The effect of partial replacement of silica fume on compressive and split tensile strength is in positive way ie there is an increase in strength, as there is increase in percentage replacement of silica fume. There is an increase in strength, up to 8-10% replacement of silica fume, then after strength loss may occur due to shrinkage and cracking.

For more details please refer this links:

Synergetic Effect of Fly Ash and Silica Fume on the Performance of High Volume Fly Ash Self-Compacting Concrete: https://doi.org/10.1080/24705314.2021.1892571

And also you can get some idea of durability from here

Article Prediction of rapid chloride permeability of self-compacting...

silica fume is only used for the high strength concrete as a replacement for an amount of cement , to decreases the temp of the concrete .

Silica fumes are very fine and react with calcium hydroxide and form CSH compounds which are final product of cement hydration. This is happening with hydration of cement and released calcium hydroxide further reacts with silica fumes. This results in to pore refinement and densification of concrete. Thereby strength will increase. Fine material having higher surface area also reduces bleeding of concrete.

Higher compressive strength and splitting tensile strength of concrete made from silica fume can be explain in twofold.

1. Generally silica fume (SF) particles are very fine (

Thank you very much Shashikant Kumar for a nice explanation but our samples reach up to 20% increment in compressive strength of some mixes

Thank you very much Dr. Om Prakash Chhangani for the precise and excellent explanation. We had tested 8 different design mixes in all mixes around 10 -15% increment were observed, but one mix reached plus 20%

Thank you very much, Dr. Arup Kumar Mohapatra for the precise and excellent explanation.

Mohammad Nadeem Akhtar , I have discussed about percentages replacement level of silica fume (8-10%), at which enhancement in strength will be more (may be 20%) due to more compact and dense microstructure formation as specific surface of silica fume is about 100 times finer than cement, resulting in dense CSH gel of hydration products which is formed due to reaction of unhydrated CH with silica fume, but after that there is a decrease in strength if higher percentage of silica fume added due the shrinkage and cracking. IS 456 also recommended that upto 10% replacement of Silica fume is beneficial for concrete‘s strength and durability properties.

Thanks for the clear explanation Shashikant Kumar

Results indicated that the use of silica fume in concrete increased the compressive strength of concrete. Increase in water-binder ratio reduced the compressive strength of concrete irrespective of the use of silica fume. Silica fume had no noticeable effect on the splitting tensile strength of concrete.

https://www.researchgate.net/publication/327258417_Effects_of_Silica_Fume_on_the_Strength_and_Durability_Properties_of_Concrete

Thanks for the question. Silica fume act like a pozzolana in a matrix, and therefore can be used as a substitute for cement in concrete and mortar. When the right quantities of silica fume are used to replace ordinary Portland cement, it helps to:

1. Improve the bond between the constituents of the concrete/mortar, thereby improving the mechanical properties;

2. Improve hydration effect on the concrete/mortar, which lead to increase physical and mechanical properties; and

3. Reduce the percentage of pores/voids development, thereby improving the water resistance and durability characteristics of the concrete/mortar.

I agree with you on Om Prakash Chhangani Yes, you will improve and increase the compressive strength.

Thank you Dr. Humphrey Danso for the nice explanation, for the right quantity optimization of silica fume must get the threshold with different doses. In most of the studies, the replacement percentage was suggested is between 8 to 15 % for optimal compressive strength. We are observing the highest increment from 10 to 12.5 % variation.

Silica fume is used in concretes to improve their mechanical properties and durability:

Silica fume reacts with lime during the hydration of the cement and promotes the formation of hydrated calcium silicates (C-S-H gel). This C-S-H phase binds the different components together to create a dense and compact cementitious matrix.

Its fineness and pozzolanic properties give silica fume a strong reactivity with Ca (OH) 2 produced during the hydration of concrete: SiO2, 3CaO + H2O → CSH + Ca (OH) 2. SiO2 + Ca (OH) 2 → CSH

Its high reactivity increases the homogeneity of the mixture and reduces the porosity and permeability of concrete.

Silica fume is 100 to 150 times finer than a cement particle and can fill the interstitial voids between fines and grains of cement. This optimal granular stacking thus contributes to greatly reducing the porosity and permeability of concrete .

l'utilisation de fumée de silice dans le béton augmentait la résistance à la compression du béton, La fumée de silice réagit avec la chaux lors de l'hydratation du ciment et favorise la formation de silicates de calcium hydratés

Dear

silica fumes exhibit a positive influence on the overall properties of concrete including compressive, tensile and shear strength, moreover you can reduce the cement content by implementation of silica fumes as partial replacement of cement.

Thank you very for a nice discussion Dr. Saad Sarsam

Thank you Dr. Achour Mokhtar for a nice discussion

Silica Fume has two effects that increase the tensile, compressive and shear strength of concrete. The first and main effect is the pozzolanic effect that everyone has alluded to. The second effect is as a filler, providing a better gradient between the sand and the cement grains. This latter effect was described by researchers at Tel Aviv in the mid 1980's.

Silica fine is 100 times finer than the cement particles. Hence, the use of silica fume results a denser concrete matrix which impacts the strength parameters positively.

Thank you Dr. Rahul Biswas for sharing your experience

Due to high pozzolanacity, reactivity and fine particles of Silica fume, It improves all hardened properties of concrete.

Dear Dr. Deep Tripathi thank you for discussion

The utilization of silica fume in concrete reduces significantly the porosity of concrete, increase the strength of concrete, protect concrete against chemical and physical deterioration, and enhance the durability.

It depends on the percentage of consumption, and usually at 10% by weight of cement used gives the best results and increases the compressive and flexural strength, but greatly increases the consumption of lubricant in concrete.

Sir

why is the compressive strength upto 10% is increasing and then decreasing