Friday, March 19, 2010

DNA of Cement

Cement seems like pretty basic stuff. It’s dusty and dirty, you mix it with sand and rock, add water and get concrete. Yes, it seems like pretty basic stuff.

For anyone who has studied cement in depth, it has remained an elusive material which has defied any definitive classification. Is it a crystal, or is it amorphous? Is it like quartz (crytalline) or is it like glass (amorphous)? And –more to the point- why should we care?

Cement is the most widely used construction material in the world. We produce 1.25 billion tons of this stuff every year. The strength of everything we make with cement and concrete relies on good quality cement. Just as important (if not more so) is the fact that cement manufacture creates a lot of CO2. Cement manufacture is one of the major contributors to greenhouse gas production, which climate scientists tell us is warming our planet. A better understanding of cement structure could provide a path toward reducing greenhouse gas generation.

Recently scientists at MIT have “decoded” the “DNA” or fundamental structure of hydrated cement. It is structured very much like a crystalline lattice, with long rows of silica tetrahedra sandwiched between layers of calcium oxide, like stacks of oranges at the grocery store, almost perfectly stacked in an ordered crystalline system.

The structure of cement has long been known to be very similar to the rare mineral tobermorite, which has these long connected chains of silica tertrahedra between calcium oxide. However, recent research by MIT scientists has shown that there are tiny gaps or flaws between the silica tetrahedra and the calcium oxide; these gaps or flaws (or interstitial sites) become occupied by water molecules upon addition of water to cement powder. Thus hydrated cement is something more like an amorphous (non-regularly repeating) structure of glass than it is like an ordered crystal.

The water forms bonds between layers of silica and CaO, helping to give hydrated cement its strength. There is some flexibility between these bonds, so that cement is less likely to suffer brittle cracking, as with a pure crystal. There is some ability for cement to move under applied stress –or strain- thus relieving the applied stress without suffering brittle failure (stress is an applied force; strain is movement under stress).

This insight into the atomic scale structure of hydrated cement was gained in September 2009. It has provided a fertile area for ongoing research and development. The hope of this new insight is that it might lead to higher strength cements (and the resulting concretes) and that ideally it may lead to an alternative chemical path for cement production which could greatly reduce the production of greenhouse gases.

Sidetracked today by this interesting scientific development, but next time we’ll look at the early history of cinder block and concrete block development.

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