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Computers and Concrete Volume 38, Number 3, September 2026 , pages 499-526 DOI: https://doi.org/10.12989/cac.2026.38.3.05 |
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Needle-shaped crystals formation after re-curing fire damaged zeolite containing concretes |
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Hamed Kharrazi, Babak Ahmadi, Mohammad Shekarchi
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| Abstract | ||
| This study investigates the microstructure of fire-damaged ordinary Portland cement (OPC) and 10% zeolite concretes after exposure to 800 oC and subsequent re-curing. Specimens were heated and then naturally cooled before being re-cured in either water or moist air at 25 oC or 60 oC for 28, 56, or 90 days. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analyses revealed that rehydration products contributed to compressive strength and volume of permeable pore voids recovery in the OPC concrete. However, re-curing the zeolite concretes resulted in the formation of needle-shaped crystals, tentatively identified as thaumasite-ettringite solid solution. While extending the re-curing period initially led to thaumasite formation, its subsequent decomposition at room temperature was observed. The combined volume expansion of thaumasite and rehydration products reduced the volume of permeable pore voids after re-curing. Re-curing at a 60 oC water bath prevented the formation of these needle-shaped crystals due to their instability at elevated temperatures. Regardless of the concrete mixture, specimens re-cured at 60 oC exhibited lower compressive strength compared to those re-cured at 25 oC. However, the re-curing temperature did not significantly affect the volume of permeable pore voids. | ||
| Key Words | ||
| microstructure; re-curing; rehydration reactions; thaumasite-ettringite solid solution; zeolite | ||
| Address | ||
| Hamed Kharrazi, Mohammad Shekarchi: School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran Babak Ahmadi: Concrete Technology Department, Road, Housing, and Urban Development Research Center (BHRC), Tehran, Iran | ||