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<p>This study investigates the effects of NaOH molarity and ground granulated blast furnace slag (GBFS) substitution on the mechanical and microstructural properties of fly ash (FA)-based geopolymer mortars activated under low-molarity conditions. Within the experimental program, NaOH molarity varied from 2 to 8&#xa0;M, while GBFS substitution ratios ranged between 0 and 25%. All specimens were cured under ambient laboratory conditions to eliminate the need for energy-intensive thermal curing and to promote a more sustainable geopolymer production approach. The workability of fresh mortars was evaluated using the flow table test, while the mechanical performance of hardened specimens was determined through flexural and compressive strength tests at curing ages of 3, 7, and 28&#xa0;days. Microstructural characteristics were further analyzed using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), and mercury intrusion porosimetry (MIP). The experimental results showed that increasing NaOH molarity significantly improved both compressive and flexural strengths, particularly in mixtures containing higher GBFS contents. The highest 28-day compressive strength of 43.38&#xa0;MPa and flexural strength of 7.33&#xa0;MPa were obtained in the mixture containing 25% GBFS activated with 8&#xa0;M NaOH. Microstructural analyses indicated that GBFS incorporation promoted the formation of hybrid C-A-S-H and N-A-S-H gel phases, resulting in a denser matrix with reduced porosity. In addition, a gradient boosting regression (GBR) model developed using the experimental dataset was used to model and visualize the compressive and flexural strength trends, showing good agreement with the experimental data (<i>R</i><sup>2</sup> = 0.985 and <i>R</i><sup>2</sup> = 0.963). The findings demonstrate that FA–GBFS geopolymer mortars can achieve high mechanical performance even under relatively low activator molarity and ambient curing conditions, highlighting their potential as a cost-effective and environmentally sustainable alternative to conventional binder systems, particularly by reducing activator consumption and eliminating the need for energy-intensive thermal curing.</p>

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Mechanical Performance and Microstructure of Low-Molarity Activated FA–GBFS Geopolymer Mortars with Gradient Boosting Regression-Based Strength Modeling

  • 期刊论文

  • 发表日期：

    2026年6月27日

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  • Gazi Günel,
  • Osman Ulusu,
  • Ali Kılınç,
  • Selahattin Güzelküçük,
  • İ. Raci Bayer,
  • Özer Sevim

摘要

This study investigates the effects of NaOH molarity and ground granulated blast furnace slag (GBFS) substitution on the mechanical and microstructural properties of fly ash (FA)-based geopolymer mortars activated under low-molarity conditions. Within the experimental program, NaOH molarity varied from 2 to 8 M, while GBFS substitution ratios ranged between 0 and 25%. All specimens were cured under ambient laboratory conditions to eliminate the need for energy-intensive thermal curing and to promote a more sustainable geopolymer production approach. The workability of fresh mortars was evaluated using the flow table test, while the mechanical performance of hardened specimens was determined through flexural and compressive strength tests at curing ages of 3, 7, and 28 days. Microstructural characteristics were further analyzed using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), and mercury intrusion porosimetry (MIP). The experimental results showed that increasing NaOH molarity significantly improved both compressive and flexural strengths, particularly in mixtures containing higher GBFS contents. The highest 28-day compressive strength of 43.38 MPa and flexural strength of 7.33 MPa were obtained in the mixture containing 25% GBFS activated with 8 M NaOH. Microstructural analyses indicated that GBFS incorporation promoted the formation of hybrid C-A-S-H and N-A-S-H gel phases, resulting in a denser matrix with reduced porosity. In addition, a gradient boosting regression (GBR) model developed using the experimental dataset was used to model and visualize the compressive and flexural strength trends, showing good agreement with the experimental data (R2 = 0.985 and R2 = 0.963). The findings demonstrate that FA–GBFS geopolymer mortars can achieve high mechanical performance even under relatively low activator molarity and ambient curing conditions, highlighting their potential as a cost-effective and environmentally sustainable alternative to conventional binder systems, particularly by reducing activator consumption and eliminating the need for energy-intensive thermal curing.