thawing [2, 3, 187, 190]. For instance, De Muynck et al. [3] have shown that the
biodeposition of CaCO 3 on concrete reduced the water absorption by 65–90%
depending on the porosity of the specimens. Achal et al. [170] have demonstrated
the difference in absorption of water between mortar cubs treated by Bacillus
sp. CT5 cells and the control without the bacterial cells. The reduced water absorption by Bacillus sp. CT5 treated specimens is accompanied by a decrease in rate of
carbonation and chloride migration by about 25–30% and 10–40%, respectively. A
similar observation made by Li and Qu [191] confirms that the biodeposited calcite
on concrete surface reduces capillary water uptake, leading to the carbonation rate
constant to be decreased by 25–40%. A good acid resistance has also been reported
upon biodeposition of CaCO 3 on the concrete surface [187, 190]. Further studies in
this line could be of great interest for concrete structures intended to be erected in
low pH environments and regions prone to acid rain.
The effect of concrete surface treatment with pure and mixed ureolytic bacterial
cultures on durability (i.e., based on capillary water uptake and gas permeability)
was studied [2]. The authors concluded that the use of pure cultures led to a more
pronounced decrease in water uptake. They also indicated that the durability performance achieved with B. sphaericus cultures was comparable to that of the conventional water repellents (silanes and siloxanes). Another interesting observation
mentioned in their study is that the biologically produced calcium carbonate is less
soluble when compared to the inorganically precipitated calcite. This indicates a
higher performance potential of the calcite obtained through microbial precipitation.
Another treatment was made by Okwadha and Li [188] who used S. pasteurii strain
ATCC 11859 cells to create a biosealant on concrete surface contaminated by PCB.
The treatment resulted in water permeability reduction by up to five orders of
magnitude. Moreover, the biodeposition on the PCB-contaminated concrete
exhibited a remarkable resistance to carbonation. Another example is the one that
results in a significant reduction in water absorption and chloride permeability of
concrete with biodeposited calcite layer [192].
The results from studies cited above are very interesting and were comparable to
those of the conventional surface treatment substances such as acrylate, silane,
siloxane, silicone, and silicate [175]. However, the procedure used in the biodeposition process, immersion, is not feasible in real application. Spraying the
bacterial culture and the precipitating precursor seems to be more attractive in the
real world. Although it might be labor intensive, application of the biodepositing
agents by brushing the surface of the concrete structures could also be another
alternative approach. Thus, field trial studies on these methods of applications
would be interesting.
The biodeposition process forms a layer of CaCO 3 crystals, and this deposited
layer functions as protective coating barrier which minimizes the penetration of
aggressive substances into the concrete structure. The protection is expected to
enhance the durability of concrete structures. However, the nature of the deposited
layer determines the effectiveness of the biodeposited matrix. The thickness, density,
cohesion, and the layer bond with the concrete matrix are some of the factors that
determine the effectiveness of the layer [57]. The layer thickness is determined by
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
313
biodeposition of CaCO 3 on concrete reduced the water absorption by 65–90%
depending on the porosity of the specimens. Achal et al. [170] have demonstrated
the difference in absorption of water between mortar cubs treated by Bacillus
sp. CT5 cells and the control without the bacterial cells. The reduced water absorption by Bacillus sp. CT5 treated specimens is accompanied by a decrease in rate of
carbonation and chloride migration by about 25–30% and 10–40%, respectively. A
similar observation made by Li and Qu [191] confirms that the biodeposited calcite
on concrete surface reduces capillary water uptake, leading to the carbonation rate
constant to be decreased by 25–40%. A good acid resistance has also been reported
upon biodeposition of CaCO 3 on the concrete surface [187, 190]. Further studies in
this line could be of great interest for concrete structures intended to be erected in
low pH environments and regions prone to acid rain.
The effect of concrete surface treatment with pure and mixed ureolytic bacterial
cultures on durability (i.e., based on capillary water uptake and gas permeability)
was studied [2]. The authors concluded that the use of pure cultures led to a more
pronounced decrease in water uptake. They also indicated that the durability performance achieved with B. sphaericus cultures was comparable to that of the conventional water repellents (silanes and siloxanes). Another interesting observation
mentioned in their study is that the biologically produced calcium carbonate is less
soluble when compared to the inorganically precipitated calcite. This indicates a
higher performance potential of the calcite obtained through microbial precipitation.
Another treatment was made by Okwadha and Li [188] who used S. pasteurii strain
ATCC 11859 cells to create a biosealant on concrete surface contaminated by PCB.
The treatment resulted in water permeability reduction by up to five orders of
magnitude. Moreover, the biodeposition on the PCB-contaminated concrete
exhibited a remarkable resistance to carbonation. Another example is the one that
results in a significant reduction in water absorption and chloride permeability of
concrete with biodeposited calcite layer [192].
The results from studies cited above are very interesting and were comparable to
those of the conventional surface treatment substances such as acrylate, silane,
siloxane, silicone, and silicate [175]. However, the procedure used in the biodeposition process, immersion, is not feasible in real application. Spraying the
bacterial culture and the precipitating precursor seems to be more attractive in the
real world. Although it might be labor intensive, application of the biodepositing
agents by brushing the surface of the concrete structures could also be another
alternative approach. Thus, field trial studies on these methods of applications
would be interesting.
The biodeposition process forms a layer of CaCO 3 crystals, and this deposited
layer functions as protective coating barrier which minimizes the penetration of
aggressive substances into the concrete structure. The protection is expected to
enhance the durability of concrete structures. However, the nature of the deposited
layer determines the effectiveness of the biodeposited matrix. The thickness, density,
cohesion, and the layer bond with the concrete matrix are some of the factors that
determine the effectiveness of the layer [57]. The layer thickness is determined by
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
313