The influence of temperature and different storage conditions on the stability of supersulphated cement

The stability of Supersulphated Cement (SSC) is investigated at 95°C when subjected to relative humidities of 100, 53 and 11% of water vapour. Previously [1] investigations at 25, 50, 75°C under the same conditions of humidity reported the stability of ettringite, one of the initial hydration products. At 95°C, decomposition of ettringite, is found at all humidities and is rapid at 100% relative humidity. The hydration products of cement pastes at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). The formation of the hydragarnet, plazolite is recorded during the decomposition/dehydration process enhanced by possible carbonation. Rehydration studies on the products after storage for up to 9 months were carried out using distilled water and the samples tested for ettringite content. It is concluded that ettringite in SSC is inherently unstable at 95°C.     

Thermogravimetric study of the reduction of CuO–WO<Subscript>3</Subscript> oxide mixtures in the entire range of molar ratios

The present study reports the results of investigation on the role of metakaolin in the formation of ettringite in a model relevant to Portland cement. The model consists of ternary system (Trio) metakaolin–lime–gypsum. Five samples of defined ternary system were cured at different temperatures 20, 30, 40, 50 and 60 °C. Conduction calorimeter TAM AIR was mainly used to capture heat evolution at different temperatures. Thermoanalytical (simultaneous TGA/DSC) and X-ray diffraction methods were used to identify different products after curing. It results that ettringite is the main hydration product supplemented by calcium silicate and calcium aluminosilicate hydrates according to sample composition. The mechanism and kinetics of hydration, as displayed by calorimetric curves, depend on composition of samples and curing temperatures. Two main types of processes have been elucidated: reaction of aluminum ions with sulfate ones in the presence of calcium ions in aqueous solution to form ettringite supplemented by pozzolanic activity leading to the formation of calcium silicate and calcium aluminosilicate hydrates. Concomitant condensation of alumina and silica species and carbonation have influenced the course of hydration. Activation energy E _a depends slightly on composition of ternary system.     

The conductance of divalent Ions in H2O at 10 and 25°C and in D2O

Precise conductance measurements are reported for potassiumm-benzenedisulfonate and ferricyanide, for calcium, strontium, barium, and manganese chloride, and for manganese and potassium sulfate in D₂O at 25°C. Measurements were also carried out in water at 10 and 25°C for all the salts with the exception of K₃Fe(CN)₆ at 10°C. Two runs are reported for sodium sulfate in water at 25°C. Limiting conductances of these ions are discussed in terms of solvent structural effects. The association constant for MnSO₄ is found to be the same in H₂O and D₂O.     

Phase evolution of sol-gel CaO-ZrO2 using sulfuric acid as hydrolysis catalyst

Several compositions in the CaO-ZrO₂ system were synthesized from zirconium n-butoxide and calcium methoxide, by the sol-gel method. Hydrolysis and gelation occurred at pH 3, using H₂SO₄ as hydrolysis catalyst. Fresh gels were annealed in air at 100 to 900°C, in 100°C steps every 20 h, for a total annealing time of 140 h. Analysis by X-ray diffraction showed the formation of hydrated calcium sulfate together with amorphous zirconia up to 400°C. At the ZrO₂ rich-end, tetragonal and monoclinic zirconia solid solutions were stabilized in the presence of Ca ions. When 20 and 30 wt% of CaO were added, cubic zirconia and CaZrO₃ solid solutions were observed above 700°C. At the CaO rich-end, the coexistence of calcium carbonate polymorphs as vaterite and calcite were observed. Anhydrite was present across the entire range of compositions studied from 300 to 900°C.     

Synthesis of homogeneous La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> solid solutions by the Pechini method and their activity in methane oxidation

Homogeneous La_{1 − x} Ca _x MnO₃ solid solutions have been synthesized by the Pechini method (using polymer-solid compositions). Their microstructure, stability at high temperatures, and catalytic activity in methane oxidation are reported. A continuous series of solid solutions stable in air up to 1100°C forms in the system, and the particle surface is enriched with calcium. A distinctive microstructural feature of the particles is their microporosity. The catalytic activity of all calcium-containing samples (except for x = 0.7) below 700°C is lower than that of lanthanum manganite and decreases under the action of the reaction medium, which can be due to the decrease in the amount of weakly bound oxygen on the surface because of the enrichment of the surface with calcium and the formation of strongly bound surface carbonates. The higher activity and stability of the La_0.3Ca_0.7MnO₃ sample (calcined at 1100°C) above 500°C can be due to the formation of nanosized areas with an Mn₃O₄ structures on the perovskite particle surface in the reaction medium.     

