Strength Development in Phosphate-Bonded Calcium Aluminate Cements

The chemical reaction at room temperature between calcium aluminate cement (CAC) and NH₄H₂PO₄-based fertilizer solution causes the formation of ammonium calcium pyrophosphate (AmCPP) as an amorphous cementitious phase. This phase was responsible for the development of strength in rapid-setting phosphate-bonded cement (PBC) specimens. Hydrothermal treatment of PBC at 200°C led to phase transformations of AmCPP and CAC into crystalline hydroxyapatite as the major phase and anorthite as the minor one. The moderate growth of these interlocked crystals in the interspaces of amorphous phases played an important role in creating a dense microstructure, thereby conferring a very high strength to the PBC. In contrast, an excessive crystal growth, which caused the formation of porous microstructure, resulted in a reduction in strength.     

Oxidation of TiB2 Powders below 900°C

Oxidation studies have been performed on titanium dibor-ide (TiB₂) powders, from room temperature up to 900°C. The studies were performed at low partial pressures of oxygen, at 10 and 0. 05 ppm of O₂ in argon, simulating furnace atmosphere with flowing neutral gas of medium and high purity, and in air. It has been found that titanium borate (TiBO₃) is formed in these processes. It has also been revealed that the oxidation process starts below 400°C and is reversible in this low temperature range from 100° to 400°C.     

Solubility and Aging of Calcium Silicate Hydrates in Alkaline Solutions at 25°C

Calcium silicate hydrate (C-S-H) gels are the principal bonding material in portland cement. Their solubility properties have been described, enabling pH and solubilities to be predicted. However, the gels also interact with other components of cements, notably alkalis. C-S-H has been prepared from lime and silicic acid in solutions of sodium hydroxide or potassium hydroxide and by the hydration of tricalcium silicate (C₃S) in sodium hydroxide solutions. Analyses of aqueous phases in equilibrium with 85 gels show that the aqueous calcium and silicon concentrations fit smooth curves over the range of increasing sodium concentrations. Where anomalous data occur, they correspond to solids with low lime contents: such gels are tentatively assumed to fall into a region where the presence of another gel phase influences the aqueous composition. Dimensional changes have been observed in the hydration products of C₃S as a function of alkali content and these may be relevant to the alkali-silica reaction. The significance of this and other data is discussed with reference to real cement systems.     

铝酸钙玻璃中羟基对红外透过性能的影响

采用不同熔化方法制备了铝酸钙红外玻璃并考察了其红外透过性能。研究了结构中的羟基对玻璃红外透过性能的影响。通过红外光谱和固体核磁共振技术对玻璃中羟基的结构特征、氢键结合状态以及红外吸收情况进行了研究和探讨。结果表明:羟基会引起铝酸钙玻璃在近红外2.9μm左右的宽带吸收,这是由于玻璃中羟基结构形式的多样化所造成的;在玻璃结构中,羟基与氢键结合会造成振动频率下降,红外吸收峰向长波移动,羟基氢键结合强度越大,长波移动越严重,羟基与氢键结合形式越多,羟基的红外吸收范围越宽;在铝酸钙红外玻璃中主要存在3种羟基存在形式:孤立羟基基团、邻近的羟基对和带有氢键的单个羟基。不同制备方法获得的玻璃结构中羟基存在形式基本相同。     

氧化镁在铝酸钙盐中的固溶性

氧化镁在水泥熟料中的存在对安定性有直接的影响,因此了解氧化镁在各熟料矿物的固溶性至关重要.铝酸三钙是水泥熟料的主要溶剂型矿物之一,本文研究氧化镁在铝酸钙中的固溶性:空白试样3CaO+Al2O3,使用0. 5 mol和1 mol的氧化镁取代对应含量的氧化钙烧制铝酸盐矿物,与空白样进行研究对比.分别采用了XRD定性、精修XRD、岩相及SEM-EDS等分析方法进行研究对比.结果显示:本实验条件下,氧化镁的加入影响矿物的产物含量,更多的氧化镁固溶于过渡相C12A7中,氧化镁在它形晶比自形晶固溶量更大,Mg2+的含量与Al3+的含量呈正相关,氧化镁的局部最大固溶量可以达到9.28%,氧化镁固溶量还会使铝酸钙盐的衬度颜色改变.     

