Sourcing Class F Fly Ash a Real Concern

“In recent years…the future availability of fly ash in the US has become a source of concern because of impending environmental regulations from the US Environment Protection Agency (EPA). Two proposals, one known as a Subtitle C classification of the Resource Conservation and Recovery Act (RCRA), would regulate coal combustion residuals (including fly ash) as a hazardous waste. A second proposal, known as a Subtitle D classification of RCRA, would consider coal combustion residuals as non-hazardous, but would enforce a higher minimum standard for CCR disposal [with] the enforcement of rules under Subtitle D left up to the states. Regardless the IPA has maintained that fly ash can still be used in concrete due to the ‘beneficial use’ exemption, which permits the use of fly ash when completely encapsulated. However, the rising cost of fly ash associated with these environmental rulings will most likely make the use of fly ash in concrete prohibitive.

“Additionally, environmental regulations, like the Clean Air Interstate Rule and Cross State Air Pollution Rule, that aim to reduce air pollution have forced coal-burning power plants to adopt emission reduction techniques that have consequently led to a lower quality of fly ash.

“As these changes in the coal power generation industry are causing considerable uncertainty for the future availability and quality of fly ash, it becomes imperative to identify and test other SCMs that can provide similar strength and durability benefits to concrete as Class F fly ash.” [ RETURN ]

—from page 1 of “Evaluating the Performance of Alternative Supplementary Cementing Material in Concrete” by Seraj, Cano, Liu et al.; University of Texas at Austin, Center for Transportation Research

Full UofTX-Austin Report: “Evaluating the Performance of Alternative Supplementary Cementing Material in Concrete”

Comparisons with and performance data on eight natural pozzolans is available in the original U of Texas-Austin/ Texas DOT report: “Evaluating the Performance of Alternative Supplementary Cementing Material in Concrete” by Seraj, Cano, Liu et al.

REPORT ABSTRACT: Uncertainty in the supply of Class F fly ash due to impending environmental restrictions has made it imperative to find and test alternate sources of supplementary cementitious materials (SCMs) that can provide similar strength and durability benefits to concrete as Class F fly ash. This project summarizes the key findings of research that was conducted to characterize and evaluate the performance of eight natural pozzolans, commercially available in Texas, to assess their potential as Class F fly ash replacements in concrete. Of the eight pozzolans tested, six were found to be viable alternatives for Class F fly ash. Methods to further enhance the performance of these SCMs were explored and guidelines are provided on the optimum SCM replacement levels for different applications. Finally, recommendations are presented on how to improve current testing practices for SCMs. [ RETURN ]

Download the entire Uof TX-Austin report (PDF file).

Download a pumice-vs-fly ash-centric summary of the Uof TX-Austin report (PDF file).

The Pozzolanic Reaction

The understood science behind the pozzolanic reaction is this: the particle-binding glue of concrete—Calcium Silica Hydrate (CSH)—is the result of combining water and Portland cement. But that same hydration reaction also produces Calcium Hydroxide (CH) by-products (up to 25% of the hydrated Portland cement) that not only do nothing to contribute to concrete strength and density, but actively work against it. This CH-induced porosity introduces a host of ills...poor strength, weak resistance to chemical attacks, high permeability, and thus, shorter life.

Replacing a portion of the Portland cement with pumice ignites a pozzolanic reaction within the hydrated paste that, via a molecular-level reclamation process, reacts to and melds with the trouble-causing CH, ultimately converting it into additional CSH. This consumptive transformation of the CH mitigates or completely eliminates the problems it spawns. And that newly-created CSH does what you’d expect: it further densifies and strengthens the concrete, welding the grout particles into a dense, durable, virtually impermeable matrix. [ RETURN ]

Pozzolan Defined by ASTM C 125

“A pozzolan, as defined in ASTM C 125, is ‘a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of water, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.’ ASTM C 618 is the standard specification for natural pozzolans, and classifies them as ‘Class N’ SCMs on the basis of several composition and performance indices, such as oxide composition, fineness, strength, and water requirement. ASTM C 618 gives some examples of natural pozzolans or ‘Class N’ SCMs such as pumice, volcanic ash, clay, shale, and diatomaceous earth. Each of these materials is discussed in detail in the upcoming sections.” [ RETURN ]

—from page 3 of “Evaluating the Performance of Alternative Supplementary Cementing Material in Concrete” by Seraj, Cano, Liu et al.; University of Texas at Austin, Center for Transportation Research

The Body of Research on Hess Pozz as a Concrete-Improving Pozzolan

More than a century ago pumice was first identified as the key ingredient in that impressively durable Roman concrete. However, modern markets didn’t demand investment in a high-quality grade of concrete, and many of the problems inherent in ordinary Portland cement were poorly understood. By the time the durability failures of Portland cement were evident and the need for a concrete-improving pozzolan was (re)understood, a cheap by-product pozzolan, in the form of coal combustion residuals (fly ash), was readily available.

When the first rumblings of trouble concerning a possible hazardous-waste classification for fly ash (adding to the sourcing problems resulting from increasingly restrictive regulations for burning coal) began to ripple through the marketplace, Hess Pumice initiated a research study at the University of Utah to quantify the performance of their pumice as a pozzolanic supplemental cementing material (SCM) for concrete. Backed by both research into Roman concrete as well as the selection of Hess' pumice by the Department of Energy's Sandia National Laboratories as the ideal pozzolanic component in an ultrafine cementitious grout (developed to seal the waste containment chambers deep underground at the WIPP site in New Mexico), Hess Pumice knew the pumice in their deposit could meet the need for a clean, economical, highly effective pozzolan. The results of the two-year study at the University of Utah detailed the significant improvements Hess Pozz made to concrete.