Conversion of phosphogypsum to potassium sulfate

Summary Solubility of calcium sulfate in concentrated aqueous chloride solutions is of particular significance in chloride hydrometallurgy and various crystallization processes, such as the production of potassium sulfate from phosphogypsum and potassium chloride. This paper examines an example of the second type of application in which gypsum and potassium chloride are reacted to form K₂SO₄. The solubility of phosphogypsum in aqueous solutions of KCl, HCl, and mixtures of both has first been measured at various temperatures and concentrations. The parameters investigated are HCl concentration up to 6M, KCl concentration up to 180 g L^-1 and temperature from 25 to 80°C. In addition, the influence of co-existing chloride salts, such as (HCl+KCl), on the solubility of calcium sulfate is estimated from 25 to 80°C. The solubility increases obviously with the temperature increment as it does initially with acid concentration, reaching a maximum of about 3M HCl, 130 g L^-1 KCl and then drops. At the same time, the solubility of CaSO₄·2H₂O decreases with increasing KCl concentration.     

The influence of temperature and different storage conditions on the stability of supersulphated cement

The stability of supersulphated cement (SSC) is investigated. The hydration products of cement pastes prepared at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). Ettringite, one of the initial hydration products, is shown to be stable under conditions of storage at 25, 50 and 75°C and when subject to relative humidities of 100, 53 and 11% of water vapour in each case. The effect of drying on ettringite stability at the higher temperatures is discussed in relation to the relative humidity. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Study of Calcium Sulfate Crystallization by the Differential Scanning Calorimetry

The state of water in calcium chloride and sodium sulfate solutions, as well as during the crystallization of calcium sulfate from solutions with concentrations of 0.68–7.08 g dm^–3 was studied by the differential scanning calorimetry. One clearly detectable endothermic peak was revealed in the DSC curves of ice melting of CaCl₂ and Na₂SO₄ solutions. An increase in the concentration of these solutions does not lead to the appearance of any additional peaks or shoulders on the DSC curves. Thermograms for sodium sulfate and calcium chloride mixed solutions change their patterns: a low-temperature peak appears and peaks with asymmetric shoulders or splitted principal peak are registered. These changes are presumably related to the water structuring by the surface of forming calcium sulfate crystals.     

Influence of borax and citric acid on the hydration of calcium sulfoaluminate cement

The hydration of calcium sulfoaluminate cement in the presence of borax and citric acid has been studied using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, Fourier transfer infrared spectrometry and mercury intrusion porosimetry. The results suggest that the formation of complex between calcium and citrate in the solution adsorbed on the solid surface delayed the C₄A₃ \( \bar{S} \) and anhydrite dissolution and AFt nucleation. At the same time, the presence of borate replaced sulfate anion in ettringite (AFt) to form B-AFt. These reactions have an effect on the setting times and the mechanical strength values. The setting times of the calcium sulfoaluminate cement increased when the mineral admixtures increased in the cement. The effect of borax and citric acid on the strength development at different hydration stages presented different results, namely a decreased strength in an early stage and an increased strength at 3 and 28 days.     

Solubility equilibria of solid solutions of hydroxyl- and chlor-apatite of arsenic

The solubility equilibria of arsenic hydroxylapatite, Ca₁₀(AsO₄)₆(OH)₂, chlorapatite, Ca₁₀(AsO₄)₆(Cl)₂, and their nine solid solutions were investigated at 37°C in the pH range, 4.6 to 7.8, in buffered systems through microanalytical determination of calcium, arsenic and chloride in their saturated solution. Stoichiometric dissolution of the samples was proved by the constancy of the ionic products of the apatite phase in the saturated solution.     

Effect of Alkaline Salts on Calcium Sulfoaluminate Cement Hydration

This work analyzes the effect of the presence of 5 wt.% of solid sodium salts (Na₂SO₄, Na₂CO₃, and Na₂SiO₃) on calcium sulfoaluminate cement (CSA) hydration, addresses hydration kinetics; 2-, 28-, and 90-d mechanical strength, and reaction product microstructure (with X-ray diffraction (XRD), and Fourier transform infrared spectroscopy, (FTIR). The findings show that the anions affect primarily the reactions involved. Ettringite and AH₃, are the majority hydration products, while monosulfates are absent in all of the samples. All three salts hasten CSA hydration and raise the amount of ettringite formed. Na₂SO₄ induces cracking in the ≥28-d pastes due to post-hardening gypsum and ettringite formation from the excess SO₄^2– present. Anhydrite dissolves more rapidly in the presence of Na₂CO₃, prompting carbonation. Na₂SiO₃ raises compressive strength and exhibits strätlingite as one of its reaction products.     

