铝酸钙水泥的异常凝结行为

硅酸盐水泥的水化速率总是随水化温度升高而增大。而在18～30℃范围内，铝酸钙水泥的水化速率随温度升高而降低，这就是所谓的铝酸钙水泥的异常凝结行为。通过比较工业铝酸钙水泥与合成纯铝酸钙的凝结行为，证明这种行为是铝酸钙水泥本身具有的，而不是掺入物引起的。铝酸钙水泥的异常凝结行为与其水化严物的结构、成核及生长等因素有关。本文还指出了对于这一现象有待进一步研究的问题。     

Porosity-strength relation in calcium aluminate cement pastes

The compressive strength and the volume porosity of calcium aluminate cement pastes have been studied in order to connect their relationship. The influence of mass fraction of lithium carbonate on compressive strength and porosity of calcium aluminate cement (CAC) has been investigated at different water-cement (w/c) ratios. The functions proposed in the literature for different technical materials were tested on obtained strength and porosity data. Those functions have been a base for further development of more general functional dependence of strength and porosity for cement materials. Thus, we propose the following equation to relate the strength and porosity for CAC pastes:

     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

化学合成高纯铝酸钙水泥的特性及应用

研究了化学合成高纯铝酸钙水泥的物理特 性,包括强度发展、凝结时间、水化结合水量和水化放 热特性,并与市售的以传统烧结法生产的水泥进行了 在典型耐火浇注料中的应用比较。结果表明:(1)化 学合成高纯铝酸钙水泥胶砂的强度发展快且充分,早 期强度高;(2)化学合成高纯铝酸钙水泥的水化放热 速率在水化2h时最大,放热快且放热量集中;(3)化 学合成高纯铝酸钙水泥的水化活性较高;(4)化学合 成纯铝酸钙水泥用于耐火浇注料中,浇注料表现出较 高的烘干强度,且浇注料的烧后强度等性能指标也与 用传统烧结水泥的大体相当。     

The correlation between compressive strength and ultrasonic parameters of calcium aluminate cement materials

Calcium aluminate cement (CAC) mortars were investigated by nondestructive ultrasonic measurement in the through-transmission mode and compressive strength measurements. The detected profile of ultrasonic signal was fitted as a sine wave modulated with the Gauss function. The linear relationship between compressive strength and the product of the amplitude and angular frequency of the signal was established. A qualitative explanation of the proposed correlation based on the existing theories was given.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

In this work the impact of Al₂O₃ amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al₂O₃ plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al₂O₃ only calcium silicate hydrates (CSH) intercalated with Al³⁺ ions were formed. While in the mixtures with a higher amount of Al₂O₃ (5.3–15.4%), calcium aluminate hydrate – C₃AH₆, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al₂O₃. It was proved that synthesized C₃AH₆ remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca₁₂Al₁₄O₃₃), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca₂Al₂SiO₇). Moreover, the synthesized C₃AH₆ addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.     

铝酸钙水泥对刚玉基浇注料性能的影响

以电熔白刚玉颗粒及细粉为主要原料,α-Al2O3微粉、铝酸钙水泥以及Alphabond300为结合系统,研究了铝酸钙水泥加入量(质量分数分别为0、0.75%、2.25%和3.75%)对刚玉基浇注料性能的影响。结果表明:1)水泥的加入使浇注料基质的粘度增大,浇注料的流动性降低。2)随着水泥加入量的增加,110℃以及800℃处理后的冷、热态抗折强度均逐渐提高;1 100℃、1 400℃和1 600℃烧后的冷、热态强度均先降低后升高,其中水泥加入量(质量分数,下同)为0.75%时值最小。3)随着水泥含量的增加,浇注料抗热震性提高。4)少量水泥的加入使浇注料的抗渣性能降低,进一步增加水泥加入量,浇注料的抗渣性能逐步改善;在本试验范围内,水泥加入量为3.75%的浇注料和不含水泥的浇注料抗渣性能基本相当。     

Effect of particle size of hydromagnesite on properties of calcium aluminate cement bonded corundum based castables

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA₆ resulted from the more uniformly distributed hydromagnesite in castables.     

