Introduction to Research: Pumice as ASR-Reducing SCM
In 2012, Hess Pumice contracted with the University of Utah’s Department of Civil and Environmental Engineering (Concrete and Materials Research and Evaluation Laboratory), to conduct a series of tests using two grades of carefully refined pumice to determine and quantify its effectiveness4 as a concrete pozzolan.
This quantitative analysis was initiated for several reasons:
1 • The on-going shortcomings of fly ash as a consistent and effective pozzolan (in terms of color, preferred-type availability, contaminant levels, and performance)2;
2 • The desire for hard numbers on the pozzolanic performance of the Hess pumice deposit4 in particular, and a definitive confirmation in general of the old-world Roman practice of using pumice as an incredibly effective concrete pozzolan;
3 • The green building industries’ need for a clean, natural SCM that improves the performance, strength, and effective lifespan of concrete and does so while replacing a portion of the needed Portland cement—produced via a very energy-extensive process—thus bringing this critical and extensively-used building material in line with responsible green building goals3.
4 • As an expansion on the pozzolanic component of a research and development effort by the U.S. Department of Energy (Sandia National Laboratories) and Atomic Energy of Canada (Whiteshell Laboratories)1 that developed a cementitious grout with the requisite strength, density and durability to seal the microfractures deep underground in the salt rock surrounding the confinement chambers of the DOE’s Waste Isolation Pilot Plant in New Mexico. This ultrafine, highly effective cement-based grout specified the use of pumice from the Hess Pumice deposit in southeast Idaho as the performance-critical pozzolanic component. (This patented grout is produced commercially by US Grout and available through and supported by Avanti International)
SEE ALSO: 1 Key Excerpts from the Whiteshell Laboratories report (PDF) | 2 Pumice Pozz vs. Fly Ash (PDF) | 3 Whitepaper: Pumice and LEED Certification (PDF)
A WELCOME SURPRISE: That University of Utah research yielded the expected results in terms of pumice being highly effective at improving concrete performance, but it also produced some unexpected data: in the presence of reactive aggregate, the Hess pumice absolutely flatlined the alkali-silica reaction. Not unexpected in that pumice (like most pozzolans) had an attenuating effect on ASR, but rather unexpected in terms of just how effective it was in mitigating, even eliminating the alkali-silica reaction.
This impressive level of ASR mitigation launched, at the University's request, a follow-up study5 that focused specifically on the use of Hess Pozz as a specialty mitigation agent to combat the plague of ASR. The data from that completed study (and others, like U of Texas Austin7) provided the validation to launch an ASR mitigation product: ASR Miti•Gator™
SEE ALSO: 4 University of Utah (Phase 1) Pumice Pozz Research Summary (PDF) | 5 University of Utah (Phase 2) Research Report Summary on Mitigating ASR Using Pumice (PDF) | 7 SlideDeck: The Potential of Natural Pozzolans to be a Class F Fly Ash Replacement in Concrete (PDF)
EXPANDING RESEARCH & VALIDATION: These findings have in turn spurred additional research by others as the quest for long-lived concrete continues. Of particular note, the Texas DOT commissioned a study by the University of Texas-Austin (beginning in 2012) to evaluate alternative SCMs in response to game-changing distruptions in the fly ash marketplace. The pumice sourced for this study was Hess Standard Pozz. The results from these comprehensive tests (results released to the public in January 2015) validated the research done by the University of Utah, particularly in terms of ASR mitigation. The U of Texas-Austin research included two tests for ASR mitigation in concrete—ASTM C1567 (accelerated mortar bar tests), and the 24-month-long ASTM C1293 (concrete test prisms). These tests showed pumice-blended cement (at 15% of replacement) delivered results well below the 0.04% ceiling set by ASTM as the acceptable limit).
To date, there have been few publications documenting the usage of pumice as a SCM in concrete—and no publications prior to this research about the usage of pumice to specifically arrest ASR expansion. Industry focus has been on utilizing (and sequestering) waste and byproduct materials in concrete. But the toxicity, performance shortcomings, restrictions in availability, and sustainability concerns with these types (like fly ash) of supplementary cementitious materials (SCMs) have spurred intense interest in clean, naturally-occurring pozzolans.
The research summarized below is the first to focus specifically on the effect pumice has on mitigating ASR. Specifically:
1 • Why, exactly, pumice is effective at short-circuiting ASR;
2 • How much pumice (optimum cement replacement level) is needed to mitigate ASR to acceptable expansion limits; and
3 • the effect pumice-blended cement mix designs have on other key concrete performance and durability characteristics—hydration kinetics, compressive strength, set time, sulfate and chloride resistance, placeability.
