# Susceptibility to Expansive Reactions of a _Greener_ Uhpc: Micro to Macro-Scale Study
###### Abstract
The application of thin layers (20-65 mm) of UHPFRC, which can be bar-reinforced, replacing carbonated and/or cracked-covered conventional concrete, promotes the improvement of the mechanical properties and the waterproofing of (part of) the structural elements exposed to severe environments and/or critical loads. Additionally, increased service life and reduced operations and maintenance costs are expected. This solution has already been applied worldwide with promising results [3; 15; 16; 17; 18; 19; 20; 21]. Although much of the focus on UHPFRC has centred on mechanical and/or structural performance, the \"in situ\" mechanical performance will be inevitably linked with durability against aggressive media. Antagonistically to conventional concrete, the dense microstructure of UHPFRC has almost no capillary pores. The UHPFRC pores are mostly between 0.1-0.001 um, and thus the pores are mainly located within the calcium silicate hydrate (CSH) gel [22; 23; 24; 25]. Nevertheless, the pores' size and interconnectivity are dependent on age, heat treatment, the w/b ratio, the type of cement, and the use of supplementary cementitious materials (SCM) or nanomaterials [26; 27; 28]. The literature survey shows that the average UHPFRC porosity corresponds to 6.4% for a w/b ratio range of 0.13-0.22 and a compressive strength of 100-279 MPa [29]. This compact microstructure is responsible for both mechanical and durability behaviour. The compact microstructure of UHPFRC is achieved through an adequate raw materials selection and mixture design. In brief, the granular skeleton mix design is optimised by the packing density theory. The first step excludes the coarser aggregate; the fine aggregate usually has a maximum particle size of 1.0 mm [30; 31; 32]. The coarser aggregate eliminating in UHPFRC has two main purposes: (i) reducing the weaknesses induced by the interfacial transition zone (ITZ) between the cementitious matrix and aggregates [33], and (ii) reducing the overall porosity in the granular matrix. Usually, the total amount of aggregate ranges from 1000-1200 kg/m\({}^{3}\), which corresponds to 38-45% of the total volume of the composite [34] and an optimum aggregate to cement ratio between 1.0 and 1.4 is recommended [31; 35] depending on performance requirements. The choice of cement is of utmost importance since it is the main ingredient of UHPFRC, accounting for almost 30% of its volume. The cement generally used is Portland cement CEM I 42.5R or 52.5R, and CEM II 42.5R or 52.5R according to EN 197-1, because of their early high strength potential [3]. The binder fraction of a UHPFRC usually contains a substantial amount of silica fume (SF). The primary function of SF is physical, i.e., SF particles fill the spaces between the cement particles, other SCM, and the aggregate. Moreover, SF presents high pozzolanic reactivity, providing additional CSH and decreasing the porosity of the cementitious mixture. A large fraction of silica fume is recommended by some authors, 20 to 35% of cement weight [3], to ensure a dense granular packing. Nevertheless, SF presents some limitations, such as availability and high cost. Thus, several studies have been conducted on the feasibility of using other supplementary cementing materials (SCM) to partially replace PC and SF in UHPFRC. These include both conventional SCM, such as limestone filler (LF) [36] fly ash or slag [37; 38], as well as unconventional, obtained from agricultural or industrial wastes, such as bottom ash [38], sugarcane bagasse ash [39], glass powder [40; 41], and RHA [42; 43]. The use of alternative SCM reduces the cement dosage and contributes to waste/by-product valorisation and it is an integrated way to promote more sustainable advanced cementitious materials.