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<p>Geopolymers have emerged as a sustainable alternative to ordinary Portland cement by utilizing aluminosilicate-rich industrial by-products while reducing energy consumption and CO₂ emissions. Although several previous studies have mainly used highly alkaline activators (e.g., NaOH and KOH), their use raises significant concerns regarding handling, safety, and the environment. Therefore, this study evaluates two lower-alkalinity activator systems, sodium silicate with calcium oxide (Na₂SiO₃ + CaO) and sodium bicarbonate with calcium oxide (NaHCO₃ + CaO), for stabilizing CL clay blended with Class F fly ash. While CaO–sodium silicate activation promotes aluminosilicate dissolution and the formation of N–A–S–H and C–A–S–H gels, resulting in higher geopolymerization and strength gain, the bicarbonate–calcium oxide system mainly enhances strength through gradual calcium carbonate precipitation. The sodium silicate–calcium oxide system demonstrated superior performance, achieving 2857&#xa0;kPa at 28 days with 20% fly ash and an activator-to-fly ash ratio of 0.4, corresponding to an improvement of more than 850% compared with untreated soil. In contrast, the sodium bicarbonate–calcium oxide system yielded lower strengths of around 630&#xa0;kPa under similar conditions. Nevertheless, it exhibited steady strength development at higher dosages, attributed to ongoing calcium carbonate precipitation, resulting in a 50% increase between 7 and 28 days and approximately a 259% improvement over untreated soil. Durability testing revealed that sodium silicate–calcium oxide mixes exhibited mass losses of 2.5%-7.4%, well below the 10% limit. Optimized sodium bicarbonate–calcium oxide blends with higher activator contents (AA/FA = 0.6, FA = 20–30%) also satisfied the same threshold after 12 wet–dry cycles. Although the obtained strength was lower than that achieved using high-alkaline activators, the proposed systems remain promising for applications requiring moderate mechanical performance, particularly in low-stress geotechnical conditions such as canal lining, shallow embankments, and non-structural soil improvement.</p>

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Low-Alkalinity Fly Ash Geopolymer for CL Clay Soil Stabilization: Exploring Alternative Alkaline Activators

  • 期刊论文

  • 发表日期：

    2026年3月7日

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  • Mohammad M. Ahmed,
  • Mustafa A. Yousif,
  • Amer H. Taher

摘要

Geopolymers have emerged as a sustainable alternative to ordinary Portland cement by utilizing aluminosilicate-rich industrial by-products while reducing energy consumption and CO₂ emissions. Although several previous studies have mainly used highly alkaline activators (e.g., NaOH and KOH), their use raises significant concerns regarding handling, safety, and the environment. Therefore, this study evaluates two lower-alkalinity activator systems, sodium silicate with calcium oxide (Na₂SiO₃ + CaO) and sodium bicarbonate with calcium oxide (NaHCO₃ + CaO), for stabilizing CL clay blended with Class F fly ash. While CaO–sodium silicate activation promotes aluminosilicate dissolution and the formation of N–A–S–H and C–A–S–H gels, resulting in higher geopolymerization and strength gain, the bicarbonate–calcium oxide system mainly enhances strength through gradual calcium carbonate precipitation. The sodium silicate–calcium oxide system demonstrated superior performance, achieving 2857 kPa at 28 days with 20% fly ash and an activator-to-fly ash ratio of 0.4, corresponding to an improvement of more than 850% compared with untreated soil. In contrast, the sodium bicarbonate–calcium oxide system yielded lower strengths of around 630 kPa under similar conditions. Nevertheless, it exhibited steady strength development at higher dosages, attributed to ongoing calcium carbonate precipitation, resulting in a 50% increase between 7 and 28 days and approximately a 259% improvement over untreated soil. Durability testing revealed that sodium silicate–calcium oxide mixes exhibited mass losses of 2.5%-7.4%, well below the 10% limit. Optimized sodium bicarbonate–calcium oxide blends with higher activator contents (AA/FA = 0.6, FA = 20–30%) also satisfied the same threshold after 12 wet–dry cycles. Although the obtained strength was lower than that achieved using high-alkaline activators, the proposed systems remain promising for applications requiring moderate mechanical performance, particularly in low-stress geotechnical conditions such as canal lining, shallow embankments, and non-structural soil improvement.