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<p>This study examines the thermomechanical behavior of concrete incorporating recycled fine and coarse aggregates 0–100% replacement derived from parent concrete containing 5% metakaolin. Specimens were subjected to temperatures between 20 and 800&#xa0;°C, and their residual strength, fracture energy, mass loss, and microstructural evolution were evaluated. Results show that moderate recycled aggregate (RA) contents 25–50% maintain performance comparable to natural aggregate concrete due to metakaolin-induced matrix densification and ITZ refinement, while higher RA levels 75–100% exhibit pronounced deterioration with losses exceeding 80–95% at higher temperatures depending on recycled aggregate (RA) content, linked to porous adhered mortar and weakened interfacial zones. Fracture energy improved up to 400–500&#xa0;°C across all mixes before declining sharply beyond 600&#xa0;°C, reflecting the onset of CSH decomposition and microstructural destabilization. Mass loss trends further confirmed reduced thermal stability in RA rich mixtures, reaching 12–13% at 800&#xa0;°C compared to &lt; 6% in the control. Response surface modeling and machine learning regression revealed nonlinear degradation thresholds and demonstrated strong predictive capability for thermomechanical performance. SEM analysis revealed a substantial increase in porosity/crack density, confirming progressive microstructural degradation. Machine learning models demonstrated high predictive accuracy, with the XGBoost model achieving an <i>R</i><sup>2</sup> value of 0.998 and RMSE of 0.79, outperforming other models. The findings highlight that metakaolin-modified recycled aggregates, when used at moderate replacement levels, can produce thermally resilient and sustainable concretes suitable for fire exposed or high-temperature applications.</p>

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High-temperature performance of metakaolin enhanced recycled aggregate concrete: mechanical, microstructural, and machine learning prediction

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月1日

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  • Zubair Yousuf,
  • Viktor Hlavička

摘要

This study examines the thermomechanical behavior of concrete incorporating recycled fine and coarse aggregates 0–100% replacement derived from parent concrete containing 5% metakaolin. Specimens were subjected to temperatures between 20 and 800 °C, and their residual strength, fracture energy, mass loss, and microstructural evolution were evaluated. Results show that moderate recycled aggregate (RA) contents 25–50% maintain performance comparable to natural aggregate concrete due to metakaolin-induced matrix densification and ITZ refinement, while higher RA levels 75–100% exhibit pronounced deterioration with losses exceeding 80–95% at higher temperatures depending on recycled aggregate (RA) content, linked to porous adhered mortar and weakened interfacial zones. Fracture energy improved up to 400–500 °C across all mixes before declining sharply beyond 600 °C, reflecting the onset of CSH decomposition and microstructural destabilization. Mass loss trends further confirmed reduced thermal stability in RA rich mixtures, reaching 12–13% at 800 °C compared to < 6% in the control. Response surface modeling and machine learning regression revealed nonlinear degradation thresholds and demonstrated strong predictive capability for thermomechanical performance. SEM analysis revealed a substantial increase in porosity/crack density, confirming progressive microstructural degradation. Machine learning models demonstrated high predictive accuracy, with the XGBoost model achieving an R2 value of 0.998 and RMSE of 0.79, outperforming other models. The findings highlight that metakaolin-modified recycled aggregates, when used at moderate replacement levels, can produce thermally resilient and sustainable concretes suitable for fire exposed or high-temperature applications.