On Sunday, 4 October 2020 00:36:00 UTC+1, David Paste wrote:
Hello all,

I have a small leak in the concrete wall in the cellar.
The rain is making it a bit of nuisance. I have tried
to fix the issue by finding the hole, drilling it out
and filling with some silicone sealant (done when there
was no rain and no water ingress). This, surprisingly,
worked to some extent, but water (as it does) finds its
way in. I suspect that the drilled hole wasn't as clean
or dry as was necessary for the sealant to work properly.

I am now thinking that perhaps a larger hole should be
drilled and filled with a cement mix. Is this in
principle something that could work? I know nothing about
cement / concrete, hence my query.

Does new cement bond with old cement to form a watertight
seal, or will another product have to be used? Or an
additive in the cement / mortar.

Thank you in advance for any help,

David Paste.

If a small hole is letting water in, drilling it out is not usually the solution. Concrete is not waterproof. And waterproof coatings are in the majority of cases not the way to keep a space dry. In short we have nowhere near enough info to address your situation reliably.