What is corrosive to pipework ?

I'd say no. Most pipes are pretty immune to most common substances or nobody would make pipes out of it. Certainly most fillers and sealants I've seen doing this job and the pipes have always been fine, assuming no leaks due to old age or mechanical straining. I I had an event in a boxed in piece in the back of the kitchen cupboard when as I later found out a hot water pipe going to the tap had a pin hole in it and made the inside of the box very, um hot and humid over some months. the inside was disgusting, growing mould and rusting everything steel and putting a gungy green mess on copper. Brian

Ok I will probably use the caulk that I have as that will allow for some expansion/contraction.

Thanks

Plaster will corrode copper if moisture is present, such as a cold water pipe with condensation forming on it.

Where are the holes that you want to fill?

If it is a hole through a wall and is hidden I tend to use a squirt of expanding foam.

If it is visible you can get collars for the pipes to cover the gap.

And if it's a pvc waste pipe, I wrap a piece of heavy sheet plastic around first - allows the pipe to slide if it wants to for expansion.

I use those for pipes through ceilings and those are very neat IMHO.

Poundland often have packets of 10, for 15mm pipe.

Pipes that pass through walls are supposed to be sleeved to be a good job. (Especially where there are temperature changes.) I usea bit of plastic pipe.