ASTM C1567 “Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)” and a modified ASTM C1293 on mortar and concrete specimens, respectively, were used to study the effectiveness of pumice in mitigating ASR.
CALCIUM HYDROXIDE (CH) CONFIRMED AS AN ESSENTIAL ASR TRIGGER: Calcium Hydroxide is a deleterious byproduct of the Portland cement hydration reaction. Not only does CH hurt concrete durability, it is also one of the four components that must be present to trigger an alkali-silica reaction. Starving concrete of any one of these components prevents ASR.
Research by others (Kilgour, 1988; Thomas, 2001, 1998; Wang and Gillott, 1991; Diamond, 1989; Chatterji, 1979) confirms that limiting calcium hydroxide (CH) arrests formation of expansive alkali-silica gel.
During the span of this research, in particular, ASR expansion and amount of CH present in test samples were compared—using Thermo-gravimetric (TGA) and X-ray diffraction (XRD) analysis—to understand and quantify the role of CH in the alkali-silica reaction. It was found with less CH in test samples produced less expansion compared to the samples with higher CH content at similar age. Hence, the anticipated reason for pumice success in mitigating ASR expansion is the reduction of CH content across time through pozzolanic reaction.
PUMICE IS A SUPPLEMENTARY CEMENTITIOUS MATERIAL (SCM) THAT CONSUMES CALCIUM HYDROXIDE: Research was conducted using the pumice from the Hess Pumice deposit in southeast Idaho. This pumice is used for the ASR Miti•Gator™ product.
Tests showed that this pumice consumed enough CH to flatline ASR, even in the presence of highly reactive aggregate. Because the pozzolanic reaction ignited by pumice in curing concrete consumes CH, the reactivity level of the aggregate was not a factor in the effectiveness of pumice as an ASR mitigator.
Low-alkali cement is not necessary in the concrete mix design, as the pumice both attacks the CH directly and reduces the alkali levels in the concrete pore solution.
THE POZZOLANIC REACTION REPURPOSES CH: Beyond simply mitigating ASR, this pumice also contributes to other factors that improve concrete durability.
Pumice reacts with the calcium hydroxide and water present in curing concrete to form additional calcium-silica-hydrate (CSH). CSH is the binder that makes concrete what it is. The more CSH present, the more tightly welded the cured concrete matrix, contributing to desirable concrete performance factors such as impermeability, strength, and durability.
HIGH SULFATE RESISTANCE: This enhanced resistance to concrete distress mechanisms includes sulfate attack. Following the procedures of ASTM C1012, all tested pumice-enhanced mix designs qualified as HS (High Sulfate resistant). Testing done by the University of Texas at Austin7 also found ASR Miti•Gator™ imparted high sulfate resistance.
This pumice-induced densification can reduce further moisture intrusion, which shields against freeze-thaw and further mitigates ASR expansion. It also means that in marine and de-icing environments, pumice-blended cement concrete is more resistant to chloride intrusion.
USE OF PUMICE ELIMINATES NEED FOR LOW-ALKALI CEMENT OR NON-REACTIVE AGGREGATE: Current and past ASR mitigation efforts involved reducing the alkali content by using low-alkali cement, and/or avoiding reactive aggregate (or sweetening it with limestone or blending in a non-reactive aggregate). ASR Miti•Gator, in a sufficient percentage-replacement of cement, works directly on reducing deleterious CH, eliminating the need to reduce the alkali or avoid reactive (siliceous) aggregate.
HOW MUCH PUMICE? As a percentage of cement replacement, as little as 10% pumice (finest grade/NCS 3/Hess UltraPozz) was needed to reduce ASR expansion to acceptable limits. ASR Miti•Gator™ (DS325 grade) was found to be fully effective at 20% replacement, even in the presence of highly reactive aggregate.
The necessary amount of pumice needed to produce the maximum acceptable expansion level can be determined based the reactivity level of the aggregate, but mortar bar test measurements clearly showed that even a small amount of pumice (5%) at any grade had a significant effect on ASR expansion compared to the 100% cement samples.
COMPRESSIVE STRENGTH: Compressive strength analysis of pumice-blended cement mix design samples show that all samples exceeded the 4000 psi (280 kg/cm2) design strength levels at 28 days, with none testing below 4800 psi.
Compressive strength repeatability tests were then run using a different cement source, different sand and aggregate sources, even a different mixer type. Low variations between test cylinders was observed. Conclusion: the addition of up to 20% pumice does not significantly change the measured strength of concrete.
