Enhancing Cementitious Component

While the Federal Highway Administration reports that 94 percent of pavements in the United States are surfaced with asphalt, without question concrete is the most widely used construction material in the nation.

Supplementary cementitious materials (SCMs) (top) for ternary or quaternary concrete mixes. From left to right, fly ash (Class C), metakaolin (calcined clay), silica fume, fly ash (Class F), slag, and calcined shale.

Conventional performance of cement and concrete can be boosted through alterations in the chemical composition of clinker – the fused lumps derived from the pyroprocessing of raw limestone (lime), clay (alumina) and sand (silica) – which is fine-ground to make cement.

But it’s easier to improve the performance of concrete via additives. Some of the additives – like coal fly ash, high-reactivity metakaolin and silica fume – are pozzolans that exhibit cementitious properties in the presence of calcium hydroxide, which is formed when cement combines with water and begins to cure.

Jet process granulator (bottom) produces slag in a glassy, amorphous state that complements portland cement concrete.

Other additives – like ground granulated blast furnace (GGBF) slag – are cementitious on their own, and in the form of GGBF slag cement actually can substitute for cement in large percentages of the cementitious component, up to 70 percent, according to the Slag Cement Association.

But because the pozzolanic reaction may be slower compared to the rest of the cement hydration reactions, the early strength of concrete made with materials such as slag cement may not be as high as concrete made only with portland cement. But GGBF cement mixes gain strength over longer periods, resulting in concrete with compressive strengths exceeding 10,000 psi or even 13,000 psi, such as in the case of Reliant Stadium in Houston.

Alternately, highly reactive pozzolans – such as silica fume and high-reactivity metakaolin – produce high early strength concretes that increase the rate at which concrete gains strength.

For the cement producer, it’s more practical to promote ternary blends of cement, as they reduce the amount of cement that’s required for a fixed amount of concrete. This equates to, all things being equal, longer lifetimes for quarry extraction sites, lower pyroprocessing costs, a lower carbon footprint for the same tonnage of blended cement, perhaps an elimination of the need to expand a plant, which is difficult in today’s permitting environment, and more portland cement to go around when the inevitable cement shortages occur in construction boom times.

There is another advantage of ternary concrete mixes incorporating industrial byproducts: their use in concrete is recognized by the Leadership in Energy and Environmental Design (LEED) system of the U.S. Green Building Council, to which the building industry has turned to evaluate the degree of “green” design a structure or development incorporates. Thus, a ternary mix can help a project earn desirable LEED certification, or help a certified project achieve coveted Silver, Gold or Platinum levels.

Reclaimed, recyclable industrial byproducts – which in past years would have been dumped in landfills – now are key components to the high-performance concrete mixes of today.

Fly Ash from Coal

Fly ash, termed a CCB (coal combustion byproduct), is the residue of the burning of pulverized coal in thermal power plants. The ash particles are collected mechanically or by electrostatic precipitators. Generally, 15 to 20 percent of burned coal takes the form of ash.

Relative appearance of fly ash and portland cement in micrographs.