Concrete: a thoroughly modern material
Concrete and cement. Words synonymous with solidity and well, staidness. Cement is probably the most ubiquitous building material of the late 20th century – yet it may provide some very 21st century surprises. Researchers at the University of Alicante have developed a cement material incorporating carbon nanofibres in its composition, turning cement into an excellent conductor of electricity. Similarly scientists from the USA, Japan, Finland and Germany have unraveled the formula for transforming liquid cement into liquid metal – opening up its use in the profitable consumer electronics marketplace for thin films, protective coatings, and computer chips.
The warmer side of concrete
Concrete is a composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material (the cement or binder) that sets and hardens independently, filling the space among the aggregate particles and gluing them together. Concrete made from such mixtures was first used in Mesopotamia in the third millennium B.C. and later in Egypt. It was further improved by the Ancient Macedonians and three centuries later on a large scale by Roman engineers. They used both natural pozzolans (such as pumice) and artificial pozzolans (ground brick or pottery) in these concretes. Many excellent examples of structures made from these concretes are still standing, notably the huge dome of the Pantheon in Rome and the massive Baths of Caracalla. The vast system of Roman aqueducts also made extensive use of hydraulic cement.
Conventional concrete is a poor conductor of electricity. To obtain a cement-like compound that is effective as a heating element, it then should have a low resistivity. This has been achieved by the addition of conductive materials such as carbon fibres, for example. This new technology, developed and patented by the University of Alicante Civil Engineering Department’s Research Group in Multifunctional Concrete Conductors, allows, among other functions, the material to heat up due to the passage of current.
The technology allows buildings’ premises to heat or prevents the formation of ice on infrastructure, such as highways, railways, roads, airstrips and other elements. In this way, a new conductive compound with much more interesting properties is achieved since it keeps the structural properties of concrete and does not compromise the durability of the structures themselves. This new product has a great versatility, since any existing structure or surface can be coated with it, keeping thermal control in it by applying continuous electric current. At present, the research group has developed trials to test the technology in plasters with carbonaceous materials. These tests have given very satisfactory results, obtaining optimal properties of heating the material with minimum energy consumption.
21st century metallic-glass cement
A team of scientists from the USA, Japan, Finland and Germany have made a metallic-glass cement. This new material has lots of applications, including as thin-film resistors used in liquid-crystal displays – basically the flat panel computer monitor that you are probably reading this from at the moment. The team have demonstrated how make and understand the cement-to-metal transformation, which has positive attributes including better resistance to corrosion than traditional metal, less brittleness than traditional glass, conductivity, low energy loss in magnetic fields, and fluidity for ease of processing and molding. Previously, only metals have been able to transition to a metallic-glass form. Cement does this by a process called electron trapping, a phenomena only previously seen in ammonia solutions. Understanding how cement joined this exclusive club opens the possibility of turning other solid normally insulating materials into room-temperature semiconductors.
This phenomenon of trapping electrons and turning liquid cement into liquid metal was found recently, but not explained in detail until now. Now that the conditions needed to create trapped electrons in materials are known, other materials can be developed and tested to find out if we can make them conduct electricity in this way. The results were reported in the journal the Proceeding of the National Academy of Sciences in the article “Network topology for the formation of solvated electrons in binary CaO-Al2O3 composition glasses.”
The team of scientists studied mayenite, a component of alumina cement made of calcium and aluminum oxides. They melted it at temperatures of 2,000 degrees Celsius using an aerodynamic levitator with carbon dioxide laser beam heating. The material was processed in different atmospheres to control the way that oxygen bonds in the resulting glass. The levitator keeps the hot liquid from touching any container surfaces and forming crystals. This let the liquid cool into glassy state that can trap electrons in the way needed for electronic conduction.
The scientists discovered that the conductivity was created when the free electrons were “trapped” in the cage-like structures that form in the glass. The trapped of electrons provided a mechanism for conductivity similar to the mechanism that occurs in metals. To uncover the details of this process, scientists combined several experimental techniques and analyzed them using a supercomputer.
These developments are sure to provide an impetus for a new look at old building material.