Enhancing the mechanical, thermal, and microstructural behavior of UHPC at elevated temperatures through amorphous coal bottom ash

초록

To date, research aimed at improving the fire resistance of UHPC (ultra-high-performance concrete) has primarily focused on mitigating explosive spalling. In contrast, studies addressing aggregate-matrix thermal mismatch-induced microcracking, suppression of hydration product decomposition, and modification of thermal properties remain relatively limited. In this study, a systematic experimental investigation was conducted to improve the fire resistance of UHPC by replacing highly crystalline silica sand (SS) with amorphous coal bottom ash (CBA). The intrinsic properties of the raw fine aggregates were first characterized in terms of morphology, porosity, crystallinity at ambient conditions, and mass and volumetric stability at elevated temperatures. UHPCs incorporating SS or CBA specimens were then heated up to 900 degrees C to evaluate mechanical and thermal behavior and microstructural evolution through TG/DTG, XRD, SEM, Micro X-CT, compressive and direct tensile tests, and thermal property measurements. The results showed that SS underwent a crystallographic phase transformation at approximately 500-600 degrees C, causing rapid volumetric expansion, severe aggregate-matrix interfacial microcracking, and accelerated mechanical degradation. In contrast, CBA exhibited superior volumetric stability without abrupt expansion, enabling UHPC to maintain interfacial integrity up to 900 degrees C. TG/DTG and XRD analyses further indicated delayed decomposition of C-(A)-S-H in CBA-UHPC due to enhanced polymerization. Consequently, CBA-UHPC showed mitigated stiffness degradation, improved deformation resistance, and significantly lower thermal conductivity (approximately 48-52% lower at ambient temperature) with lower temperature dependence. These findings demonstrate that CBA effectively enhances the fire resistance of UHPC by mitigating microstructure degradation and improving thermal properties at the material level.

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