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Journal ArticleDOI

Crystal morphology of calcium aluminates hydrated for 14 days

01 May 1997-Journal of Materials Science Letters (Kluwer Academic Publishers)-Vol. 16, Iss: 9, pp 735-736
TL;DR: In this paper, the authors investigate exactly what the crystal habit and morphology are on the fourteenth day of hydration of calcium alumi-nates, and what their role is in the development of compressive strength of the hydrated samples.
Abstract: phase [2–4]. Many researchers [5–7] have investi-gated the development of these hydrated phases byelectron microscopy. The effects of crystal morphol-ogy on the hydration characteristics and setting timehave also been studied by some workers [8–9]. Theaim of the work reported here is to investigateexactly what the crystal habit and morphology are onthe fourteenth day of hydration of calcium alumi-nates, and what their role is in the development ofcompressive strength of the hydrated samples.Calcium aluminates such as CA, CA
Citations
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Journal ArticleDOI
TL;DR: The ability of calcium aluminate cement (CAC) to encapsulate toxic metals (Pb, Zn and Cu) was assessed under two curing conditions, allowing the use of CAC for waste management as proved by the leaching tests.
Abstract: The ability of calcium aluminate cement (CAC) to encapsulate toxic metals (Pb, Zn and Cu) was assessed under two curing conditions. Changes in the consistency and in the setting time were found upon the addition of the nitrates of the target metals. Both Pb and Cu caused a delay in CAC hydration, while Zn accelerated the stiffening of the mortar. Compressive strengths of the metal-doped mortars, when initially cured at 60 °C/100% RH, were comparable with that of the free-metal mortar. Three different pore size distribution patterns were identified and related to the compounds identified by XRD and SEM. Sorbent capacities of CAC for the toxic metals were excellent: a total uptake was achieved for up to 3 wt.% loading of the three metals. In this way, CAC mortars were perfectly able to encapsulate the toxic metals, allowing the use of CAC for waste management as proved by the leaching tests.

78 citations


Cites background from "Crystal morphology of calcium alumi..."

  • ...These crystals appeared as twofaced basal pinacoids of the hexagonal system [31]....

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  • ...AH3 appeared in the micrograph as needles while some hexagonal prisms could be attributable to the remaining metastable hydrates [31] (Fig....

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Journal ArticleDOI
TL;DR: In this paper, the authors investigated the hydration products and thermal decomposition mechanism of hydrated CaAl 2 O 4 and Ca 7 ZrAl 6 O 18 compounds by X-ray diffraction, SEM/EDS and thermal analysis.
Abstract: The hydration products and thermal decomposition mechanism of hydrated CaAl 2 O 4 and Ca 7 ZrAl 6 O 18 compounds were investigated by X-ray diffraction, SEM/EDS and thermal analysis. The processes of crystal hydrate nucleation and precipitation were preceded by the evolution of the X-ray amorphous phase during the first 24 h of hydration. DTA–TGA–EGA techniques allowed the study of the detailed decomposition and identification of intermediate and stable to be performed. The differential thermal analysis (DTA) curves of hydrated CaAl 2 O 4 and Ca 7 ZrAl 6 O 18 compounds show five similar endothermic peaks due to crystal water desorption. According to the quantitative TGA–EGA analyses performed on hydrated CaAl 2 O 4 and Ca 7 ZrAl 6 O 18 compounds, it was found that C 2 AH 8 , C 3 AH 6 and Al(OH) 3 phases are the main hydration products of CaAl 2 O 4 . Under the same laboratory conditions, the hydration of Ca 7 ZrAl 6 O 18 proceeds with the formation of mainly CAH 10 and AH 3 -gel phases. We provide the original illustrations of the hydrate crystals formation via amorphous phases.

22 citations

Journal ArticleDOI
TL;DR: In this paper, the internal moisture of twelve hardened cement pastes fired at high temperature, made with four aluminous hydraulic binders and three different Water/Binder ratios, was registered under diverse thermo-hydrous conditions, including at very high temperature.
Abstract: Refractory castables based on aluminous hydraulic binders are commonly used in aluminium casthouses (furnaces, ducts, etc.). Their selection is based on their good mechanical strength, thermal behavior and compatibility with molten aluminium. However, few studies focus on their hydrous evolution in operation, whereas this property can also have an influence on the produced metal quality. In this article, the internal moisture of twelve hardened cement pastes fired at high temperature, made with four aluminous hydraulic binders and three different Water/Binder ratios was registered under diverse thermo-hydrous conditions, including at high temperature. The water trapped by physisorption and chemisorption can be significant for some products, and it strongly depends on the mineralogy and porosity of the hardened cement paste. The more the binders contain alumina phase, the more the hardened cement pastes mobilize and render moisture.

