Hatakka (Aalto University) for help with pulp composition determinations as well as Hannu Revitzer (Aalto University) for elemental analysis. We thank Dr. Isabelle Morfin (ESRF) for assistance at the D2AM beam line, and ESRF (Grenoble, France) for the provision of beam time. At the D2AM beam line, the WAXS Open for SAXS detector (WOS) was funded by the French National Research Agency (ANR) under the "Investissements d'avenir" program, grant number: ANR-11-EQPX-0010.

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All-cellulose composites (ACCs) in which both matrices and reinforcing fibers are made from lignocellulose were successfully obtained from alfa (Stipa tenacissima L) and wood (Genus Betula) fibers. Matrices were prepared either from alfa or wood pulps that were dissolved in 8 wt% NaOH-water, while reinforcing fibers were alfa in both matrices. Prior to the dissolution, alfa and wood pulps were acid hydrolyzed in order to decrease cellulose degree of polymerization to 149 and 145, respectively. The degree of cellulose dissolution was 95-96 % for both alfa and wood dissolving pulps. X-ray diffraction (XRD) analysis confirmed the coexistence of cellulose I and II. The adhesion between the reinforcing fibers and both matrices was investigated using Scanning Electron Microscopy (SEM) and was found to be excellent. The density of ACCs decreased with the increase of the reinforcing fiber content for both types of matrices and ranged from 0.6 to 1.0 g.cm-3 leading to remarkably low-density ACCs. Young's modulus reached 3.8 and 4.2 GPa for the composites based on alfa and wood dissolving pulps, respectively, while yield stress reached 16 and 13.9 MPa, respectively.

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