Pozzolanic materials like fly ash, silica fume which is much more finer than cement is capable to improve the permeability of concrete by formation of dense concrete matrix and CSH gel.

For more details please refer these articles:

Article Synergetic Effect of Fly Ash and Silica Fume on the Performa...

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

Article Pulse velocity–strength and elasticity relationship of high ...

Thank you

Silica Fume;

Water to cement ratio influences concrete permeability,

Etc.

W/C ratio I am using 0.2 only. It should not affect much

By increasing the 30 day Target Strength from, say, 25 MPa to, say, 45 MPa by increasing the cement powder proportion and reducing the water/cement ratio, you will dramatically reduce the ability of concrete to allow water to pass through it because you will be microscopically "closing up" the pores within the concrete that water travels through.

High degree of uniform immersion vibration of the poured concrete is recommended. I have had success with external vibrators rigidly mounted to the formwork in regular spacings along the wall.

Also, if the double the percentage of steel Y bar reinforcing and add 20% to lap lengths, this will also assist significantly as you will have "no crack" concrete.

If you have a concrete wall of a concrete tank holding water like a reservoir and you decide to use concrete as the wall material, best concept is to post tension the concrete with tension cables through Fressinet ducts cast into the wall.

Finally, there are fine particle powder additives claiming to reduce permeability of concrete but seek reviews from people who have used them prior to purchase.

fly ash and pozzolana and GGBS (GROUND GRANUAL BLAST SLAG)

Most of the time crystalline hydrophilic additives enhances permeability of concrete. And also, Supplementary cementitious materials (SCMs) are also added to decrease permeability of concrete like fly ash. Water reducing chemicals or admixtures are also great impact in permeability of concrete.

Fly Ash in Portland Cement Concrete

even you use from additives , the concrete still a prose materiel , so you have to coat it by a epoxy morter