Natural Pumice Pozzolan: the Ideal Replacement for Fly Ash in Concrete Mix Designs

THE TROUBLE WITH FLY ASH

The future of fly ash as a readily available and quality-effective concrete pozzolan is murky and troubled. As a concrete pozzolan, fly ash was never ideal. But it was cheap (the by-product of burning coal) and, within a restricted class type (Class F), effective. The knocks against fly ash—ever-growing environmental regulations that impact suitability, cleanliness, color, inconsistent performance, and especially availablity—pushed the concrete industry to find and quantify better alternatives [01]. The writing-on-the-wall fly ash supply shortage has now become a reality as coal-fired power plants are being taken offline and scrapped. [02].

01 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.”

—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

02 2020 to 2070 and Beyond: Transitioning from Production to Post-Production Coal Ash Use: Over the last 50+ years, production coal ash has successfully transitioned from a “resisted use” product to a “required component” in cement mixtures to be used in infrastructure material.

The market is now witnessing the beginning of the end of one era and the genesis of a new economic reality. The “21st century commercial paradigm” of the construction and building products industry is already evident in regional production coal ash supply shortages around North America, the United Kingdom, and Europe. Additional demand is forecast over the next 20 years with the significant infrastructure upgrades required in most countries. Combined with new infrastructure requirements in fast-growing countries like China and India, this will create significant pressures on the collective coal ash market’s future.

Gone are the days of readily available excess, compliant production coal ash. In all major markets, coal is being displaced in favor of lower-cost and less-carbon-emissions-intensive fuels. The UK and an increasing number of EU member states will eliminate coal by 2025, and Canada will follow by 2030. It is further evident that the U.S. will continue to see accelerated, ahead-of-schedule coal plant retirements.

As a result, what is old becomes new again. As with the formulas developed during the Roman Empire circa 312 BC to 500 AD—which used volcanic ash as a natural pozzolan in the construction of some of its most durable structures—new formulas will need to be considered in the search for a low-carbon reality for the 21st century’s infrastructure buildout and refresh.

—Ash at Work, Issue 1, 2019; Bruce Sifton, P. Eng., and Brad MacKenzie

RESEARCH TO FIND THE IDEAL NATURAL POZZOLAN

Over a two-plus year period, at the behest of the Texas DOT, the University of Texas at Austin conducted an extensive suite of tests to determine alternative SCMs as alternatives to using fly ash as a pozzolan in the state’s concrete transportation infrastructure [03]. The data from that study, specific to pumice (sourced from Hess Pumice Products and available commercially as Hess Standard Pozz), is summarized and graphed below. The research team also ran the same tests with Class F fly ash, providing a performance baseline with which to compare to pumice-enhanced and fly ash-enhanced concrete. Those comparative tests are the basis of this pumice pozz vs. fly ash site.

03 The University of Texas-Austin/Texas DOT report titled “Evaluating the Performance of Alternative Supplementary Cementing Material in Concrete” (by Seraj, Cano, Liu et al) provides comparisons with and performance data on eight natural pozzolans.

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.

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 CONCRETE PERFOMANCE CATEGORIES THAT MATTER

Concrete is such a vital part of civilization’s infrastructure, yet ordinary Portland cement (OPC) is not nearly good enough for the job. To last beyond a few decades and withstand attacks from chloride ions (deicing salts, seawater), sulfates, freeze-thaw, alkali-silica reaction (a slow-motion shattering of concrete from within), heat of hydration cracking (which amplifies the above) and other ills, modern concrete needs the benefit of a secondary chemical reaction [04] during the curing process, a reaction that supplements and amplifies the primary water-cement hydration reaction. A reaction that is triggered by the introduction of an additional material—a supplementary cementitious material—into the concrete mix design. A pozzolan [05]. The right pozzolan.

04 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.

05 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.”

—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

NATURALLY CALCINED PUMICE AS A POZZOLAN

Pumice as a concrete pozzolan is nothing new—the Romans were using naturally calcined pumice [06] (they first sourced pumice from Pozzuoli in Italy, which subsequently became the origin of the term “pozzolan”) in their concrete infrastructure 2000 years ago.

The role of a pozzolanic SCM is to ignite a reaction within curing concrete that improves performance—by both consuming deleterious compounds (like calcium hydrate) and amplifying concrete characteristics like density and resistance.

Unlike fly ash, a by-product scrubbed from the stacks of coal-burning power plants, pumice is a consistent, naturally-occurring mineral that is simply mined, refined (crushed), and packaged for shipment [07].

