<![CDATA[The role of borax as a setting-time modifier in Class F fly ash-based geopolymer concrete is not yet well understood. Particularly with respect to its effects on mechanical and fresh properties. This research investigates the influence of borax incorporation on the properties of geopolymer concrete. Class F fly ash from the Tanjung Jati B power plant was used. Borax was added at fly ash weights of about 0%, 5%, 10%, and 15%. Tests were arranged to observe setting time, compressive strength, elastic modulus, and slump value to assess mechanical performance and workability. The results represent, borax effectively prolongs the initial and final setting times, with greater effectiveness in Class F fly ash (low CaO) than in Class C fly ash (high CaO). The addition of 5% borax resulted in a higher compressive strength and modulus of elasticity. However, higher borax dosages reduced mechanical properties by inhibiting geopolymerization. An increase in borax content reduced slump values, reflecting lower workability due to higher mixture viscosity. While borax can effectively regulate setting time in Class F fly ash–based geopolymer concrete, its dosage must be carefully optimized to prevent negative effects on strength and fresh concrete performance.]]>
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