Increasing the recovery of concrete by cyclic recovery

A recent article published in mATERIALS examined the strength and microstructure restoration of high performance concrete (HPC) through water and water-CO₂ cyclic recovery, which is promising for the rehabilitation of thermally damaged concrete (TDC).

Background

Concrete is known to be inherently fire resistant; however, its mechanical performance degrades when exposed to high temperatures. This is attributed to the decomposition of hydration products, roughening of the pore structure, thermal cracking and phase transformations at elevated temperatures.

Thus, efforts are made to develop the self-healing properties of concrete after fire damage. Various post-fire recovery methods are used to rehabilitate TDC. However, the efficiency of any such process depends on the choice of curing method, rehydration stages and extent of thermal damage.

Water-CO₂ The cyclic recovery method has considerable potential in the recovery of thermally damaged HPC. However, the efficiency of strength recovery at different healing times and the microstructural causal mechanisms are not fully understood. Thus, this study aimed to demonstrate the micromechanisms behind the strength development of thermally damaged HPC using water and water-CO₂ cyclic repeater.

methods

A mortar mix for HPC was prepared using Portland cement, silica fume (SF, 10 wt.% cement) and standard sieved quartz sand. The water-binder ratio was set at 0.36 and a polycarboxylate superplasticizer was added to achieve the desired consistency and workability. Additionally, polypropylene fibers were incorporated to prevent detonation at high temperatures.

HPC specimens were prepared using 20 L of this mortar mixture, cast into cubes (50×50×50 mm³) in two layers. After initial curing for 24 h, the samples were deformed and immersed in saturated lime water at 20±3 °C for 89 days.

The 90-day cured HPC samples were heated to 600 and 900 °C at a rate of 1 °C/min in a muffle furnace and held at this temperature for one hour to ensure uniform temperature exposure. These were then allowed to cool naturally to room temperature.

Rewatering was performed by immersing the heated samples in saturated lime water for 3, 6, 18 and 30 days. Alternatively, for water-CO₂ recovering cyclically, they were immersed in saturated lime water for 3 days and transferred to an air chamber for the next 3 days. This cyclic process was repeated for 3, 6, 18 and 30 days. The compressive strength of HPC samples was investigated after high temperature exposure and recovery.

Furthermore, their phase composition was analyzed before heating, after heating and during recovery using X-ray diffraction (XRD). Alternatively, the microstructure changes during each process were observed through scanning electron microscopy (SEM). Finally, mercury intrusion porosimetry (MIP) was performed to record the pore structure distribution of the HPC samples.

Results and discussions

The compressive strength of the HPC samples recovered significantly through both curing processes, with cyclic curing exhibiting higher strength recovery than water curing. This was supported by XRD, SEM and MIP analysis.

While the calcium silicate hydrate (CSH) gel in HPC was completely decomposed after exposure to 600 °C, C₃S largely remained unaffected due to its limited reaction with SF. During the water recovery process, Ca²⁺ and OH⁻ ions dispersed from saturated lime water in HPC samples through microcracks. At the same time, C₃S ions leach from the cement, promoting the production of calcium hydrate (CH) in microcracks and disturbed pores.

Thus, CSH gel rapidly rehydrates during recovery, filling microcracks and disturbed pores. This resulted in improved mechanical properties of the HPC samples, which achieved a compressive strength comparable to undamaged HPC after a 3-day water-drying period.

Otherwise, CO₂ infiltrated the microcracks during cyclic curing and reacted with Ca²⁺ ions to precipitate CaCO₃ within the microcracks. Consequently, the HPC samples exhibited a 10.1% higher compressive strength after 6 days of cyclic curing than the water-cured samples. Furthermore, the compressive strength recovery after 18 days of water and cycling exceeded 95% of the total recovery observed after 30 days.

Otherwise, the compressive strength of HPC samples damaged at 900 °C showed a slow improvement after 30-day rehydration, while cyclic recovery accelerated the compressive strength recovery. Overall, 18 days was concluded to be optimal for the recovery of HPC samples damaged at 600 or 900 °C.

Conclusion and future perspectives

The researchers comprehensively analyzed the compressive strength recovery of thermally damaged HPC using water and water-CO₂ cyclic recovery methods. Consequently, optimal regeneration regimes and periods for HPCs damaged at 600 and 900 °C were identified, elucidating the underlying mechanisms.

Both recovery methods can effectively increase the compressive strength of TDC due to the filling of microcracks and disturbed pores by hydration products. However, cyclic recovery exhibited accelerated recovery rates due to the formation of carbonation products in the microcracks and cement paste.

The researchers suggest increasing the carbonization depth during cyclic recovery to improve the strength recovery of thermally damaged HPC. Furthermore, the effect of temperature variations on strength recovery should be analyzed to improve the application of the proposed iterative methods in practical engineering.

Journal reference

Li, Y., Wang, H., & Lou, H. (2024). Strength recovery of thermally damaged high-performance concrete during recovery. mATERIALS, 17(14), 3531. DOI: 10.3390/ma17143531, https://www.mdpi.com/1996-1944/17/14/3531

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