This aims to examine the engineering and environmental properties of C-type fly ash (FA-C)-ferronickel slag 4 (FNS 4)-based geopolymer mortar under heat curing is examined. The series of C-type fly ash-based geopolymer mortar was generated with 0%, 10% and 20% ferronickel slag 4. The heat curing was applied at a temperature of 60 ± 5 oC for 6 hours. Fresh geopolymer mortar developed more workability with the addition of FNS 4. The porosity of geopolymer mortar was determined by immersing the sample and vacuuming it at a pressure of 0.5 bar for 24 hours. To study the formation of calcium-aluminate-silicate-hydrate (CASH) gel of RPC, the scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) mapping was carried out. The life cycle assessment is used for measuring the environmental impact of FNS 4 in FA-C-based geopolymer mortar. The results obtained that the addition of FNS 4 increased the workability of geopolymer mortar. FNS 4 has a higher effect on the development of compressive strength in geopolymer mortar under normal curing. Under heat and normal curing, the bending strength of geopolymer mortar raises for all curing ages. The porosity of the geopolymer mortar containing FNS 4 decreased. The formation of CASH gel in geopolymer was formed in large quantities as the Ca and Si were detected in higher amounts. The use of ferronickel slag 4 has the potential to decrease ozone depletion, fossil fuel depletion, and global warming impact.