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

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.

Assessment of the Composition Effect of a Bio-Cementation Solution on the Efficiency of Microbially Induced Calcite Precipitation Processes in Loose Sandy Soil

“…The type and concentration of chemicals used in MICP play a crucial role in controlling the calcium and carbon source [22] as well as contribute to the types and amount of precipitation. The most commonly selected chemicals for MICP are urea, which is hydrolyzed to carbonate ions [23], and calcium chloride, as a source of calcium ions reacting with carbonate ions (Table 1). S. pasteurii growth medium (in 0.13 mol/L tris buffer, 10 g/L (NH4)2SO4, 20 MgCl 2 (0, 0.05, 0.1, 0.25, 0.5 M).…”

Rehabilitation of Porous Building Components and Masonry by MICP Injection Method

“…Moreover, the potential of the recycling ability could be affected if non-mineral grout materials were used. Mu et al [4] summarised further advantages and disadvantages of various conventional grouting materials, i.e., they suggested that mortar-based grouts could cause efflorescence of the masonry due to the high alkalinity.…”

“…However, there is still a huge demand for research on proper injection methods which achieve an effective CaCO 3 distribution [8]. Some studies have already focused on possible MICP applications on bricks, stones or masonry [4,[9][10][11][12]. Jimenez-Lopez et al [12] developed biomineralization experiments by activated bacteria inhabiting the stone for consolidation and increasing the resistance to the deterioration of stone pieces.…”

Influence of Fiber Type and Length on Mechanical Properties of MICP-Treated Sand

“…MICP has been used for many purposes, such as improvement of the physical and mechanical properties of soil [ 3 ], remediation of contaminated soil [ 4 ], repair of concrete cracks [ 5 ], restoration of ancient buildings [ 6 , 7 ], reinforcement of liquefied foundations [ 8 ], suppression of flying dust [ 9 ], prevention of rock and soil seepage [ 10 ], and mitigation of the greenhouse effect [ 11 ]. MICP is a popular research direction in geotechnical engineering, and the strength of sand reinforced by MICP is considerably improved, although shortcomings of brittle failure are observed [ 12 , 13 , 14 , 15 ], leading to the limitation of large-scale promotion and use of the technology.…”