Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85916
RoadScience ecule, generally 10 times the diameter of a water molecule, one-billionth of a meter, called a nanometer. Concrete is a porous material, ranging from air voids to nanometer-scale pores produced by the cement-water chemi- cal reaction, say Ken P. Chong, National Science Foundation, and Edward J. Garboczi, National Institute of Standards and Technology, in their paper Smart and Designer Structural Material Systems (download at http://fire.nist.gov/bfrlpubs/build03/PDF/ b03006.pdf). "Since these nanoscale pores control the properties of the calcium-silicate-hydrate hydration product, which is the main 'glue' that holds concrete together," they say, "concrete is in some ways a nanoscale material." The new fi eld of nanotechnology is giving us a better view of what is happening at the physio-chemical level as cement hydrates and concrete cures. Nuclear resonance reaction analysis (NRRA) is being used to study cement hydration at a nanoscale level. The result is a better idea of what takes place on the surface of the cement particle as it hydrates, leading to improved indus- try standards and guidelines for mixing and curing concrete. NRRA uses a beam of nitrogen atoms to probe a reacting cement grain to locate hydrogen atoms, a necessary compo- nent of water, or its reaction products. The results of the probe are plotted in a graph called a hydrogen depth profi le, which shows the rate of penetration of the water. This also indicates the arrangement of the various surface layers formed during the reaction. The 20-nanometer-thick surface layer acts as a semi-perme- able barrier that allows water to enter the cement grain and calcium ions to leach out. However, the larger silicate ions in the cement are trapped behind this layer. As the reaction continues, a silicate gel layer forms beneath the surface layer, causing swelling within the cement grain and eventually leading to breakdown of the surface layer. This breakdown releases the accumulated silicate into the sur- rounding solution, where it reacts with calcium ions to form a calcium-silicate hydrate gel, which binds the cement grains together and sets the concrete. The evolution of the hydrogen profi le shows the time of breakdown of the surface layer. This information can be used to study the concrete setting process as a function of time, temperature, cement chemistry, and other factors. For example, researchers used NRRA to determine that in cement hydrating at 30 deg C (86 deg. F), the breakdown occurs at 1.5 hours. Mineral Admixtures: Fly Ash Mineral admixtures like fl y ash, silica fume or microsilica, and slags are usually added to concrete to enhance the workability Write 147 on Reader Service Card 12 May 2012 Better Roads