The development of environmentally friendly construction materials is stimulated by the need for construction with a minimal carbon footprint. The development includes the usage of abundant industrial wastes such as Mill Scale waste (MS) and Fly-ash (FA) to replace Ordinary Portland Cement (OPC) concrete which greatly contributes to the reduction of carbon emission. The MS waste produced from rolling mills of steel bars can be used as partial replacement for FA based geopolymer mix. In this study, partial replacement of 20 %, 30 % and 40 % MS by weight of low calcium (Class F) FA for developing geopolymer paste with varying sodium hydroxide-to-sodium silicate (NaOH-to-WG) ratio of 1:2.5, 1:2 and 1:1, in 50 mm x 50 mm x 50 mm cube specimens were tested under unconfined compressive strength (UCS) test to evaluate their mechanical performance. It is observed that the specimens with 20 % MS-to-FA ratio and 1:2.5 NaOH-to-WG ratio gives the higher UCS value. The setting time of the geopolymer mixes is also recorded to show its relationship to the varying design mixes. Regression models are generated through Response Surface Methodology (RSM) to provide coefficients in obtaining desired UCS values.