Make production is the gear and wrench, and make construction a building, be it factory or otherwise.
Could you explain why the gear/wrench would be better for production than the factory?
I tested my icons (the bulk no-gear production icon and constuction icon), using my sister as a test subject, she said that its clear that the factory is for producing things and the tools, the wrench and gear, to construct things.
Because the resource icon for "Industry" is a gear. And industry is used for "Production" of many things, including buildings and ships and possibly other things.
Ideally, for me, the "Production" tech category icon would have a pickaxe or mineral chunk to represent mining, as well as a gear to represent industry, since the category contains techs for both.
Rather than using a factory as the production/construction icon, make it a more generic looking building perhaps? So that it's not a factory that is itself producting / constructing things, but the building itself that is being constructed...
Also, consider my previous suggestion for the construction icon of a hammer and incomplete building, similar to my icon for "buildings under construction" on a particular planet for the buildings window.
HOWEVER: If you wanted to redo all the resource icons (I don't really like them anyway), you could
a) make "Industry" not a gear anymore, thus making "gear" refer exclusively to construction
b) make research blue food green and production grey or somesuch, rather than food yellow (wheat) and research green and production blue as they are now
c) make a similarly styled trade or money icon, which we currently don't any version of...
I really don't understand what a gear has to do with construction though... A wrench, sure... but a gear? I don't see the connection...
Edit: actually, just a hammer alone, w/o any building might be a decent construction icon...