669 cm−1 arise due to splitting of a triply degenerate antisymmetric bending vibration (ν4) of the sulfate tetrahedron in free sulfate-ion into a doublet in gypsum. Similarly, the strong bands observed as a doublet at 1123 and 1142 cm−1 are due to the splitting of S−O asymmetric stretching vibration (ν3) which is triply degenerate in free sulfate-ion.9,10,13,14 Additionally, absorptions corresponding to the first overtone of asymmetric stretching vibration (2ν3) of SO42− anion and binary combination band of SO42− internal fundamentals (ν3 + ν1) are also observed in the FTIR spectrum of gypsum in the range 2100−2300 cm−1 (Figure 2b).9 Absorption peaks in the spectral region near 3500 and 1600 cm−1 are attributed to the presence of water molecules in the gypsum. Two strong bands at 3405 and 3545 cm−1 correspond to the O−H···O stretching vibrations, ν1 (Bu) and ν3 (Au),
start to fall off and settle down at the bottom of the cell holder. This process continues until the entire nano-CaFeO2.5 pellet converts into gypsum in about 48 h (Figure 1d). X-ray diffraction study on the as-obtained gypsum confirmed the phase purity of the compound, and the average crystallite size was found to be ∼96 nm using the Scherrer formula.8 Rietveld refinement of the powder X-ray diffraction data of gypsum performed with FullProf suite revealed a monoclinic structure with space group C2/c (No. 15). The result of the Rietveld fit is shown in Figure 2a, which reports both the experimental pattern (red dots) and the calculated profile (black curve). Expected Bragg positions (vertical green bars) and the residual fit profiles (experimental−calculated pattern) as blue curve are also shown in the Figure 2a. The monoclinic unit cell parameters obtained from the refinement of powder XRD data are a = 6.285 Å, b = 15.210 Å, c = 5.677 Å, and β = 114.09°, which matches well with the JCPDS data (#33-0311). The final agreement factors converged to Rwp = 12.5%, RBragg = 7.33%, Rf = 5.4%, χ2 = 4.3%, and goodness-of-fit indicator (GOF index) = 2.1. Other refined parameters and reliability factors are given in Supplementary Table S1. Fourier transform infrared (FTIR) spectrum of nano-gypsum spans the internal vibrational modes of SO42− ion in gypsum, internal modes of water molecule, and the overtone of sulfate B
DOI: 10.1021/acs.cgd.6b00216 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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emission scanning electron microscopy (FESEM) micrographs (Figure 3).
respectively. There are two absorption peaks for in plane O− H···O bending vibrations in the gypsum spectrum, i.e., ν2 (Bu) bend at 1621 cm−1 and ν2 (Au) bend at 1685 cm−1. In addition, a small absorption peak attributed to the first overtone of O−H bending vibration (2ν2) is observed at 3245 cm−1 as shown in Figure 2b.9,13,15−18 The vibrational spectrum of gypsum may be divided into three parts including the external low frequency modes, the internal vibrational modes of sulfate groups, and the internal vibrational modes of water molecule.19 Out of the 21 Raman active external modes (translational, sulfate rotational, and water rotational), only one mode at 176 cm−1 was observed clearly as shown in Figure 2c. Four vibrational bands of SO42− are around 1000 cm−1 (ν1), 450 cm−1 (ν2), 1150 cm−1 (ν3), and 650 cm−1 (ν4) in aqueous solutions. The vibrational bands of H2O were observed around 3410 and 3498 cm−1. In Figure 2c, ν 1 corresponds to SO 4 symmetric stretching mode, ν 2 corresponds to SO4 symmetric bending mode, ν3 corresponds to SO4 antisymmetric stretching mode, and ν4 corresponds to SO4 antisymmetric bending mode.20−22 Above investigation of vibrational modes corroborates the formation of phase-pure gypsum. Simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis of the as-obtained nanogypsum compound is shown in Figure 2d. The obtained TG curve of nanogypsum indicates single weight loss between 100 and 150 °C with the loss of about 20.6% of the initial mass, corresponding to the full dehydration of gypsum (removal of crystallization water). However, the first derivative of weight loss plot indicates two closely overlapped peaks, confirming the two-step process (eq 1 and eq 2) of dehydration of gypsum to soluble anhydrite (γ-CaSO4) at 115 and 127 °C, which correspond to calcium sulfate dihydrate to hemihydrate (CaSO4·0.5H2O) and hemihydrate to soluble anhydrite (γCaSO4) conversion, respectively, as shown in the inset of Figure 2d.23,24 The temperatures of these steps and dehydration behavior are influenced by various experimental conditions such as heating rate, atmosphere (water vapor partial pressure, PH2O), sample impurities, and particle size.25−27 Two stages of dehydration of gypsum are 1 3 CaSO4 · 2H 2O → CaSO4 · H 2O + H 2O 2 2
(1)
1 1 CaSO4 · H 2O → CaSO4 + H 2O 2 2
(2)
Figure 3. Morphological changes during different stages of electrochemical oxidation of Brownmillerite nano-CaFeO2.5. (a) FESEM of Brownmillerite nano-CaFeO2.5 pellet at an intermediate step of electrochemical oxidation in 0.1 M H2SO4. (b) Magnified image of unreacted CaFeO2.5 phase. (c) Formation of gypsum can be seen within the microcracks. (d) Morphology of the as-formed nanogypsum after complete electrochemical oxidation of Brownmillerite nanoCaFeO2.5 (images are in false color).