Hydraulic activity of C4A3Cr in presence of C4A3S

This paper deals with synthesis and assessment of the hydraulicity of C₄A₃Cr, analog phase C₄A₃S to , relevant to the phase chemistry and properties of sulfoaluminate cements. C₄A₃Cr, synthesized at 1250 °C is well crystallized phase, latently hydraulic, with hydration accelerated in the presence of C₄A₃S, or CS. Calorimetric curves show reciprocal influence of sulfate and chromate phase in hydration of C₄A₃S-C₄A₃Cr system. The total heat expressed at hydration is nearly the same for all specimens, but the rate of heat evolution depends on the ratio of C₄A₃S/C₄A₃Cr phases. X-ray diffraction pattern and DTA curves showed that, increasing content of C₄A₃Cr in hydrating mixture results in a coexistence of two types of ettringite (chromate and sulfate ettringite) hydrogarnet, gibbsite and monosulfate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of iron sulfides on the physicochemical processes occurring during solidification of synthetic calcium aluminosilicate monolith

Phase formation processes occurring during solidification of two samples of silicate materials obtained by filtration combustion of a mixture of fly ash, limestone, and sand were studied. During the solidification of the material produced in the combustion mode in which impurity pyrite FeS₂ completely decomposed by the interaction with atmospheric CO₂ and water, dicalcium silicate, melilite, and glass phase gradually transformed into a mixture of serpentine-type binder (Ca, Fe, Mg)₃[(Si, Al)₂O₅](OH)₄ · nH₂O and calcium carbonates. The narrowing of the combustion wave front allows one to preserve pyrite particles encapsulated in the glass phase. In this case, the mechanism of solidification of the combustion product changes: the expose to sulfuric acid released by pyrite oxidation of the silicate components of the material results in the formation of gypsum and hydrosilicates filling the pore space. This enables one to reach high strength characteristics of the material without prepressing. The presence of a lower-pH medium in the pore space inhibits ettringite crystallization, thus preventing mechanical destruction.     

Thermal Decompositions of Pure and Mixed Manganese Carbonate and Ammonium Molybdate Tetrahydrate

The thermal decompositions of pure and mixed manganese carbonate and ammonium molybdate tetrahydrate in molar ratios of 3:1, 1:1 and1:3 were studied by DTA and TG techniques. The prepared mixed solid samples were calcined in air at 500, 750 or 1000°C and then investigated by means of an XRD technique. The results revealed that manganese carbonate decomposed in the range 300–1000°C, within termediate formation of MnO₂, Mn₂O₃ andMn₃O₄. Ammonium molybdate tetrahydrate first lost its water of crystallization on heating, and then decomposed, yielding water and ammonia. At 340°C,MoO₃ was the final product, which melts at 790°C. The thermal treatment of the mixed solids at 500, 750 or 1000°C led to solid-solid interactions between the produced oxides, with the formation of manganese molybdate. At 1000°C, Mn₂O₃ and MoO₃ were detected, due to the mutual stabilization effect of these oxides at this temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydration mechanisms of supersulfated cement

Considerable attention has been given to special cements, capable of reducing CO₂ emissions, energy and limestone consumption. Supersulfated cements are made of blast furnace slag (GBFS), calcium sulfate (CS), and small quantities of activator, but achieving their optimal proportions is complex. In this paper, the effects of the both CS and alkali activator (KOH) contents were studied. The main results showed that the compressive strength, heat of hydration, and consumption of anhydrite phase were strongly influenced by the alkaline content, while low calcium sulfate or alkaline content increased the formation of CSH. The instability of ettringite was verified: with low CS, the probable hypothesis was its conversion into monosulfate due to the scarcity of sulfate; with high CS, it was associated with intense, rapid consumption of anhydrite with high KOH content, followed by the precipitation of ettringite on the surface of slag grains and its conversion into monosulfate.     

