Effect of alkalies on portland cement hydration: I. Alkali oxides incorporated into the crystalline lattice of clinker minerals

Na₂O and K₂O in absence of SO₃ are incorporated preferentially into the C₃A phase of portland clinker altering its modification from cubic to orthorhombic. The hydration of Na₂O doped C₃A is retarded and that of K₂O doped C₃A accelerated in the resultant cements. The progress of C₃S hydration and strength development are not altered noticeably. The setting time is extended in Na₂O-doped and shortened in K₂O-doped cements.     

Crystal structure refinement and hydration behaviour of 3CaO·SiO2 solid solutions with MgO,Al2O3 and Fe2O3

In this paper, analytical evidence on crystal structure and hydration behaviour of 3CaO·SiO₂ (C₃S) solid solutions with MgO, Al₂O₃ and Fe₂O₃ is presented. Samples were prepared using an innovative sol–gel process as precursor. The Rietveld refinements of XRDs show significant changes in the crystal structures for C₃S solid solutions with MgO and Al₂O₃, but only small changes for Fe₂O₃. Low concentrations of MgO do not change the hydration of C₃S, but 1.4 wt.% increases the reactivity after some days. With Al₂O₃ the initial and long-term hydration is activated, but the main reaction of C₃S decreases. Fe₂O₃ retards the hydration for several days, the long-term reaction is not affected or even activated. Altogether changes in hydration activity are more dominated by the type of oxide than by the height of changes in lattice parameters. The results can help to interpret the reactivity of different Portland cements and improve the quality control of the cement production.     

Influence of MgO on the formation of Ca3SiO5 and 3CaO·3Al2O3·CaSO4 minerals in alite-sulphoaluminate cement

The influence of MgO on the formation of Ca₃SiO₅ and 3CaO·3Al₂O₃·CaSO₄ minerals in alite-sulphoaluminate cement is reported in this paper. The results show that adding a suitable amount of MgO can lower the clinkering temperature, promote the formation of Ca₃SiO₅ and 3CaO·3Al₂O₃·CaSO₄ minerals, and help in the coexistence of the two minerals in the clinker. MgO may obviously decrease the formation of Ca₃Al₂O₆, and increase the SiO₂ content incorporated into the interstitial phase.     

Influence of additives on the autoclave reactions of β-Ca2SiO4 and Ca3SiO5 at 170 °C

The products formed from β-Ca₂SiO₄ and Ca₃SiO₅ in the autoclave at 170°C are influenced by the presence of Al₂O₃, Fe₂O₃, Na₂O and B₂O₃, whether in solid solution in the starting material or added to the mix in suitably reactive compounds. The most important conclusion is that Al₂O₃, Fe₂O₃ and Na₂O all favour the formation of the orthorhombic α-2CaO.SiO₂.H₂O as against hillebrandite or tricalcium silicate hydrate. Addition of Na₂O, or especially of B₂O₃, retards the hydration of both β-Ca₂SiO₄ and Ca₃SiO₅.     

Chemical zoning of calcium aluminoferrite formed during melt crystallization in CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₂(Al_xFe_1−x)₂O₅, with x<0.7 (ferrite), Ca₂SiO₄ (belite), Ca₃Al₂O₆ (C₃A) and Ca₁₂Al₁₄O₃₃ (C₁₂A₇), were crystallized out of a complete melt during cooling at 8.3 °C/min. Upon cooling to 1370 °C, both the crystals of ferrite with x=0.41 and belite would start to nucleate from the melt. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.7. At ambient temperature, the ferrite crystals had a zonal structure, the x value of which successively increased from the cores toward the rims. The value of 0.45 was confirmed for the cores by EPMA. The chemical formula of the rims was determined to be Ca_2.03[Al_1.27Fe_0.68Si_0.02]_Σ1.97O₅ (x=0.65). As the crystallization of ferrite and belite proceeded, the coexisting melt would become progressively enriched in the aluminate components. After the termination of the ferrite crystallization, the C₃A and belite would immediately crystallize out of the melt, followed by the nucleation of C₁₂A₇. The C₁₂A₇ accommodated about 2.1 mass% Fe₂O₃ in the chemical formula Ca_12.03[Al_13.61Fe_0.37]_Σ13.98O₃₃, being free from the other foreign oxides (SiO₂ and P₂O₅).     

