Ultrafine Fly Ash

A class of highly reactive pozzolans offers more durability for aggressive environments.

By Kevin Foody
Photos courtesy Boral Material Technologies Inc.

Today’s concrete consists of various chemical and mineral admixtures to meet the rising demands for constructability, service life and performance. In addition to portland cement, supplementary cementitious materials such as fly ash, slag and silica fume have become commonplace in concrete, particularly in situations where the concrete is exposed to aggressive environments. More recently, engineered pozzolans have been introduced that give early age reduction in permeability similar to silica fume, while contributing to long-term refinement of pore structure and reduction in porosity. These products have been termed “highly reactive pozzolans” and include ultrafine fly ash, metakaolin, rice husk ash, zirconium fume and others.

An ultrafine fly ash supplied by Boral Material Technologies Inc. is being used in the reconstruction of the Interstate-10 bridge over Lake Pontchartrain in New Orleans. The bridge was severely damaged during Hurricane Katrina. Gulf Coast Pre-Stress located in Pass Christian, Miss., is the supplier of the high-performance precast concrete for the project and chose to use an ultrafine fly ash due to its ease of handling and the cost savings.

Ultrafine fly ash is a processed ash generated from a Class F fly ash source. The parent ash is passed through a classifier where the coarse particles are removed and the fines are collected and stored separately. While the average particle size of raw or unprocessed ash is around 20 to 30 microns and the largest at 100 microns, an ultrafine fly ash has a top size of 10 microns and an average particle size of 2 to 4 microns. It is this finer size that provides the reactive surface area that generates the higher early strengths and lower permeabilities in the concrete mix.

When developing the mix for the I-10 project, Gulf Coast Pre-Stress had two requirements: Generate enough early age strengths to allow quick removal of the forms to keep production moving along, and develop the specified 56-day permeability of less than 1,000 coulombs. The final mix that was developed generated the 24-hour strength required for form removal, and the 56-day permeability was met within 28 days.

While the performance of an ultrafine fly ash is seen in the first days, the additional benefits and reactions are not complete. The ultrafine fly ash continues to increase the strengths and decrease the permeability as the concrete continues to cure beyond 28 and 56 days. This benefit was shown on the I-10 project through additional testing as the permeability dropped on some of the samples to as low as 150 coulombs.

In addition, the use of an ultrafine fly ash for mitigating the adverse effects of marine salts has been studied extensively. One recent study found that ultrafine fly ash improved the permeability and chloride diffusion rate, along with improving the abrasion resistance of concrete exposed to magnesium chloride salt applications. Other long-term ponding tests have shown a 75 percent to 90 percent reduction in diffusion when measured at one year and beyond compared with cement-only mixes. Several state DOT offices have specified the use of ultrafine fly ash in conjunction with other pozzolans and/or corrosion inhibitors on projects located in aggressive environments.

Ultrafine fly ash imparts many of the desirable hardened concrete traits associated with highly reactive pozzolans without increasing workability and placement demands. In fact, ultrafine fly ash reduces the water demand and superplasticizer demand compared with cement-only mixes. No special batching, transportation or placement and finishing operations are required with the use of ultrafine fly ash. Recent research (“Influence of Ultrafine Fly Ash on Shrinkage and Cracking Tendency of Concrete and the Implications for Bridge Decks,” Transportation Research Board Annual Meeting 2007 Paper #07-0022) has shown ultrafine fly ash to provide additional resistance against early age stress development and restraint cracking. This makes ultrafine fly ash particularly attractive to applications such as bridge decks, overlays, parking garages and pavements.

When looking for a longer-lasting structure in a marine or aggressive environment, low permeable concrete is important. If the concrete has a crack, the effective cover over the reinforcing steel is reduced to the distance from the bottom of the crack to the reinforcing steel. A recent study by the University of South Alabama (“Influence of Ultrafine Fly Ash on the Early Age Response and the Shrinkage Cracking Potential of Concrete,” Journal of Materials in Civil Engineering 2005, Vol. 17, No. 1, pages 45-53) indicated that the propensity of autogenous and plastic shrinkage cracking is reduced when ultrafine fly ash is used.

When putting together a concrete mix with ultrafine fly ash, it is not necessary to preload the ultrafine fly ash into the mixer. This material does not require any additional or special mixing procedures to ensure that the concrete is homogenous throughout the concrete mix. Ultrafine fly ash can be added along with the normal dosage of fly ash and will disperse throughout the mix.

Typical dosages of ultrafine fly ash will range from 8 percent of total cementitious to as high as 12 percent depending on the particular performance desired. In a high-strength application, the typical additional rate is 8 percent to 9 percent. If low permeabilities are required, then the dosage rate typically ranges from 9 percent to 12 percent depending on how low the permeability target is for the project.

The particle size of the ultrafine fly ash has been optimized so that water can be removed from the mix and slump is still retained. This aspect allows for a lower water-cement ratio and stronger concrete. Less water in the mix also results in fewer bleed channels leading to lower permeability.

Durable concrete is concrete that resists weathering, chemical attack, freeze-thaw abrasion, the ingress of salts and other service conditions. It is the salts that corrode the reinforcing steel. The rusting reinforcing steel expands and causes the concrete to crack. The longer you can keep the salts away from the steel, the longer the service time before cracks form and maintenance is required.

To meet the demands for durable concrete without slowing production, Gulf Coast Pre-Stress chose to use ultrafine fly ash as a highly reactive pozzolan. The required overnight strengths were met and the resulting low permeability was well below the required values.

Kevin Foody is manager of the Technical Service Eastern Region, Boral Material Technologies Inc. Boral, based in San Antonio, is a marketer of fly ash and all coal combustion products. Contact Boral at (800) 964-0951 or visit www.boralmti.com.