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
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
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