On 21/01/2025 17:06, David Paste wrote:

If I lay fresh cement on old (50 or so years), will that interface be watertight?

If not, what do I need to do to make it so? Use some acid to etch away
at the top layer of the old cement?

Use some sort of chemical mixed into the new cement?

Paint on some sort of chemical to the old cement? If this, do I wait til
it has dried to add the new cement, or paint it on and pour the new
cement right away?

Thanks for any help.

I assume you mean concrete

https://www.youtube.com/watch?v=ElNThXvtJ18

When he says glue substitute SBR which can also be used in the mix.

https://www.sealantsandtoolsdirect.co.uk/pub/pdfs/everbuild-503-sbr-bond-technical-data-sheet.pdf

https://www.youtube.com/watch?v=Qm4TLiM5go8

Plenty more Youtube videos

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