Fly Ash in Concrete Mixes

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Author Message J. Peter Jordan Senior Member Username: jpjordan Post Number: 517 Registered: 05-2004 Posted on Monday, November 05, 2012 - 10:06 am:    On a couple of projects recently, we have discussed using concrete mixes with high fly ash content (both are LEED projects). On one project, we will be doing stained concrete, and the designer wonders if the fly ash would compromise the color. A discussion with the concrete color rep indicates that the fly ash's affect on concrete color depends on where the fly ash comes from, and in this market, the fly ash tends to lighten the concrete. On the other, we will be doing polished concrete, and the owner says that he has had trouble polishing concrete with high fly ash concrete, that the polishing wheels tend to tear up the surface. This sounds to me like the polishing guy did not let the concrete cure long enough before starting polishing operations. Any thoughts out there? ken hercenberg Senior Member Username: khercenberg Post Number: 364 Registered: 12-2006 Posted on Monday, November 05, 2012 - 10:49 am:    There seems to be a lot of confusion in the industry about the various replacement pozzolans available. I suffer from this confusion as much as anyone. To my understanding, fly ash darkens concrete and granulated slag lightens the color of concrete. I've never heard of fly ash lightening the color, but maybe Texas fly ash reacts differently. I've seen a lot of people use 'fly ash' to mean replacement pozzolan, not knowing that there is a difference. You may want to see what credentials the concrete polisher has. There are many people out there who 'do' polishing that really don't have a clue. The 'good' polishers usually have a pretty good handle on what's going on in the industry and can be excellent resources. Jim Cuviello in the Mid-Atlantic is a great resource. I'd be happy to put you in touch with him if you'd like. He is very interested in promoting positive experiences with polished concrete. I'd check out http://www.concretepolishingassociation.com/ for starters and see what, if anything, they say about using replacement pozzolans. They have an accreditation process for concrete polishing craftsmen, including a location in Grapvine, TX. I've had some less than optimal results with a couple of color manufacturers. I'd be happy to have a sidebar with you about that. I'm not looking to disparage anyone, but I would stongly advocate specifying extensive mockups by the actual crew prior to acceptance of anything. Once this product is in, you're living with it. you might even want to specify that an area of one mockup be damaged and repaired so your client (and designer?) knows what to expect. BTW, if you're thinking about using this technology in toilet rooms, stay away from concrete stains and make sure the coloring process can hold up. Detailing around area drains can end up looking pretty bad, so that may be another mock-up consideration. Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 203 Registered: 08-2004 Posted on Monday, November 05, 2012 - 01:46 pm:    Peter, At an excellent SGH seminar (Peter Nelson and Emily Hopps), I learned that very high amounts of fly ash affect the hygroscopic qualities of concrete, making the new concrete hold moisture much longer than concrete with less fly ash. Perhaps this also affects the success of hardeners, sealers, stains, or polishing procedures? Again I ask, why did no one advise me to take chemistry in college? - Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 521 Registered: 10-2007 Posted on Monday, November 05, 2012 - 04:37 pm:    While fly ash may resulting in the concrete holding onto water this does not necessarily mean that the water is free to escape causing flooring and other problems. ken hercenberg Senior Member Username: khercenberg Post Number: 367 Registered: 12-2006 Posted on Monday, November 05, 2012 - 05:27 pm:    Probably more than the amount of water being 'held' are the changes to the chemical properties of concrete containing fly ash vs. cement. The types of hardeners, stains, sealers, etc. being used then also becomes a factor. As noted in another thread, products often used in polished concrete and other processes use various types of silicates (there are many variations). To work, topically applied silicates usually react with the free calcium found in Portland cement that has not hydrated within the concrete mix (Google 'calcium silicate hydrate' for some basic information). Replacing other pozzolans for cement can mean a change in the chemical and physical properties of the concrete. These pozzolans also hydrate and help create a concrete matrix when used with Portland cement as part of the concrete mix. Because of their rates of reaction and the densities of the concrete, use of these pozzolans can potentially slow or block reaction of the cement if the cement hydrates slower than the pozzolans. I haven't heard about fly ash or other pozzolans affecting the drying times of concrete but perhaps they could affect it in some way, especially if rate or completion of cement hydration is affected. Whether it is significant or not, I will leave that to others. Perhaps the denser matrix resulting from the use of pozzolans is what is preventing the topically-applied silicates from reaching the cement content; the problem may be mechanical and not chemical, I don't know.