*2.1. Ordinary Portland Cement (OPC)*

The OPC used for the production of HPC and UHPC can be the same for the production of CC. However, several authors recommend the use of OPC with higher clinker contents and lower amounts of mineral additions. This is necessary because the production of HPC/UHPC uses mineral additions with greater reactivity than those usually considered in the production of commercial cement, such as blast furnace slag and fly ash [13]. Different denominations are used for these special OPCs. According to the main international standards, these cement are known as CP-I. CP-V-ARI cement according to the Brazilian standards NBR 16697 [47]. CEM I cement according to the European standard EN 197-1 [48]. Type I cement according to the American ASTM C150 [49]. All these cement have in common their high clinker content, usually above 90% or 95%, and also high fineness. However, it should be noted that CP-I cement is not a commercial product, which is why research-based on NBR 16697 [47] focuses on the use of CP-V-ARI.

In particular, the use of one of these types of OPCs, the Brazilian CP-V-ARI, is cited as an example. Silva et al. (2020) [50] studied the effect of high temperatures on the mechanical performance of HPC/UHPC containing recycled aggregates and CP-V-ARI. This cement had a compressive strength at 7 days of 37.4 MPa and at 28 days of 43.7 MPa. The reference concrete compositions presented a compressive strength of 65.2 MPa, proving that it is an HPC. Roberti et al. (2021) [13] studied the autogenous shrinkage effects and the fresh and hardened state properties of an HPC. They used cement CP-V-ARI, with a strength of 38 MPa at 7 days and 45.8 MPa at 28 days. The evaluated compositions showed compressive strength from 68.3 to 84.3 MPa. Viana et al. (2020) [51] used CP-V-ARI

to evaluate the influence of the incorporation of carbon nanotubes in HPC, obtaining compressive strength for concrete around 80 MPa at 28 days of cure. De Matos et al. (2020) [52] used CP-V-ARI to produce UHP cement pastes with 28-day strength of around 130 MPa. Pilar et al. (2021) [53] studied the rheological behavior of HPC/UHP using cement type CP-V-ARI, however, the authors did not report the results of compressive strength since the focus of the work was to evaluate the properties of the fresh state.

It is also important to mention other investigations on special processing and aggregates. Sohail et al. (2021) [1] studied the durability characteristics of HPC and obtained concrete with compressive strength of a little less than 100 MPa at 28 days. Storm et al. (2021) [54] evaluated the ways in which different fibers were pulled out in highperformance concrete. Li and Zhang (2021) [55] studied the thermal stresses in UHPC containing polypropylene and steel fibers, obtaining a compressive strength of 141.5 MPa. Liu et al. (2021) [56] evaluated the application of steel slag as a complementary material in UHPC, obtaining strength between 120 to 150 MPa. Suescum-Morales et al. (2021) [57] evaluated the effect of temperatures on high-performance concrete performing a microstructural analysis. Rashid et al. (2020) [58] evaluated the effects of using magnetite sand in UHPC, obtaining compressive strength of 134 MPa at 28 days. Olawuui et al. (2021) [59] evaluated the development of initial and long-term strength of HPC containing polymer incorporation, obtaining a strength of approximately 80 MPa at 28 days. Zhang et al. (2021) [60] valuated the fragmentation strength and mechanical properties of UHPC at high temperatures, obtaining strength around 130 MPa.

On the use of type I OPC, it is worth mentioning the following works. Choi et al. (2021) [61] evaluated the effect of TiO2 as a filler in concrete, obtaining compressive strength above 150 MPa at 28 days, typical of UHPC. Khan et al. (2020) [62] developed an ultra-high performance concrete for shielding from nuclear radiation, with a strength greater than 160 MPa. Kim et al. (2021) [63] used an OPC with a strength of 42.5 MPa to evaluate the benefits of curved steel fibers in the pullout strength of HPC. Yoo et al. (2021) [64] evaluated the effect of glass powder on the mechanical properties of UHPC, obtaining compressive strength greater than 200 MPa. Manigandan et al. (2021) [26] evaluated the use of treated banana fibers in the compressive strength of HPC, obtaining a value of approximately 52 MPa. Other researchers such as Yoo et al. (2021) [65], Kareem et al. (2021) [66], and Bae and Pyo (2020) [67] also used type I cement in their research with HPC and UHPC.

Regarding the chemical composition of the OPCs considered for the production of HPC and UHPC, it is observed that there are no major differences between the OPCs used for the production of CC, as shown in Table 2. The typical OPC base is essentially the same: between 60 to 72% of CaO, between 14 and 22% of SiO2, resulting in approximately 80% of the cement composed of CaO + SiO2. One can note that virtually all the cement presented in Table 2 had appreciable values (up to ~4%) of loss on ignition (associated with the CO2 thermal decomposition) and/or MgO. This can be explained by the presence of carbonaceous fillers, e.g., calcite (CaCO3) and/or dolomite (CaMg(CO3)2), or periclase (MgO). However, from a mineralogical point of view, it is relevant that the cement used for the production of HPC must present higher amounts of alite (C3S) and belite (C2S), which are the constituents of the OPC responsible for the formation of C-S-H, the main strength product of concrete [30]. Therefore, the CaO content must be analyzed coupled with the loss of ignition value, since this element can be present either as calcium silicate (i.e., C3S and C2S) or calcium carbonate filler. The content of C3A must be reduced because this mineral is incompatible with the conventional water-reducing chemical additives used since it mainly forms ettringite which adsorbs the additive molecules and tends to increase the additive content required to reach proper flowability [68,69]. This is the reason why cement must have the lowest possible amounts of Fe2O3 and Al2O3.


**Table 2.** Chemical composition of Ordinary Portland Cement (OPC) used for HPC and UHPC.

Regarding other important properties of OPCs, it is noteworthy that the density of OPC for HPC and UHPC and for conventional applications is the same, around 3.10 to 3.15 g/cm3. This is due to the low content of additions, since silica fume and fly ash, for example, have a specific mass around 2.2 to 2.4 g/cm3. The fineness, in general, is greater, due to the use of high initial strength OPCs, which are generally thinner, and due to the low amount of water used, generally less than necessary for the hydration reaction. So, greater fineness helps in a considerable degree of hydration, which is why OPCs need to be thinner. The same happens with the percentage of material retained in the #200 sieve, considered an important parameter for post-production OPC grinding control. This information is summarized in Table 3, where it is observed that the fineness of the OPC varies in values above 3500 cm2/g by the Blaine method. In general, the OPC used for conventional concrete presents a fineness close to 3000 cm2/g. Regarding the percentage retained in the #200 mesh sieve, it is observed that the maximum value observed was 2%, much lower than other types of OPC used for CC production, such as CP-II, CP-III, CP-cement V, cement type II and III and CEM II. This is necessary because the larger specific area allows these cement to react faster than those used for conventional applications.


**Table 3.** Physical properties of OPC used in HPC and UHPC.

Based on this, it is possible to establish that the OPC used in HPC and UHPC must be chemically rich in calcium and silica, mineralogically rich in C3S and C2S, with the least number of mineral additions possible, aiming at the incorporation of more reactive pozzolans than those used in OPC commercial.
