## 1 Introduction  
The physical, chemical, mechanical and biological actions affecting concrete structures throughout their service life compromises their durability and compliance with the ultimate limit state and service standards to which they were designed. The result is substantial yearly repair and maintenance costs, which in 2016 came to \(\sim\)5123 \(\times\) 10\({}^{9}\) worldwide according to ASCE 2017 data on infrastructures [1]. [PERSON] et al. [2], in turn, reported that maintenance costs amount to 14% to 20% of design costs in the UK, Hong Kong and Australia. The presence of physical-chemical pathological processes in such structures is associated with parameters (choice of materials, type of environmental exposure, w/c ratio in concrete, structural shapes, etc.) defined in project stages ranging from design and construction to operation [3]. When choosing materials, the type of binder (cement), as the concrete component that interacts most readily with aggressive media (sulphates, chlorides, seawater, etc.) in the environment is of particular relevance to durability. Such interaction directly and indirectly induces the formation of expansive products that prompt cracking and/or surface spalling to the detriment of the bearing section of the concrete. Given that cement susceptibility to such action depends on the chemical composition, crystalline or gelatinous nature and porosity of cement hydration products, those factors play a decisive role in the reactions with aggressive agents. The presence of calcium silicate or aluminate hydrates and especially portlandite is therefore a determinant in cement durability [4]. Along those lines, blending supplementary cementitious materials (SCMs) into cement or concrete is known to improve concrete structure durability [5]. Pozzolanic SCMs in particular favour durability due to their effect on pore system refinement and reduction of pore solution alkalinity and portlandite content in cement matrices [4,6]. In addition to those technical advantages, the use of SCMs carries social and environmental benefits associated with lower natural resource consumption and the valorisation of industrial by-products, as well as a significant decline in CO2 emissions. CEM II/A-LL or L manufacture generates 16.2% and CEM III/B manufacture 73.0% less of that greenhouse gas than CEM I [7]. Against that backdrop, the scientific community has shown a growing interest in the last 10 years in identifying SCMs that may serve as alternatives to the silica tune, fly ash, blast furnace slag, natural or calcined pozzolans and limestone envisaged in European standard EN 197-1 [8]. One of the most prominent lines of research underway involves the assessment of the mechanical viability of new SCMs sourced from construction and demolition (C&DW) and agroforestry waste in the formulation of more sustainable binary eco-cements. In 2016, the European Union alone generated 923.9 Mt of C&DW, accounting for around 35% of all the solid waste produced by its 28 members [9]. Those by-products can be separated, crushed and sieved to yield recycled aggregate (RA), in turn classified under three categories depending on concrete (Rc) and masonry (Rb) content [10]: concrete RA (Rc \(\geq\) 90%; Rb \(\leq\) 10%), mixed RA (Rc \(<\) 90%; Rb \(\leq\) 30%) and mixed masonry RA (Rc \(<\) 70%; Rb \(>\) 30%). Whilst the coarse (\(>\)4 mm), and especially the concrete, fraction have been extensively researched [11,12] for use as the granular skeleton in recycled concrete, little scientific-technical knowledge has been collected on the fine (\(<\)4 mm) and ultrafine (\(<\)0.125 \(\upmu\)m) fractions. The valorisation of those materials, comprising 30% to 40% of all C&DW [13], is a challenge to be confronted if circular economy principles are to be instituted in the construction industry.