21 citations

Journal ArticleDOI
TL;DR: The presente trabajo es una revision sobre el proceso de hidratacion de los cementos de aluminatos de calcio de alto contenido in alumina as mentioned in this paper.
Abstract: El presente trabajo es una revision sobre el proceso de hidratacion de los cementos de aluminatos de calcio de alto contenido en alumina. En el se describen los principales productos de hidratacion de estos materiales en funcion de las relaciones de agua/solidos y temperatura. Se exponen los mecanismos de reaccion que han sido establecidos por distintos autores en la literatura y se describe la microestructura tipica de estos materiales.

18 citations

Journal ArticleDOI
TL;DR: In this paper, the potential of calcium aluminate cement (CAC) to solidify/stabilize Cr(VI) (Na2CrO4) was investigated by means of isothermal calorimetry, XRD, TG/DTG and SEM-EDS.
Abstract: This study investigated the potential of calcium aluminate cement (CAC) to solidify/stabilize Cr(VI) (Na2CrO4). The hydration and microstructure development of CAC pastes in the presence of Na2CrO4 were examined by means of isothermal calorimetry, XRD, TG/DTG and SEM-EDS. The leaching properties of Cr(VI) were evaluated by TCLP and GANC methods. The results demonstrated that addition Na2CrO4 significantly retarded the early-age hydration of CAC depending on the dosage, while caused an acceleration of long-term hydration. The formations of CAH10 and C2AH8 were inhibited, instead AH3, CrO4-C3AH6 and CrO4-U phase were found as the main hydrates. These changes of hydrate assemblages led to a strong decrease in compressive strength. Complying with the TCLP regulatory limit of 5 mg/L, up to 1.0% of Cr(VI) could be effectively immobilized in the CAC paste. Two binding phases, CrO4-C3AH6 and CrO4-U phase, were responsible for controlling Cr(VI) leaching in the CAC pastes.

11 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors studied the hydration of CaAl2O4 (CA) by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy.
Abstract: Hydration of CaAl2O4 (CA) was studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy. During the induction period, the solution remains almost unchanged and is equilibrated temporarily with both superficially intrusion-hydrated CA particles and Al(OH)3 gel formed by dissociation of Al(OH)4– ions, the solubility of the Al(OH)3 gel being 10–4.24 molkg–1 at 25°C, while the intrusion-hydrated layer on the CA particles grows following a nearly linear law to reach a critical thickness (∼3 nm at 10° to 20°C, or 12 nm at 30°C). At this point destruction of the layer occurs, nuclei of hydrous compounds are generated, and the induction period terminates. Subsequent reaction proceeds in accordance with the rate equation of Schiller based on the dissolution-crystallization mechanism.

54 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined the conversion reaction in high alumina cement/concrete samples by scanning electron microscopy and differential thermal analysis and found that the conversion process is strongly influenced by the original water/cement ratio, both in facility with which it occurs and in the ultimate scale and distribution of the conversion products.
Abstract: The conversion reaction in high alumina cement/concrete samples is examined by scanning electron microscopy and differential thermal analysis. The transformation, which involves the dissolution of platy/acicular crystals of the metastable hydrates CAH 10 and C 2 AH 8 and the crystallisation of the lower hydrate C 3 AH 6 (in the form of faceted icositetrahedra), is seen to be strongly influenced by the original water/cement ratio, both in facility with which it occurs and in the ultimate scale and distribution of the conversion products. In commercial concrete samples it is illustrated how inevitable inhomogeneities in the water content of the cement can give rise to a very uneven and localised pattern of conversion. The observations indicate that conversion must occur by a “through solution” mechanism and the conclusion is that although the process is thermodynamically favoured by increasing temperatures of hydration, kinetically it is governed by the availability of free (liquid) water within the cement microstructure.

48 citations