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.

Portland Cement

Cement is often confused with concrete. Cement is the glue that holds the other concrete ingredients together (sand, aggregate and rock). Cement is the world’s most widely used construction material, with around 1.25 billion tons produced each year.

Ancient civilizations sought to bind stone together into a solid mass. Assyrians, Babylonians and other civilizations used mud for this purpose. Egyptians began to use lime and gypsum to improve their mortar to a material more durable than simple clay. Romans developed cement to a much higher degree, by including volcanic ash known as Pozzolanic material (named after the town of Puozoli, at the foot of Mt. Vesuvius). This material was used in Roman concrete, or “Opus Caementicium.” Pozzolonic material acts as a cement in the presence of cement. It is basically just ash, and does not work as a cement by itself.

Romans developed cement to a high state, as described by Vitruvius, around 25 BC in his work “Ten Books of Architecture.” With the fall of the Roman Empire, the art of cement making and use was lost. The key feature of Roman cement is that it was hydraulic cement, and would cure or set underwater.

Hydraulic cement was not rediscovered until late in the eighteenth century, when the scientific method led to its rediscovery, as discussed in this article. “Repeated structural failure of the Eddystone Lighthouse off the coast of Cornwall, England, led John Smeaton, a British engineer, to conduct experiments with mortars in both fresh and salt water. In 1756, these tests led to the discovery that cement made from limestone containing a considerable proportion of clay would harden under water.
Making use of this discovery, he rebuilt the Eddystone Lighthouse in 1759. It stood for 126 years before replacement was necessary.

Other men experimenting in the field of cement during the period from 1756 to 1830 include L. J. Vicat and Lesage in France and Joseph Parker and James Frost in England.

Before portland cement was discovered and for some years after its discovery, large quantities of natural cement were used. Natural cement was produced by burning a naturally occurring mixture of lime and clay. Because the ingredients of natural cement were mixed by nature, its properties varied as widely as the natural resources from which it was made.

In 1824, Joseph Aspdin, a bricklayer and mason in Leeds, England, took out a patent on a hydraulic cement that he called portland cement because its color resembled the stone quarried on the Isle of Portland off the British coast. Aspdin's method involved the careful proportioning of limestone and clay, pulverizing them, and burning the mixture into clinker, which was then ground into finished cement.
Portland cement today, as in Aspdin's day, is a predetermined and carefully proportioned chemical combination of calcium, silicon, iron, and aluminum.”

Portland Cement is widely used in construction today. Tomorrow we’ll look at cinder blocks and concrete blocks, and what the difference is between them.