Isolation of Rare-Earth Elements from Mixtures of Calcium and Lanthanide Oxalates

The temperature regions for separate crystallization of rare-earth element (REE) oxides of the cerium group in the presence of CaCO₃ have been determined using X-ray diffraction, differential thermogravimetric analysis, inductively coupled plasma mass spectroscopy, and X-ray fluorescence. The possibility to separate REE oxides from CaCO₃ in H₂SO₄ solutions after heat treatment (450–600°C) has been studied. The solid phase of the precipitate is represented by slightly soluble calcium sulfate, whereas the REE oxides pass into the liquid phase in the form of highly soluble sulfates. After heat treatment of the test mixture of REE oxalates and calcium oxalate at a temperature higher than 750°C, calcium compounds pass into 1–2% HNO₃ liquid phase in the form of nitrates, whereas lanthanide oxides remain in the insoluble phase of REE oxide solid solution having CeO₂ structure.     

Solubility of α-and β-gypsum in aqueous solutions of calcium,magnesium, and sodium chlorides

The interaction of multicomponent water-salt systems including calcium, magnesium, and sodium chlorides, gypsum, air, and water were studied by the methods of thermodynamic modeling. The solubility of α-CaSO₄ · 2H₂O and β-CaSO₄ · 2H₂O were determined in aqueous solutions of calcium, magnesium, and sodium chlorides and their mixtures at 25°C and carbon dioxide partial pressure of 10^?1.53 kPa, metal chloride concentrations of 10^?4 to 1.0 mol/kg H₂O, and their molar ratio of 1: 1.     

Determination of low degrees of hardness in water using a soap solution according to clark. I

1. The determination of low degrees of hardness in water according to the usual methods does not yield satisfactory results. Therefore an investigation was carried out with a soap solution of sodium, oleate according to clarke, and also nephelometric measurements were made. 2. Measurements were cairied out at degrees of hardness from 0.5 to 39.0 p.p.m. (i.e. 0°.005–3°.09 FH; or 0°.003–2°.18 DH), made out of pure calcium chloride solutions, at different pH values; also with calcium chloride solations, containing NaCI and alcohol. Magnesium, chloride solutions were investigated in the same way. 3. The calcium oleate, that precipitates when the hardness is determined, has a composition of 5 CaOl₂-2NaOl. The magnesium oleate has a corresponding composition. Dependent on the circumstances also 5 CaOl₂-3NaOl and calcium oleates. containing yet more sodium oleate may be formed. This phenomenon occurs especially at very low degrees of hardness. 4. No turbidity of calcium oleate arises when precipitation takes place at degrees of liardness lower tlian 1.2 p.p.m. CaCO₃(0°.07 DH), Magnesium olcate does not precipitate at degrees of hardness lower than 3.2 p.p.m. (0°.18 DH). 5. As a result of the phenomenon referred to in. point 3 the determination according to clarke for degrees of hardness below 5 p.p.m. CaCO₃(0°.3 DH) are only reliable if very important errors (up to 30%) are tolerated. In order to determine these degrees of hardness it is therefore advisable to add so much concentrated CaCl₂ solution as to reach a handy hardness of the water sample.     

Kinetic and thermal studies of removal of CrO 4 2− ions by ettringite

Ettringite was prepared in the presence of various dosages of CrO ₄ ^2? ions at three different temperatures. X-ray diffraction (XRD) and differential scanning calorimetery (DSC) results showed shift in the peaks characterized to ettringite as the result of formation of Cr-substituted ettringite. Substitution of SO ₄ ^2? by CrO ₄ ^2? ions in ettringite crystal resulted in delay in the rate of its formation, as well as reduction in the crystal diameters. However, the crystal diameters and rate of formation of both ettringite and Cr-substituted ettringite were increased by increasing temperature of preparation, and this was confirmed by scanning electron microscope (SEM) micrographs. The thermodynamic parameters: the enthalpy, entropy, and Gibbs free energy changes for substitution process were studied. It is concluded that the removal of chromate ions by ettringite is an endothermic spontaneous process. The positive value of ΔS° suggests increasing in randomness during the removal process.     

Solubilities of Magnesium Sulfite

Literature data of solubility of MgSO₃ in water and in aqueous solutions of MgSO₄ have been correlated. Magnesium sulfite forms hexahydrate (stable below 40°C) and trihydrate (above 40°C), nevertheless, metastable hexahydrate can precipitate at temperatures significantly higher than this transition temperature. Magnesium sulfate increases the solubility of the sulfite. This revised version was published online in August 2006 with corrections to the Cover Date.