*2.3. Aggregates*

Aggregates are defined as inert, granular materials without defined shape and volume, used in concrete for economic and technological reasons [144]. They are classified according to their origin as artificial (or industrialized), natural, or recycled. Crushed stones are cited as an example of artificial aggregates. Other examples are natural sand washed from the river and construction and demolition waste used after recycled [145,146].

As for their own weight, aggregates are classified as light, such as expanded clay, conventional, such as crushed stone, or heavy, such as hematite aggregates [144,147]. From a granulometric point of view, they are classified into coarse, those whose grains pass through the 152 mm opening sieve and are retained in the 4.75 mm sieve, and in giblets, those whose grains pass through the 4.75 mm opening sieve and remain retained in the 0.075 mm aperture sieve [145,148]. The particle size distribution of the materials plays a major role in the fresh and hardened performance of HPC and UHPC. While the aggregates correspond to the macroscale components, the binder materials (i.e., OPC and mineral additions) correspond to the microscale fraction of concrete. In addition, the very small silica fume particles can improve the particle packing of the binder fraction, leading to higher compactness due to physical effects besides the pozzolanic contribution [149]. By optimizing the particle size distribution and mix proportions, one can achieve maximum particle packing, therefore improving the fresh and hardened properties of concrete. This strategy has been widely used for HPC and UHPC design over the last years [150–152].

Dealing specifically with the aggregates used in HPC, it is observed that there is no specificity regarding the use of these materials when compared to conventional concrete. However, care must be taken with regard to the particle size of the materials, in order to find the packing of all aggregates, following a continuous distribution, which presents the smallest possible void volume. This characteristic is even more important in the production of UHPC, which usually do not have coarse aggregates in their composition, due to the possible presence of micro-cracks caused by crushing or because the strength of the coarse aggregate is usually inferior to that of the cement matrix, making the aggregate the fragile point of the material. Owing to the above-described reasons, it is common for authors to use more than one type of fine aggregate or different combinations of fine and large aggregate to obtain the best packing. Arunothayan et al. (2021) [70], for example, uses three different types of sand as fine aggregate for HPC production used in 3D printing applications. With the different combinations proposed, the authors obtained compressive strength ranging from 110.1 to 152.5 MPa.

In addition to aspects related to packaging, some physical parameters need to be analyzed. Regarding the coarse aggregate used for HPC, the following points are worth mentioning. The content of fines, passing through the 75 mm sieve, should be limited to 1%, as this material is generally attributed to silt and clay particles, which can increase the aggregate water absorption. The D/d shape ratio, which relates to the largest size and smallest aggregate size, should be limited to 3 to avoid anisotropy in concrete. Water absorption should also be limited to 7%, as should abrasion wear strength measured by the Los Angeles method, which should be restricted to 50%.

On the D/d ratio, Zhao et al. (2021) [153] evaluated the influence of three coarse aggregate geometry on the mechanical properties of HPC. The authors used lamellar, irregular, and rounded aggregates. The strength results obtained by the authors are highlighted in Figure 3, where it is possible to observe that the best results are obtained with irregular aggregates. This is attributed by the authors to greater adherence between the cementitious paste phases and the aggregates, which improves the behavior of HPC.

Regarding the physical characteristics of small aggregates, some information is pertinent. For example, the number of fines is also limited, but to a total of 3%. The limit water absorption is 7%, while the swelling coefficient must be as low as possible, to avoid excessive volume increase. There are still some recommendations related to concrete durability problems, which apply to both coarse and fine aggregates. The content of chlorides and sulfates, for example, should be limited to 0.1% of the chemical composition of the aggregates.

This is necessary to avoid the occurrence of oxidation in the concrete reinforcement and to avoid the formation of late ettringite in the concrete, causing an expansive reaction that destabilizes the material volume and generates internal stresses [154]. Problems related to alkali-aggregate reaction (AAR) must also be verified through the mortar bar test. The maximum expansion allowed in this test is 0.05% after 3 months and 0.10% after 6 months. In AAR, a gel is formed that absorbs water and tends to increase the volume of the concrete, which can generate cracking and disaggregation of the aggregate paste [148,155].

**Figure 3.** Compressive strength results as a function of aggregate shape [153].

Analyzing the researches published with HPC and UHPC, it is observed that the authors traditionally use quartz sand washed from the river [1,3,13,70] or quartz sand from dunes [62,156] as the main fine aggregate. As a coarse aggregate, it is common to use crushed stones [25] and gravel [157]. The stones used are limestone [114,115,158] and diabase [159], mainly.

There are also researches using heavy aggregates such as barite, magnetite, and hematite for HPC in nuclear protection applications [147,160,161]. Figure 4, for example, shows the compressive strength results obtained using 3 types of fine aggregate: silica sand (97.32% SiO2), barite (58.69% BaO), and hematite (71.71% Fe2O3). The silica sand used has a specific mass of 2.7 g/cm3, while the barite has 3.0 to 4.4 g/cm3. Although the compressive strength results are better for the composition with silica sand, the radiation absorption values of cobalt and cesium were much higher for heavy concrete manufactured with barite, justifying its use in this type of application.

There are also researches that use light aggregates, mainly expanded clay, for the production of HPC. Angelin et al. (2020) [162] evaluated the packing of lightweight concrete containing expanded clay and rubber as aggregates, obtaining strength at 28 days of 58.5 MPa. Lu et al. (2021) [163] also obtained compressive strength results compatible with HPC using expanded clay aggregates. However, Garcia et al. (2021) [164] reported that the use of expanded clay in high-performance concretes is problematic, due to defects arising from the calcination of these aggregates. As a result, it is usual to increase OPC consumption and use a high number of additives, in addition to reducing the w/c factor, which is only possible using considerable amounts of plasticizer additive. This makes concrete too expensive and is therefore not recommended.

**Figure 4.** Compressive strength due to different types of aggregate [147].

Research that use recycled aggregates for the production of HPC and UHPC are also mentioned. These researches use aggregate from concrete waste [114,115] and ceramic waste [157,165]. Using these residues, which present the particle size curve within the normative limits of the ASTM C33 standard, the authors obtained a compression strength after 28 days of curing of 77.3 MPa, proving the feasibility of using recycled aggregates, as long as they meet the stipulated granulometry parameters.

Based on this, it is observed that the choice of aggregates for application in HPC and UHPC should be more carefully selected for application in CC, due to the need to obtain greater packaging. It is possible to use not only coarser and finer but also conventional and heavy aggregates, as well as lighter and recycled aggregates for the production of HPC and UHPC.