Studies on oxidative coupling of methane using Sm2O3-based catalysts

The effects of Mn/Na₂WO₄, Li, and CaO loading on the monoclinic Sm₂O₃ catalyst were investigated for the oxidative coupling of methane using O₂ or N₂O as an oxidant. The catalysts were prepared by wet impregnation method and characterized by XRD, BET, CO₂-TPD, and XPS analysis. Impregnation of Mn/Na₂WO₄ on monoclinic Sm₂O₃ resulted in the formation of Sm_2?xMn_xO₃ phase, decreasing the catalytic performance. Li impregnation increased the C₂ selectivity but decreased the catalytic activity. The SmLiO₂ formation increased the catalytic activity and selectivity. High amounts of CaO impregnation increased the C₂ selectivity of monoclinic Sm₂O₃ without a loss in catalytic activity. 6Li/m-Sm₂O₃ were found unstable due to the Li loss from the catalyst. The 15CaO/m-Sm₂O₃ was quite stable and showed 8.2% ethylene yield with N₂O use, much higher than that was obtained with the well-known 2Mn/5Na₂WO₄/SiO₂ and 4Li/MgO catalysts. N₂O was more selective than O₂ as an oxidant and enhanced ethylene formation.     

A study of thermodynamic stability of oxide phases in heating multicomponent ceramic binders

A computer simulation of the thermodynamic stability of phases in firing ceramic silicate binders in the SiO₂-A1₂O₃-B₂O₃-K₂O-Na₂O system for abrasive tools based on grained normal electrocorundum is described. The thermodynamic data for the computation are chosen from the HSC 203 data base of the Outokumpu concern and TAPP of the E. S. Microwave Inc. It is established that when the binder (67.73% SiO₂,20.0% A1₂O₃,5.47% B₂O₃, 5.78% K₂O,2.84% Na₂O with CaO, MgO and Fe₂O₃ impurity oxides, the remainder TiO₂) is fired at 1250°C, the most stable phases bear potassium (KAlSi₃O₃ and KAlSi₂O₆), the amount of the sodium-bearing phases diminishes substantially after 900–1100°C with only the Na₂O · A1₂O₃ · 2SiO₂ phase remaining stable. This seems to explain the fact that the proportion of K₂O to Na₂O in feldspar pegmatites and ceramic binders for abrasion tools is regulated by standards.     

PRELIMINARY STUDY ON PORTIONS OF SYSTEMS Na2O-CaO-AI2O3-Fe2O3 AND Na2O-CaO-Fe2O3-SiO2*

Portions of the quaternary system Na₂O-CaO-Al₂O₃-Fe₂O₃ have been studied by the exploration of (1) the plane CaO-4CaO.Al₂O₃°Fe₂O₃-(Na₂O + 3Al₂O₃) and (2) planes above the base system CaO.5CaO.3Al₂O3–2CaO.Fe₂O₃ which contain successively increasing amounts of Na₂O up to 6%. A portion of the quaternary system Na₂O-CaO-Fe₂O₃-SiO₂ has been studied by the exploration of a plane containing 5% of Na₂O above the base system CaO-2CaO.SiO₂-CaO.Fe₂O₃. In the pseudosystem CaO-4CaO.Al₂O₃.Fe₂O₃-(Na₂O + 3A1₂O₃) the compound Na₂O.-8CaO.3A1₂O₈ was found to exist as a primary phase, and the area in which the plane cuts the Na₂O.8CaO.3A1₂O₃ primary-phase volume was established. Three points on uni-variant curves were located. The iron phase (4CaO.A1₂O₃.Fe₂O₃ solid solution) was observed to exist in a solid-solution series. In the system Na₂O-CaO-5CaO.3Al₂O_3--2CaO.Fe₂O₃ it was found that the compound Na₂O.8CaO.3Al₂O3 appears at an Na₂O concentration of 4.2%. As soda, however, is taken into solid solution by other phases, it was not feasible at this time to determine the invariant point for Na₂O.8CaO.3A1₂O₃, 3CaO.Al₂O₃, 5CaO.3A1₂O₃, and 4CaO.Al₂O₃.-Fe₂O_a solid solution. In the system Na₂O-CaO-2CaO.SiO₂-CaO.Fe₂O₃ no ternary compounds were observed up to the 5% limit of Na₂O employed. A soda-containing phase occurred in solid solution with α-2CaO.SiO₂, which may precipitate on cooling, forming inclusions in the ß-2CaO.SiO₂, or enter into reaction with the glassy phase.     

