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Silicate-rock-based material can replace flyash in cement, lowering costs and CO2

| By Scott Jenkins

Flyash, a waste byproduct from coal burning, has been used effectively as a supplementary cementitious material (SCM) in Portland cement to displace a portion of the cement and lower the CO2 emissions associated with cement making. But because coal burning is becoming less common, supplies of flyash are more limited and its quality more variable.

The company Terra CO2 (Golden, Colo.; has developed a process for making a flyash replacement from silicate-based igneous rock. The product, known as Opus SCM, promises to alleviate issues with flyash supply and lower the costs associated with transporting flyash for cement making.

Opus SCM can be blended into portland cement to displace between 10 and 30% of the cement. Terra CO2 plans to break ground on the first commercial plant to produce Opus SCM in late 2023.

Manufacturing Opus SCM involves widely available, alluvial and granite-based construction aggregates, which are milled to a size of 6–7µm. This size is similar to that of the limestone used to make portland cement. The powder is then pneumatically conveyed to a proprietary reactor, where it undergoes air-suspension melting. “We suspend the particles in a flame,” explains Bill Yearsley, president and CEO of Terra CO2. “It’s a dust cloud at that particle size, so a good portion of our trade secrets are around what we call ‘cloud management.’”

The material “cloud” is melted in milliseconds inside the reactor, resulting in glassy particles that are quenched with ambient air. The resulting reactive microspheres can replace flyash in concrete formulation.

“The materials we use to make Opus SCM are widely available at scale, so they don’t require extensive transportation to deliver to concrete makers, and don’t require opening new mines,” Yearsley points out. The product would be delivered to a silo at a ready-made concrete site, similar to how flyash is currently used.

Using the same manufacturing process, Terra CO2 plans to introduce a blended cementitious material (BCM) that will be used with an additive at levels of 40% or higher in cement, and eventually, a geopolymer that can be a full replacement for portland cement, Yearsley says.