Interdiffusion of Calcium in Soda-Lime-Silica Glass at 880° to 1308°C

The interdiffusion of calcium in soda-lime-silica glass under the action of a concentration gradient was studied. Pairs of glass blocks differing by 2.9 mole % in CaO content were fused together to form diffusion couples and were held at 880° to 1308°C. The couples were allowed to cool to room temperature and the diffusion which had taken place was measured by optical interferometry and by an electron microprobe. The diffusion coefficients varied from 4.4 × 10⁻¹⁰cm²/sec at 880°C to 8.0 × 10⁻⁸ cm²/sec at 1308°C. The activation energy was 42,000 cal/mole. It is concluded that oxygen diffuses simultaneously with the calcium, maintaining the electrical neutrality of the glass.     

Characterization of C─B─N Solid Solutions Deposited from a Gaseous Phase between 900° and 1050°C

The composition, the structure, and some physical properties of boron–carbon–nitrogen solid solutions prepared by chemical vapor deposition (CVD) have been investigated. Both crystalline films and amorphous granular materials resulting respectively from heterogeneous and homogeneous nucleation were characterized by X-ray diffraction, XPS, RBS, and TEM. The compositions of these single-phase materials are gathered in two main domains located in the B/N > 1 part of the C─B─N composition diagram. It is stated that the carbon-rich domain results from structural disorder of an ideal C₅B₂N composition. The thermal behavior of these films indicates that no mass loss can be detected after 1 h at 1700°C but a graphitization and a formation of small amounts of B₁₃C₂ are observed. Density and preliminary electrical conductivity measurements were also performed.     

Reaction between Sodium Carbonate and Silica Sand at 874°C < T < 1022°C

This paper is concerned with the mechanism of the reaction between sodium carbonate and silica sand at 874° to 1022deg;C. A reaction path which evolves in four stages, i.e., decarbonization, dissolution of orthosilicate, dissolution of metasilicate, and final dissolution of silica, is dealt with in detail, including mass balances. Formulas for dissolution rates in the first two stages are suggested, and the effect of wetting the grains by the melt, amount and distribution of the melt, and nonuniform dissolution (e.g., segregation) on the dissolution rate of silica grains are discussed. The need for more detailed experimental studies is stressed.     

Hot Corrosion Behavior of Barium Aluminosilicate-Coated C/SiC Composites at 900°C

Barium aluminosilicates (BAS) were coated on the carbon fiber-reinforced silicon carbide composites (C/SiC) as environmental barriers. The hot corrosion behavior of the coated composites was studied at 900°C in dry air and water vapor, respectively. The molten Na₂SO₄ was used as the corrosion reactant. The results indicate that the BAS coatings can effectively block the attack of molten Na₂SO₄ to C/SiC composites in dry air. However, the coated composites degrade rapidly when exposed to molten Na₂SO₄ coupled with water vapor. It is found that the BAS is corroded by Na₂SO₄ melt with the formation of BaSO₄, resulting in the destruction of BAS structure, which makes the coating lose its protection to the C/SiC composites in water vapor.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Synthesis of Pure Monolithic Calcium, Strontium, and Barium Hexaluminates for Catalytic Applications

Calcium, strontium, and barium hexaluminates have been prepared via a chemical route. In light of their interest as catalytic substrates the relation between attrition-milling time and density was studied, with the scope of obtaining fired bodies with suitable pore structure. It was found that all hexaluminates grew in elongated structures; longer milling times resulted in higher aspect ratios for calcium hexaluminate, with less than 75% of the theoretical density. Strontium and barium hexaluminates were prepared with even lower densities, but the reason is the inherent poor densification, and not the growth of mutually interfering elongated structures, as in the case for calcium hexaluminate.     

低温烧成纯铝酸钙水泥的机理研究

在常压条件下水热合成水化铝酸钙，然后经过低温烧成制备了纯铝酸钙水泥。通过力学性能的测试，并采用XRD，DTA等分析方法，探讨了低温烧成纯铝酸钙水泥的机理。研究结果表明：采用水热合成处理方法制备纯铝酸钙水泥，烧成温度低（1250℃，制得熟料活性好，强度高，是替代传统烧结法制备纯铝酸钙水泥的新方法。     

Formation Kinetics of Calcium Aluminates

The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X-ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA₂) in a 1:2 reaction mix of CaO and Al₂O₃. CA reacted further with Al₂O₃ to form CA₂. The formation of CA₂ obeyed the rate law equation 1 - (1 - x )^1/3= Kt / r ². The activation energy for the system (140 kJ·mol⁻¹ (33.4 kcal · mol⁻¹)) was determined by the Arrhenius equation.     

