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This study investigates the geopolymerization process of fly ash–based geopolymer paste specimens using acoustic emissions; the activating solution is a combination of silica fume and sodium hydroxide, made with two different water: solid weight ratios (w/s), 0.30 and 0.35. The specimens are cured in ambient conditions (22+ 3 C). The acoustic emission data are processed through pattern recognition (PR), and two clusters are identified and assigned to a specific mechanism depending on their characteristics. Results show that there is a predominant difference in the acquisition data between the two water: binder weight ratios. Pattern recognition shows that the greatest number of geopolymerization mechanisms, including dissolution of Si and Al atoms, forming bubbles, and microcrack initiation, occur at roughly the same time for the 0.3 w/s samples, whereas in 0.35 w/s samples the mechanisms occur sequentially. Final setting time of Vicat penetration tests, which assume a starting point of microcrack initiations, is 60 min after the test onset for 0.3 w/s ratio samples, whereas it occurs after approximately 600 min for 0.35 w/s samples. Microcrack initiation predicted by the pattern recognition technique coincides with the final setting times. Scanning electron microscope observations also coincide with the pattern recognition findings. DOI: 10.1061/(ASCE)