It really depends on what you are looking for, there is no simple answer unfortunately.

Silica fume is great in low additions (up to 10% cement replacement) and can produce very high compressive strength concrete (and high early strength too) thanks to its very fine particles that not only act as a filler but exhibits a high degree pozzolanic activity too. But silica fume, despite the fact it is an industrial by-product, is in demand and so is not cheap.

Fly ash (I presume you mean coal fly ash) can be very useful or terrible. It really depends on the type of coal burned and the exhaust gas treatment systems used at the coal power plant. The good quality coal fly ash is in demand and exhibits a moderate pozzolanic activity. It can be used up to 55% cement replacement although normally contents do not exceed 35%. It is especially useful in large concrete structures because it dilutes the degree of cement hydration reactions at the beginning, which reduces heat released (too much heat in a large structure may not be able to escape and lead to thermal stressing and thus cracks). The downside is that the more fly ash you add, the lower your early strength, but after 28 days the strength is generally similar to a concrete without fly ash.

Finally, SCC is a very special type of concrete, for SCC alone, because the flowability of the mix is so important, silica fume additions are going to be problematic because they increase the water demand of the mix, so I would urge caution with sliica fume in SCC. Fly ash should not be such a problem and you could add much higher quantities without worrying too much about the flowability of the mix.

Sorry if I have rambled on a bit!

Hello Shane!

Thank you for the tips. Were very enlightening.

Do you consider interesting a comparative study between the use of these two mineral additions? For example, which of the two has better performance as the resistêcia compression?

Perhaps also consider a 120 grade GGBFS. This would have the particle size you require, provide long term strength and compression resistance, increase initial flowability without requiring as much additional water compared to silica fume.  Negative would be the setting time and you would not achieve your strength targets early without additional admixtures.

Hello Andressa,

Glad to help. If you are focussed on SCC, then maybe it would be interesting to do a study about optimising the mix design. This could be down from one of two angles:

1 - introducing silica fume or fly ash as a replacement for fine sand filler in SCC.

2 - introducing silica fume or fly ash as a replacement for cement in SCC.

I'm not sure what the state of the art is but both silica fume and fly ash are well studied as potential cement replacements. The influence on the properties of SCC could be much more novel in my opinion. Especially if you compare results to a control with sand added of an equivalent particle size. As pointed out above, you could also look at blast furnace slag which is another very relevant mineral addition to cement.

Poling and Donatello, I thought of the blast furnace slag some time ago. What is the maximum percentage of their use in SCC?

Good day Andressa ,   You would really need to look at the application and the overall mix design itself to properly answer what the maximum percentage would be. As my colleagues from the UK often said " you want the right horse for the right course "

I am aware of some GGBFS mixes with values as high as 70% Portland cement replacement.  In California the specification requires a minimum of 25% cementitious for any DOT work used in bridges, roadways, and other concrete decks.  Somewhere in between those values is probably your answer.

It depends really on what are your targets. Both silica fume and fly ash have pros and cons. Me personally I prefer silica fume because with the right regulation on the superplastisizer dosage results in SCCs with better consistency, less prone to bleeding and good mechanical properties.

If you are after lightweight SCC then you should probably consider lightweight aggregates.

 Have to agree with rest of the crowd, both have pro's and con's. If you are interested, check attached file, interesting paper about SCC containing fly ash.

silica fume is better addetive than flyash.