Flyash can be added with higher percentage but the silica fumes are very fine and adding large volume is not possible. both are equally good in making pore refinement in concrete as they reach with lime released during hydration process and produce C-S-H compounds which are produced during hydration of cements. As such concrete becomes more dense internally and become impermeable. However, as the lime is consumed the hydroxide ion concentration is reduced and it can reduced the pH value but as the concrete internal pores are blocked the diffusion and permeability is reduced.

However, there was a long debate in the past that reduction of ph in case of silica fumes is less in comparison to flyash and therefore adding silica fumes may be a better choice to certain extent.

Both the SCMs have positive impact on durability such as water permeability, sorptivity, RCPT etc. Silica fume has greater impact on durability in compression to fly ash.

As fineness of fly ash is about ten times more than the cement so it fills the voids and more compacted microstructure has been formed and improved the durability of concrete. But it has low heat of hydration so gives later age strength.

If we talk about silica fume, it is 100 times more finer than cement and it forms denser microstructure in comp to fly ash replacement.

Overall Silica fume replacement is better option

For more details please refer the article

Prediction of rapid chloride permeability of self-compacting concrete using Multivariate Adaptive Regression Spline and Minimax Probability Machine Regression

https://www.sciencedirect.com/science/article/abs/pii/S2352710219326385

Fly ash can prove to be a better SCM than the silica fume.

Thank you Shashikant Kumar for a nice explanation

By partially replacing OPC with SCMs, better performance is observed in terms of durability, strength, sorptivity, and reduce the air and water permeability of concrete due to denser microstructure. Pozzolanic reaction is a slow reaction and when fly ash is added lesser amount of CSH gel produced as compared to OPC (at early age). So the compressive strength of fly ash replaced concrete is low due to less dense microstructure in early age. As a result of less dense microstructure ingress of aggressive ions could be higher at early age. Once these ions are penetrate into concrete they start reacting and durability problem may arise at later age. However, when silica fume is added we get higher compressive strength at early age due to denser microstructure (i.e. more CSH gel) than both OPC and fly ash replaced concrete. Moreover, if we add more percentage of silica fume it will give good particle packing than fly ash due to higher fineness of silica fume. From both the point of view silica fume concrete give more denser microstructure from early age than fly ash concrete. It indicates that very less or no chance of penetration of aggressive ions into the concrete and has very good durability performance.

By taking everything into account we can say silica fume concrete has better long term durability performance than fly ash concrete.

I suggest you use ultrafine GGBS in place of microsilica (I have found better results at 56 days )and crystaline growth admixture for making durable concrete

very nice sugeestion Dr. Sandip Uddhaorao Sonule

Thank you very much, Dr. Arup Kumar Mohapatra for a clear and technical explanation

Thank you very much for a clear and technical explanation Dr. Om Prakash Chhangani

Both are good.

% additions are different.

Silicafume is mostly necessited in concretes of grade M 50 and above.

Also Silica fume can be added in ternary blends alongwith fly ash & cement.

- - Dr. K C Tayade.

Thank you Dr. Keshav C Tayade for the nice discussion .....

Agree with@shahikant.

And the best result is achieved from a blend of about 15% fly ash and 4% silica fume. This gives about the best packing and the best pozzolanic action.

The work was done at UNSW in collaboration with Roads and Traffic Authority NSW. (UNSW.edu.au)

Thank you Dr. Gordon Chirgwin for this technical data

It depends on the grade of concrete you want to achieve. Generally, for HSC the silica fume is more suitable as the concrete matrix becomes more homogeneous and also the production of CSH gel is more for Silica fume concrete which makes more durable concrete.

Rahul, I am talking long term durability, not short term strength. Also, if you want longer term strength and high durability, make sure you cure the OPC-FA-SF blend a little longer and don't look for a very high proportion of strength till about 90 days.

We all became fixated on 28-day strength in the 1960's, as it was a convenient measure for ensuring the suppliers were controlling their production. Unfortunately, this led to specifications and codes being written around 28-day strength, and ignored the long term durability of the product.

If you need 100 MPa concrete for strength, then SF is the pozzolan of choice.

Thank you Dr. Gordon Chirgwin for a nice technical discussion on the topic

Thank you Dr. Gordon Chirgwin for a niceanswer on the topic.

I think most of the structures are not loaded untill 56 days , so that strength at 56 days time period can be considered..

Thank you Dr. Keshav C Tayade for the discussion

Silica fume is a very fine filler which can improve early strength. It is thought there is a limit of 10% in OPC after which strength is reduced. Fly ash is also a fine filler. It is not as fine as silica fume and seems not to give the early strength benefits but appears to give at addition rates of 5 to 10% higher strengths at 90 & 120 day, because it gives workability improvements, than OPC. Silica fume was used in the past in some countries where it was a waste product. I would suggest that in most countries it is no longer available. Health problems with silicosis would be a consideration. Unfortunately for no good reason, fly ash is being phased out in most developed countries.