This research work aims to develop a rapid hardening lightweight composite including flax shives, foaming agent and earthern materials. The results show a satisfying workability just after the mixing step and a rapid hardening of about less than thirty minutes. The mixes with a large amount of flax shives show a thermal conductivity inferior to 0.1 W.m−1.K−1 and porous-like acoustic absorption coefficients. Even if the mechanical performances are low (fc28days = 0.1–1 MPa), the developed mixes, in association with a structural material, can be used with advantage the 3D printing technique.