• Reduced permeability. As fly ash reacts with free lime and alkalis, it creates cementitious compounds that block up concrete’s porous structure.

• Reduced susceptibility. Class F fly ash resists concrete ASR and sulfate attack, as fly ash “locks up” alkalis and free lime, making it less available to react to sulfates.

• Lower cost. As fly ash generally is half the price of cement, replacement rates of 15 to 25 percent can reduce the price of concrete per ton equivalently, with even higher replacement rates and savings in mass pours.

‘BescherBalls’ in Sulfoaluminate Cement

In April, a new nanostructural component of a proprietary, ultra-fast setting non-portland hydraulic cement mix with fly ash – with the working name of BescherBalls – was articulated by CTS Cement Manufacturing Corp.

Calcium sulfoaluminate cements are used in ultra-high early strength cements, and in green “low-energy” cements. Its physical properties – such as expansion or rapid curing, valuable for pavement repairs under traffic or emergency bridge structural repairs – are obtained by adjustment of the availability of calcium and sulfate ions. Manufacturing energy requirements are lower because of the lower kiln temperatures required for reaction, and the lower amount of decarbonated limestone in the mix.

Interaction of cement grains (left) and microsilica fume (right) takes place at the nanoscale.

Discovered in mixtures of coal fly ash and hydrated calcium sulfoaluminate cement, BescherBalls consist of micron-sized glass spheres upon which needles have grown radially. Whereas these needles usually grow randomly in CTS Rapid Set cement, in fly ash/cement mixes they organize themselves as spines on round fly ash particles. It is possible these structures grow and develop only in calcium sulfoaluminate/fly ash mixtures.

According to Dr. Eric Bescher, CTS vice president for cement technology, this is the first time these complex structures have been seen. “We are excited about discovering these new self-organized inorganic architectures,” Bescher said. “Think of these structures as micron-sized sea urchin shells embedded in cement paste. We have some indications that they may play a beneficial role in the reinforcement of concrete or in shrinkage mediation. Our work is in progress and we are investigating the influence they could have on other properties of construction materials.”

Silica Fume or Microsilica

Another mineral admixture, silica fume, also called microsilica, provides a big improvement in durability of concrete structures exposed to deicing salts.

Silica fume makes a more durable concrete, but too much will make concrete brittle, as a number of state DOTs found in the early 1990s as bridge decks containing large amounts of silica fume began to disintegrate.

An industrial byproduct of glass manufacture, silica fume admixture operates at the nanoscale. Because the silica fume particles are much, much smaller than the cement particles – with a surface area in the neighborhood of 20,000 sq. m./kg. – they can “pack” between the cement particles and provide a finer pore structure.

In the early stages of hydration, silica fume can help accelerate the hydration process because its tiny particles provide nucleation sites for hydration, much the same way that microfine dust particles, or cloud seeding, induce formation of rain droplets.

In the nucleation process, a silica fume particle provides a site on which material in solution can “nucleate” or “center,” which helps the material precipitate sooner than it might otherwise do. And once it precipitates, the concentration of that material in solution is reduced, which tends to get more material into solution from elsewhere, speeding the process.

And like fly ash, silica fume can reduce bleeding, but for a different reasons: the silica fume introduces a lot of surface area per particle into the mix, which helps hold the water in place.