The FESEM micrographs reveal the formation of microcracks during electrochemical oxidation, formation of nanogypsum along the microcracks (Figure 3a−c), and finally, conversion of the whole CaFeO2.5 pellet to nanogypsum (Figure 3d). A clear difference in morphology of CaFeO2.5 and gypsum can be seen from Figure 3b,d. The electrochemical process of conversion from CaFeO2.5 to gypsum is very fast due to the high surface reactivity of the nanoparticles which are exposed in more numbers at the defect regions (microcracks). This enhanced reactivity helps nanoCaFeO2.5 (unlike bulk-CaFeO2.5) to readily undergo electrochemical oxidation and produce nano-gypsum and also suppresses the formation of larger and uniform crystallites of gypsum. During electrochemical oxidation, microcracks appear and spread throughout the pellet surface (Figure 4a,b). Initial nucleation starts along the periphery of these microcracks, coalescing and forming nano-gypsum particles as the reaction proceeds (Figure 4c). Upon completion of this electrochemical oxidation, the entire pellet is converted into nano-gypsum (Figure 4d). On the basis of the above discussion, we propose a schematic of nano-gypsum formation as shown in Figure 4e. As can be seen from the schematic, the initial nucleation at the defect region extends throughout the surface following a pattern governed by the binding energy at these sites and helps convert CaFeO2.5 to gypsum. The formation of nano-gypsum in the microcracks can be explained on the basis of the theory of nucleation,29,30 crystal growth,31 and the Burton−Cabrera−Frank (BCF) model.32 According to the theory of heterogeneous nucleation, cracks and defects (like grain boundaries or dislocations) require less
It can be determined from the DSC curve as well, in which two peaks overlapped showing an endothermic peak in the temperature range (100−150 °C), which corresponds to the dehydration of gypsum to soluble anhydrite (γ-CaSO4). The simultaneous TG/DSC analysis was performed in dry nitrogen atmosphere and in the open crucible (negligible PH2O) which could be the reason for single endothermic peak in the DSC curve.27 In addition, a small exothermic peak is also observed below 400 °C (∼340 °C) in the DSC curve, indicating phase transition of soluble anhydrite (γ-CaSO4, hexagonal structure) to insoluble anhydrite (β-CaSO4, orthorhombic structure).25 The oxidation process can be associated with the formation of microcracks in order to relieve the growth stress generated during oxidation.28 Interesting morphological changes in Brownmillerite nano-CaFeO2.5 during electrochemical oxidation in 0.1 M H2SO4 can be observed in false-colored fieldC
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gypsum formation are within the cracks (defect regions) leading to further growth. Bright-field TEM, selected-area electron diffraction (SAED), and high-resolution TEM (HRTEM) studies were performed to observe the morphology and structure of the as-formed nano-gypsum (CaSO4·2H2O). Figure 5a shows the bright-field
Figure 4. Schematic of nano-gypsum formation. (a) Annealed Brownmillerite nano-CaFeO2.5 pellet after few minutes of electrochemical oxidation showing formation of microcracks at the periphery of the pellet. (b) CaFeO2.5 pellet at intermediate step of electrochemical oxidation showing microcracks propagation throughout the surface, during oxidation. (c) FESEM of the pellet shown in (b), exhibiting formation of nano-gypsum within the microcracks. (d) FESEM of gypsum obtained after complete oxidation of nanoCaFeO2.5. (e) Schematic representation of (a, b, c, d), displaying formation and propagation of microcracks during electrochemical oxidation, formation of nano-gypsum along these cracks and finally, conversion of whole CaFeO2.5 to nano-gypsum. (f) Schematic showing various nucleation sites (black and white) on a surface (blue) with cracks, here nucleation will be preferred at sites in the sequence of 1, 2, 3, 4, 5, 6.
