De-aluminated Metakaolin (DK) and Metakaolin (MK) were studied as partial replacement of slag binder using 7 and 4 different ratios respectively to produce alkali activated slag (AAS ) mortar. Different material characterization tests were carried out to investigate the properties of the Slag, DK and MK by using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FTIR). A comparable study between DK and MK partial replacement on the AAS mortar physical and mechanical properties was carried out. Alkali activator used, was a mixture of Sodium Hydroxide Na(OH) solution and soluble Sodium Silicate using 4 different mixing ratios. Three different percentages of additive water were also checked. Flow rate and initial setting time were tested for the fresh modified AAS mortar samples. The compressive strength of modified AAS hardened mortar samples were tested at 3 and 7 days, under 2 different curing regimes namely; ambient and wet curing to investigate the effect of both curing on both mortars. The percentage of natural water absorption was tested for wet curing regime. The increase in DK and MK content had led to decrease in the flow rate, the initial setting time and percentage of natural water absorption. The increase of additive water increased the compressive strength until a certain ratio and then reduced the compressive strength beyond that. The effect was the same for both DK and MK modified mortar. The optimum ratio of the alkaline activator composition for DK modified mortar differed from MK modified mortar.