Characterisation of iron inclusion during the formation of calcium sulfoaluminate phase

The iron distribution among the sulfoaluminate clinker phases and its ability to enter the calcium sulfoaluminate lattice in solid solution can have a significant influence on manufacturing process and reactivity of calcium sulfoaluminate (CSA) cements. X-ray diffraction (XRD) analysis, Mössbauer spectroscopy, scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analysis system (EDAX) and infrared spectroscopy were used to identify the mineralogical conditions of iron inclusion during the formation of calcium sulfoaluminate (C₄A₃S) phase from different mixtures in the CaO-Al₂O₃-Fe₂O₃-SO₃ system. The mixtures, heated in a laboratory electric oven, contained stoichiometric amounts of reagent grade CaCO₃, Al₂O₃, Fe₂O₃ and CaSO₄·2H₂O for the synthesis of Ca₄Al_(6 − 2x)Fe_2xSO₁₆, where x, comprised between 0 and 3, is the mole number of Al₂O₃ substituted by Fe₂O₃. With x increasing from 0 to 1.5, both the iron content of C₄A₃S phase and the amounts of side components such as C₂F and CS increased. For x values included in the range of 1.5-3.0, at temperatures higher than 1200 °C, melting phenomena were observed and, instead of the C₄A₃S solid solution, ferritic phases and anhydrite were formed.     

Tricalcium aluminate hydration in the presence of lime,gypsum or sodium sulfate

The influence of Ca(OH)₂, CaSO₄·2H₂O and Na₂SO₄ on the C₃A hydration was examined in order to study the retardation mechanism of C₃A hydration caused by lime and/or gypsum additions. When C₃A hydrates in the presence of gypsum, the results do not confirm the retardation mechanism based on sulfate ions adsorption or C₄AH_x impervious coating. They substantially confirm the mechanism based on ettringite crystals coating C₃A grains. In the absence of gypsum C₃A hydration is retarded by C₄AH_x formation coating C₃A grains.     

Elaboration and thermal study of iron-phosphorus-substituted dicalcium silicate phase

In the present work, we examine the simultaneous effect of iron and phosphorus additions on the calcium carbonate decomposition in CaCO₃, SiO₂, Fe₂O₃ and P₂O₅ mixtures at the molar ratio CaCO₃/SiO₂=2. The formation of the dicalcium silicate Ca₂SiO₄ is also studied. The temperatures of the decarbonation and the enthalpy evolution during the heating of the mixtures are measured. The additions of Fe₂O₃ and P₂O₅ oxides decrease the onset temperature of the CaCO₃ decomposition. The energy consumption of decarbonation at about 835 °C is lowered when the dopant concentrations increase. Synthesized solid solutions are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The free-lime quantity is determined by chemical analysis. The mineralogical analysis at room temperature of the products of the reaction shows the presence of iron-phosphorus-doped β, α′ and α-C₂S modifications.     

Influence of various promoters on the basicity and catalytic activity of MgO catalysts in oxidative coupling of methane

Addition of promoters, such as Li₂O, Na₂O, PbO, La₂O₃, MgCl₂ and CaCl₂, to MgO causes a large increase in its surface basicity (particularly strong basic sites) and catalytic activity/selectivity in oxidative coupling of methane, but the correlation between the basicity and C₂-yield is poor, indicating that factors other than basicity are also important in deciding catalytic performance.     

