Abstract
A series of binary Al2O3–CaO and Al2O3–CaO–CaCO3 systems with Ca/Al molar ratios of 0.05, 0.1, 0.25, 0.5 and 1.0 have been synthesised by the sol–gel technique from aluminium isopropoxide and metallic calcium powder. The rate of the metal reaction is used as a limiting factor to control the binary gel formation. The proposed modification of the traditional sol–gel method was used to examine the influence the effect of the metallic form of the second component as an oxide precursor on the form of the final product. By applying acetic acid instead of mineral acid, calcium acetate is formed and then decomposed to calcium carbonate upon thermal processing. During the synthesis of the binary systems, metallic calcium acts both as a precursor of calcium acetate and as a secondary pH modifier of the gel system. Calcination in air at 600 °C did not produce systems containing only oxides and the calcium carbonate phase was still present. Due to particle size reduction, the CaCO3 to CaO decomposition temperature was lowered. The systems were characterised by X-ray powder diffraction, low-temperature nitrogen adsorption, transmission and scanning electron microscopy (TEM, SEM and SEM/EDS), thermogravimetric analysis (TGA) and FTIR spectra.
Highlights
Al2O3–CaO systems in the form of cement or oxides have excellent flame retardant qualities that are important from the point of view of industrial applications, e.g., for production of flooring, mortars resistant to chemicals and concretes, for construction of sewers, production of tile adhesives, or protective laminates [1,2,3]
Modification of alumina gel network employing a metallic precursor of calcium oxide may have numerous implications
The proposed method employing metallic calcium was used as an alternative to traditional methods of co-precipitation or carrier impregnation with salts which often lead to a considerable decrease in the carrier surface area
Summary
Al2O3–CaO systems in the form of cement or oxides have excellent flame retardant qualities that are important from the point of view of industrial applications, e.g., for production of flooring, mortars resistant to chemicals and concretes, for construction of sewers, production of tile adhesives, or protective laminates [1,2,3]. A new synthesis method has been proposed, in which calcium metal powder is used as alternative source of the second component of the gel matrix. Our previous work has shown that the introduction of a metal precursor could lead to a change in the parameters of the final product [12] and to the formation of new (carbonate) intermediate phases upon thermal treatment [13]. J Aust Ceram Soc (2018) 54:679–690 procedure was to maintain large initial surface area after modification, which is of importance for application of the systems as a carrier for metallic phase catalysts or for other purposes. The introduction of metallic calcium was a fast, pure and efficient method for incorporation of the second component of the alumina gel matrix. The replacement of mineral acids in the sol–gel synthesis, such as hydrochloric [14, 15], nitric [16, 17] or sulphuric acid [18], by acetic acid allows the formation of a carbonate form that is stable over a relatively wide temperature range
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