The research from the University of Texas-Austin (as well as the University of Utah and others) has fully quantified the super-charging performance pumice lends to modern concrete formulations.

06 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.

07 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.

CONSISTENCY MATTERS

When it comes to pouring hundreds of yards of concrete, consistency is a big deal. Consistency in fresh properties and long-term performance, load after load. Consistency in availability. Consistency in chemical make up. Consistency in supply-chain availability. Research data [08] in pumice-blended cement mix designs quantifies a performance consistency that fly ash rarely can.

08 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.

ECONOMICS

Pumice pozzolan is used as a percentage of replacement—as little as 15% will give an impressive performance boost across the entire spectrum of key categories—and so the cost is off-set in part by cement savings. And, by virtue of its volcanic origin, pumice is naturally “green.” [09]

09 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.

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

A UNIQUE PUMICE POZZOLAN

All pumice is not created equal [10]. The pumice used in the referenced research and available commercially as Hess Standard Pozz is sourced from a unique pumice deposit in Southeast Idaho, USA—recognized as the purest commercial deposit of white pumice in the world—the result of volcanic ash landing on a prehistoric mountain lake, washing out the heavier impurities, leaving behind a deposit of 99% pure pumice. Its unique physical and chemical characteristics are what make this carefully refined pumice an excellent pozzolan.

10 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.

CONSIDERING THE SOURCE: RELIABILITY AND SUPPLY CAPACITY

Concerning reliability: Hess Pumice Products was founded in 1958 and has been in continuous operation for over 6 decades. As a result of attention to detail and a philosophy of “On-spec, On-Time”, Hess Pumice has become a major player in the natural pozzolan and pumice industries. Through advanced grinding and blending technologies Hess currently has over 100 products that ship to 26 countries around the globe and has a reputation for superior quality, logistical expertise, and unmatched customer service.

With regards to capacity, Hess is currently producing and selling ~20,000 tons per year of quality natural pozzolan (Hess Standard Pozz) with the ability to expand in order to meet future market demands.

PERFORMANCE DATA: PUMICE VS. FLY ASH

COMPRESSIVE STRENGTH

Twelve 4-in. x 8-in. cylinders were cast for compressive strength testing at 7, 28, 56, and 90 days. At the appropriate ages, three cylinders were removed from moist storage and tested in a Forney FX-700 compression machine according to ASTM C 39. End caps per ASTM C 1231. Average compressive strength was calculated from three cylinders.

RESULTS: The pumice pozzolan concrete mixture gained strength slowly at first, reaching 95% of the control strength at 90 days with a 15% replacement and 99% of the control strength with 25% replacement. This is similar to the trend seen during the mortar studies (below). It should be noted that although the pumice SCM specimens gained strength slower than the control and the Class F fly ash specimens, at 28 days scored strengths greater than 4500 psi.

compressive strength of pumice SCM in concrete at 7, 28, 56, and 90 days

Mortar Bar Studies—Shows the compressive strength at a w/cm of 0.5 and 20% SCM replacement. Except for the (slightly higher) 3-day strength of pumice pozz, all ranges are within the limits prescribed by ASTM C109.

RESULTS: The pumice pozzolan and fly ash mortar mixtures gained strength slowly at first, but by 28 days had surpassed control (OPC). At 365 days, the pumice-enhanced specimens had significantly higher compressive strength, proving the long-term strength and durability benefits of using a pumice SCM.

compressive strength of pumice SCM in mortar bars at 1 year

FRESH STATE PROPERTIES OF CONCRETE MIXTURES

Concrete slump was measured according to ASTM C 143. The air content of the fresh concrete mixtures was measured according to the pressure method described in ASTM C 231. The unit weight of the mixture was found using the procedures described in ASTM C 29. The setting time of concrete mixtures was found using the procedures of ASTM C 403. In addition to the penetration resistance tests, ultrasonic tests were investigated to continuously monitor the setting process on concrete samples and sieved mortar samples. (This study aims to develop a field applicable nondestructive testing method for in-situ monitoring of the setting and hardening process of concrete.)

RESULTS: The pumice SCM concrete mixture was able achieve the target slump with the help of a superplasticizer and achieved final set in 3.4 hours, compared to the final set of the control at 4.5 hours.

fresh state properties of pumice-enhanced concrete

MITIGATING ALKALI SILICA REACTION

Resistance to ASR was measured according to the procedures of ASTM C 1293, except for the concrete mixture design used, which is detailed in the MIXTURE DESIGN table below. The average expansion for each mixture was calculated from three or more bars, and the range was checked to see whether it was within the limits stated in ASTM C 1293.

