Gasifier slag (GS), a by-product of the manufacture of substitute natural gas, is a potential new pozzolanic cement replacement material. A small-scale durability study of a series of blended cements as paste systems has been carried out using two Portland cements blended with two gasifier slags, and also with a granulated blastfurnace slag and a pulverized fuel ash for comparison. The compressive strength results indicate that GS could be used successfully as a blending component for cement, generally producing good durability (especially with high replacement levels in aggressive environments). Analyses of the hydrates and their modification by the presence of GS could help to explain these effects.