A Case Study: Bacterial Surface Treatment of Normal and Lightweight Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 357

The distributions of the calcium carbonate precipitation on the surfaces of the
concrete specimens were photographed using a digital camera and a scanning
electron microscope (SEM). The energy dispersive spectroscopy (EDS) spectra
were evaluated to verify the chemical compositions of the crystals on the surfaces
of the concrete specimens. Moreover, the amount of capillary water absorption of
the concrete specimens was measured to evaluate the effects of the bacterial
coating on the water transport properties. The experimental details of the present
work can be found in Kim et al. ( 2013 ).
16.3 Results and Discussion
16.3.1 Precipitation of Calcium Carbonate on Concrete
Figure 16.2 shows the surfaces of normal and lightweight concrete specimens with
various surface treatments.
As shown in Fig. 16.2 , calcium carbonate was formed on the surfaces of the
concrete specimens treated with the cell-free medium, similar to the cases of
concrete specimens treated with a medium containing bacteria. However, it was
found that small, light-brown spots covered the top surface of the lightweight
concrete specimens treated with the cell-free medium. These were not found on the
surfaces of lightweight concrete specimens treated with the medium containing
bacteria. As marked by the red circle in Fig. 16.2 d, these small spots have a hollow
dome-shaped structure.
The assumed development process of the small spots by the cell-free medium
on the surfaces of the lightweight concrete specimens is illustrated in Fig. 16.3 a.
The lightweight concrete had more pores due to the porous lightweight aggregate
material used. Thus, some of the liquid medium was absorbed in the pores when
the medium was poured on the surface of the lightweight concrete; consequently, a
corresponding volume of air was simultaneously released from the pores.
Although some amount of air also rose to the surface of the liquid medium, some
remained on the substrate and formed air bubbles at the openings of the pores due
to the surface tension which arose. Calcium carbonate then formed at the interface
between the liquid medium and the air bubbles. After the medium on the concrete
specimens was removed, dome-shaped calcium carbonate structures formed and
the remaining medium under these structures dried out.
These dome-shaped structures, however, were easily broken due to their thin
layer. However, the respiration characteristics of the bacteria were assumed to be
hindered during the development of the dome-shaped structures on the lightweight
concrete specimens. As shown in Fig. 16.3 b, the air bubbles remaining at the
openings of the pores due to the surface tension may have been consumed by the
respiration of the bacteria. Therefore, calcium carbonate crystals could precipitate
inside the pores.