Hess Pozz Flatlines the Alkali Silica Reaction (ASR) for Pennies a Yard

The graph below talks big. But it’s backed up with ASTM standards-research providing definitive data that Hess Standard Pozz (a natural, carefully refined pumice pozzolan) mitigates or eliminates the Alkali Silica Reaction (ASR) in curing concrete in the presence of even the most reactive aggregate. And that “pennies a yard” statement? It’s calculated by combining the savings from using less Portland cement with the cost of the HessPozz replacement.

The dramatic affect pumice has on mitigating alkali silica reaction ASR results of a two-year ASR mitigation concrete prism study The above graphs talk big. But they are backed up with ASTM standards-research providing definitive data that Hess Standard Pozz (a natural, carefully refined pumice pozzolan) mitigates or eliminates the Alkali Silica Reaction (ASR) in curing concrete in the presence of even the most reactive aggregate.

How Pumice Pozzolan Flatlines ASR

The science behind the amazing pozzolanic reaction ignited by pumice pozzolan is this: the aggregate-binding concrete glue—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, introducing grief like the integrity-destroying alkali silica reaction as well as a swarm of other future ills that result from CH-induced porosity.

Replacing some of the Portland cement with natural pumice pozzolan sets off a reaction within the hydrated concrete paste that consumes trouble-making CH and converts it into additional CSH. The pozz also reduces the alkali content of the concrete mix as well as the pH of the concrete pore solution.

Think of it as molecular reclamation: concrete is densified and strengthened while trouble-making CH is consumed and repurposed. ASR is flat-lined.

Also, a major reduction in concrete permeability (porosity) means sulfate and chloride attacks are mitigated. Reinforcing steel is protected without the need to thicken the cover concrete. The water-infiltration freeze-thaw cycle is impeded. Efflorescence is practically eliminated. Compressive strength gets a significant boost.

Benefit after benefit. Structure after structure. Mile after mile.

Alkali Silica Reaction in Brief

There is nothing simple about ASR. Despite the extensive damage it does to the civilization's infrastructure and the years of studies and testing standards and mitigation recommendations, the mechanisms driving the expansive gels that do the damage continue to defy definitive understanding.

Basically, ASR damage results from the meet-up of three essential components: reactive silica (present in the aggregate), alkalies (mostly from the portland cement), and moisture. When these three compenents are present in sufficient quantities, they form moisture-holding gels that relentlessly expand within and around the aggregates, shattering the concrete from within. Long before the symptomatic cracking is visually evident in the concrete structure, the integrity of the concrete microstructure has already been wrecked.

ASR can be prevented by eliminated one of the three, and the go-to move is to avoid using reactive aggregate. But that's not as simple as it would seem. While aggregates can be classified generally as reactive and non-reactive, not all siliceous aggregates are prone to ASR and not all so-called and classified non-reactives are completely immune to ASR. Too many factors are in play—aggregate mineralogy, degree of crystallinity, solubility.

Instead of running that gauntlet of maybe-maybe-not, a project engineer can turn to a supplementary cementing material (SCM), like Hess' pumice pozzolan. Fly ash is cheaper, sure, but its effectiveness is a crap-shoot (see Pumice Pozz Instead of Fly Ash, below). Laboratory tests done using highly-reactive cullet glass proves that choosing HessPozz as the mitigating SCM shuts down the alkali silica reaction, even when dosed at as little as a 20% portland cement replacement.

Pumice Pozzolan Instead of Fly Ash

Although fly ash can have a mitigating effect on the alkali silica reaction, it is critical that the right fly ash is used. Because fly ash is the byproduct of burning coal, the effective qualities of the ash vary widely.

Quoting for a U. S. Federal Highway Adminstration report (FHWA-RD-03-047): “Perhaps the most important parameter affecting the ability of fly ash to control ASR expansion is the CaO content of the ash. Specifically, fly ash lower in CaO produces a calcium silicate hydrate (C-S-H) structure with a lower calcium to silicon ratio, which imparts a negative surface charge, leading to the absorption of cations, especially alkalies. In addition, low-calcium ashes are more effective in controlling ASR because the alkalies contained in the ash generally are not available to the pore solution, whereas high-calcium ashes tend to make their alkalies more readily available. Although higher-lime ashes can still be used to combat ASR, significantly higher dosages may be needed, especially when using highly reactive aggregates.”

Hess Pozz/ASR Miti•Gator™ has no such performance variance. It works consistently and predictably. No need to test to find out what you're going to get and how much youre going to have to dose.

Concerns with ongoing availability and performance quality of Class F fly ash spurred the Texas Department of Transportation to commission a two-year study (2012-2014) by the University of Texas-Austin to evaluate commercially available pozzolans to take the place of fly ash in improving concrete performance. Hess Standard Pozz was used in the study and performed exceptionally well in every test category—making it the ideal pozzolan.

•DOWNLOAD a summary of that University of Texas-Austin research quantifying the performance of pumice pozz in critical concrete performance categories.

•WEBSITE: That University of Texas-Austin study also compares the perfomance of pumice pozzolan with Class F fly ash in concrete over a two-year period.

Hess Pozzolan Grades

We offer two pozz grades: Hess StandardPozz (DS-325) and Hess UltraPozz (NCS-3). The DS-325 grade is also packaged and sold under the ASR Miti•Gator™ product label.

Why choose one over the other? If you have a high-end (typically precast) application, the UltraPozz will provide the best result in every concrete-performance category. Standard pozz is the best fit for large-scale applications—infrastructure, roads, buildings.

Hess Standard Pozz DS-325 Hess UltraPozz NCS-3
Dx Micron Size Dx Micron Size
D50 14-16 D50 2-4
hess pozz logo ASR MitiGatorTM logo


Our extensive logistical expertise allows us to ship pumice pozzolan anywhere on the planet—packaged in 44lb/20kg bags, 1-ton super sacks, or in bulk pneumatic rail cars and pneumatic trucks.


•We have provided an online sample request form to expedite the processing and delivery of your pumice sample.


We know pumice pozzolans. We’ll be glad to answer your questions, provide you with testing data, or provide a sample for your own testing—whatever you need.

•SALES: If you need to talk availability, pozz grades, logistics, costs...contact Mike Hess Jr., Sales Manager, at extension 147 or email salesmgr@hesspumice.com

•RESEARCH: Perhaps you are exploring the possibilities—thinking pumice pozz may fit your particular need, thinking you’d like to see the existing research data, thinking you’d like to do some testing of your own—then we invite you to contact Brian Jeppsen, VP Research and Development, at (208) 766-4777 x111 or email: rd@hesspumice.com


Flatlines ASR even in the presence of the most reactive of aggregates.

White Hess UltrapozzHess UltraPozz Powder


ASR cracking in concrete supportsConcrete shattered from within by ASR

Typical ASR-induced damage in unrestrained concrete results in the classic map-cracking seen above.
[ASR crack-damage photos from www.fhwa.dot.gov]

Micrograph of ASR cracking in concrete

Micrograph of polished concrete core sections showing cracks and dark reaction rims around reacted aggregate particles.
[Micrograph of ASR damaage from www.fhwa.dot.gov]

Control Alkali-silica reaction with ASR MitiGator product - Website
Extensive information on alkali-silica reaction and how pumice-blended cement mitigates ASR is found on the ASRmitigator.com website.
Hess Pozz Website
Extensive pozz information over on the HessPozz.com website. 208.766-4777  •  Malad City, Idaho 83252