This paper focuses on the development of a geopolymer (GP) mortar that can be cured under ambient conditions using flash-calcined dredged sediment (FCS) and flash-calcined excavated clay (FCC), as a substitute for Metakaolin (MK). Granulated blast furnace slag (GBFS) is also added to some of the GP mixes to study its effect on GP performance. Potassium silicate (K2SiO2) was used as an alkaline reagent (AR). Therefore, this study investigates the effect of binder composition on the mechanical and durability performance of the GP mortar samples. Acid attack test, high-temperature resistance test, and microstructural analysis tests such as X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were conducted to compare the reference mix with other substitute mixes containing FCS and FCC. Finally, the nuclear magnetic resonance (NMR) test is done on all GP mixes to verify their network classification. The results show that using FCS/FCC and only 10–20 % wt. of GBFS in the GP mortar mixes can result in similar or better properties in terms of strength and durability, while the NMR test identifies their tetrahedral network can vary between formulations.