Large chalk deposits are to be found in the North of France, and, as a result, developing chalk as a load-bearing aggregate is considered by French road building companies. Due to its limited mechanical performance, chalk is subjected to a medium heat treatment process (up to 500 °C). In this context, microstructure and poro-mechanical performance of Haubourdin chalk are characterized before and after heat treatment. The investigation of the so-called water weakening effect is associated to that of heat-treatment, mainly to show to what extent the strength of Haubourdin chalk decreases at a given water saturation state, prior to or after heat-treatment. In terms of microstructure analysis, SEM and FIB/SEM observations show the initial weak cementation of Haubourdin chalk, in direct relation with its moderate mechanical performance. After heat treatment, minor re-crystallisation of calcium carbonate is observed, which should be confirmed further. The specific behavior of Haubourdin chalk is highlighted under partial water saturation and after heat treatment, by using uniaxial compressive tests, and triaxial drained and undrained compressive tests. Pore collapse is quantified directly under increasing hydrostatic stress by measuring coupled gas permeability and porosity change. A heat-hardening effect is identified, which is counter-balanced by the water-weakening effect. The latter makes natural Haubourdin chalk unsuitable as load-bearing road aggregate, without further binding matter.