Mixing lignocellulosic fibres with a mineral binder to produce fibreboards is a challenging process due to their large volume per unit mass and their susceptibility to agglomeration (balling effect). The main objective in the dry mixing-spraying process presented in our study is the uniform distribution of the geopolymer binder in the lignocellulosic material. In this work, we compare the properties of two types of composites processed by implementing the abovementioned technique. Geopolymer-bonded fibreboards were produced using up to 50 wt% seagrass or wood fibres. Microscopy and X-ray micro-tomography investigations of the geopolymer composites indicated that their mechanical and physical properties depend on the size of incorporated fibres. Large seagrass fibres were appropriately mixed with the mineral binder matrix forming solid fibreboards that were able to reach the standard requirements for cement boards. More specifically, seagrass-based fibreboards exhibit up to 42% higher bending strength (up to 9.4 MPa) compared to fibreboards composed of wood fibres. In addition, their low thickness swelling and low mean heat release rate in a cone calorimeter (varying from 21.5 to 26.6 kW m⁻²) indicated a high resistance to water and fire. Considering the resulting properties of the produced fibreboards, the dry-mixing spraying process can be an appropriate technique for producing geopolymer composites containing large amounts of relatively long fibres.