The Amazing Nacre (Mother of Pearl)

 

Nacre, also known as mother of pearl, is an organic–inorganic composite material produced by some molluscs as an inner shell layer. It comprises 95% calcium carbonate (aragonite) which, due to its layered microstructure, yields tensile strength, toughness and ductility levels as well as barrier and durability characteristics that far exceed those of monolithic aragonite or artificial inorganic materials such as concrete.

The superior engineering properties of nacre result from its controlled microstructure fored during a biomineralization process. This process involves controlled through-solution crystallization of amorphous materials (mostly calcium carbonate) to form intricate microstructures tailored towards serving structural, protective and other purposes.

Can we learn from biomineralization of nacre to produce improved material for construction of longer-lasting, safer and more sustainable infrastructure systems?

More Durable Concrete: The Need & The Rationale

 

Concrete is the most widely used material of construction, with a consumption rate of 22 billion tons/yr. Production of concrete accounts for more than 4 billion tons/yr CO₂ emissions and 20 billion GJ/yr energy use, most of which is associated with the manufacturing of cement. The concrete-based infrastructure supports diverse economic and social activities. The sustained growth in cement consumption over several decades reflects upon the buildup of an immense concrete-based infrastructure. Aging of this infrastructure has mounting economic and safety implications. 

While concrete occupies a significant volume of vast infrastructure systems, the expenditures on concrete materials constitute a relatively small fraction of the total infrastructure costs. Deterioration of concrete and corrosion of reinforcing steel under weathering, thermo-mechanical and chemical effects are key factors governing the maintenance and repair requirements and service life of the concrete-based infrastructure. More durable concrete materials would thus yield improportionally large life-cycle benefits. The improved service life of the concrete-based infrastructure would also translate into reduced CO₂ emission and energy use, which would be magnified if Portland cement is replaced with a cementitious binder of reduced carbon footprint and energy content.

Deterioration of concrete and corrosion of the reinforcing steel are complex phenomena. Concrete is prone to microcracking at young age, primarily due to the internal and external restraint of shrinkage movements. The pore structure of concrete together with these microcracks facilitate transport of moisture, aggressive solutions and gases into concrete. These transport phenomena facilitate various mechanisms of concrete deterioration, including freeze-thaw damage, alkali-aggregate reaction, sulfate attack, carbonation, acid attack, and corrosion of the reinforcing steel. These deterioration mechanisms together with the growth of microcracks and cracks under mechanical (including fatigue) loading and restrained dimensional movements compromise the barrier qualities and accelerate the degradation of concrete. Abrasion, erosion and cavitation are also among the mechanisms of concrete degradation in such concrete-based infrastructure systems as pavements and hydraulic structures.