水泥加入量对刚玉浇注料性能的影响

讨论了使用电熔白刚玉做骨、粉料,纯铝酸钙水泥作结合剂时,纯铝酸钙水泥加入量对制品性能的影响。结果表明,刚玉浇注料的重烧膨胀是由于刚玉与水泥发生反应生成了密度较低的CA6。     

Calcium silicate/calcium aluminate composite biocement for bone restorative application: synthesis,characterisation and in vitro biocompatibility

Pure β-dicalcium silicate and monocalcium aluminate powder were prepared by Pechini method. A series of calcium silicate/calcium aluminate cements (CSC/CAC) were prepared. The setting time, crystalline phases, microstructures, compressive strength, cells attachment and silicon release of the cements were investigated. The results indicate that the setting time of CSC/CAC was shorter than that of either CSC or CAC. The hydration products in CSC/CAC composite are gehlenite (Ca₂Al₂SiO₇·8H₂O), calcium aluminate hydrate (Ca₃Al₂O_6?×?H₂O), and katoite (Ca₂Al₂O₆·6H₂O). Platelike crystals were found in the microstructure. The liquid to powder ratio has a significant effect on the porosity and the strength of CSC/CAC. The MC3T3 cells attached well to the surfaces of CSC/CAC. However, the cells proliferation on the surface of 7S3A was better than that of 3S7A due to its higher silicon release. In general, CSC/CAC exhibits good biocompatibility and relative high strength, and may be suitable for some non-load bearing bone restorative applications.     

铝酸盐水泥熟料对粉煤灰硅酸盐水泥性能的影响

以铝酸盐水泥熟料、硅酸盐水泥熟料和粉煤灰为原料,探讨了掺加少量铝酸盐水泥熟料对硅酸盐水泥及粉煤灰硅酸盐水泥复合体系水化、凝结和硬化性能的影响。结果表明,在硅酸盐水泥及粉煤灰硅酸盐水泥中掺加铝酸盐水泥熟料,可以明显缩短水泥的初、终凝时间,但复合体系的需水量增加;掺加少量铝酸盐水泥熟料(≤3%)可明显提高硅酸盐水泥的早期强度,但后期强度(28d)有所降低;当铝酸盐水泥熟料的掺量达5%时,水泥的各龄期强度均明显降低。少量铝酸盐水泥熟料掺加到粉煤灰硅酸盐水泥中,复合体系的各龄期强度都明显提高,且早期强度的提高幅度较大。     

Effect of nanosilica addition on bioactivity and in vivo properties of calcium aluminate cement

This study aims at assessing the influence of nanosilica on the bioactivity and mechanical properties of calcium aluminate cement. For this purpose, nanosilica was applied as a replacement for calcium aluminate cement at 0, 2, 5 and 8 wt%. The main components were analyzed by scanning electron microscope coupled with surface imaging and elemental analysis, fourier transform infrared spectroscopy and X-ray diffraction analysis. To estimate the bioactivity of specimens, hydroxyapatite formation on the surface of cement paste was investigated in the simulated body fluid solution. In addition, in vivo evaluation of calcium aluminate cement was performed in subcutaneous tissue of rats. To investigate the mechanical properties, both compressive and flexural strengths were also measured. The results revealed that by increasing nanosilica up to 8 wt%, the strength enhanced. Moreover all cement paste samples with various amounts of nanosilica represented good bioactivity because of formation of bonelike apatite layer on the surface of specimens within 28 days after soaking in simulated body fluid. In vivo experiments indicated that the cement sample was absorbed by the tissue and there was no infection at the implant site. Based on the in vitro and in vivo results, the specimen with 2 wt% nanosilica represented the highest bioactivity.     

Liquid-phase self-diffusion in hydrating cement pastes — results from NMR studies and perspectives for further research

The changes in the pore structure of hydrating cement are accompanied by changes in the dynamics of liquid phases contained in the pore system of the hydrating matrix. Dynamic NMR methods (relaxometry, diffusometry) allow the non-destructive observation of these changes. Relaxometry can be performed using quite simple equipment and has been widely used in studies of the kinetics of cement hydration. Diffusion studies, by contrast, require much more sophisticated equipment. On the other hand, the diffusion coefficient has a direct relevance for the transport of moisture or contaminants in the cement matrix while relaxation time measurements provide more indirect information. The purpose of the present paper is to review the possibilities of field gradient NMR in diffusion studies on hydrating cement and to provide an outlook on how this information can be used for improving our understanding of the properties and microstructure of hydrating cement. As an example, new results on the relationship between the diffusive exchange length in the sample and non-exponential relaxation in cement are discussed at the end of the contribution.     

水泥和水合氧化铝对铝镁浇注料性能的影响

固定电熔白刚玉、电熔镁砂粉、αAl2O3微粉、SiO2微粉等原料的含量不变,分别加入不同量的水合氧化铝和纯铝酸钙水泥作结合剂(ρAl2O3加入量为2%、3%和4%,水泥加入量为3%、5%和7%),对比研究了水泥结合和水合氧化铝结合铝镁浇注料的显气孔率、体积密度、耐压强度、抗折强度、抗热震性和抗渣性能。结果表明:与水泥结合铝镁浇注料相比较,水合氧化铝结合浇注料具有更好的抗热震性、抗渣侵蚀和渗透性能;水合氧化铝在浇注料中的合适加入量为3%;水泥结合铝镁浇注料具有较高的常温强度,但其抗渣侵蚀和渗透性能随着水泥含量的增加而逐渐降低。     

