Mineral Composition of Alumina Cement with Special Reference to the Shrinkage of Castable Refractories

Abstract

The mineralogical constitution of the alumina cements of various composition (Table 1) were investigated by comparing the peaks of the X-ray patterns with those of the mixture of the synthetic minerals, CA, C5A3, C3A5, C2AS and glassy substance (Fig. 1, Table 2).The samples, except C and D (Table 1) were produced by an electric furnace. They, therefore, contained a small amount of FeO. The following results were obtained:(1) CA was found to be the principal mineral of the cement.(2) The samples with more than 35% CaO and less than 56% Al2O3 contained C5A3.(3) The samples with more than 57% Al2O3 and less than 34% CaO contained C3A5.(4) All samples contained C2AS. The samples with more than 35% CaO contained a small amount of C2AS together with β-C2S.(5) Fairly large crystals of C2AS were found in the samples with more than 5.2% SiO2, while no such large crystals did not exist in the samples with less than 5% SiO2.(6) The residue insoluble in N/2 HCl was CaTiO3. Also C3A5 was slightly soluble in N/2 HCl (Fig. 2).Shrinkage on heating the castable refractories prepared from these cement and usual or light weight chamotte was tested.At 1200°C the component of the cement acted upon the cristoballite of the chamotte to form C2AS, while above 1300° CAS2 was formed together with glassy substance. The more the CaO content of the cement the more was the amount of CAS2 thus formed.When 72 parts of the chamotte and 28 parts of the CaO rich (37-39% CaO) cement were used the refractories swelled in the case of usual chamotte, or shrinked slightly in the case of light weight chamotte. However, such cement caused the softening of the refractories at lower temperature. When the refractories were heated under small load, i.e. 110-190g/cm2, those prepared with CaO rich cement deformed easily.