thirty years ago construction workers discovered that concrete pumped
through aluminium nozzles expanded.  Later, it was found that making
ice cream in an aluminium ice cream bucket greatly accelerated the
process. What is causing these two things to happen, and are there any
common conclusions or facts other than that aluminium is involved?

Could this be what you mean with concrete and aluminum:
http://www.toolbase.org/Technology-Inventory/Foundations/autoclaved-aerated-concrete

I didn't find anything about aluminum nozzles having any effect on concrete.

Certainly when making ice cream, an aluminum bucket will conduct the
heat of the makings better than most other metals, though this would
hardly seem to be something discovered just in the last 30 years.

Most super soft commercial ice cream makers use stainless steel buckets 
and other componets that are in contact with the icecream.
I dont know of any other metal in use
the eceptions use plastics

Kemlo