Quantifying pumice as the ideal SCM replacement for fly ash in concrete

Over a two-year period (2012-2014), the Center for Transportation Research at the University of Texas-Austin tested and quantified the performance of eight commercially available natural pozzolans to determine a viable replacement for fly ash in concrete.

A downloadable summary of that research report details the impressive performance of pumice pozzolan (sourced from Hess Pumice Products and available commercially as Hess Standard Pozz) across the entire suite of those tests, proving pumice to be the ideal natural pozzolan for significant improvements to concrete in critical infrastructure applications.

In every important concrete performance category, pumice pozzolan has proven to be the ideal SCM replacement for problematic fly ash.

Supplementary Cementitious Materials (SCMs) are recognized and widely used to improve the performance of ordinary Portland cement (OPC) concrete, primarily in terms of strength and durability.

SCMs improve the density and quality of the concrete matrix by amplifying the production of calcium silicate hydrate (CSH), the cementitious binder that makes concrete, and by consuming deleterious compounds produced by the cement water hydration reaction and otherwise alter OPC chemistry so as to contribute to the concrete’s ability to resist sulfate, chloride, and alkali-silica attacks, heat-of-hydration cracking, freeze-thaw damage.

Fly ash, a by-product of coal-burning power plants, has been used extensively as a readily-available SCM to improve concrete performance. But fly ash has an uncertain future in terms of necessary quality, availability, and practical cost. This concern motivated the Texas Department of Transportation to commission the research (summarized in this report) in order to expand its options for sourcing cost-effective and performance-quantified SCMs.

The linked report is a focused distillation of the research conducted by the University of Texas-Austin (August 2012 to August 2014) to quantify the performance of natural pozzolanic SCMs in concrete and establish the viability of these pozzolans in critical concrete infrastructure. In particular, this summary document focuses specifically on the results delivered by the pumice pozzolan used in the study, sourced from Hess Pumice Products of Malad City Idaho and commercially available as Hess Standard Pozz.

The pumice pozzolan performed consistently and effectively across the suite of tests, proving to be the ideal natural pozzolan for improving concrete in critical infrastructure applications.

Two rounds of studies were conducted-the first round consisted of tests run on pastes and mortars to identify optimal replacement level for concrete mixtures and for rapid assessment of how natural pozzolans could affect important fresh and hardened state properties of cementitious mixtures. The second round of studies used concrete prisms, with some of the durability testing taking up to two years.

In both studies, samples were also prepared using Class F fly ash so as to compare performance results with the more commonly understood performance results of Class F fly ash. Those results are also included in this linked summary.

Of particular note were the results of pumice-enhanced concrete in tests quantifying sulfate resistance (Class 3 Severe Sulfate Exposure Environment) and Alkali-Silica Reaction (ASR) mitigation (reaction remained flatlined in concrete test prisms-well below the 0.04% ceiling set by ASTM C1293 as the acceptable limit)-out to 24 months, which supported the results from the round one ASTM C1567 accelerated mortar bar tests).

In every other performance category tested, Hess Standard Pozz delivered results well within ASTM limits-heat of hydration, drying shrinkage, coefficient of thermal expansion, set times, compressive strength, chloride ion penetration. All of these performance results were delivered using cement-replacement dosages of 15%.


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