Cationic substitution in tricalcium aluminate

Cubic and orthorhombic crystals of tricalcium aluminate doped with Na₂O, Fe₂O₃ and SiO₂ were prepared and examined using an electron probe microanalyzer. The cationic ratios based on six oxygen atoms were derived from the oxide compositions. These data, together with those in previous studies for clinker aluminates containing Mg²⁺ and K⁺, provided excellent correlations between Al+Fe and Si (Al+Fe=2.001−1.03Si) and Ca+Mg and Na+K+Si [Ca+Mg=3.006−0.51(Na+K+Si)]. The chemical variation that is constrained by these equations is well accounted for by the general formula (Na,K)_2x(Ca,Mg)_3−x−y[(Al,Fe)_1−ySi_y]₂O₆, where x is the amount of Ca substituted by Na and K (0≤x<0.158), and y is the amount of Al substituted by Si (0≤y<0.136). The crystal system changed from cubic to orthorhombic with increasing x value. The substitution of Si and Fe for Al extended the solid solution range of the orthorhombic phase to lower values of x, while its effect on the solid solution range of the cubic phase was reversed.     

The role of sulphates in cement clinkering reactions: phase formation in the system CaOAℓ2O3Fe2O3SiO2CaSO4K2SO4

The CaO-rich portions of the systems CaOA?₂O₃SiO₂K₂SO₄, CaOA?₂O₃K₂SO₄, CaOA?₂O₃SiO₂K₂SO₄CaSO₄ and CaOA?₂O₃Fe₂O₃CaSO₄K₂SO₄ have been studied experimentally. Schemes are presented showing phase assemblages compatible at subsolidus temperatures. Melting commences at about 825°C in assemblages containing K₂SO₄. Silicate, aluminate and ferrite phases are comparatively insoluble in molten alkali-rich sulphates. At clinkering temperatures, two immiscible liquids form; one is rich in K₂O and sulphate, the other is a silicate liquid. Some features of vapour-liquid-solid equilibria relevant to the S cycle in cement kilns are discussed.     

The Effects of Na2O and SiO2 on Liquid Phase Sintering of Bayer Al2O3

To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na₂O and 603 ppm SiO₂ and heated at 1525°C up to 8 h. Na₂O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al₂O₃, Na₂O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO₂ concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20?400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na₂O/SiO₂ ratios, densification of Bayer Al₂O₃ at 1525°C is controlled by diffusion of Al³⁺ through the grain‐boundary liquid, whereas for high Na₂O/SiO₂ ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al₂O₃ or diffusion of Al³⁺. Increasing Na₂O in SiO₂‐doped samples increases diffusion of Al³⁺ and Al₂O₃ solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al₂O₃ densification can be manipulated by adjusting the Na₂O to SiO₂ ratio.     

The catalytic conversion of CO2 to hydrocarbons over Fe–K supported on Al2O3–MgO mixed oxides

Al₂O₃–MgO mixed oxides prepared by a co-precipitation method have been used as supports for potassium-promoted iron catalysts for CO₂ hydrogenation to hydrocarbons. The catalysts have been characterized by XRD, BET surface area, CO₂ chemisorption, TPR and TPDC techniques. The CO₂ conversion, the total hydrocarbon selectivity, the selectivities of C₂–C₄ olefins and C₅₊ hydrocarbons are found to increase with increase in MgO content upto 20 wt% in Fe–K/Al₂O₃–MgO catalysts and to decrease above this MgO content. The TPR profiles of the catalysts containing pure Al₂O₃ and higher (above 20 wt%) MgO content are observed to contain only two peaks, corresponding to the reduction of Fe₂O₃ to Fe⁰ through Fe₃O₄. However, the TPR profile of 20 wt% MgO catalyst exhibits three peaks, which indicate the formation of iron phase through FeO phase. The TPDC profiles show the formation of three types of carbide species on the catalysts during the reaction. These profiles are shifted towards high temperatures with increasing MgO content in the catalyst. The activities of the catalysts are correlated with physico-chemical characteristics of the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

