I love the idea of using the scraps of the burgeoning cannabis industry to build houses, but the embodied energy in the lime cements most mixes call for is a real turn off. Anyone have any ideas for an alternative hempcrete binder?

Clay, although lime is pretty carbon neutral.

Plaster of Paris?  However, I started roasting some drywall, and by the time I was satisfied it was cooked I think I accidentally made lime... oops.  blooper.

lime is carbon neutral only in the the fact that it absorbs all the carbon dioxide it emits during its transformation. it does not take into account the emissions from the mining, or the transportation. which is pretty huge.
clay, on the other hand is dug out from your neighbours yard. local always wins!

Well consider earth blocks instead

Earth blocks are often stabilised with an amendment of up to 10% lime. I have seen this with rammed earth as well.

I would also go the earth block/rammed earth route, but I think that there are areas of concern with regards to carbon footprint that are orders of magnitude more serious than a little lime in your mix.

Incidentally, I don't think it has to be an either-or situation. Some experimentation would need to be done, but using hemp hurds in rammed earth or earthblock would basically entail making a mix with a moisture content more in-line with that of rammed earth or compressed earth block, either for compaction in the wall forms or in the brick machine; not much would change.

The embodied energy has to be weighed against the effect it has on the lifespan of the project itself. If your goal is to have a structure that dissolves without a trace in a generation, the embodied energy would be a waste, and would mean longer-lasting rubble after the structure has collapsed.

If, on the other hand, adding 10% lime to your mix means that the structure outlasts your grandchildren and allows you to heat the structure with passive solar gain and a candle, that embodied energy, rather than being a waste, will have been spread over three generations or more.

I think there are many more useful ways to cut one's carbon budget, and ones that don't run the risk of almost-hempcrete walls collapsing on you.

-CK