Influence of Citric Acid on Reactions in the System 3CaO·Al2O3-CaSO4·2H2O-CaO-H2O

The influence of citric acid on paste hydration of 3CaO· Al₂O₃ in the presence of CaSO₄·2H₂O and Ca(OH)₂ was studied using X-ray diffraction, scanning electron microscopy, and conduction calorimetry. The time at which the citric acid is added (either prior to or with the mixing water) determines how it affects the reactivity of the aluminate. Immediately after the paste is gaged citric acid promotes a more rapid reaction, but later reactions are retarded. Hexagonal calcium aluminate hydrates, ettringite, and monosulfate were all detected as early hydration products. The influence of citric acid on the hydration of 3CaO·Al₂O₃ slabs immersed in saturated CaSO₄·2H₂O solutions was also studied and a reaction scheme proposed.     

Kinetics of the Early Hydration of Tricalcium Aluminate in Solutions Containing Calcium Sulfate

The rates of reaction of 3CaO. Al₂O₃, in sulfate-containing solutions of three compositions were investigated. It was observed that the rates of calcium and sulfate uptake decreased with increasing calcium hydroxide concentration. In a further experiment using a calcium sulfate solution, which also contained NaOH, it was established that the kinetics of calcium sulfoaluminate hydrate formation are strongly dependent on the hydroxyl ion concentration. The rate of sulfate ion consumption per unit surface area of 3CaO·Al₂O₃ was observed to be constant during the period in which a calcium sulfoaluminate hydrate is a reaction product. The ratio of calcium-to-sulfate ions consumed in the hydration reactions investigated exceeded unity suggesting the formation of 4CaO·Al₂O₃· n H₂O in addition to ettringite.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Coordination of Aluminum Ions in Tricalcium Aluminate

The coordination of aluminum ions in tricalcium aluminate (3CaO·Al₂O₃) was examined using infrared absorption, X-ray emission spectroscopy, and molar refraction techniques. Each of these methods indicated the presence of aluminum in fourfold coordination. No evidence was found for aluminum in sixfold coordination. The isomorphous substitution of Sr²⁺ for Ca²⁺ and Ga³⁺ for Al³⁺ and a comparison between the absorption frequencies for an SiO₄ group and an AlO₄ group were used to identify the infrared absorption bands in tricalcium aluminate.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Conversion of CaO·Al2O3·10H2O to 3CaO·Al2O3·6H2O

The morphological changes accompanying the conversion of the hexagonal CaO·Al₂O₃·10H₂O phase to the cubic 3CaO·Al₂O₃·6H₂O phase were studied by scanning electron microscopy. The hydration and conversion reactions were monitored by X-ray diffraction analysis. From the micrographs, it was inferred that changes in the pore structure and the presence of large cubic crystals of questionable adhesive value were probably the principal factors responsible for the loss of strength in converted calcium aluminate cement pastes.     

The Effects of Prehydration on the Liquid Hydration of 3CaO·Al2O3 with CaSO 4·2H2O

Liquid hydration and water-vapor hydration of 3CaO·Al₂O₃, were studied. Variable parameters were hydration time, temperature, relative humidity, and amount of gypsum. The hydration products (gel, ettringite, hexagonal hydrates, and 3CaO·Al₂O₃·6H₂O) were studied by electron microscopy, X-ray diffractometry, and thermal analysis. A reaction scheme is proposed. The degree of water-vapor hydration influenced the sequence of the subsequent liquid hydration which, however, was independent of the composition of the water-vapor hydration products. Below a critical degree of water-vapor hydration (≊3% combined water) the reaction with liquid water occurred as if no water-vapor hydration had taken place. Above this value the reaction gave hydration products suggesting a change of the 3CaO·Al²O³ reactivity. A possible correlation with the retardation of strength development of prehydrated cement is suggested.     

