In an effort to make structures more sustainable and durable, supplementary cementitious materials are often used to replace cement. Ground granulated blast furnace slag, for instance, is an industrial by-product of iron refinement and is frequently used in concrete mixture design to not only reduce cost, but also increase later-age strength as well as durability. However, published literature indicates that slags with a high alumina content may have a detrimental effect when concrete is exposed to a sulfate environment. ASTM standard C989 does not suggest any information or guidelines regarding using slags with an alumina content between 11-18%. Therefore, the objective of this study was to fill in the gap of this standard by studying slags of variable alumina content as high as 16 percent. This study presents data collected for compressive strengths of mortar cubes exposed to lime and 5 percent sodium sulfate solution at ages of 7, 28, 91, and 182 days from the date of mixing as well as expansion data for mortar bar specimens exposed to 5 percent sodium sulfate solution up to 120 days. Slag replacement levels used here were 0, 30, 50, and 70%. Mortar bar specimens showing deterioration were analyzed using x-ray diffraction coupled with Rietveld refinement to assess the mechanism of deterioration. Cubes were stored in lime and sulfate solutions abiding by ASTM C1012 in order to analyze the resistance to sulfate attack. Sulfate resistance was measured in terms of decalcification of the CSH gel as well as expansion. The results suggest using high alumina slags at a low percentage adversely affects sulfate resistance since the acquired strength at 182 days fell below that of 28 day strength, which is often used in the industry as the parameter which constitutes whether a mixture is adequate. It was also seen that increasing alumina content of the slag resulted in increased expansion. X-ray diffraction analysis indicates that the mechanism of deterioration, of the control as well as the blended mortar, is due to secondary gypsum and secondary ettringite formation. Therefore, it is recommended that slags having a high alumina content should be further analyzed in laboratory tests to examine their performance especially if concrete will be subjected to a sulfate environment during its service life.