Bioinspired Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 295

in the final cured cement were also present in the starting materials. Many
examples exist, mostly sedimentology studies focused on beachstone, a term used
to describe the lithification of sands or sediments by carbonate mineral formation ,
resulting in carbonate binding the sediments into a hardened mass. This is also
commonly termed cemenation. For the discussion herein, we will focus on
aqueous or ''hydraulic'' cements, where mineral particles, are combined in an
aqueous solution to form a paste that will set to a solid mass, then cure to acquire
structural integrity.
13.3 Bioinspired Cements
Cement can be ''bioinspired'' in various ways. The first is the process of cement
formation and secondly is the resulting cemented composition and structure, in
some cases termed concrete and mortar. In the former case, organisms form
mineralized structures in ambient temperature environments by making use of
unstable precursor materials (commonly ionic solutions) to engage a mineralizing
reaction. Almost ubiquitously, organisms begin with amorphous precursors that
attain their final structure and composition through crystallization to more stable
forms that involve an aqueous cementing reaction.
13.3.1 Unstable Calcium Carbonate Cement Components
Calcium carbonate formation is well known to occur in aqueous solution through the
initial formation of amorphous phases, including precursors to metastable phases
which also convert to more stable phase polymorphs through a continuous pathway.
Lowenstam first described amorphous calcium carbonate precursors at the California
Institute of Technology in the 1950s, and summarized them in the classic paper
on biomineralization published in SCIENCE in 1981 (Lowenstam 1981 ), and further
in his topic, 'On Biomineralization' published in 1987 (Lowenstam and Weiner
1989 ). Several amorphous calcium carbonate and calcium phosphate precursors
where described: including ACC-calcite precursor, ACC-vaterite precursor, ACC,
as well as ACP-brushite precursor, ACP-octacalcium phosphate precursor, and ACP-
dahlite precursor. All of these compounds were found to be synthesized by organ-
isms, existing in there metastable states in vivo, and to transform to their eventual
more stable crystalline polymorph when explanted (Gunasekaran et al. 1994 ).
Metastable precursors such as proto calcite ACC and proto vaterite ACC have
been investigated as highlighted in Gebauer et al. ( 2010 ). Elucidating the devel-
opment of the precursor chemistry, resulting in final carbonate polymorphs, has
been largely limited by the resolution of spectrographic and imaging techniques
employed to observe the long and short-range order of the crystal lattice. In depth
analyses of the binding chemistry of the carbonate chemical components along