Sizing up nano-scale materials

For the third year in a row, we visit a pavement project VCNA Prairie Material helped convert from all asphalt to partial (base slab) or 100-percent roller compacted concrete. More than a routine switch, the Maryhill Cemetery maintenance shop (pages 28-31) marks the commercial debut of RCC Surface Pro, a silica agent emanating from the field of nanotechnology, where particles are measured in billionths of a meter.

RCC Surface Pro is Lythic Solutions’ latest nano-silica agent, joining a series of successful concrete polishing or slab-finishing products of similar formulation. Nano-silica is also referred to as colloidal silica, owing to the material’s suspension within an “ultra-low surface tension liquid.” Lythic tests find that a) colloidal silica reacts with lime formed during portland cement hydration, much like fly ash and other pozzolans; and, b) the lime imparts no structural value in a normal portland cement mixture, but in combination with colloidal silica promotes additional calcium silicate hydrate—the chief binding and strengthening compound in concrete.

The RCC Surface Pro demonstration at Maryhill yielded a troweled, broom-finished slab. Instead of an RCC base placement one day, followed by asphalt wear course placement the next, the owner realized the economy of monolithic RCC finished in hours. The project is the latest example of nano-scale materials’ emerging role in concrete practice.

A principal in New Jersey-based Intelligent Concrete LLC, Jon Belkowitz examines nano-silica admixture technology in “Colloidal Silica,” a July 2014 Concrete International report: “The small CS [colloidal silica] particle can accelerate cement dissolution and nucleation as well as provide a much larger surface area of free silica for pozzolanic reaction. The CS admixture can thus provide rapid early-strength development and binding of the calcium hydroxide so that it does not participate in alkali silica reaction.”

Colloidal silica consists of ultra-pure silicon dioxide particles in a size range 1/1,000 that of Class F fly ash, he adds. Tested in a concrete mix with 20 percent ASTM C618 material, a colloidal silica admixture shows the potential to enhance fresh concrete properties; reduce high range water reducer and air-entraining agents; increase early and 28-day compressive strengths as measured against control specimens; and, allow a reduction in cement content while maintaining target, hardened-concrete properties. A chart accompanying the Belkowitz article paints tested materials’ reactivity potential through jaw-dropping differences in fineness (m2/kg): Type I/II portland cement, 392; Class F fly ash, 420; nano-silica, 500,000.

Observation of 5-nanometer silica and other atomic-scale subjects is part of the Concrete Science program at the Massachusetts Institute of Technology-hosted Concrete Sustainability Hub. Researchers have recently outlined the potential for “Gorilla Cement,” a nod to the thin, high strength Gorilla Glass that Corning Inc. has parlayed for smartphone, mobile device and laptop screens the world over. Viewing cement at the nano-scale, CSHub investigators aim to pinpoint clinker phase chemistry changes netting higher silica content in calcium silicate hydrate. The effect would likely parallel concrete performance characteristics observed with colloidal silica admixtures or slab finishing agents.

Gorilla Cement research stands to benefit from the recently announced MIT.nano, a 200,000-sq.-ft. facility supporting research with nano-scale materials and processes. “The tools of nanotechnology will play a critical part in how many engineering disciplines solve the problems of the 21st century, and MIT.nano will shape the Institute’s role in these advances,” says MIT School of Engineering Dean Ian Waitz.

Whether they emanate from Cambridge, Mass., or companies like Vancouver, Wash.-based Lythic Solutions, nano-scale materials are sure to extend the value proposition of concrete in the decades ahead.