Production of Bacteria for Structural Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 311

in form of pellets to mortars. The mortars were cured in solution containing urea
and calcium chloride for 7 days. When tested for compressive strength, it was
recorded about 65 MPa in the presence of S. pasteurii, which was relatively higher
than control mortars (55 MPa). As P. aeruginosa is not reported to induce calcite
precipitation, it couldn't improve the compressive strength of mortars.
Other than bacteria of genus Bacillus, Shewanella sp. was used to enhance the
compressive strength of mortars by 17 and 25 % at 7 and 28 days, respectively;
however, they cured mortar specimens in air (Ghosh et al. 2005 ). Just like
P. aeruginosa, there was no increment in the compressive strength with Esche-
richia coli (non-urease producing bacterium). Later, while mixing spores of
Bacillus pseudofi rmus and Bacillus cohnii, an increment of 10 % mortar com-
pressive strength was recorded (Jonkers and Schlangen 2007 ).
One of problems associated, while studying compressive strength, with
microbial concrete is cost factor used in growing bacterial cells or curing in
nutrient media. To overcome this, Achal et al. ( 2009b ) replaced the commercially
available nutrients with some industrial by products such as lactose mother liquor
(LML) and corn steep liquor (CSL). An improvement of 17 % in the compressive
strength of mortar cubes was noticed with S. pasteurii grown and cured with LML
medium compared to control (23.2 MPa) at the end of 28 days. Later, Achal et al.
( 2010a ) reported 35 % improvement in the compressive strength of mortar at
28 days with S. pasteurii prepared mortar cubes with CSL-urea medium. The
similar experiment resulted into 36 % improvement in the compressive strength,
when CSL was replaced with commercial nutrient medium (Achal et al. 2011b ).
The effect of different media on the compressive strength of cement mortar using
S. pasteurii has been summarized in Fig. 14.4 . Their experiments were of great
potential with respect to economization of microbial concrete preparation.
The overall trend of an increase in the compressive strength was very much
dependent on calcium carbonate precipitation induced by bacterial cells and the
behavior of bacterial cells within the cement mortar matrix. As cement mortar
remains still porous during the initial curing period, though bacterial cells get good
nourishment; but growth might not be proper due to the completely new envi-
ronment for bacteria, especially high cement pH (Achal et al. 2011b ). As the
curing period proceed, bacterial cells started growing and start precipitating cal-
cium carbonate within mortar matrix. The bacterial growth and curing period led
to plugging of pores in the matrix and the flow of the nutrients and oxygen to the
bacterial cells stops that causes the cells either died or turned into endospores and
acts as an organic fiber that may enhance the compressive strength of the mortar
cubes (Ramachandran et al. 2001 ).
Further, to confirm MICP process in the improvement of compressive strength,
researchers analyzed mortar specimens with techniques such as X-ray diffraction
(XRD) and visualized with scanning electron microscope (SEM).