4.3 Temperature
The influence of temperature on bacterial and enzymatic activity, reactant solubility,
and reaction rate is well known. A modest increase in temperature is often accompanied by an increase in bacterial growth and activity. The activity is related to the
effect of temperature on the chemical and enzymatic reaction rates, and hence
temperature directly affects MICCP. For instance, most ureases are optimally active
in the range of 20–37
C, and their activity on urea is temperature dependent
[241, 245, 248]. An increase in treatment temperature (up to its optimum) increases
the rate of calcite precipitation [249]. On the other hand, the efficiency of calcite
precipitation can drop with temperature. Thus, temperature is one of the important
parameters that determine the success of microbial-based applications in concrete.
It appears that with rise in temperature, the rate of calcite precipitation increases,
and with decreasing temperature, the rate of calcite formation falls. Both fast and
slow rate of calcite deposition is not desirable. If CaCO 3 precipitates too fast, it will
loosely accumulate, and this will not bring the desired effect [124]. If it is slow, it
will take very long time to seal the cracks and that may incur loss. Thus, it is
important to consider a moderate pace with lasting instead of transient carbonation
process to achieve an effective treatment that brings the desired quality on the
concrete. As different countries have different environmental temperature and
humidity, it is very important to formulate the most suitable microbial agents that
can optimally work in specific region of the world. In cold environments, it may be
good to use organisms that are active at relatively low temperature. Whereas, since
tropical regions are warmer in most of the year, the use of microbes that optimally
grow around 30
C could be rewarding.
4.4 Urea, Ca
2+ , and CO 3
2À Concentrations
From a chemical perspective, the concentrations of Ca
2+ and CO 3
2À directly determine the amount of CaCO 3 precipitation [232]. Concrete contains a high amount of
calcium; however, the amount of Ca
2+ leached from the concrete is very small and
often not enough to make good precipitation [250]. Thus, it is necessary to add an
external Ca
2+ source to the mixture. Different calcium sources like Ca-lactate,
Ca-formate, Ca-gluconate, and Ca-nitrate have been considered. Even CaCl 2 has
been used in many laboratory studies [165] despite the fact that it cannot be applied
to concrete. In ureolytic process, urea is the source of CO 3
2À and hence can influence
the precipitation process. However, the efficiency of CaCO 3 precipitation relies
more on Ca
2+ than urea concentration [241]. This could be partly due to the fact
that unlike urea, the calcium added seems not metabolically utilized, but it accumulates outside the cell and remain readily available for precipitation of CaCO 3
[251]. Moreover, the activity of urease is stimulated by the presence of Ca
2+ .
Compared to Ca
2+ -free system, a tenfold increase in the activity of urease has
326
G. Mamo and B. Mattiasson
The influence of temperature on bacterial and enzymatic activity, reactant solubility,
and reaction rate is well known. A modest increase in temperature is often accompanied by an increase in bacterial growth and activity. The activity is related to the
effect of temperature on the chemical and enzymatic reaction rates, and hence
temperature directly affects MICCP. For instance, most ureases are optimally active
in the range of 20–37
C, and their activity on urea is temperature dependent
[241, 245, 248]. An increase in treatment temperature (up to its optimum) increases
the rate of calcite precipitation [249]. On the other hand, the efficiency of calcite
precipitation can drop with temperature. Thus, temperature is one of the important
parameters that determine the success of microbial-based applications in concrete.
It appears that with rise in temperature, the rate of calcite precipitation increases,
and with decreasing temperature, the rate of calcite formation falls. Both fast and
slow rate of calcite deposition is not desirable. If CaCO 3 precipitates too fast, it will
loosely accumulate, and this will not bring the desired effect [124]. If it is slow, it
will take very long time to seal the cracks and that may incur loss. Thus, it is
important to consider a moderate pace with lasting instead of transient carbonation
process to achieve an effective treatment that brings the desired quality on the
concrete. As different countries have different environmental temperature and
humidity, it is very important to formulate the most suitable microbial agents that
can optimally work in specific region of the world. In cold environments, it may be
good to use organisms that are active at relatively low temperature. Whereas, since
tropical regions are warmer in most of the year, the use of microbes that optimally
grow around 30
C could be rewarding.
4.4 Urea, Ca
2+ , and CO 3
2À Concentrations
From a chemical perspective, the concentrations of Ca
2+ and CO 3
2À directly determine the amount of CaCO 3 precipitation [232]. Concrete contains a high amount of
calcium; however, the amount of Ca
2+ leached from the concrete is very small and
often not enough to make good precipitation [250]. Thus, it is necessary to add an
external Ca
2+ source to the mixture. Different calcium sources like Ca-lactate,
Ca-formate, Ca-gluconate, and Ca-nitrate have been considered. Even CaCl 2 has
been used in many laboratory studies [165] despite the fact that it cannot be applied
to concrete. In ureolytic process, urea is the source of CO 3
2À and hence can influence
the precipitation process. However, the efficiency of CaCO 3 precipitation relies
more on Ca
2+ than urea concentration [241]. This could be partly due to the fact
that unlike urea, the calcium added seems not metabolically utilized, but it accumulates outside the cell and remain readily available for precipitation of CaCO 3
[251]. Moreover, the activity of urease is stimulated by the presence of Ca
2+ .
Compared to Ca
2+ -free system, a tenfold increase in the activity of urease has
326
G. Mamo and B. Mattiasson