**1. Introduction**

With the advancement of cement science and technology and due to the necessity for slender and bolder structures, conventional concrete (CC), also named as normal strength concrete [1], no longer meets the requirements for the execution of these works. In this context, other concretes and other cementitious mixtures, with properties superior to CC, emerged to meet this need. This is the case for high-strength concrete (HSC) [2], highperformance concrete (HPC) [1,3] and, more recently, ultra-high performance concrete (UHPC) [3,4].

Another important aspect of using HPC and UHPC is the environmental issue, as these concretes provide lower values of binder intensity (*bi*). This index measures the total amount of binder needed to provide one unit of a given performance indicator, for example,

**Citation:** Marvila, M.T.; de Azevedo, A.R.G.; de Matos, P.R.; Monteiro, S.N.; Vieira, C.M.F. Materials for Production of High and Ultra-High Performance Concrete: Review and Perspective of Possible Novel Materials. *Materials* **2021**, *14*, 4304. https://doi.org/10.3390/ma14154304

Academic Editors: Rossana Bellopede and Lorena Zichella

Received: 6 July 2021 Accepted: 29 July 2021 Published: 31 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

compressive strength in MPa, as per Equation (1). In this sense, the application of these types of concrete provides an ecological gain, due to the reduction in cement consumption to reach the same level of compressive strength [5,6].

$$bi = \frac{b}{\sigma} \tag{1}$$

where *<sup>b</sup>* is the total consumption of binder materials (kg·m−3) and *<sup>σ</sup>* is the compressive strength (MPa) at 28 days.

It is very common to confuse these terms, due to the absence of technical standards that provide satisfactory definitions for these materials. Some standards for concrete structures, such as NBR 8953 [7], NBR 6118 [8], ACI 363 [9], ACI 318 [10], and BS EN 1992 [11], differentiate two classes of concrete, as a function of characteristic compressive strength (fck). For instance, class I concretes have a compressive strength between 20 and 50 MPa, while class II concretes have a strength between 55 and 100 MPa. It can be considered that class I concretes be associated with the CC while class II concretes are related to HSC.

While the definition of HSC is based only on compressive strength, the definition of HPC and UHPC is related to performance, which includes not only mechanical strength but also workability, aesthetics, finish, integrity, and durability [12,13]. Despite not being unanimous, a satisfactory definition for the HPC is that this concrete presents a compressive strength equivalent to the HSC, that is, fck above 50 or 55 MPa [2,14], but presents a workability equivalent to self-compacting concrete (SCC), that is, a spread between 455 to 810 mm by the slump-flow test [1,15,16]. Regarding the water/binder factor (w/b), some authors suggest that this factor should be less than 0.40 [17–19], contrary to what is observed in conventional concretes, where the w/b factor ranges from 0.45 to 0.65 [20–22]. Regarding cement consumption, it is usual for it to be between 400 to 700 kg/m3 [23–26], while for CC, the cement consumption is usually between 260 to 380 kg/m<sup>3</sup> [25,27,28].

The UHPC, on the other hand, presents even greater requirements. Some authors suggest a minimum strength of 120 MPa [13,29], while others stipulate a minimum of 150 MPa [30,31], with a fluidity equivalent or greater than HPC, in addition to low porosity. In theory, a concrete that presents a strength above class II could be considered a concrete with a mechanical performance superior to HSC or HPC. In other words, these materials would compose a kind of strength class III, where the UHPC fits. These properties are achieved using a w/c ratio between 0.2 to 0.3 [32,33] associated with a very high cement consumption, around 800 to 1000 kg/m<sup>3</sup> [34,35]. In addition, UHPC is usually produced without the use of coarse aggregates, which theoretically turns it into a mortar, allowing the evaluation of workability to be carried out by the flow table test. Some authors recommend that flow table measurements be greater than 260 mm, without the application of blows to the material [36]. All this information is summarized in Table 1, which presents the main characteristics of the concretes discussed in this literature review.



NA—Not applicable.

Another way to understand the difference between the concretes presented in Table 1, from the point of view of compressive strength. This is one of the reasons for the need to apply fibers in UHPC, which will be further discussed in this paper. In addition to conventional (i.e., steel and polymeric) fibers, the search for alternative natural fibers, such as basalt, sisal, and banana fibers has grown in recent years (detailed in Section 3.3) mainly due to their low cost and good fiber-matrix bond. Finally, despite the great interest in the application of fibers in HPC and UHPC, few standards are available to specify it [37].

The main applications of HPC and UHPC initially were in the construction of highrise residential and/or commercial buildings, mainly between the 60 and 80 s. Examples are the Lake Point Tower and Water Tower Place buildings, in Chicago—the USA, built, respectively, in 1965 and 1970 [23,38]. Later, these classes of concrete started to be applied more frequently in infrastructure works, through the construction of bridges, viaducts, and other special buildings of art [39–41]. Since then, HPC and UHPC have been also applied to road pavements, industrial floors, and underground buildings [42,43]. More broadly, these types of concretes can be applied in any works requiring high compression loads as well as structures subjected to aggressive environments that need high durability and in cases of emergency or recovery works [12,13,44]. However, the cost must be taken into account when analyzing the feasibility of the building construction, since in general high-performance concrete is more expensive than conventional concrete [45,46].

In this context, the objective of this literature review is to present some concepts related to the main materials used for the production of HPC and UHPC. The materials named in this research as "classics" will be highlighted, as they are the same ones traditionally used to elaborate the CC. In addition, it is also an objective of this review to identify new materials used to obtain (ultra) high-performance concretes; in particular through the inclusion of fibers and the use of alkali-activated cement.

#### **2. Classic Components Used for HPC and UHPC Production**

In this section, the main materials that make up the structure of HPC and UHPC will be described. These materials are named classics as they are also used for CC production. These are ordinary Portland cement (OPC), mineral additions, aggregates, and chemical additives.
