Basics of Construction Microbial Biotechnology - Biotechnologies and Biomimetics for Civil Engineering - page 42

2.5.4 Calcium- and Magnesium-Based Biocementation

The biocement for this biocementation can be produced through dissolution of
dolomite, which is a common raw material for the production of cement, in
hydrochloric acid:

Ca 2 þ þ Mg 2 þ þ CO NH 2 Þ#þ 2NH 4 : ð Þþ UPB ! CaMg CO 3 ð ð 2 : 18 Þ

Our experiments showed that this biocementation produced the unconfined
compressive strength of the biocemented sand column 12.4 MPa at the content of
precipitated Ca and Mg 6 % (w/w). However, the hydraulic permeability of the
biocemented sand was high, 7 9 10 -4 ms -1 . For calcium-and urea based bioce-
mentation the hydraulic conductivity and strength are correlated and high strength
is accompanied with low hydraulic conductivity of the biocemented sand. Prob-
ably, high strength but at the same time high permeability of sand after magne-
sium-based biocementation is due to coating of whole surface of sand grains with
crystals, while calcium- and urea-based biocementation produced crystal mainly in
the contact areas of the sand grains (Fig. 2.7 ).
Combined calcite and struvite (NH 4 MgPO 4 ) precipitation using triple super-
phosphate and magnesium salt to avoid formation of soluble ammonia and release
of ammonia to atmosphere during biocementation:

Þ 2 þ 2Mg 2 þ þ CO NH 2

Ca H 2 PO 4 ð ð Þ 2 þ H 2 O þ acid urease ! 2NH 4 MgPO 4 #

þ CaCO 3 # :
ð 2 : 19 Þ

2.5.5 Calcium-Phosphate Biocementation

Calcium phosphate precipitation from calcium phytate (myo-inositol hexakis-
phosphate, calcium salt) solution, (the main storage form of phosphorus in the
plant seeds) using phytase activity of microorganisms (Roeselers and van Loos-
drecht 2010 ) producing a mixture of the crystal forms such as monetite (CaHPO 4 ),
whitlockite [Ca 9 (Mg,Fe 2+ )(PO 4 )6HPO 4 ], and hydroxyapatite [Ca 5 (PO 4 ) 3 OH] with
the Ca-to-P molar ratio 1.55. The problem of this type of biocementation is a low
solubility of calcium phytate (in the described study the concentration was
5.6 mM), so big volumes of solution must be pumped through soil.
Triple superphosphate, which is a relatively cheap commodity, has also low
solubility about 0.08 M, and can be used for calcium phosphate precipitation using
acidotolerant urease-producing microorganisms as shown below.