Research Examines Limestone in Concrete

Research Examines Limestone in Concrete

The initial age strength of high volume fly ash concrete can be amazingly increased by the replacement of limestone with a small amount of fly ash.

In a recent laboratory test, it is observed that a concrete mix with 50% cement replaced by the fly ash can perform well as compared to conventional concrete while keeping the carbon savings and cost constant. The performance level is increased by the replacement of 25% fly ash with limestone powder, by volume, or with Type III Portland cement.

Dale Bentz, a chemical engineer at the institute of National Institute of Standards and Technology (NIST), says “A lot of the industry is using 15, 20, even 25 percent fly ash in their mixtures, but this research demonstrates that you can go beyond that and maybe replace 40 up to 60 percent of your cement with fly ash and limestone.”

Research Methodology

The researchers initially prepared nine mixtures: mixtures with 40% fly ash by Volume, Mixtures with 60% fly ash by Volume and control concrete. The fly ash used for this project is of two types i.e. ASTM Class C and ASTM Class F. A mixture with replacement of limestone powder with 25% fly ash is also prepared.

Fly Ash

Fly ash is obtained as a by-product during the combustion process. For commercial purpose, it is mostly obtained from coal burning power plants. The properties of fly ash largely depend on the properties of burning coal. If bituminous and anthracite coal is burned, Class F fly ash forms, while Class C fly ash is obtained from the burning of sub-bituminous coal or Lignite.

“These are two vastly different fly ashes in terms of their calcium oxide contents and also in terms of the way they behave in cementitious systems,” Bentz says. “The F ash is basically inert at early ages and the C ash produces extensive retardation. So it’s really a problematic ash to use.”

Results

After testing the samples, it is observed that the conventional concrete had a strength of 2,870 psi at one day, as compared 3,880 psi compressive strength, of the highest-strength mixture of the research. Where as for 28 days strength, the C fly ash concrete mixture gave strength of 8,840 psi as compared to conventional concrete having value of 6,750 psi. The cost test was also against the conventional concrete. The conventional concrete cost $53.61 per cubic yard where as the fly ash concrete costs $48.41 per cubic yard.

Researchers Discussions

The limestone performs an important role in a concrete mixture, offering clear advantages and synergisms, Bentz notes. “A pert of the cement grains are dissolved, and form a solution, which can later give cement grains from a chemical process,” Bentz explains. “Some of them have increased in their size helping the pores to be filled. The purpose of limestone is to increase the area of precipitation.”

“Limestone Powder serves as a backbone, because basically it is the increase of hydration process, from which we get both initial and final strength of our material,” Bentz says. “The limestone increases the process of hydration by increasing the surface area of precipitation as well as it reacts with aluminates present in the fly ash or cement.”

Keywords: Cement, fly ash, limestone.