In this study, F-class fly ash-based geopolymers containing various amounts of Portland cement were fabricated by a high-pressure forming method and the compressive strength levels of the samples were assessed by analyzing the nano-microstructures and the types of phases which formed. In addition, two types of activators to harden the starting materials, water and a NaOH solution, were assessed to analyze how the nano-microstructures and the mechanical strength of the samples formed. When the specimen is hardened by water, the alkaline Ca(OH)(2) generated by the hydration of the cement induced a pozzolanic reaction of fine silica in the fly ash, thereby developing the mechanical strength of a specimen consisting of an admixture of fly ash and cement. When the specimen is hardened by the NaOH solution, the fly ash is geopolymerized while most of the constituents of the cement remain without hydration due to the low water content. With a nano-size structure, the compressive strength of the geopolymer was highest at 38 MPa when needle-shaped zeolite or calcium silicate hydrate (C-S-H) crystals of 50 similar to 150 nm in diameter were generated. This geopolymer containing 30 similar to 50 cement has potential as a substitution material for cement for applications in building materials given that its compressive strength measurement only after 1 day aging showed much higher value as 18 similar to 37 MPa compared to that of ordinary pure cement of below 10 MPa.
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