Experimental Determination and Thermodynamic Calculation of the Zirconia–Calcia–Magnesia System at 1600°, 1700°, and 1750°C

Phase relations in the ternary system ZrO₂–CaO–MgO were experimentally established at 1600°, 1700°, and 1750°C. The investigation was based on powder X-ray diffractometry, scanning electron microscopy–energy dispersive spectroscopy, and electron probe microanalysis, on 24 ternary compositions. The compositions were prepared using attrition milling of respective oxides and carbonates as raw materials. The results obtained allowed construction of the corresponding isothermal sections, which verified the existence of the cubic-ZrO₂–CaZrO₃ phase compatibility field at the three temperatures. Finally, experimental results also were compared with the thermodynamic assessment previously reported of the system ZrO₂–CaO–MgO.     

高炉铝酸钙炉渣浸出过程动力学

研究了高炉铝酸钙炉渣的浸出动力学,考察了搅拌强度、浸出反应温度、浸出剂初始浓度及炉渣粒度对浸出速率的影响. 结果表明,浸出过程符合一级反应的收缩未反应核模型,宏观动力学方程为1+2(1-xB)-3(1-xB)2/3= 1.108exp(-1906/T)t,表观活化能为15.84 kJ/mol,过程速率为固膜内扩散速率控制. 通过实验数据验证,表明所得模型能较好地描述炉渣的浸出过程.     

Al2O3含量对燃烧合成铝酸钙粉体物相组成的影响

本文以CaO、Al和Al2O3为原料,在氧气气氛下,采用燃烧合成法(CS)制备铝酸钙粉体,计算了CaO-Al-Al2O3-O2体系的绝热温度,结合物质自由能函数的相关理论、X衍射法(XRD),研究了Al2O3含量对燃烧合成铝酸钙粉体物相组成的影响.热力学计算及XRD物相分析结果表明:体系绝热温度随Al2O3含量的增多而降低,但均大于1800 K,说明体系反应可自持;当Al2O3/(Al2O3+ CaO) (mole)分别为0.3,0.47和0.55时,物相组成分别为C3A和C12A7,C12A7和CA,CA和CA2;热力学数据显示C12A7-C3A、C12A7-CA和CA-CA2之间物相可实现转化,但由于燃烧合成反应速度过快及不可控性导致物相之间尚未完成转化,致使燃烧合成铝酸钙的物相生成量与理论量有较大差异.     

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO₂–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y₂O₃ partially stabilized ZrO₂ in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as-sintered composites were characterized by X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO₂, resulting in the formation of α-Ti(O, Zr), Ti₂ZrO, and/or TiO. These oxygen-containing phases extracted oxygen ions from ZrO₂, whereby oxygen-deficient ZrO₂ was generated. For relatively small Ti/ZrO₂ ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO₂. For the specimens with ≥50 mol% Ti, lamellar Ti₂ZrO was precipitated in α-Ti(Zr, O), with no TiO being found. Both m -ZrO_{2− x} and t -ZrO_{2− x} were found in specimens with ≤50 mol% Ti; however, only c -ZrO_{2− x} was formed in the specimen with 70 mol% Ti. As ZrO₂ was gradually dissolved into Ti, yttria was retained in ZrO₂ because of the very limited solubility of yttria in α-Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO₂ increased with the Ti content. The complete dissolution of ZrO₂ into Ti was followed by the precipitation of Y₂Ti₂O₇ in the specimen with 90 mol% Ti.     

Mechanochemical Synthesis of Magnesium Aluminate Spinel Powder at Room Temperature

MgAl₂O₄ spinel was successfully synthesized using a mechanochemical route that avoided the formation and calcination of its precursors at high temperatures. The method involved a single step in which γ-Al₂O₃–MgO, AlO(OH)–MgO, and α-Al₂O₃–MgO mixtures were milled at room temperature under air atmosphere. The formation of MgAl₂O₄ occurred faster with γ-Al₂O₃ than with AlO(OH) or α-Al₂O₃. After 140 h, the mechanochemical treatment of the γ-Al₂O₃–MgO mixture yielded 99% of MgAl₂O₄.