This work aims to investigate the possibility of reusing tunnel excavated material to produce an earth-based mortar. The elaborated earthen mortars will be used as interior partition walls or coating to replace plaster materials. The challenge of this technic is multiple, in the context of a strategy to achieve a neutral carbon footprint in building: (1) to avoid the use of gypsum-based materials which, during demolition, can pollute the waste destined for re-use; (2) to reduce as far as possible the need to landfill excavated earth; (3) to develop panels or coating materials with improved thermal efficiency and environmental impact. The mechanical, thermal and hydric properties of excavated earth mortars were characterized after 28 days. A previous study pointed out that the increase of cement content improves the mechanical characteristics but at the same time affects negatively the thermal properties as well as the environmental factor. Hence, all mortars developed were stabilized with cement at 5% by mass. In total, 9 formulations were developed by varying the amount of hemp fibres and tannin powder. It is established that the addition of 1% tannin and 1.2% hemp fibres provided the best solution for optimum mechanical, thermal and hydric properties. This study not only facilitates the development of a material with favourable thermal properties and other suitable usage characteristics but also addresses a crucial issue concerning the management of excavated materials from major infrastructure projects. By proposing a solution aligned with the principles of the circular economy and sustainable development, it offers a practical approach to handling excavated materials in an environmentally responsible manner.