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<p>This study explores, for the first time, the synthesis of geopolymers using low‑calcium fly ash (FA) blended with a very high calcium FA containing 41.58% CaO. The aim is to enhance the compressive strength of geopolymers by combining low- and very high-calcium fly ashes, and to investigate the effect of mechanical activation (MA) of the raw materials on geopolymer synthesis. NaOH solution was employed as an alkaline agent, and curing was conducted using heat treatment. The replacing 20% of low-calcium FA with the very high-calcium FA significantly increased the compressive strength of geopolymers. Given that the strength of geopolymers based on low-calcium FA without additives is 100%, the addition of very high-calcium FA allowed the strength of 28-day samples to be increased to 285% (31.6&#xa0;MPa) and 148% (42.5&#xa0;MPa) for 30&#xa0;s and 180&#xa0;s of MA, respectively. The diminished efficacy of the very high-calcium FA additive with prolonged MA duration is presumably explained by the presence of low-hardness compounds within very high-calcium FA. During the initial milling phase, these compounds form a coating on the low-calcium FA particles, thereby hindering further grinding and activation. Consequently, applying MA to low-calcium FA prior to incorporating very high-calcium FA is likely to optimize geopolymer synthesis. Thermal analysis, XRD analysis, FTIR spectroscopy, and SEM–EDS were used to elucidate the mechanism underlying the synergistic interaction between FA with low and very high calcium content. This interaction accelerated formation of aluminosilicate hydrogel, the primary cementitious phase of the binder.</p>

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Recycling of low-calcium coal fly ash into environmentally sustainable geopolymer material: effect of blending with very high-calcium fly ash and mechanical activation

  • 期刊论文

  • 发表日期：

    2026年7月12日

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  • Alexander M. Kalinkin,
  • Elena V. Fomenko,
  • Elena V. Kalinkina,
  • Ekaterina A. Kruglyak,
  • Alla G. Ivanova,
  • Galina V. Akimochkina,
  • Vladimir V. Yumashev

摘要

This study explores, for the first time, the synthesis of geopolymers using low‑calcium fly ash (FA) blended with a very high calcium FA containing 41.58% CaO. The aim is to enhance the compressive strength of geopolymers by combining low- and very high-calcium fly ashes, and to investigate the effect of mechanical activation (MA) of the raw materials on geopolymer synthesis. NaOH solution was employed as an alkaline agent, and curing was conducted using heat treatment. The replacing 20% of low-calcium FA with the very high-calcium FA significantly increased the compressive strength of geopolymers. Given that the strength of geopolymers based on low-calcium FA without additives is 100%, the addition of very high-calcium FA allowed the strength of 28-day samples to be increased to 285% (31.6 MPa) and 148% (42.5 MPa) for 30 s and 180 s of MA, respectively. The diminished efficacy of the very high-calcium FA additive with prolonged MA duration is presumably explained by the presence of low-hardness compounds within very high-calcium FA. During the initial milling phase, these compounds form a coating on the low-calcium FA particles, thereby hindering further grinding and activation. Consequently, applying MA to low-calcium FA prior to incorporating very high-calcium FA is likely to optimize geopolymer synthesis. Thermal analysis, XRD analysis, FTIR spectroscopy, and SEM–EDS were used to elucidate the mechanism underlying the synergistic interaction between FA with low and very high calcium content. This interaction accelerated formation of aluminosilicate hydrogel, the primary cementitious phase of the binder.