Effects of Different Calcium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al-Alloy ESMAEIL ADABIFIROOZJAEI, ALI SAIDI, AHMAD MONSHI, and PRAMOD KOSHY Andalusite containing low-cement castables (LCCs) have been used in aluminum casthouses for several decades. CaF 2 is commonly added to the refractory to improve its corrosion resistance mainly because of its role in the formation of anorthite (CaAl 2 Si 2 O 8 ); the latter has been reported to decrease the penetration of molten aluminum alloys into refractories. This article investigates the effect of the addition of different calcium containing compounds (CaO, CaCO 3 , CaSO 4 , CaF 2 , Clinker white cement, calcia feldspar, wollastonite, and Ca 3 (PO 4 ) 2 ) on reactions with the refractory constituents to form anorthite as well as the effect of the additives on both the subsequent physical properties and the corrosion resistance of andalusite LCC refractories. Corrosion tests using the Alcoa cup test at temperatures (1123 K [850 °C] for 150 hours and 1433 K [1160 °C] for 72 hours) were conducted to determine the extent of penetration, whereas immersion tests in boiling water were conducted to determine the extent of open porosity in the material. Scanning electron microscopy coupled with energy dispersive spectrometer, optical microscopy, and X-ray diffraction techniques were employed to characterize the phase forma- tions in the materials after the tests. The study demonstrated that both calcia feldspar and clinker white cement had the potential to be used as new additives for decreasing the penetration of molten Al-alloy into the refractory materials. Anorthite formation (in the refractory matrix), along with the absence of glassy phases, were responsible for the improvement in the corrosion resistance of the castables containing calcia feldspar. However, in the sample containing cement, the presence of calcium silicate phases were observed to resist reactions with molten aluminum. The observed results were validated using thermodynamic calculations, which indicated that tricalcium silicates (3CaO.SiO 2 ) and dicalcium silicate (2CaO.SiO 2 ) phases were more resistant than wollastonite (CaSiO 3 ) for applications involving contact with molten aluminum. DOI: 10.1007/s11663-010-9468-z Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION MELTING and holding furnaces in aluminum cast- houses generally have used aluminosilicate refractories containing different chemical additives from the late 1970s. [1] Several additives have been used in the past to improve the resistance of the refractory to corrosion by molten aluminum, such as zircon, barium sulfate, calcium fluoride, glass frits, [1] wollastonite, [2–4] alumi- num fluoride, aluminum borate, [5] chromium oxide, aluminum titanate, cryolite, [6,7] barium celsian, [8–10] and ceria. [11] The additives work by decreasing the wettabil- ity of the molten aluminum alloys with the refractory, thereby decreasing the extent of corrosion and associ- ated molten metal penetration. [12] Although these addi- tives have been used for several decades, ambiguity exists in regard to the mechanisms responsible for the improvement in the corrosion resistance. The nature and quantity of the materials used is believed to be influ- enced by the silica level of the refractories [13] and the operating temperatures. However, the addition of these materials to aluminosilicate refractories also influences the physical, mechanical, and corrosion properties of the refractories. [12,14] Previous research on calcium-containing additives has focused on CaF 2 , which has been effective in improving the wetting resistance of aluminosilicate refracto- ries. [5,8,12–14] The efficiency of CaF 2 addition has been shown to depend on the quantity used and the temper- atures involved. [14] The addition of CaF 2 to an alumi- nosilicate composition (close to mullite) was shown either to increase or to decrease the corrosion resistance. This response is dependent on the amounts of additive used as a result of the formation of noncrystalline phases that easily undergo reactions with the molten aluminum alloy. [5] Previous studies have shown that the presence of both high CaF 2 contents and higher firing temperatures tend to favor the formation of larger quantities of anorthite, leading to an increase in the corrosion resistance. However, the use of 3.5 wt pct calcium fluoride resulted in extensive cracking, which ESMAEIL ADABIFIROOZJAEI, M.Sc. Student, ALI SAIDI and AHMAD MONSHI, Professors, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 8415683111, Iran. Contact e-mail: e.adabifiroozjaei@ma.iut.ac.ir PRAMOD KOSHY, Doctor, is with the School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia. Manuscript submitted June 11, 2010. Article published online January 11, 2011. 400—VOLUME 42B, APRIL 2011 METALLURGICAL AND MATERIALS TRANSACTIONS B