Experimental investigation of calcium silicate hydrate (C-S-H) nucleation

Due to the importance of calcium silicate hydrate (C-S-H) in cement chemistry, its nucleation mode and parameters influencing it were investigated. It has been observed that the C-S-H nucleation follows the general laws governing the nucleation. The degree of supersaturation has been found to be the main parameter controlling homogeneous nucleation rates. The lime concentration in solution, well known to be the most important parameter determining the kinetic, morphological and structural features of C-S-H, also controls the nucleation characteristics of heterogeneous nucleation, i.e. during hydration of cement. The correlation between heterogeneous nucleation of C-S-H and possible final mechanical properties of cement paste has been well established.     

Effect of sulphate ions on iron precipitation from aqueous solutions

In the present work, the effects of sulphate ions on the iron precipitation from aqueous solution were investigated. It was shown that sulphate ions delayed the iron precipitation when this ion was added in form of Na₂SO₄. This effect became less significant in presence of magnesium or calcium. The iron precipitates were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). In all experiments iron oxide hydroxide (FeOOH) precipitates were obtained. The sulphate ions were adsorbed on the surfaces of the iron precipitates. The effect of temperature on these precipitates was also studied. At 237 °C, the iron oxide hydroxide precipitates obtained from NaCl solution was transformed in crystallized hematite, Fe₂O₃. At 793 °C, the hematite was partially transformed into magnetite (Fe₃O₄). In presence of sulphate ions, this transformation was not detected. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of calcium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Calcium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate and hydroxoaluminate-excess hydroxide solutions at ambient temperature (at C_Al = 0.1 to 0.3 M and at XS OH/Al = 0 to above 8). The precipitations were monitored by potentiometric (pH) measurements. Precipitate morphologies were examined by optical microscopy and precipitate compositions were determine by chemical analysis, infra-red spectrophotometry and thermal analysis. Generally at OH/Al ratios of 4 to 4.5 (XS OH/Al = 1 to 1.5), the compound 2 CaO · · Al₂O₃ · 8 H₂O (C₂AH₈) was precipitated with some aluminium hydroxide; then at OH/Al ratios of 5 to above 11 (XS OH/Al = 2 to above 8), the compound 2 CaO · Al₂O₃ · 8 H₂O was precipitated with increasing amounts of the compound 4 CaO · Al₂O₃ · 13 H₂O (C₄AH₁₃).     

Morphologies of calcium carbonate crystallites grown from aqueous solutions containing polyethylene glycol

Calcium Carbonate has been precipitated from aqueous solutions containing different concentrations and different molecular weight of Polyethylene Glycol (PEG). The precipitations were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR) and X‐ray diffraction (XRD). The results demonstrated that PEG has profound influence on the nucleation and crystal growth of CaCO₃, under condition of low PEG6000 (refer to PEG M_W=6000) concentration, it favored the formation of calcite, while high PEG6000 concentration promoted vaterite formation. Additionally, low molecular weight PEG can stabilize vaterite phase. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interactions between silica sand and sodium silicate solution during consolidation process

This work is based on the knowledge of the consolidation of silica sand with an alkaline solution in order to determine the mechanisms that occur during the drying of sand and various alkaline solution mixtures. The investigations concern effects of sand distribution size, dilution of sodium silicate solutions and drying temperature of the mixtures on consolidation behaviour. The thermal analysis performed on fresh mixtures show a release of free-water from diluted silicate solution during the consolidation. SEM observations and compressive strength tests results indicate that interactions between sodium silicate binder and silica sand depend on drying temperature. Consequently two consolidation mechanisms are proposed: at low drying temperature (70 °C), sodium silicate acts as a thin layer of glue covering sand grains and bind them to each other, while at high temperature (150 °C), dissolution-precipitation reaction occurred in the mixture consolidating more strongly the granular system. The increase of Si/Na molar ratio in a sodium silicate solution containing silica sand is in accordance with the proposed mechanism.     

