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Description
Alkali-activated composites based on fly ash (FA) and metakaolin (MK) were designed as sustainable low-carbon construction materials. The fly ash was mechanically activated in a planetary mill for 20 min to enhance its reactivity, while metakaolin was produced by the thermal treatment of kaolin at 700 °C for 2 h. Sodium hydroxide and sodium silicate solutions were used as alkaline activators. The curing regime consisted of 24 h at 50 °C, followed by curing at room temperature up to 28 days. Different alkali-activated composites were designed using as-received fly ash, mechanically activated fly ash, and mechanically activated fly ash with the addition of metakaolin at 10, 25, and 50 wt.%. The raw materials and the synthesized alkali-activated composites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). In addition, bulk density, capillary water absorption, and compressive strength were determined for all alkali-activated composites. The physical and mechanical properties of the alkali-activated composites were evaluated after 28 days of curing. The bulk density of the hardened products ranged from 1.05 to 1.50 g/cm³, depending on the precursor composition and metakaolin content. The compressive strength varied between 8 and 32 MPa. Among the investigated formulations, the composite containing 25 wt.% metakaolin exhibited the highest compressive strength (32 MPa) and the most compact microstructure, indicating a synergy between mechanically activated fly ash and metakaolin. The results demonstrate that the combined use of mechanically activated fly ash and metakaolin is an effective approach for producing alkali-activated materials with enhanced performance. The study supports the green transition and the principles of the circular economy by valorizing fly ash, an industrial by-product, as a secondary raw material for the design of sustainable low-carbon construction materials.