RECENT ADVANCES IN CaSO4 OXYGEN CARRIER FOR CHEMICAL-LOOPING COMBUSTION (CLC) PROCESS

Chemical-looping combustion (CLC) is a novel combustion technology with inherent separation of the greenhouse gas CO₂ and low NOx (NO, NO₂, N₂O) emissions. In CLC, the solid oxygen carrier supplies the stoichiometric oxygen needed for CO₂ and water formation, resulting in a free nitrogen mixture. The performance of oxygen carrier is the key to CLC's application. A good oxygen carrier for CLC should readily react with the fuel (fuel reactor) and should be re-oxidized upon being contacted with oxygen (air reactor). In this case, the behavior of CaSO₄ as an oxygen carrier for a CLC process, reacting with gas fuels (e.g., CO, H₂, and CH₄) and solid fuels (e.g., coal and biomass), has been analyzed. The performance of the oxygen carrier can be improved by changing the preparation method or by making mixed oxides. Generally, Al₂O₃, SiO₂, etc., which act a porous support providing a higher surface area for reaction, are used as the inert binder to increase the reactivity, durability, and fluidizability of the oxygen carrier particles. Further, simulation analysis of a CLC process based on CaSO₄ oxygen carrier was also analyzed. Finally, some important tendencies related to CaSO₄ oxygen carrier in CLC technology are put forward.     

Effect of ZnO/MgO ratio on the crystallization and optical properties of spinel opaque glazes

The effect of the ZnO/MgO ratio on the crystallization and optical properties of glass‐ceramic glazes from the SiO₂–Al₂O₃–ZnO–MgO–CaO–K₂O–Na₂O–B₂O₃ system was studied. The glazes with different ZnO/MgO ratios were characterized by differential scanning calorimetry, X‐ray powder diffraction, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy analysis and a spectrophotometer. The results reveal that the A glaze without ZnO content contains forsterite and sapphirine. The B and C glazes with intermediate ZnO/MgO ratio contain enstatite and spinel solid solution. The D to F glazes with higher ZnO/MgO ratio crystallize spinel solid solution as the only crystalline phase. The amount of spinel solid solution, lightness values (L*), gloss values and the reflectance of the studied glazes increase with the ZnO/MgO ratio.     

Thermodynamic calculation of the equilibrium composition of the gasification products of oil shale from the Kendyrlyk deposit

Based on the thermodynamic calculations of the equilibrium composition of the gasification products of shale from the Kendyrlyk deposit (Republic of Kazakhstan) (air blast coefficient α = 0.3; pressure, 0.1 MPa) with consideration for the chemical composition of the ash of mineral components (SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, K₂O, and Na₂O) and the concentrations of trace elements (Cu, Sr, Zn, Cr, Ti, Mn, and Ni) in it, the constituents of a gas phase (the most probable of 500 substances) were determined depending on the process temperature. The equilibrium compositions of gas and condensed phases at a temperature of 1185.65 K were calculated using the HSC Chemistry software.     

Synthesis of magnetic Pb/Fe3O4/SiO2 and its catalytic activity for propylene carbonate synthesis via urea and 1,2-propylene glycol

To facilitate the recovery of Pb/SiO₂ catalyst, magnetic Pb/Fe₃O₄/SiO₂ samples were prepared separately by emulsification, sol-gel and incipient impregnation methods. The catalyst samples were characterized by means of X-ray diffraction and N₂ adsorption-desorption, and their catalytic activity was investigated in the reaction for synthesizing propylene carbonate from urea and 1,2-propylene glycol. When the gelatin was applied in the preparation of Fe₃O₄ at 60°C and the pH value was controlled at 4 in the preparation of Fe₃O₄/SiO₂, the Pb/Fe₃O₄/SiO₂ sample shows good catalytic activity and magnetism. Under the reaction conditions of a reaction temperature of 180°C, reaction time of 2 h, catalyst percentage of 1.7 wt-% and a molar ratio of urea to PG of 1:4, the yield of propylene carbonate attained was 87.7%.