This work aims to compare the mechanical and mineralogical properties of coal waste and fly ash based geopolymers obtained in the same conditions using sodium carbonate (Na₂CO₃) and phosphoric acid (H₃PO₄) solutions as diverse activators. Na₂CO₃ solutions with 3.5% and 5% mass ratios were prepared by dissolving Na₂CO₃ powder (99.9%) in deionized water. On the other hand, three H₃PO₄ solutions with molar concentrations of 4, 6, and 8 M were prepared by dilution of the commercial H₃PO₄ (85%) in distilled water. Both types of alkaline and acidic solutions were used to produce geopolymers from the coal waste which is systematically replaced by fly ash (0, 15, 25, and 35 wt.%). The geopolymers were obtained by adding each of the prepared solutions to the blend with a liquid/solid ratio of 0.5 and 0.6 for the alkaline and acid solutions, respectively. The mechanical tests of cubic samples after 28 days of curing showed that the compressive strength of phosphate-based geopolymers is 5.33 MPa while that of alkali-based geopolymers is 5.14 MPa. The strength increases with the increase of the activator concentration. The mineralogical analysis of cured samples after 28 days showed the formation of berlinite (AlPO₄) in the structure of the phosphate-based geopolymer which is dispersed in the matrix and contributes to the strength. For alkali geopolymers, X-ray diffraction results showed the existence of Portlandite crystalline phase and the N-A-S-H gel type.

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