The global reliance on Ordinary Portland Cement (OPC) has raised significant environmental concerns, with cement production contributing to 5–7% of global CO₂ emissions. Geopolymer concrete (GPC) has emerged as a promising sustainable alternative, leveraging industrial by-products like fly ash (FA) and ground granulated blast furnace slag (GGBFS) to mitigate the ecological footprint of traditional construction materials. This research focuses on the slump, slump retention, and durability performance of geopolymer concretes under chemical conditions, comparing three distinct mixes: a geopolymer concrete with 25% GGBFS and 75% FA (UoM), a 100% FA geopolymer mix (MU), and conventional OPC concrete. Durability tests included acid resistance, sulphate resistance, and supported by advanced microstructural analyses such as X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDX). The findings demonstrate that geopolymer concretes exhibit enhanced resistance to chemical degradation, lower permeability, and reduced mass and strength loss compared to OPC based concrete. The slump retention values show that the geopolymer concrete can retain slump values for up to 40 minutes; this positions geopolymer concrete as a viable solution for sustainable construction in aggressive environments.