Effect of Zn(OH)2 on properties of corundum based castables bonded with calcium aluminate cement

In this work, the effect of Zn(OH)₂ on properties of corundum based castables bonded with calcium aluminate cement (CAC) was investigated. The phase composition and microstructure of castable matrixes containing Zn(OH)₂ after firing 800 °C, 1100 °C and 1550 °C were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectra, respectively. The results indicate that a small amount of Zn(OH)₂ can dramatically improve the medium temperature strength of castables because the generation of zinc aluminate spinel increases the ceramic bonding of castables. In addition, the addition of Zn(OH)₂ also improves the volume stability of CAC-bonded castables due to the enhanced formation of pores from Zn(OH)₂ decomposition in castables.     

Alkali-activated blends of calcium aluminate cement and slag/diatomite

The present study deals with the formulation of new cementitious materials via the alkaline activation of an industrial by-product (blast furnace slag) or a natural rock (diatomite) in the presence of reactive aluminium sourced from calcium aluminate cement (CAC). Two blends, one containing 20% CAC and 80% slag and the other 20% CAC and 80% diatomite, were prepared and activated with sodium sulphate or a sodium hydroxide solution. The hardened materials were characterised with X-ray diffraction (XRD) as well as ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) and tested for their 2-day mechanical strength. The main reaction product was a cementitious gel that precipitated with crystalline phases such as ettringite, U phase and katoite. While the slag blend reacted to generate a C–(A)–S–H-like gel under moderately alkaline conditions, diatomite reactivity proved to be very low under such conditions. The greater reactivity of both slag and diatomite at high pH (high alkalinity) favoured their interaction with CAC.     

A simple semiempiric model for NMR relaxometry data of hydrating cement pastes

In this contribution, results from NMR relaxometry experiments on hydrating white cement pastes are presented. Nonexponential effects are discussed and quantified by stretched-exponential analysis. From the magnetization decay curves in transverse relaxation experiments, the degree of hydration is estimated and a semiempiric model is proposed describing the relationship between the microstructure development in the hydrating cement and the degree of hydration.     

Petrographical analysis of calcium aluminate cement mortars: Scanning electron microscopy and transmitted light microscopy

The samples studied for this paper were obtained from experimentally induced carbonation and alkaline hydrolysis of calcium aluminate cement (CAC) mortars. The mineralogical composition of phases present was studied from X-ray diffraction (XRD) spectra. Scanning electron microscopy (SEM) and transmitted light microscopy (TLM) were used to obtain some morphological (size and shape) and fabric data of mineralogical phases to improve the interpretation of damage processes. SEM is a useful tool to examine the evolution of mineral phases in cavities and discontinuity zones. The interpretation of the degree of carbonation and the distribution of carbonated phases in the samples, the presence of C₃AH₆ and CAH₁₀, as well as possible alterations of aggregates are well differentiated and defined using TLM method. The main objective of the present paper is to validate the use of both microscopic techniques (TLM and SEM) for the study of phase evolution in the CAC mortar during curing and degenerative processes.     

铝酸钙水泥对焦宝石基喷涂料性能的影响

以焦宝石颗粒及细粉为主要原料，铝酸钙水泥为结合系统。分别研究了不同铝酸钙水泥含量对焦宝石基喷涂料性能的影响。结果表明．焦宝石基喷涂料的抗折强度随着铝酸钙水泥含量的增加呈上升趋势。但经过600℃、1000℃和1300℃烧结后的焦宝石基喷涂料的耐压强度随着铝酸钙水泥含量的增加呈先增加后减小的趋势。     

Calcium aluminate cement in castable alumina: From hydrate bonding to the in situ formation of calcium hexaluminate

Calcium hexaluminate (CA₆) is an intrinsically densification-resistant material, therefore, its porous structures are key materials for applications as high-temperature thermal insulators. This article reports on the combination of calcined alumina and calcium aluminate cement (CAC) in castable aqueous suspensions for the in situ production of porous CA₆. The CAC content (10–34 vol%) and the curing conditions ensure structural integrity prior to sintering and maximize the development of hydrated phases. Changes in physical properties, crystalline phases, and microstructure were investigated after isothermal treatments (120–1500 °C), and three sequential porogenic events were observed. The hydration of CAC preserved the water-derived pores (up to 120 °C), and the dehydroxylation of CAC hydrates (250–700 °C) generated inter-particles pores. Moreover, the in situ expansive formation of CA₂ and CA₆ (900–1500 °C) hindered densification and generated intra-particle pores. Such events differed from those observed with other CaO sources, and resulted in significantly higher pores content and lower thermal conductivity.