Question: Our ready-mix producer is thinking about trying a new lightweight aggregate made from flue gas desulphurization (FGD) sludge. Is it permissible to use synthetic aggregate produced from FGD sludge in lightweight concrete?

Answer: It depends. For those not familiar with the concept, the questioner is referring to material that is made from a waste product of the coal-burning process designed to reduce power plant emissions. Power plants, in many areas of the country, use a forced-oxidation process to reduce the sulfur dioxide concentration of their emissions. In this process, they place limestone into bins through which they pass the outgoing combustion exhaust. In most cases, this aggregate is a very pure limestone, meaning it approaches 95% calcium carbonate.

The stack emissions containing the sulfur residue from the burning process pass through the aggregate bed and the sulfur is absorbed into the limestone. The stone undergoes a chemical change due to the heat and absorption. The final product's chemical composition depends on several factors: the type of coal used in the power plant, the original limestone's chemistry, and the temperature of the bed at the time of absorption. Depending on the efficiency of the scrubbing process, a significant portion of the limestone aggregate is converted to products such as gypsum and, to a lesser extent, calcium sulfite. With so many variables influencing the limestone's make-up, most experts in the field warn that it may not be acceptable for use in ready-mixed concrete as a substitute for other lightweight materials.

Prof. Prasad Rangaraju, a member of ACI 221, Aggregates, and an assistant professor at Clemson University's Department of Civil Engineering, has studied the use of scrubber material in concrete. Rangaraju studied the potential use of scrubber material from a power plant in eastern Iowa, as part of his thesis work at Iowa State University.

Rangaraju's work provides some reasons to avoid using the material as an aggregate. He notes that gypsum, obtained as a by-product of the FGD process, is also a component of normal portland cements. Gypsum is specifically added to portland cement clinker to control the rate of initial hydration and setting time. For these reasons, the amount of gypsum in portland cement is carefully proportioned by the cement manufacturing companies to achieve a desired window in time of setting. Too little gypsum in cement will cause problems such as flash setting (that is, instant hardening of paste); too much could lead to problems such as unsoundness due to late formation of ettringite (delayed ettringite formation or DEF), leading to cracking and failure of the concrete.

In his research, Rangaraju found that addition of scrubber material containing gypsum and anhydrite to portland cement mixes creates an imbalance in the available sulfate for hydration of clinker phases, in particular C3A. This results in excessive formation of ettringite at delayed ages (after final set), causing significant volume change and cracking in concrete. He suspects that the use of synthetic aggregate from the FGD process in normal strength concrete will lead to similar issues.

But there other uses for this material. In many cases it can meet many of the requirements found in ASTM C-33 (coarse aggregate) and ASTM C-331 (lightweight aggregate) and has then been found to be a good aggregate for products like controlled low strength fill, cement stabilized road base, and structural fill for embankments.

For more information on FGD waste material visit the Web site posted by the Turner-Fairbanks Highway Research Center, (TFHRC) a federally owned and operated research facility, and run a search for FGD ( TFHRC is the home of the Federal Highway Administration's (FHWA's) Office of Research, Development, and Technology.