29Si and 27Al MASNMR Study of Stratlingite

Strätlingite (2CaO·Al₂O₃·SiO₂·8H₂O) is a complex calcium aluminosilicate hydrate commonly associated with the hydration of slag-containing cements or other cements enriched in alumina. Strätlingite can coexist with the hydrogarnet solid solution [hydrogarnet (3CaO·Al₂O₃·6H₂O)-katoite (3CaO·Al₂O₃·SiO₂·4H₂O)] and calcium silicate hydrate (C-S-H). Since Strätlingite is present in many blended cements, the knowledge of strätlingite's characteristic silicate anion structure and how aluminum is accommodated by the structure is important. Phase pure Strätlingite samples have been synthesized from oxides in the presence of excess water and from metakaolinite, calcium aluminate cement, CaO, NaOH, and water. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and then further examined using ²⁹Si, with and without cross-polarization (CP), and ²⁷Al solid-state magic angle nuclear magnetic resonance spectroscopy (MASNMR). For the most part, NMR data for these strätlingites corroborate structural information available in the literature. The aluminum atoms are both tetrahedrally and octahedrally coordinated, and the silicon atoms exist predominantly as Q₂, Q₂(1Al), and Q₂(2Al) species. The presence of alkali affects the structure of strätlingite in subtle ways, significantly reducing the Al^IV/A1^VI ratio.     

Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum

The formation of ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) from monosulfate (3CaO·Al₂O₃·CaSO₄·12H₂O) and gypsum (CaSO₄·2H₂O) was investigated by isothermal calorimetry and X-ray diffraction (XRD) analyses. Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 0.2 M , 0.5 M , and 1.0 M sodium hydroxide (NaOH) solutions. Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations. A sodium-substituted monosulfate phase was formed as a hydration product in the 1.0 M sodium hydroxide solution regardless of temperature. XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction. The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration. Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium-substituted monosulfate phase competes with ettringite formation.     

Reaction Characteristics of Magnesia–Spinel Refractories with Cement Clinker

The adherence ability of cement clinker on magnesia–spinel refractories is investigated, using a sandwich test, at 1550°C for 30 min under a load of 5.3 kPa. Fractional factorial experiments determine that the silica ratio (SR)—SiO₂/(Al₂O₃+Fe₂O₃) and particle size of raw meal, as well as heating rate, have a significant effect on adherence ability. Microstructural analyses indicate that the adherence ability depends upon reactions between clinker and refractories at high temperature. Only spinel reacts with CaO and 3CaO·SiO₂ from clinker to form n -calcium aluminate (such as 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃), but there is no reaction between MgO and the clinker. Fine crystalline spinel, evenly distributed in magnesia-based brick, is prone to reacting with lime-containing phases from clinker to form low melting phases and a belite-enriched zone at the clinker/brick interface. This reaction positively contributes to the high adherence on a magnesia−spinel brick. The high content of liquid in clinker with low SR accelerates reactions between spinel and clinker, while a limited reaction occurs at the brick/clinker interface with high silica.     

Influence of Gluconate, Lignosulfonate, and Glucose Admixtures on the Hydration of Tetracalcium Aluminoferrite in the Presence of Gypsum with or without Calcium Hydroxide

The influence of 0.3 wt% gluconute, lignosulfonate, or glucose on the hydration of 4CaO·A1₂O₃-Fe₂O₃ in the presence of gypsum with or without Ca(OH)₂, was examined. In the absence of Ca(OH)₂ all the admixtures retard both ettringite production and subsequent conversion of ettringite into the monosulfate in the decreasing order glucose >lignosulfonate >gluconate. In the presence ofCa(OH)₂ all the admixtures accelerate early ettringite production but do not affect subsequent conversion of ettringite into the monosulfate, at least up to 28 d.     

Hydration Characteristics of Portland Cement after Heat Curing: I, Degree of Hydration of the Anhydrous Cement Phases

The degree of hydration of the four major anhydrous cement phases in three U.K. portland cement mortars has been observed during the period of water storage at room temperature after an initial short-term heat cure. Such a heat cure at 85° or 100°C for 12 h generally accelerated the initial hydration of the four major anhydrous minerals in portland cement. Subsequent retardation of the degree of hydration of the alite, tricalcium aluminate, and ferrite phases was observed when these heat-cured mortars were stored at ambient temperature. General similarity but some differences in hydration behavior were observed between the three cements. The hydration of belite in the heat-cured mortars during storage at room temperature produced porous inner products that favored deposition of ettringite and reduced the risk of expansive ettringite formation. The substantial retardation in hydration of the aluminate-bearing phases, especially the ferrite phase, during the storage at room temperature raised the overall SO₃/Al₂O₃ ratio of the cement hydrates formed, bringing about a potential for ettringite formation and hence the risk of expansion through delayed ettringite formation.     

