Browse Forums General Discussion Re: fly ash and concrete 2Mar 23, 2015 6:22 pm Fly ash is a material with variable properties depending on the original source coal. It can be used to replace around 30% of the cement and have a more workable concrete. Although not applicable to domestic work in very large pours there is a real advantage of replacing cement with flyash because it reduces the heat of the chemical reaction. The Harder You Try - the Luckier You Get ! Web site http://www.anewhouse.com.au Informative, Amusing, and Opinionated Blog - Over 600 posts on all aspects of building a new house. Re: fly ash and concrete 3Mar 23, 2015 7:13 pm Flyash also makes the concrete harder, stronger, denser and more waterproof by filling in the smallest voids.Once a waste product from coal fired power stations now sort after for high strength concrete manufacture Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: fly ash and concrete 4Mar 23, 2015 8:39 pm StructuralBIMGuy Flyash also makes the concrete harder, stronger, denser and more waterproof by filling in the smallest voids.Once a waste product from coal fired power stations now sort after for high strength concrete manufacture + 1 it also increases it durability and protects the reinforcement. If in a saline environment I suggest the addition of flyash Re: fly ash and concrete 5Mar 23, 2015 9:42 pm It's also cheaper than cement so, in NSW at least, you'll find flyash in almost every batch. Re: fly ash and concrete 6Mar 23, 2015 11:13 pm Seems everyone has been sold the message. I disagree with a few points: The strength, which has been compromised by the reduction of cement by 25-30%, This also reduces the chance of the surface being closed off as nicely without the full load of cement being used(cement is very fine) medical waste can also be burnt in furnaces which includes plastics and who knows what. Did the cost of concrete come down in line with the saving of using a waste by-product that ordinarily costs to dispose of? ...and hiding it in concrete, is that a good idea? Also what 'deal' has been made with the concrete companies to manage disposal. Long ago when 25 mpa concrete was batched and a decent curing method was used the concrete will develop a strength above 25 mpa. Now, only if the correct 'method of curing' is used, will the concrete reach its' desired strength potential. The addition of water at placement stage, will increase the concrete slump to 'fat', the fly ash will allow hydration more rapidly and the dehydration becomes rapid as the fly-ash substituted concrete releases bleed water rapidly and the concrete will snap 'gel set' and the plastic stage will develop early 'crazing' if the bleed water is not allowed to evaporate, prior to 'finishing'. A lot of cement will be drawn to the surface and the weaker underlying surface becomes brittle and the top hard and then you will see 'dusting off' as the immediate slurry surface is worn of off. Footings ok I don't have a problem but if something goes wrong on the day if it is a bit warm and there is a delay it will compound the issue. INHO only Where you are coming from is where you are going to... Re: fly ash and concrete 7Mar 24, 2015 12:10 am Yes, fly ash takes a little longer to gain it's strength. So, although the EARLY day strength of fly ash concrete may be lower than a similarly rated (MPa) Portland cement concrete, by 28 days (the age at which concrete's strength is rated) fly ash concrete will match portland cement concrete, & over time it will exceed the strength of portland cement concrete. This lower early day strength is not an issue for house slabs, & really only has to be considered for load bearing applications like multi-story apartments & carparks. Here, an earlier 7 day (or 14 day) strength test may be required. This is obviously factored into the concrete design process, so if a fly ash concrete is used it will have a significantly higher strength over time than a similarly rated 7 day portland cement concrete. Fly ash particles are spherical in shape. This makes the concrete more workable. So water levels, which improve workability can be lower for fly ash concrete. Water makes concrete more porous, so lower levels of water will improve a concrete's strength & significantly improve it's durability. Fly ash concrete is less prone to shrinkage & cracking. Along with silica fume & blast furnace slag, the addition of fly ash will improve a concrete's durability in saline environments. Bleeding is generally the result of poor concrete design, inadequate mixing or over vibration, & is possible for all concretes, not just fly ash concrete. Generally fly ash reduces the chance of bleeding, due to the lower levels of water required. As has been stated above, fly ash is a waste product of coal fired power stations. So you won't be finding 'medical waste' & other plastics in it. You may be confusing fly ash with silica fume & blast furnace slag, which are also supplementary cementitious materials (portland cement replacements), & are produced in the manufacture of steel & other metals. I was a concrete scientist in a previous life. Re: fly ash and concrete 8Mar 25, 2015 10:55 am Gee all those years that we did bathroom and laundry renos in old terraces and pre-war houses where the "slab" was fly ash and after digging them up we took it to the tip. I could have sold it to the concrete suppliers and made a motza ! Bummer. Quote: I was a concrete scientist in a previous life. I knew you were smarter than the average bear ddarroch ! Stewie Re: fly ash and concrete 10Mar 25, 2015 6:07 pm ddarroch in a previous life? I wonder if we have met? Who were you with? I've been around a few test labs, since a little way back circa 1981 was when I started for pioneer concrete(wa) back in the bad old days. Where you around then? (I was previous of Northern beach Sydo) Where you are coming from is where you are going to... Re: fly ash and concrete 12Mar 27, 2015 10:53 am So ddarroch were you involved in the testing?...and integration of widespread use of fly ash and the re marketing a bit over ten years ago? Where you are coming from is where you are going to... Re: fly ash and concrete 13Mar 27, 2015 8:22 pm CSIRO was R & D. Main project was investigating the use of fly ash as an aggregate in lightweight concrete (using a sintering process). Also investigating sulphate attack & shrinkage of concretes containing fly ash, blast furnace slag & silica fume. Among other things. CSR was mainly testing, & some mix design. Seems like a lifetime ago. Re: fly ash and concrete 14Mar 27, 2015 9:25 pm A concrete scientist.... Hmmm would love to pick your brains about a few things. I've had a lot of fun with light weight and thin high strength concrete stuff.. A bit of a passion of mine... but I guess if you are into skiing then concrete is no longer an interest and you must be young enough to change trades. fantastic Where you are coming from is where you are going to... 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