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Flyash and Concrete

There has been a lot of talk about flyash in New Mexico lately.  If you listen to the talk you have probably heard our market has been put on a Flyash allocation by the sole supplier, Salt River Materials Group. So what does that mean to you the concrete customer short term and long term?

If you have been around this market for a while you know the evolution of flyash here in Albuquerque. If not, a quick story.  Flyash, a byproduct of coal burning power plants, was introduced to us in the early 80’s.  At that time we replaced up to 20 percent with flyash. At that time it was called “Hamburger Helper” among other things since it allowed the supplier to use less cement but still achieve strengths, although it took a little longer. It is much finer than cement but it is spherical rather than crystal shaped.  This acts as ball bearings and helps workability and pump ability.  The complaints at the time were that it was sticky and hard to finish with the increase of super fines.

About this time, middle to late 80’s, the interstate concrete in the area was starting to fall apart after only 15 or so years. Upon further investigation ASR, Alkali Silica Reactivity, was found to be the culprit.  The Alkali in the cements were interacting with the Silica in the aggregates of this area causing a gel in the concrete attaching itself to particles in the mix.  When water migrated into the concrete after final set it would cause this gel to expand and break up the concrete from the inside.

Testing with another Class of Flyash , F, with lower Alkali, proved to mitigate the ASR. The Class F flyash would soak up great amounts of the Alkali in the cement thus reducing half of the equation.  Today the City of Albuquerque and NMDOT both require 25% Class F flyash in their approved mixes just for this reason.  We here at Duke City have been a big proponent of using Class F flyash in all exterior applications to mitigate ASR influencing architects and engineers not familiar with the ASR issue in our area to consider specifying Class F flyash.

Fast forward to today. Because of maintenance issues, lower natural gas prices, lower usage of electric power among other factors, the supplier of area has allocated flyash to its customers based on prior usage. Four Corners PNM coal burning power plant is the main provider for Class F flyash for Colorado, New Mexico, Arizona and California.  We credit Salt River Materials Group for classifying a consistent flyash out of this plant.  The other sources, not so much.

This allocation actually has an end date, April 10, 2016, but the writing maybe on the wall for the future supply of Class F flyash. You have all heard the attack on coal from the environmental sector and if clean natural gas pricing remains low flyash maybe be a thing of the past.

What then do we do to mitigate ASR?

Since the ASR “Gel” requires water, interior mixes wouldn’t have the same risks as exterior mixes exposed to moisture, therefore those mixes could go to straight cement. The risks would be greater for subgrade applications, but not as severe as exposed concrete to the elements.

As you may or may not know, Duke City Redi-Mix coarse aggregate is Basalt. The other coarse aggregate used in our market is the natural river rock in the area which is considered very reactive, with a  high silica content. Basalt is considered non-reactive.  We do use natural sand, as do all producers in the area, which is also very reactive. Duke City is half way to resolving the issue due to the fact our coarse aggregate is not reactive.

The current alternatives to mitigate ASR are through chemical admixes, namely Lithium, using alternate sources of aggregates or other pozzolans. Duke City is currently testing a lot of the available alternatives.  The availability, workability of the mixes and cost of these alternatives will make this transition a challenge for all of us.

  1. Lithium cost is in the $25 per gallon range. The dosage averages one gallon per yard depending on the severity of the reaction. AND Lithium is known to accelerate set times.

  2. Since all natural sands in this state are silica based we must crush a non-reactive aggregate down to use as a fine aggregate. Natural sand is spherical, great for pumping and finishing. Crushed fines are angular. Not really pump able and very tough to finish. AND, get this, it will cost $10 to $12 more per yard.

  3. Other pozzolans are being considered including pumice. Testing is underway.

In light of all of this I believe Duke City will be able to get through this initial shortage of flyash without resubmitting mixes or losing any integrity in any of our mixes. Looking to the future we will be able to keep the increase in costs to a minimum using the alternatives mentioned earlier due to the fact that we use a non-reactive coarse aggregate and must just deal with the natural fine aggregate.  Please be assured Duke City Redi-Mix will address these issues with the highest integrity and commitment to quality you have grown to expect.

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