Effect of Lime on Hydration of Pastes Containing Gypsum and Calcium Aluminates or Calcium Sulfoaluminate

Chemical reactions responsible for ettringite formation in the commercial expansive cements are reviewed. X-ray diffraction analyses of the hydrated expansive compounds showed that in the presence of Ca(OH)₂ and CaSO₄ the tricalcium aluminate hydrated very slowly and the monocalcium aluminate quickly, whereas the calcium sulfoaluminate hydrated at a uniform rate during the early hydration period. Scanning micrographs are presented which show that ettringite formed in the presence of Ca(OH)₂ is colloidal. It is proposed that colloidal ettringite can adsorb large quantities of water, thereby causing slump loss in fresh concrete and expansion in hardened concrete.     

The System CaO-Al2O3-H2O

Phase equilibria have been determined in the system CaO-Al₂O₃-H₂O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)₂, 3CaO·Al₂O₃·6H₂O, and 4CaO·3Al₂O₃-3H₂O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al₂O₃ and 3CaO·Al₂O₃ have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed.     

Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2

We investigated the effect of characteristics of raw Al₂O₃ powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF₂ was added as a catalytic material. Four types of Al₂O₃ powders were selected. An Al₂O₃/C molar ratio of 0.29 was fixed, and the amount of CaF₂ was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N₂ flow. The nitridation rate tended to increase with decreasing particle size of raw Al₂O₃ and was found to depend on the Al₂O₃ synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al₂O₃ powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al₂O₃ and CaO·2Al₂O₃ were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al₂O₃ with CaF₂ addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF₂-CaO·6Al₂O₃-CaO·2Al₂O₃.     

Dissolution in Ceramic Systems: III, Boundary Layer Concentration Gradients

By use of electron microbeam probe analysis on quenched samples, the concentration distribution of CaO, A1₂O₃, and SiO₂ was determined across the boundary layer between molten calcium aluminum silicates and dissolving or growing sapphire and fused silica. A definite shift in the concentration ratios of the solvent components was found near the interface. Analysis of diffusion flux equations for a ternary system successfully related the shift in concentration ratio to the intrinsic diffusion coefficient for each component. For alumina dissolution in a melt rich in CaO, evidence of incongruent dissolution was observed with the formation of new phases, CaO· 6Al₂O₃ and CaO· 2A1₂O₃.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Forming and Sintering of in Situ Alumina Composite with Hydraulic Inorganic Binder

Two types of barium aluminate binders were prepared by heat treatment of barium aluminate precursors, synthesized by using solution processes, at low temperatures ( T < 500°C). One was barium monoaluminate (BaAl₂O₄), and the other was barium aluminate binder (BAH binder) composed of amorphous phase, barium aluminate monohydrate (BaAl₂O₄·H₂O), and BaAl₂O₄. The setting time of the BAH binder was controlled by adjusting the heat-treatment temperature of the BAH binder precursor. The addition of the synthesized BaAl₂O₄ powders to Al₂O₃ powders improved the bending strength of Al₂O₃ matrix green bodies. The synthesized BaAl₂O₄ powders led to the in situ forming of barium hexaaluminate (BaO· x Al₂O₃, x = 6.9: BA6) platelets in the matrix by reacting with Al₂O₃ during sintering. The formed BA6 platelets inhibited the grain growth of the matrix Al₂O₃ grains.     

Contactless Density Measurement of Liquid Nd-Doped 50%CaO–50%Al2O3

The density of neodymium-doped calcium aluminate (<1 mol% Nd₂O₃·50% CaO·50% Al₂O₃) liquid was measured over a wide temperature range using an electrostatic levitation furnace. The density was obtained using an UV-based imaging technique that allowed excellent illumination throughout all phases of processing, including elevated temperatures. Over the 1560–2000 K temperature range, the density could be expressed as ρ( T ) = 2.83 × 10³– 0.21( T – T _m) (kg·m⁻³) (±2%) with T _m= 1878 K, which yielded a volume coefficient of thermal expansion α( T ) = 7.5 × 10⁻⁵ K⁻¹.     

The System CaO–"CaCr2O4"–CaAl2O4 in Air and under Mildly Reducing Conditions

The system CaO–chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca₅Cr₃O₁₂, Ca₃(CrO₄)₂, and Ca₅(CrO₄)₃) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, δ -CaCr₂O₄, is observed. In the CaO-rich section of the CaO–Al₂O₃–Cr₂O₃ system a ternary intermediate phase, chrome-haüyne, Ca₄[(Al,Cr³⁺)₆O₁₂](Cr⁶⁺O₄), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca₆Al₄Cr₂O₁₅, coexists with CaO, CaCr₂O₄, chrome-haüyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO–Al₂O₃ system is insignificant.