Microsilica vs Flyash 2

[SShinde]

Which is superior in terms of
performance/cost?

Does anybody have bad experiences with flyash cement?

 

[JedClampett]

Fly ash is literally dirt cheap. It is a waste product from coal fired power plants. It is commonly used as a substitute for cement. It adds many advantageous properties, in particular regarding chemical resistance. In our area (Arizona) it is routinely added to concrete mixes in percentages up to 20% of cement. I've never had a bad experience with it, although I've heard some clients don't like it. I think they got a bad batch of fly ash once.
Microsilica is very expensive, adding up to $50/cy. It isn't that common, so batch plants don't like to deal with it. It is ultrafine, so it's hard to handle. It is used to greatly increase the strength and/or density of concrete.
Unless you have a special need, I would stay away from microsilica. Fly ash, on the other hand, makes a cheaper and easier to handle mix.

 

[TPNY]

I agree with JedClampett about cost. To answer your question about performance, microsilica will result in a higher compressive strength than the fly ash. For example, long term strength will increase up to ~25% with 30% Class C fly ash mix, whereas 15% microsilica mix will increase strength by ~30%. Microsilica also has a significant advantage over fly ash when permeability is a factor. But like Mr. Clampett recommended, unless you have a need for it, don't go with microsilica.

 

[Rjeffery]

Don't use flyash in cold weather or when the componants of the concrete are not above 50 degrees ( f ) as the set and strength gain will be markedly delayed!

 

[dicksewerrat]

Use fly ash in hot weather as a cement substitute up to 15% avoid premature set.

 

[DaveFlax]

The key ingredient in each prodcut is silicon dioxide, which reacts with the calcium hydroxide by-product of Portland cement hydration. Fly ash is about half the size of cement and is about 50% silicon dioxide. Silica fume is about 1% the size of cement and is 85% plus silicon dioxide; both its size and concentration make it much more reactive.

The big advantage of silica fume is the very high strengths, but it requires a lot of water. One pound of silica fume has over 2 acres of surface area that must be wetted. It is sticky and it is hard to keep it from shrinking and cracking.

The big advantage of fly ash is it lowers the cost of the concrete and, because fly ash is basically small spheres, it assists in both finishing and pumping. The drawback is that it delays strength gains so early strengths will be lower.

 

[Bbird]

From my line of work people use fly ash to reduce heat from a large pour or improve the chemical resistance of the concrete.

Microsilica is more used for protection of the rebar. As Daveflax said it is 100th the size of a cement grain and so the concrete matrix is less permeable than OPC or the one mixed with fly ash. Micro silica does need a lot of water in the mixing and the high performance superplasticers have been developed to solve this problem. Microsilica needs water for curing as well and finish is generally superior to OPC.

As far as I am aware the French Charles de Gaulle airport is mixed with it and finsih quality is presented as an architectural feature.

Apart from better protection to the rebar, higher strength (though not usually the prime motivation behind as a bit of more cement can achieve the same result) microsilica is one of the rare material one can add to the mix to improve the abrasion resistance of the concrete. Thus it is popular in areas where water flow at velocity too high for the normal concrete.

 

[BigH]

CANMET in Canada has done a lot of work on fly ash in cement. Suggest you might want to research this.