HYDRATION KINETICS: Measured using an isothermal heat conduction calorimeter, the heat generated by the exothermic hydration reaction of pumice-blended cement was found to be less than 100-percent Portland cement. This makes pumice-blended cements advantageous in mass concrete placements.
WATER REQUIREMENT: A higher water-to-cement ratio is necessary in pumice-blended cement mix designs due to the porous nature of pumice and, in the ultrafine grade, an increase in surface area. Common water-reducing admixtures are used to maintain optimal slump for placement and/or reach setting time equivalents of 100% ordinary Portland cement (OPC).
SETTING TIME: Without admixtures, there is an increase in initial and final setting time for mixtures containing pumice. These increases are well within the limits of ASTM C595; setting-time increases are attributed to the increased water demand.
PUMICE GRADES: While an ultrafine grade of pumice pozzolan (Hess UltraPozz) showed the best grade-to-quantity-needed ratio (due to its greater surface area), the use of the DS200 and DS325 (ASR Miti•Gator™) grades at 20% of the total cementitious material produced excellent durability characteristics and the required strength gain, while flatlining ASR.
Hess UltraPozz is a very fine grade (particle size 4 times finer than ordinary Portland cement). Results show Hess UltraPozz mix designs (80C20Ultra) delivered the top results in testing for sulfate resistance, initial set (high-early strength), hydration, compressive strength, and ASR resistance.
PUMICE AS A POZZOLAN: When exposed to the saturated lime, the disordered alumino-silicate structure of pumice does not remain stable. This instability in the presence of lime is the basis for the pozzolanic property of pumice (and other volcanic glasses).
UNIVERSITY OF TEXAS - AUSTIN. The Texas Department of Transportation went to the University of Texas at Austin for a look at the effectiveness of natural pozzolans, like pumice, as replacements for Type F fly ash for the critical task of improving the long-term durability of concrete. The objective was to identify and specify which natural materials provide the necessary concrete-enhancing and ASR-mitigating performance properties. Based on previous research done using pumice from the Hess deposit (DS325 grade standard pozz/ASR Miti•Gator) the researchers requested testing samples. The results returned by the research7 found concrete mix designs using ASR Miti•Gator to be very effective in resisting sulfate attack (Sulfate Exposure Qualification: Class 3 severe environment exposure), developing needed compressive strength, mitigating alkali-silica reaction (well below the 0.04% expansion limit at 24 months), and chloride ion penetration tests (at 32 weeks) indicated very low penetration.
SEE ALSO: 7 Research Summary: Summary of Research Evaluating Supplementary Cementitious Materials (U of TX-Austin) (PDF)
GEORGE WASHINGTON UNIVERSITY (ST. LOUIS). Commissioned by Nippon Electric Glass America, a multi-national corporation and major supplier of materials and expertise to manufacturers of Glass-Fiber Reinforced Concrete (GFRC) panels, GWU St. Louis ran tests using Hess Ultrapozz™ (a more finely processed grade of the ASR Miti•Gator™ product) to determine performance of pumice as an effective SCM in the concrete mix designs containing reactive glass fiber reinforcement6. Per the results of this study and the work done by the University of Utah, NEG America subsequently became the exclusive distributor of ASR-mitigating, performance-super-charging Hess Ultrapozz™ to the GFRC industry worldwide.
SEE ALSO: 6 Whitepaper: Ultrafine Pumice Pozzolan for Glass Fiber Reinforced Concrete Products (PDF)
UNITED STATES BUREAU OF RECLAMATION. The Bureau has initiated (6/2015) a series of tests to certify the use of pumice (in particular, the ASR Miti•Gator product) as an alternative/replacement option for current specifications that require lithium silicates and fly ash as SCMs to mitigate ASR and enhance concrete durability in US regions with reactive aggregate.
Lithium, while proven effective in mitigating ASR, has some significant drawbacks—primarily initial cost, which is too often exacerbated by the unpredictable performance of lithium in wet concrete, causing batch rejections when the concrete fails pre-placement tests on site. Also, prescriptive specifications for the use of lithium compounds in concrete are, by necessity, complicated. A variety of national and local specifications are in use and continue to be developed, but generally vary from being either totally prescriptive, or vary in the specification of performance testing. To complicate matters further, lithium is generally specificed to be used in concert with fly ash, which has performance-consistency issues of its own.
It is fully anticipated that the Bureau's testing of ASR Miti•Gator will track with previous research-quantified results and so provide a clean, cost-effective, simple ASR-mitigating solution for future Bureau of Reclaimation projects involving concrete placements in reactive-aggregate regions.
TECHNICAL: Brian K. Jeppsen, VP - R&D
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