Effects of cadmium on crystallization of calcium phosphates

The precipitation of calcium phosphates in the presence of increasing cadmium amount was studied at 25 °C in dilute ammoniacal solutions ([Ca] = [P] = 0.005 M). An amorphous precipitate, an apatite‐like calcium phosphate and the compound Cd₅H₂(PO₄)₄·4H₂O are found according to pH and Cd concentration. The nucleation of hydroxyapatite is greatly delayed by cadmium, which influences also its crystallinity, but the effect tends to vanish with time. The role of cadmium is discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The precipitation of magnesium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Magnesium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate and hydroxoaluminate-hydroxide solutions at ambient temperature, at C_Al = 0.1 M, OH/Al ratios = 4–9 and XS OH/Al ratios = 1–6. The precipitations were monitored by potentiometric (pH) measurements while the final precipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis. At solution OH/Al ratio = 4, the main precipitate phase at 20°C was Mg(H₂O)_n[Al(OH)₄]₂ admixed with some Al(OH)₃; at solution OH/Al ratio = 5, the main phase was Mg₂(H₂O)₄[Al₂(OH)₁₀]; at solution OH/Al ratio = 7, the main phase was Mg₄(H₂O)_n(OH)₄[Al₂(OH)₁₀] while at solution OH/Al ratio = 9, the main phase was Mg₆(H₂O)_n(OH)₈[Al₂(OH)₁₀] admixed with some Mg(OH)₂. These hydrates were dehydrated at 60–100°C probably to the compounds Mg₂[Al₂O₃(OH)₄], Mg₄(OH)₄[Al₂O₃(OH)₄] and Mg₆(OH)₈[Al₂O₃(OH)₄], respectively.     

The precipitation of barium hydroxoaluminate hydrate powders from sodium hydroxoaluminate solutions: Precipitate phases and precipitation mechanisms

Barium hydroxoaluminate hydrates were precipitated from different sodium hydroxoaluminate solutions at 20 °C; C_Al varied from 0.1 to 0.5 M and initial Ba/Al₂ ratios ( = excess OH/Al ratios) varied from 1 to 7. Precipitate compositions were determined by chemical analysis, infra-red spectrophotometry and thermal analysis. The compound BaO · Al₂O₃ · 7 H₂O was precipitated at initial Ba/Al₂ ratios of one to well above two while the compound 2 BaO · Al₂O₃ · 5 H₂O was only precipitated over a narrow range of concentrations. The compound Ba(OH)₂ · 8 H₂O was precipitated from solutions of high hydroxide and barium ion concentrations. The ionic equilibria and precipitation mechanisms in different solutions are discussed.     

Kinetics of precipitation of nickel ammonium sulphate hexahydrate from aqueous solutions

The precipitation of nickel ammonium sulphate [Ni(NH₄)₂(SO₄)₂·6H₂O] prepared by mixing of equimolar aqueous solutions of (NH₄)₂SO₄ and NiSO₄ has been studied at 25 °C. Precipitation induction periods and the number of crystals formed per unit volume have been determined experimentally as functions of supersaturation for values of S_{a, H} < 2.6. The mechanisms of nucleation and crystal growth are assessed by comparing the experimental results with theoretical predictions. In this system, heterogeneous nucleation and crystal growth are assessed by comparing the experimental results with theoretical predictions. In this system, heterogeneous nucleation appears to operate at S_{a, H} < 1.3, whereas at S_{a, H} > 1.6 crystals are generated predominantly by homogeneous nucleation and grow by a polynuclear mechanism. The interfacial tension of nickel ammonium sulphate crystals in contact with their supersaturated solution is estimated to be about 20 mJ/ m².     

Crystal growth from concentrated aqueous solutions of calcium nitrate

The crystal growth kinetics was studied in the system of calcium nitrate – water at concentrations 14.3, 11.1, and 9.1 mole% of Ca(NO₃)₂ within the supercooling region ΔT = 0–65 K. The mechanism of the growth was suggested and the concentration dependence of the crystal growth rate was discussed.     