energy for the critical nuclei formation, and they can also increase the phase transition rate since the filling of these cracks can decrease the surface energy.29,30 In addition, the theory of crystal growth suggests that the crystal forming elements (growth species: atoms, ions, or molecules) with the highest number of nearest neighbors are bound most strongly to the surface, therefore most energetically favorable incorporation of growth species will be at a kink site.31 Moreover, the BCF model suggests preferential growth along screw dislocations at low supersaturation, which is based on a defect in the structure of the crystal lattice formed by stress inside the crystal lattice.32 In the present case, microcracks formed during the oxidation process are defect regions which provide energetically favorable positions for nucleation. Figure 4f shows the schematic based on Kossel crystal model.33 As indicated, adsorption of the growth species (depicted as black and white crystals) on the surface structure (highlighted in blue), having cracks (highlighted in orange), may occur at six possible sites, marked as 1, 2, 3, 4, 5, 6. According to the theory of nucleation, crystal forming elements (ions) prefer atomic sites with maximum binding energy, which itself depends on the coordination number (i.e., number of nearest neighbors). The binding energy of an ionic growth species (and generally for nonionic species too) is given by, E = Φ (e2/d); here, e is the charge of the ion, d is the lattice spacing, and Φ is a numerical factor which depends on the site of the deposition.33 Thus, in Figure 4f, nucleation will be preferred at atomic sites in the sequence of 1, 2, 3, 4, 5, 6, since Φ1 > Φ2 > Φ3 > Φ4 > Φ5 > Φ6. Here, sites 1, 2, and 4 are located within the cracks. Thus, the theory recommends that the energetically favorable atomic sites for
Figure 5. Electron microscopy analysis of as-obtained gypsum. (a) Bright-field TEM image of gypsum particles. (b) SAED of marked area in (a), showing the single-crystalline pattern of gypsum. (c) HRTEM image of the same area exhibiting the conversion of gypsum into CaO nanoparticles (of average diameter ∼6−10 nm), due to e−-beam irradiation during the data collection of HRTEM. (d) Bright-field TEM image of marked area in (a) after HRTEM data collection. (e) SAED of encircled area of (d) showing the presence of both, polycrystalline rings of CaO and single crystalline pattern of gypsum.
TEM micrograph of gypsum particles (with diameter 200−300 nm) and SAED of marked portion of one of the gypsum particles shows single crystalline pattern (Figure 5b). Further investigation shows the formation of calcium oxide (CaO) nanoparticles with average diameter of 6−10 nm (Figure 5c) which were formed due to the e− beam irradiation during the HRTEM data collection. Selected area in the TEM image, after this e− beam irradiation (Figure 5d), confirms the presence of both, polycrystalline rings from electron diffraction of CaO nanoparticles, and single crystalline pattern of gypsum (Figure 5e).34 Further TEM study on this electron beam-induced phase transformation of gypsum to CaO nanoparticles is presented in Figure S1 of the Supporting Information. Previous studies suggest that the interlocking of crystals, crystalline texture, intercrystalline interaction, shape and size of the crystals, and porosity, all influence mechanical properties of gypsum.4,6,7 In this regard, gypsum mixtures were formed with 1, 2.5, and 5 wt % of nano-gypsum in commercial gypsum. D
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pellet. Better interlocking between the gypsum crystals, after addition of nano-gypsum, provides higher hardness. The compression tests were conducted on gypsum mixtures with 0, 2.5, and 5 wt % nano-gypsum in the micron-sized commercial gypsum as shown in Figure 6b. Compressive strength of the mixture with 5 wt % of nano-gypsum added to micron-sized commercial gypsum was found to be 92.05 MPa, which was almost 1.7 times higher than that of the commercial gypsum. Moreover, on the basis of the comparison of slopes of compressive stress−strain curves for three mixtures (inset of Figure 6b), it can be proposed that the elastic modulus of the mixture with 5 wt % nano-gypsum is higher than the commercial gypsum.
