Microbial-Induced Carbonate Precipitation Improves Physical and Structural Properties of Nanjing Ancient City Walls.

Baogang Mu,Evangelos Petropoulos,Yongjie Yu,Fei Lu,Zheyi Gui

doi:10.3390/ma14195665

Abstract

The preservation and restoration of heritage sites have always been of key focus in the field of cultural relics. Current restoration methods mainly involve physical or chemical techniques, which are in many cases intrusive, destructive, and irreversible. Hereby, we introduce a novel biological strategy (microbial-induced carbonate precipitation (MICP)) to repair natural and simulated surface cracks on six hundred years’ old wall bricks (part of the Nanjing City Min Dynasty ancient wall, China). X-ray micro computed tomography (X-ray micro-CT) was employed to non-destructively visualize the internal structure of the MICP-treated brick cubes. The results showed that MICP can effectively repair both natural and simulated cracks present on the brick’s surface. The compressive strength of the MICP-treated brick cubes was significantly higher than that of the untreated control cubes (33.56 ± 9.07 vs. 19.00 ± 1.98 kN, respectively). MICP significantly increased the softening coefficient and decreased the water absorption rate (p < 0.05), indicating that the water resistance of the wall bricks can be improved after treatment. The 3D images from X-ray micro-CT, a method that could non-destructively assess the internals of such cultural structures, showed that MICP can effectively repair ancient relics, promoting durability and limiting degradation without affecting the structure. X-ray diffraction analyses showed that MICP generates the same calcite form as that of original bricks, indicating that MICP filler is compatible with the ancient city wall brick. These findings are in line with the concept of contemporary heritage preservation.

Highlights

For the cubes with simulated cracks, only the obvious cracks made by the circular saw were Microbial-induced carbonate precipitation (MICP) treated (Figure 2B)
These findings show that the metal oxides in ancient brick included silica (SiO2 ), calcium oxide (CaO), alumina into consideration
These findings show that the metal oxides in ancient brick included silica (SiO2), calcium oxide (CaO), alumina (Al2O3), and iron oxide (Fe2O3)

Summary

Introduction

As per the evolution of civil architecture, numerous traditional underground cultural relics (mausoleums) and magnificent above-ground ancient buildings were constructed of masonry with sintered clay bricks. Among them is the Nanjing City Wall, an outstanding representation of above-ground architecture, one of the longest, largest, and most authentic ancient city walls worldwide. Apart from the prominent risk of systemic load-bearing capacity, the most common problem of the Nanjing City Wall is the persistent deterioration of the main brick material and the development of cracks on its surface driven by weathering. The persistent deterioration will eventually lead to a reduction in load-bearing capacity increasing the risk of failure. The city wall bricks urgently require the establishment of means for weathering mitigation or other deterioration prevention measures to eliminate the likelihood of failure

Methods

Results

Discussion

Conclusion