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Calcium aluminates

About: Calcium aluminates is a(n) research topic. Over the lifetime, 490 publication(s) have been published within this topic receiving 9200 citation(s).

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Journal ArticleDOI
Abstract: Synthesis of fine particle α-alumina and related oxide materials such as MgAl2O4, CaAl2O4, Y3Al5O12 (YAG), Image , β′-alumina, LaAlO3 and ruby powder (Image ) has been achieved at low temperatures (500°C) by the combustion of corresponding metal nitrate-urea mixtures. Solid combustion products have been identified by their characteristic X-ray diffraction patterns. The fine particle nature of α-alumina and related oxide materials has been investigated using SEM, TEM, particle size analysis and surface area measurements.

569 citations

Journal ArticleDOI
Abstract: The AFm phase of Portland cements refers to a family of hydrated calcium aluminates based on the hydrocalumite-like structure of 4CaO.Al{sub 2}O{sub 3}.13-19 H{sub 2}O. However OH{sup -} may be replaced by SO{sub 4} {sup 2-} and CO{sub 3} {sup 2-}. Except for limited replacement (50 mol%, maximum) of sulfate by hydroxide, these compositions do not form solid solutions and, from the mineralogical standpoint, behave as separate phases. Therefore many hydrated cements will contain mixtures of AFm phases. AFm phases have been made from precursors and experimentally-determined phase relationships are depicted at 25 deg. C. Solubility data are reported and thermodynamic data are derived. The 25 deg. C stability of AFm phases is much affected by the nature of the anion: carbonate stabilises AFm and displaces OH and SO{sub 4} at species activities commonly encountered in cement systems. However in the presence of portlandite, and as carbonate displaces sulfate in AFm, the reaction results in changes in the amount of both portlandite and ettringite: specimen calculations are presented to quantify these changes. The scheme of phase balances enables calculation of the mineralogical balances of a hydrated cement paste with greater accuracy than hitherto practicable.

482 citations

Journal ArticleDOI
Abstract: 27 Al magic-angle spinning (MAS) NMR spectroscopy of the central and satellite transitions is shown to be a powerful technique for the characterization of calcium aluminates and aluminate hydrates, which are of importance in the chemistry of high alumina and Portland cements. Synthetic samples of CaO.2Al 2 O 3 , CaO.Al 2 O 3 , 12CaO.7Al 2 O 3 , 3CaO.Al 2 O 3 , CaO.Al 2 O 3 .10H 2 O, γ-Al(OH) 3 , 6CaO.Al 2 O 3 .3SO 3 .32H 2 O (ettringite), 4CaO.Al 2 O 3 .13H 2 O, 4CaO.Al 2 O 3 .SO 3 .12H 2 O (monosulfate) and hydrated samples of high alumina and Portland cements are investigated. For the synthetic samples 27 Al quadrupole coupling constants (C Q ), asymmetry parameters (η) of the electric field gradient (EFG) tensors, and isotropic chemical shifts (γ( 27 Al)) are determined with high accuracies from computer simulations of the complete manifolds of spinning sidebands (ssb's) for the satellite transitions or of the central transition (centerband and ssb's). Simulations of the ssb manifolds show that some of the samples cannot be characterized by a single set of C Q and η values, but rather by a distribution of these values. The ranges of the distribution in C Q and η are estimated by summation of simulated ssb patterns for the satellite transitions employing sets of slightly different C Q and η values. The quadrupole coupling parameters are shown to be an important supplement to γ( 27 Al) in the characterization of structurally different Al sites and for identification of the individual aluminates in 27 Al MAS NMR spectra of hydrated cement samples. Linear correlations between C Q and the mean bond angle deviations from ideal tetrahedry symmetry, as well as between C Q and a calculated estimate for the geometrical dependency of the EEG tensor are reported, for the anhydrous calcium aluminates. These correlations allow assignment of the C Q values for CaO.2Al 2 O 3 , 12CaO.7Al 2 O 3 , and 3CaO.Al 2 O 3 , all of which contain two nonequivalent AlO 4 tetrahedra in the asymmetric unit

217 citations

Journal ArticleDOI
Abstract: Commercial calcium aluminate cements for refractory use are known to contain various phases, the hydration behavior of which is interdependent and not fully understood. Hydration studies normally require that pure oxides be synthesized. Solid-state synthesis of single aluminates requires high temperatures and full conversion is not guaranteed. Moreover, it is difficult to obtain a compositionally homogeneous product with this method. The present work discusses a straightforward combustion synthesis technique to prepare submicron calcium aluminates using the corresponding metal nitrates-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizer/fuel in the redox mixture was investigated, namely, to find out if urea contents below stoichiometry were enough to trigger the explosive combustion of the fuel and the subsequent decomposition of the salts, and how that ratio affected the phase formation and the characteristics, e.g., morphology and grain size, of the powder produced.

215 citations

Journal ArticleDOI
Abstract: A chemomechanical mathematical model is presented to simulate the response of concrete exposed to external sulfate solutions. The model is based on the diffusion-reaction approach, and several mechanisms for the reaction of calcium aluminates with sulfates to form expansive ettringite are considered. Fick’s second law is assumed for diffusion of the sulfate ions. A second-order chemical reaction between reacting calcium aluminates and ingressing sulfates depletes the sulfate concentration. The products of the second-order reaction between the aluminates and sulfates are chosen among several competing mechanisms, and a rule-of-mixtures approach is used to relate the expansive nature of the products with the prescribed specific gravity of the compounds. It is furthermore assumed that the crystallization pressure of products of reaction results in a bulk expansion of the solid. The constitutive response of the matrix and the expansive stresses are calculated from the imposed volumetric strain. Microcracks are initiated when the strength of the matrix is reached, leading to changes in the diffusivity and a reduction in the elastic properties of the matrix. The variation of diffusivity is linked to the scalar damage parameter due to cracking of the matrix. Due to the changes in the diffusivity, the problem is treated as a moving boundary problem, and a methodology is proposed to adapt the solution of the 1D case to the 2D problem of a prismatic specimen. Theoretical expansion-time responses are obtained and compared with a variety of data available in the literature.

204 citations

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