Due to the growing population, drought, and the contamination of conventional water sources, the need for clean water is rising worldwide with high demand. The application of nanomaterials for water purification can provide a better water quality, by eliminating toxic metals and also decomposing organic contaminants. Exploitation of industrial coal-burned byproduct, fly ash, through nanomodification has been developed in this exertion for the treatment of wastewater along with heavy-metal remediation and dye degradation. The fly ash was sintered at 1000 °C with addition of hydrothermally synthesized iron oxide nanoparticles to make a cementitious composite (FA10C) using an alkali activator (NaOH + Na2SiO3) at ambient temperature. Chemical investigations of the fly ash and the FA10C composites were done by X-ray fluorescence techniques. Analysis of FA10C by X-ray diffraction, Fourier transform infrared, field emission scanning electron microscopy, energy-dispersive spectrometry, and dynamic thermal analysis/thermogravimetric techniques revealed that nanodimensioned rod-shaped mullite formation and its interlocking textures enhance the strength of the building composite. Furthermore, the cementitious composite (FA10C) has been used as an adsorbent to remove heavy metals (lead, chromium, cadmium, copper) and carcinogenic dyes (methylene blue, Congo red, and acid red-1) from their aqueous solutions. The mineralogical features of the composite FA10C and its adsorption capacities/efficiencies were studied by systematic investigation of different parameters, and the adsorption data have been analyzed using Langmuir isotherm. The experimental findings suggest that the iron oxide nanoparticles facilitated fly ash can be implemented as a substitute cementitious composite (greenhouse effect) in construction technology being an energy-saving, low cost, and eco-friendly process in adsorbent manufacturing.