Three composites that were 96% alumina were mixed and uniaxially dry-pressed into bars and pellets; all had monoclinic SrAl2Si2O8 as an intergranular phase. The diffraction patterns, microstructure, density, dielectric properties, strength, and toughness were measured, The first composition, which contained crystalline SrCO3, Al2O3, and SiO2, in a 1:1:2 molar ratio, as the 4% component, densified but was generally inferior to the second and third compositions, which contained strontium aluminosilicate (SrAlxSiyOz, SAS) glass as the 4% component, in terms of mechanical properties, defects, and monoclinic SrAl2Si2O8 transformation, The second composition, which lacked B2O3, was very tough and was comparable to commercial alumina, in terms of the dielectric constant. The third, which contained 0.068% of B2O3 that was dissolved in the SAS glass as a sintering aid, had high strength and toughness and no macroscopically visible defects. Mullite formed, in addition to monoclinic SrAl2Si2O8 in all three composites. Alumina-monoclinic SrAl2Si2O8 composites of the third composition had room-temperature properties that were comparable to commercial aluminas that contained 96% alumina and also had potential for mechanical and refractory applications.