## 1 Introduction  
The recycling of wastepaper sludge ash (WSA) has been the focus of extensive research around the world [1; 2; 3; 4; 5]. Numerous reviews on the recycling potential of waste paper sludge ash have shown that the most successful reuses are in the cement industry, construction and road engineering, geotechnical and agriculture applications. In agriculture, wastepaper sludge, whether incinerated or not, can be applied as a soil amendment when it is free of contamination [6]. However, very small volumes of WSA are spread on land. In mortar and concrete production, wastepaper ash replaced a part of cement. In these studies, the amount of WSA was optimized mainly according to the measurements of mechanical strengths. Typically, the WSA amounts range from 5 wt.% to 10.5 wt.% of the cement weight to obtain standard unconfined compressive strength (UCS) values for concrete [7; 8; 9; 10; 11]. Beyond that, the UCS resistance decreases but may be suitable for other construction and road materials [12; 13] and to increase the volume of recycled WSA. This is also the case for geotechnical applications such as soil stabilization [14], soil erosion protection and backfill materials [15; 16] where the required strengths are no longer those of construction materials. In order to recycle a larger volume of wastepaper sludge ash while maintaining UCS strengths in line with standard concrete and mortar values, recent studies [17; 18; 19; 20; 21; 22] have focused on the activation of WSA alone or in combination with other industrial by-products such as granulated ground blast-furnace slag (GGBS). Wastepaper sludge ash and GGBS act as precursors. Different activators are often used separately or in combination for geopolymer concretes, sodium hydroxide and sodium silicate, NaOH and Na\({}_{2}\)SiO\({}_{3}\), respectively [17; 18; 19; 20; 21; 22]. Precursors replace Portland cement, providing green materials. These recent studies mainly concern so-called green concretes, but few consider mortars and even fewer dry mortars. Dry mortars based on activated wastepaper sludge ash have the particularity of avoiding the use of cement, i.e., reducing the carbon footprint, recycling an industrial by-product (WSA) and ensuring water savings. Therefore, the possibility to use waste paper sludge-ash to be used as raw materials for supplying cementing materials is investigated. The mechanical aspects of waste paper sludge as dry mortar have been studied previously by [PERSON] et al. [23]. The previous study demonstrates the potential of use of paper fly ash and blast-furnace slag as a substitution for standard cemented material. The present study will be focused on a deep analysis of the contribution of the waste paper sludge ash and the blast-furnace slag on the microstructure evolution of the dry mortar matrix stabilized with paper fly ash and blast-furnace slag. In fact, the chemical reaction that takes place during the hydration of the binder leads to a greater generation of mesopores, i.e., a structural modification. This structural modification modifies the mechanical properties of the matrix in the short or long term. In order to allow a detailed analysis of the changes that take place, we need a suitable analysis method that allows access to information on the pore structure generated by the hydration of the binders tested. Currently, there is no direct method; therefore, indirect measurement methods are used. The most common method is the fluid or gas injection method: mercury, nitrogen, etc. The mercury intrusion porosimetry (MIP) and the Barrett-Joyner-Halenda (BJH, [24]) pore size and volume analysis are two of the available methods. The MIP method allows for the analysis of pore distribution, especially macropores, and is less accurate in the mesopore area, whereas the BJH method allows a fine analysis of the mesopores. The BJH method has the advantage of providing important information about the adsorption isotherm and the specific surface area of the materials.