Hardness of these gypsum mixtures was evaluated with Vickers indentation technique (Figure 6a) using the formula HV = 1.8544F /d 2
(3)
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CONCLUSIONS In summary, we have reported on a direct way of synthesis of nanocrystalline calcium sulfate dihydrate (gypsum), at room temperature by electrochemical oxidation of Brownmillerite nano-CaFeO2.5 that also yielded high hardness in the mixture (5 wt % nano-gypsum in micron-sized commercial gypsum). Morphological changes during electrochemical oxidation of CaFeO2.5 pellet indicated the formation and propagation of microcracks, and nucleation of nano-gypsum within the microcracks. High purity nano-gypsum was investigated further using Rietveld refinement of XRD pattern, FT-IR, simultaneous TG/DSC analysis, and Raman studies. Electron beam irradiation during HRTEM study suggested the conversion of gypsum to CaO nanoparticles of particle sizes 6−10 nm. MicroVickers hardness measurement showed that the hardness improved by a factor of 1.5 (63% higher) by the addition of a mere 5 wt % of nano-gypsum in micron-sized commercial gypsum. Moreover, in comparison with earlier work of Osterwalder et al., in which nanogypsum was synthesized via an indirect route of flame synthesis and hydration,5 our sample exhibited almost 200% higher hardness. Compressive strength of the mixture with 5 wt % of nano-gypsum added to micronsized commercial gypsum was found to be 1.7 times higher (92.05 MPa) than that of the commercial gypsum (54.23 MPa). These improved mechanical properties of gypsum mixtures (hardness and compressive strength) after adding a mere 5 wt % of nano-gypsum are noteworthy.
Figure 6. Micro-Vickers hardness and compression tests of gypsum mixtures. (a) Average micro-Vickers hardness (HV) values of gypsum mixtures are plotted with the nano-gypsum content (data shown with standard deviation on 15−20 measurements) and compared with the hardness of nano-gypsum obtained by Osterwalder et al. (dashed plot). Inset shows optical micrograph of a typical Vickers indent at 25 gf load with dwell time of 10 s. (b) Compressive strength of gypsum mixtures are plotted with the nano-gypsum content. Inset shows compressive stress plots with compressive strain for gypsum mixtures.
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Here F denotes the force (load) applied in kg and d is the diagonal length of indentation (mm), micro-Vickers hardness (HV) was calculated for each observation. The micro-Vickers hardness (HV) tests were done for over 25 indents for each nano-gypsum mixture and the average HV values for these mixtures were determined. Micro-Vickers hardness of the mixture with 5 wt % of nano-gypsum added to micron-sized commercial gypsum was determined to be 75.6 kgf/mm2, which was enhanced by a factor of 1.5 (63% higher) compared to commercial gypsum and almost 200% higher than the reported Vickers hardness of nano-gypsum synthesized via an indirect route of flame synthesis and hydration (Figure 6a).5 This is a remarkable increase in the hardness of commercial gypsum by the addition of a mere 5 wt % of nano-gypsum. Moreover, the density of the commercial gypsum pellet increased from 2.156 g/cm3 (theoretical density, 2.322 g/ cm3), to 2.205 g/cm3, after adding 5 wt % of nano-gypsum. This indicates that the hardness of gypsum is significantly affected by the bonding among the particles in the compact
MATERIALS AND METHODS
The nano-gypsum (CaSO4·2H2O) compound was synthesized via electrochemical oxidation of Brownmillerite nano-CaFeO2.5. This nano-CaFeO2.5, with an average crystallite size of 60 nm, was prepared using a modified Pechini route,35 and corresponding pressed cylindrical pellet (10 mm diameter and ∼1 mm thickness) was annealed at 700 °C. The XRD pattern of Brownmillerite nanoCaFeO2.5 is shown in Supplementary Figure S2. Electrochemical oxidation of Brownmillerite nano-CaFeO2.5 was performed at room temperature by means of CHI660D electrochemical workstation in the acidic medium of 0.1 M H2SO4. The pellet of nano-CaFeO2.5, with the use of platinum wire attached to the periphery of the pellet, was used as a working electrode for electrochemical oxidation, and Pt wire was used as counter electrode. A schematic of the electrochemical setup and a typical time dependence of the electrode potential of nanoCaFeO2.5 are given in Supplementary Figure S3. The resultant compound after oxidation of Brownmillerite nano-CaFeO2.5 was found to be gypsum (CaSO4·2H2O). The phase purity of obtained gypsum (CaSO4·2H2O) was studied using X-ray powder diffraction (XRD) on PANalytical X’Pert Pro with CuKα radiation. Rietveld refinement was performed using FullProf Suit. Pseudo-Voigt profile functions were employed and background polynomial parameters, zero-point parameter, scale factor, overall E