Follow-up research by the University of Utah focused on and further quantified the impressive data showing how completely effective pumice is at mitigating the alkali-silica reaction (ASR), even in the presence of the most reactive aggregates.

About this same time, the Texas Department of Transportation commissioned a research study (by the University of Texas at Austin) to evaluate the effectiveness of commercially available natural pozzolans as replacement options for problematic fly ash. Pumice from the Hess deposit was chosen for this study—and performed impressively in every category, proving to be the ideal all-around pozzolan.

Nippon Electric Glass America, a major player in the Glass-Fiber Reinforced Concrete (GFRC) industry, also commissioned research on the use of Hess Pumice as a pozzolan—and subsequently became the exclusive distributor of Hess UltraPozz to the GFRC industry.

Download the Pumice Pozz vs. Fly Ash slidedeck (contains the research table and chart data).

Whitepapers, research summaries and copies of the original research can be downloaded from the Hess Pumice website or the Hess Pozz website. ASR-focused information can be found on the ASR MitiGator™ website. [ RETURN ]

The Volcanic Origin of Pumice

The word pumice is derived from the Latin word pumex, meaning foam.

Pumice is a textural rock formed from volcanic eruptions. Deep underground, molten rock incorporates water and other gases, and when the magma erupts from a vent, the gases and water flash off, leaving behind a frothy, vesicle-riven structure that quickly cools, solidifying the foamy structure. The magma has now transformed into an amorphous aluminum silicate, or pumice.

Pumice is found in pyroclastic flows or accumulated into drifts, piles and banks (deposits, in mining terms) by wind or wave action.

If the newly formed pumice falls on or erupts beneath a body of water, wave action will deposit the floating mass on or near the shore as it saturates and sinks. This centuries-long process results in a very pure pumice—the heavy minerals dropping to the sea or lake floor, while the purified pumice is washed to the shoreline. This is the case with the pumice deposit (see below) that is sourced by Hess Pumice Products in Southeast Idaho. [ RETURN ]

Naturally Calcined Means Green

The “green” tag is generally given to products and processes that are not energy intensive or do no harm to the environment. That is, of course, a very basic definition...the term “green” is now a widely applied marketing term for just about anything.

Pumice pozzolan is “green” by the simple fact that the most crucial step in the manufacturing process—the superheating that gives pumice its amazing pozzolanic properties—has already been taken care of by Mother Nature. Pumice is “naturally calcined,” by virtue of the fact that it has already had it’s time in the furnace: a volcanic furnace. Unlike metalolin, pumice does not need to be heated or calcined to change its chemical makeup to make it useful as a pozzolan, nor is it the by-product of an industrial furnace process, like silica fume and fly ash.

Contrast, for example, Portland Cement—the key ingredient for concrete. Portland Cement is manufactured by mining limestone, crushing it, and then (critical!) heating it in a furnace to change its physical properties. The resulting “clinkers” are then ground down to about -325 mesh. The result is ever-so useful cement powder. So, in terms of a carbon footprint—the energy needed to produce the final useful product—Portland Cement, is a major concern of industry and governments alike. Cement production constitutes a major portion of all major green house gases production. [ RETURN ]

Download a Whitepaper: LEED Certification and Pumice Products (PDF file).

Mining Pumice

The various commercial pumice producers use their own variants on the process, but generally, a pumice deposit is surface-mined, meaning the overburden of soil covering the pumice is pushed aside and stockpiled for later reclamation. The pumice is then scraped up using bulldozers and pushed to the crusher for preliminary processing and stockpiling for loading and hauling to the plant for processing. When weather permits, the mining goes on year-round, otherwise, sufficient quantities are stockpiled for refining during winter months.

Pumice is then typically dried to the necessary moisture content ideal for refining before it is lifted to the top of the plant structure and via gravity, begins its journey down through the crushers, screens, and separators until it is finally bagged in sacks or bulk bags or loaded in bulk container cars or blown into pneumatic transport trailers or pneumatic rail cars. [ RETURN ]

The Hess Pumice Deposit

In the southeast corner of the State of Idaho, 23 miles northwest of Malad City, lies a vast reserve of white, pure pumice that is in demand all over the world. That demand is the result of two factors: the quality and brightness of the pumice, and the company that mines and refines it: Hess Pumice.

The pumice deposit is located on the shoreline of an ancient lake known as Lake Bonneville...a vast, freshwater lake that once covered much of North America’s Great Basin region (most of Utah and parts of Idaho and Nevada). The Great Salt Lake is all that currently remains of Lake Bonneville.

The volcano that produced the pumice is about a mile to the north of the Hess Pumice mine. The volcanic ash (pumice) was deposited in the lake, where it was washed and stratified. This process cleaned the pumice of the undesirable heavy minerals that are often found in other pumice deposits.

In 1958, a local farmer, frustrated by his inability to grow anything in the thin soil over what is now the Wright’s Creek Area pumice mine, leased the ground to Marion Hess, who ripped up the white volcanic rock and sold the crushed pumice to a building block manufacturer in Salt Lake City. From those beginnings, the now carefully refined, pure and white pumice from the Hess mine is in demand by industry worldwide. [ RETURN ]