Ceramic matrix composites are interesting materials due to their good mechanical properties and their good damage resistance, even at high temperature. Nevertheless, their fabrication requires usually expensive constituents, and processing routes with long duration and expensive too. Geopolymers offer advantageous ways to produce ceramic matrix composites reinforced by long woven fibres: less expensive constituents and processing routes, fast fabrication, and low environmental impact. The objective of this project is to develop and optimize the fabrication methods of a sandwich composite material composed of long oxide-ceramic fibres (basalt, alumina) reinforced-skins with geopolymer matrix and a highly porous geopolymer core, and to study its mechanical behaviour and damage mechanisms. This type of sandwich composite is not classical and not well known, but could be useful to produce, for aeronautic or terrestrial transportation applications, structural lightweight parts with high thickness and good mechanical resistance even under temperatures of several hundreds of Celsius degree. The aim of this project is to develop the formulations of the geopolymer suspensions to produce on one hand the skins with an optimised impregnation of the fibres, on the other hand a highly porous geopolymer foam by an optimization of the processing routes to generate high rate of pores, and to perform the skin/core assembly by the simplest fabrication methods. Preliminary studies for this project have shown that this type of assembly is possible but needs to be optimized. A specific study of microstructures, physical and mechanical properties at room temperature will be done in help to reach these optimisations. The sandwich composite will be studied from a thermomechanical point of view to determine its mechanical behaviour at room and at high temperature, and to analyse the damage mechanisms associated. Bending and tensile tests instrumented with acoustic emission monitoring, digital images correlation and thermographic cartographies will be performed for that purpose. They will allow to follow the multi-cracking of the skins and of the cores according to the level and the type of mechanical loading, but will allow also to determine the interaction mechanisms between skins and cores at the interfaces of these parts of the sandwich. Thermomechanical fatigue tests will be analysed to evaluate the mechanical resistance of the composite under periodic loading, and to determine if additional damage mechanisms are induced by fatigue (for example, if microcracks are created by fatigue in the cores and could propagate progressively inside them and along the skins due to cyclic loading). The monitoring of the tests by acoustic emission and the various imaging techniques, but also the microstructural observations by electronic microscopy and X Ray tomography will allow to identify and analyse the damage mechanisms and to introduce them in finite element models in order to simulate the macroscopic mechanical behaviour from a numerical description of the composite at a mesoscopic scale, and to link these simulations to the macroscopic mechanical behaviour determined experimentally. The design and use of this composite will be done in a sustainable development context. That is the reason why the questions concerning recycling of this composite, especially by reusing ground composites as filler in new geopolymer pastes or suspensions, but also the problems linked to repairability of the composite by foam filling or uses of patches on the skins, will be investigated.