3.2 Use of Microbes for Concrete Surface Coating:
Biodeposition
Most deterioration of concrete structures is due to water coming from surface to the
interior through cracks and openings. Therefore, diminishing the uptake of water is a
very important measure in protecting concrete against severe damage [176]. The
environment and the concrete permeation properties determine the risk of damage
and the rate at which the damage can develop. Without repair, the service time of a
concrete structure is expected to be relatively short if the environment is moist,
aggressive substances such as acid and chlorides are present, and the concrete is
permeable to liquid. In such environments, surface treatments that reduce the
infiltration of water and dissolved aggressive substances into concrete play a major
role in extending the service time of concrete structures. An array of chemical-based
products is available on the market to choose from to treat concrete surfaces which
includes water repellents, coatings, and pore blockers. Although these chemicalbased surface-treating agents are useful in minimizing the concrete water uptake,
they suffer from some setbacks such as (1) they are subject to chemical, physical,
and biological degradation; (2) their thermal expansion coefficients are different to
that of concrete; and (3) some of these materials contain solvents that pollute the
environment [97, 177]. These setbacks have led to the search for alternative methods
that are environmentally benign. It is within this framework that MICCP has been
proposed as a novel and eco-friendly strategy for surface treatment of concrete
structures [178, 179].
The process of microbially induced precipitation of CaCO 3 on concrete or other
building material surfaces is known as biodeposition. Initially, biodisposition was
used for surface protection and consolidation of historical buildings and repair of
limestone monuments [96, 100, 124]. The main desire to use biodeposition in
treating these valuable specimens emanates from the limitation of the existing
traditional treatment methods and incompatibility problems with organic coatings
and consolidants [180–182]. The pioneer strains for these applications were mostly
isolated from carbonate producing environmental samples such as calcareous sludge
[100] and calcareous stones [183, 184].
The biodeposition process was initiated by applying the microbial agent and the
precipitation precursors on the stone surface by immersion, spraying, or brushing
[18]. The biodeposition forms a layer of CaCO 3 precipitate which serves as a barrier
to resist degradation or as a binder to consolidate the loose particles on the surface of
the stones [124]. The precipitate is found firmly attached to the stone surface and
brought a consolidating effect [2, 3, 185]. It is believed that the remarkable cohesion
of the precipitate may be due to incorporation of organic materials in the CaCO 3
crystals that influence the crystals’ epitaxial growth [124]. It was in 1993 the first
on-site microbially mediated bioconsolidation treatment was done on 50 m
2 of the
Thouars Saint-Medard Church tower [97]. Later, biodeposition was also used for
on-site conservation of decayed stones [126, 186]. Analysis of the treated limestone
revealed that it was evenly strengthen at thickness of 30 mm with at least a similar
performance to that of the conventional surface treatments such as ethyl silicates
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
311
Biodeposition
Most deterioration of concrete structures is due to water coming from surface to the
interior through cracks and openings. Therefore, diminishing the uptake of water is a
very important measure in protecting concrete against severe damage [176]. The
environment and the concrete permeation properties determine the risk of damage
and the rate at which the damage can develop. Without repair, the service time of a
concrete structure is expected to be relatively short if the environment is moist,
aggressive substances such as acid and chlorides are present, and the concrete is
permeable to liquid. In such environments, surface treatments that reduce the
infiltration of water and dissolved aggressive substances into concrete play a major
role in extending the service time of concrete structures. An array of chemical-based
products is available on the market to choose from to treat concrete surfaces which
includes water repellents, coatings, and pore blockers. Although these chemicalbased surface-treating agents are useful in minimizing the concrete water uptake,
they suffer from some setbacks such as (1) they are subject to chemical, physical,
and biological degradation; (2) their thermal expansion coefficients are different to
that of concrete; and (3) some of these materials contain solvents that pollute the
environment [97, 177]. These setbacks have led to the search for alternative methods
that are environmentally benign. It is within this framework that MICCP has been
proposed as a novel and eco-friendly strategy for surface treatment of concrete
structures [178, 179].
The process of microbially induced precipitation of CaCO 3 on concrete or other
building material surfaces is known as biodeposition. Initially, biodisposition was
used for surface protection and consolidation of historical buildings and repair of
limestone monuments [96, 100, 124]. The main desire to use biodeposition in
treating these valuable specimens emanates from the limitation of the existing
traditional treatment methods and incompatibility problems with organic coatings
and consolidants [180–182]. The pioneer strains for these applications were mostly
isolated from carbonate producing environmental samples such as calcareous sludge
[100] and calcareous stones [183, 184].
The biodeposition process was initiated by applying the microbial agent and the
precipitation precursors on the stone surface by immersion, spraying, or brushing
[18]. The biodeposition forms a layer of CaCO 3 precipitate which serves as a barrier
to resist degradation or as a binder to consolidate the loose particles on the surface of
the stones [124]. The precipitate is found firmly attached to the stone surface and
brought a consolidating effect [2, 3, 185]. It is believed that the remarkable cohesion
of the precipitate may be due to incorporation of organic materials in the CaCO 3
crystals that influence the crystals’ epitaxial growth [124]. It was in 1993 the first
on-site microbially mediated bioconsolidation treatment was done on 50 m
2 of the
Thouars Saint-Medard Church tower [97]. Later, biodeposition was also used for
on-site conservation of decayed stones [126, 186]. Analysis of the treated limestone
revealed that it was evenly strengthen at thickness of 30 mm with at least a similar
performance to that of the conventional surface treatments such as ethyl silicates
Alkaliphiles: The Emerging Biological Tools Enhancing Concrete Durability
311