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temperature factor, lattice parameters, atomic displacements of Ca, S, and O, profile parameters U, V, W, Shape, X, Y and asymmetry parameters, were among the parameters which were refined to produce Rietveld fit. In addition, 12-coefficient background fit with 12 parameters was employed in the Rietveld refinements. Fourier transform infrared spectroscopy (FTIR) was carried out on PerkinElmer Spectrum One in the range of 450−4000 cm−1. Raman spectroscopy study was performed on HORIBA Jobin Yvon HR800 UV (with 488 nm Ar-ion laser). Bright-field transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies were carried out on FEI Tecnai F20 operating at 200 kV. Sample for TEM related studies was prepared by dispersing the sample in ethanol and drying a drop on carbon-coated copper grid. Field-emission scanning electron microscopy (FESEM) images were acquired using FEI Quanta 3D at room temperature. Simultaneous thermogravimetry/ differential scanning calorimetry (simultaneous TG/DSC) analysis was performed by TA Instruments SDT Q600 in the range of 28−500 °C with a heating rate of 10 °C/min. Thermal analysis was performed in nitrogen atmosphere with a gas flow of 200 mL/min and cylindrical alumina crucible (without lid) was used as sample holder. Micro-Vickers hardness (HV) measurements were performed on different mixtures of as-prepared nano-gypsum in commercial gypsum (0, 1, 2.5, and 5 wt %). These gypsum mixtures were prepared by mixing and grinding the appropriate amount of nano-gypsum into commercial gypsum and pressed into pellets. Later, these pressed pellets were hardened by first soaking in water and then left for drying under ambient conditions. Dry pellets were crushed and obtained powder was then pressed again at 12 ton pressure for 5 min resulting in the final cylindrical pellets with the dimensions of 10 mm × 20 mm. Micro-Vickers hardness measurements were performed on these cylindrical pellets using a Wolpert Wilson Instruments 402 MVD hardness tester, operated at an indentation load of 25 gf (245 mN) at ambient conditions using a dwell time of 10 s. The diagonal lengths of the indentation were measured using a digital micrometer attached to the instrument, digital encoders were used for data input, and the average of diagonals was taken for the micro-Vickers hardness calculations (eq 3). The compressive testing was conducted on the pellets of gypsum mixtures on a 30 kN Universal Testing machine (Instron 3367) at room temperature. The rate of displacement was kept constant at 0.12 mm/min. Load was applied until fracture and loads and displacements were continuously recorded using Instron Bluehill software. The compressive stress and strain were calculated at yield (zero slope). Moreover, TG analysis was performed on gypsum mixtures (with 0 and 5 wt % nano-gypsum added to commercial gypsum), which is given in Figure S4 of the Supporting Information.
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ACKNOWLEDGMENTS K.G. would like to thank IFCPAR (Indo French Centre for the promotion of Advanced Research), New Delhi for providing a fellowship during the course of this work (Project No. IFC/ 4108-1/2009/1605). M.S.R. and K.G. would like to thank funding from Department of Science and Technology, New Delhi, that facilitated the establishment of Nano Functional Materials Technology Centre (Grant: SR NM/NAT/02-2005). The authors also thank Mr. Subhrakanti De for helping in the synthesis process.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00216.
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Article
Rietveld refinement parameters, TEM images, XRD, schematic of electrochemical setup (PDF)
AUTHOR INFORMATION
Corresponding Authors
*(K.G.) E-mail:
[email protected]. *(M.S.R.R.) Tel.: +91-44-22574872. E-mail:
[email protected]. Present Address §
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan. Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.cgd.6b00216 Cryst. Growth Des. XXXX, XXX, XXX−XXX
