Bioinspired Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 294

Therefore, it is evident that a replacement for Portland cement as the binding
component of concrete would benefit mankind in a number of different ways.
Large concrete structures pre-date the invention of Portland cement by millennia,
including Egyptian and Roman concretes that are known for their durability in
large structures still standing today. Taking a biologically inspired or biomimetic
approach to understanding how nature builds structure of mineral with cement and
concrete-like properties may provide insight about what other possibilities may be
used in concrete.
13.2 Overview
Earth's solar system is composed principally of a few anionic complexes—sili-
cates, carbonates, phosphates, sulfates, and nitrates. While many other anionic
complexes are also important, they are not in abundance. Silicates are present in
abundance in high temperature forms such as seen in igneous and metamorphic
rocks and are the source rock for some sedimentary rocks which are all used for
aggregate in concrete. At the same time Portland cement is also a high temperature
silicate. Silicates are also present as well as low temperature hydrated forms, such
as the skeletons of sponges, and are the reaction products of Portland cement.
Elements scavenged by organisms are often found in the ocean at depth, where
light does not penetrate to drive photosynthesis and form the basis of a food chain.
In surface oceanic waters, versus deep ocean waters, silica, nitrates, and phos-
phates are biolimiting elements in this photic zone of the ocean because they are
taken up by marine phytoplankton, so they are not readily available in low tem-
perature systems. By contrast, carbonates are by far the most broadly employed
and abundant materials seen in massive biological structures such as the skeleton
of massive mineralizing organisms like coral comprising coral reefs. Carbonate
forms from the interaction of carbon dioxide gas with water, in a carbonate buffer
system, such as seawater. Carbonate rocks comprise about 10 % of the lithospheric
crust of Earth, and are mined for aggregates used in concrete as well as for the raw
materials for the formation of Portland cement via high temperature processing
that drives off carbon dioxide into the atmosphere. Phosphates and nitrates are
biolimiting elements, used as fertilizer, and tend to be expensive since they are
relatively rare. However, there appear to be some applications of phosphate-based
cement in concrete construction stemming from their initial development in bone
cement applications. Sulfates and nitrates are more soluble than carbonates, sili-
cates, and phosphates and do not appear to have broad applications in concrete
construction as a primary component.
We define a cement as the result of combining ingredients that when mixed
form a moldable paste that will set to a hardened mass and cure to a structurally
stable solid. The process of cementation can occur in an open system, where new
elements are introduced, from molding to setting to curing, but in general most
common cements will be relatively closed systems where most of the components