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

i was reading about some compressed hemp fibre earth blocks ,where a test was being done based on 5 formulas /mixes to determine which or how many solutions could work , one was woodash paste ,clay and chopped whole hemp stalk

Im going to start experimenting with hemp hurds and clay. Ive read some articles on it and it seems like it works, even more structurally load bearing than hempcrete/hemp lime...?? It sounds like the biggest factor is just drying time. I want to rip out all the sheet rock & insulation in my home and replace the insulation and walls with this natural product and put 3 coats of clay plaster/paint (?) on it. I have a brick exterior on the lower half of my home, timber framed house (also needs some plumbing and electrical work). I want to make cobb showers/baths, I had read somewhere that cobb can be waterproofed, so if anyone has any information on remodeling an existing home to a cobb or clay hemp home I am very interested. I just want to live in a natural healthy home, that breathes. I have a lot of clay soil right in my backyard. I also want to permaculture the front and back yards and have a food farm. I live in Cary NC if anyone has resources, ideas , I would appreciate the help. I also have some trees that need to be removed, but dont have a budget for that yet, I would like the trees to be used for wood projects/products or given to someone who needs them.
I read about a company in NY that is putting hemp clay or cobb on walls inside old brick homes...covering up the brick on the inside...is that doable? Does there need to be a gap/vapor barrier  between the brick and the clay/hemp??

Cob is a product that needs fibers which create a structural matrix that holds the mixture together. Hemp fibers would be fairly great for that or you could incorporate it into adobe bricks and plaster.