Influence of citric acid on calcium sulfate dihydrate crystallization in aqueous media

The crystallization of Calcium sulfate dihydrate produced by the reaction between pure Ca(OH)₂ suspension and H₂SO₄ solution was investigated at different pH values, temperatures and citric acid concentrations. Crystal size distributions, filtration rates and zeta potentials of gypsum were determined as a function of citric acid concentrations at pH 3.5 and 65°C. The influence of citric acid on the morphology of gypsum was also investigated and discussed. The average particle size of gypsum was reached to maximum in the presence of approximately 2500 ppm citric acid concentration, where the minimum cake resistance and maximum filtration rate were obtained. In the presence of citric acid, various crystal morphologies such as tabular, plate‐like, double‐taper leaf‐like and flower‐like, etc., were obtained. The change of morphology is related to the preferential adsorption of citric acid on different crystallographic faces. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of metastable zone width of ammonium oxalate aqueous solutions

The metastable zone width of pure ammonium oxalate aqueous solutions, as represented by maximum supercooling ΔT_max, is investigated as functions of cooling rate R and saturation temperature T₀ by the polythermal method. The experimental results are discussed by using two recently advanced approaches: (1) self‐consistent Nývlt‐like approach based on a power‐law relationship between nucleation rate J and maximum supersaturation lnS_max, and (2) a novel approach based on the relationship between J and lnS_max described by the classical three‐dimensional nucleation theory. Analysis of the experimental data revealed that both approaches describe the experimental data on metastable zone width by the polythermal method reliably and provide useful information about the physical processes and parameters involved in nucleation kinetics. The values of various physical quantities predicted by both of these approaches are reasonable for a fairly‐soluble compound. A careful examination of the data on ΔT_max as a function of T₀ obtained by polythermal method and from density measurements showed that ΔT_max has a slight tendency to decrease with increasing saturation temperature T₀. The values of lnS_max at saturation temperature 303 K suggest that the metastable zone width of ammonium oxalate aqueous solutions is determined by primary nucleation in the polythermal method and by secondary nucleation during density measurements. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Large-scale simulations of sodium silicate glasses

Half million and million atom systems of sodium silicate glass have been simulated using classical molecular dynamics, and their structures have been examined. Although the structural features are reasonable for the intended glasses, and have a lower configurational energy than smaller systems, they contain features which are indicative of an effective internal, or configurational, temperature higher than the target temperature. This observation is ascribed to the use of a thermostat which controls only the kinetic energy, or momenta, of the particles, and because of the non-equilibrium nature of the simulations, there is no requirement that the potential, or configurational, energy and kinetic energy be coupled. The use of alternative thermostats is suggested.     

The precipitation of lead sulphate from aqueous and aqueous alcohol solutions: Nucleation,final sizes and morphology

The precipitation of lead sulphate was studied from 0.0001 to 0.01 M aqueous solutions (supersaturations 3 to 600) and from 20% aqueous ethanediol, methanol and ethanol solutions, in polypropylene beakers, at ambient temperature: the experimental techniques were conductivity measurements and optical microscopy. The precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at intermediate to high supersaturations. New crystal morphologies generally developed at some what higher supersaturations in the aqueous alcohol systems. The final crystal lengths at first increased with increasing initial metal salt concentration and then decreased with this parameter; the largest crystals at any concentration were obtained from solutions in which lead sulphate solubility was highest. The critical supersaturations (for the onset of homogeneous nucleation) increased from 36 (in water) to 50 (in 20% aqueous ethanol): the surface energies for the formation of nuclei correspondingly increased from 90 to 110 mJ m⁻² in good agreement with the Nielsen-Söhnel relation. The nucleation and crystal growth processes are taking in an aqueous environment of similar water activity to that of the bulk solutions.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

The precipitation of calcium carbonate via a bubbling method in the presence of Mg2+ and glucose

Abstract

The effect of Mg²⁺ on the crystallization of precipitated calcium carbonate (PCC) via a bubbling carbonation method and the mechanism of eliminating its influence by glucose were investigated. The polymorph and morphology of crystals were characterized by field emission-scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. These results demonstrated that Mg²⁺ and Mg/Ca molar ratio played significant roles on the morphology of PCC. When the Mg/Ca molar ratio was below 0.5, only spindle-like calcite formed. The rod-like aragonite started to form when the ratio was 0.6. As the ratio increased, the amount of aragonite increased and the length of rod-like aragonite became longer. Notably, the effect of Mg²⁺ could be eliminated efficiently when the 1.5?wt% glucose was added into the carbonation system, in which system, the PCC crystals were all spindle-like calcite. Furthermore, the mechanism of the glucose to eliminate the influence of Mg²⁺ on PCC crystallization was proposed.     