RESULTS: The pumice SCM concrete mixture performed very well, and kept expansions below the 0.04% limit of ASTM C 1293, validating the results found from the ASTM C 1567 Accelerated Mortar Bar Test for ASR. The table below lists the average expansion of the concrete prisms at 24 months, along with the range of the data.

pumice is an effective supplementary cementing material for mitigating alkali-silica reaction

RESISTANCE TO SULFATE ATTACK

The yellow dots in the graph represent the “ACI 201: Guide to Durability” limits of Class 1, Class 2 and Class 3 sulfate exposure. An SCM qualifies for a Class 1 mild sulfate exposure if it can keep expansions below 0.1% for 6 months, when tested for sulfate attack using ASTM C 1012. Similarly, an SCM qualifies for a Class 2 moderate sulfate exposure at expansions below 0.1% for 12 months. Finally, a Class 3 severe sulfate exposure requires the SCM to keep expansions below 0.1% for 18 months.

RESULTS: The pumice pozzolan qualifies for use in a Class 3 severe sulfate exposure environment at both 15% and 25% replacement levels.

pumice pozz provides concrete with class 3 extreme environment exposure rating

RESISTANCE TO CHLORIDE ION PENETRATION

The chloride penetrability of concrete cylinders that were cured for 32 weeks was measured according to ASTM C 1202. Although the standard does not require repeat testing, three or more cylinders per concrete mixture were tested for rapid chloride penetrability. The range of the data was checked to see whether it was within the limits prescribed by ASTM C 1202.

RESULTS: There are no well-established paste and mortar tests for this measurement, but this durability property is crucial, as the ingress of chloride ions can depassivate the steel in concrete and cause corrosion, without needing a drop in the pH content. At 32 weeks, all the SCM-concrete samples had less than 1000 coulombs of total charge passing through them when tested, which indicates very low chloride ion penetrability, according to ASTM C 1202. The overall results indicate that increasing the SCM content also increased the resistance to chloride ion penetrability.

resistance to chloride ion penetration enhanced with pumice SCM

DRYING SHRINKAGE

The drying shrinkage of the concrete bars was measured according to the procedures of ASTM C 157.

RESULTS: The pumice SCM concrete mixture had negligible differences in shrinkage compared to the control when used at a replacement dosage of 15%. However, the amount of shrinkage increased to more than 0.010% of the control as the replacement dosage was increased to 25%.

drying shrinkage with pumice-enhanced concrete

COEFFICIENT OF THERMAL EXPANSION

The CoTE value was measured according to the TxDOT procedures of Tex-428-A. CoTE measurement is another test that was not performed during the paste and mortar phase of the project. However, it is an important durability property to know, especially when considering the performance of concrete pavements, as concrete with a high CoTE can cause early age cracking and joint spalling. In continuously reinforced concrete pavements, a high CoTE value of the concrete may increase the crack spacing and width, affecting the crack load transfer efficiency. Although CoTE is primarily dominated by the aggregate type and source, the SCM type and content could also have smaller effects on the value. As such, these experiments were conducted to ensure that these pozzolans did not have any detrimental effects on the CoTE value of concrete.

RESULTS: The pumice SCM specimen tested for CoTE showed compliance with Tex-428-A. The difference in CoTE values between the control specimen and the pumice SCM-concrete specimen was small, indicating that the use of pumice as an SCM would not cause any detrimental effects to the CoTE value of concrete.

Coefficient of thermal expansion with pumice-enhanced concrete

CALCIUM HYDROXIDE CONTENT OF PASTES

The calcium hydroxide contents of the pastes were measured to observe whether the pozzolanic reaction was occurring in the SCM pastes.

RESULTS: Graphing the test results shows that the calcium hydroxide contents of the SCM pastes are generally always lower than that of the control with no SCM, suggesting that the calcium hydroxide in the SCM pastes is being depleted through the pozzolanic reaction. In particular, the 7-day calcium of the pumice paste was similar to that of the control paste with no SCM, however, by 90 days, the pumice paste demonstrated a significant reduction in its calcium hydroxide content (20% or more), indicating that pozzolanic reactions are taking place in the mixture. The decrease of calcium hydroxide content in the TGA results also suggested that pozzolans contributed to long-term strength by converting the calcium hydroxide in the paste to C-S-H.

calcium hydroxide content in concrete reduced with pumice pozz

SUMMARY OF PUMICE SCM

summary of enhancements pumice gives concrete

An Ideal Alternative: Pumice Pozzolan Instead of Fly Ash for Concrete Improvement

Extensive ASTM-standard research quantifies powdered pumice as a remarkable concrete pozzolan.