Thermal and infrared studies of calcium malate crystals grown in diffusion limited medium

Single crystals of the title compound Ca (C₄H₅O₅)₂.10H₂O are grown in silica gel using controlled chemical reaction method. Multifaceted single crystals of size up to 8 × 4 × 2 mm³ are obtained. Powder X‐ray Diffraction (XRD) pattern of the grown crystal and the Fourier Transform Infra‐Red (FTIR) spectrum in the range 400–4000 cm^–1 are recorded. The vibrational bands corresponding to different functional groups are assigned. Thermal decomposition stages observed in the Thermo‐gravimetric (TG) and Differential Thermal Analysis (DTA) studies are discussed. A six‐stage thermal decomposition scheme is proposed for the compound. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization kinetics of ZnS precipitation; an experimental study using the mixed‐suspension‐mixed‐product‐removal (MSMPR) method

The precipitation kinetics of zinc sulfide were studied using a lab scale mixed‐suspension‐mixed‐product‐removal (MSMPR) precipitation reactor. The vessel was operated at different feed concentrations, molar ratios, stirrer speeds, pH‐values, feed injection positions and residence times. Primary nucleation and volume average crystal growth rates as well as agglomeration kernel were determined. Relationships were found between the rates of the different crystallization steps on the one hand and supersaturation, stirrer speeds, pH‐values, Zn²⁺ to S^2‐ ratio, feed positions on the other. These show that larger crystals are obtained at high supersaturation, moderate stirrer speeds, small residence times, a pH‐value of around 5 and high Zn²⁺ to S^2‐ ratios. One should realize though that the applied MSMPR method is not the most optimal technique for examining fast precipitation reactions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The coprecipitation of calcium aluminium hydroxide (calcium hydroxoaluminate hydrate) powders from aqueous solution with sodium hydroxide: Precipitate compositions and percipitation mechanisms

Calcium aluminium hydroxides were coprecipitated from different mixed metal cation solutions — at total C_M = 0.1 M and Ca/Al₂ ratios from 1 to 4 — with sodium hydroxide solution at ambient temperature. The coprecipitations were monitored by potentiometric (pH) titration and the final coprecipitate compositions were examined by chemical analysis, infra-red spectrophotometry and thermal analysis Generally, microcrystalline aluminium hydroxide was first precipitated at pH about 4; this then redissolved on further addition of sodium hydroxide to form hydroxoaluminate anion and polyanion and calcium aluminium hydroxide coprecipitates were formed continuously at pHs from about 9 to above 12. Their compositions were similar to the calcium hydroxoaluminate hydrates formed by direct precipitation from high pH sodium hydroxoaluminate solutions. At Ca/Al₂ ratio = 1, the main phase was probably Ca₂(H₂O)_h[Al₂(OH)₄]₂ with some Al(OH)₃; At Ca/Al₂ ratio = 2, the main phase was probably Ca₂(H₂O)_h[Al₂(OH)₁₀] dehydrating to Ca₂[Al₂O(OH)₈]; At Ca/Al₂ ratios = 3–4, the main phase was Ca₂(H₂O)_h[Al₂(OH)₁₀] with increasing amounts of Ca₄(H₂O)_h(OH)₄[Al₂(OH)₁₀] and 5–10 percent adsorbed or post-precipitated Ca(OH)₂.     

The coprecipitation of magnesium nickel hydroxide solid solutions from aqueous solution: Precipitate compositions and precipitation mechanisms

Magnesium nickel hydroxides (solid solutions) were coprecipitated from different mixed metal cation solutions (overall concentration 0.1 M) and from hydroxide solution (0.1 M). The course of different coprecipitations was monitored by potentiometric (pH) titrations. Final Coprecipitate compositions were determined by chemical analysis, infra-red spectrophotometry and thermal analysis. The ionic equilibria involved in different coprecipitations and the precipitation mechanisms are discussed.