*4.4. Activator Solution*

The activating solution is produced by dissolving hydroxide and/or silicate of some alkaline metal in water as well as varying parameters of molarity and mass concentration. In general, the most used activators are sodium and potassium hydroxide, in addition to sodium silicate [258]. They play the same role as water in the hydration reaction of OPC, that is, starting the chemical reaction process of hardening the binder.

Some authors highlight the importance of using activated agents obtained by dissolving silicates since, in the alkaline activation of precursors rich in aluminosilicates, alumina dissolves before silica [232]. Thus, the use of silicates increases the reaction kinetics because it already provides, in a faster way, the necessary silica species for the activation

reaction [233,275]. However, it favors the occurrence of efflorescence. In the case of alkaline activation of calcium-rich precursors, the need for the formation of tobermorite (C-A-S-H).

One of the disadvantages of using silicates is the high cost associated with this material. This highlights the need for alternative activated agents, which, in addition to reducing the cost of alkali-activated cement, contribute to the sustainable development of this type of HPC and UHPC, as indicated by Mendes et al. (2021) [284]. It is suggested, for example, the use of activated rice husk ash, glass waste, and silica fume.

#### **5. Conclusions and Suggestion for Future Work**

The main objective of this review article is to evaluate relevant concepts related to precursor materials used for the production of HPC and UHPC. Although there is no normative definition, these concretes can be understood as materials with high mechanical strength, good workability parameters, and high durability. After consulting the bibliography, a limit of 50 MPa for HPC and 100 MPa for UHPC was established, analyzing the compressive strength at 28 days.

Initially dealing with classic HPC and UHPC, which generally uses the same construction materials as conventional concrete, it was observed that the main type of OPC used is one that is richer in clinker and with few mineral additions. This is necessary so that pozzolans with superior quality and reactivity than those used in OPC production are used in the production of HPC and UHPC. From a chemical and mineralogical point of view, it is preferable to use OPCs rich in C3S and C2S, that is, with low levels of oxides of Al2O3 and Fe2O3.

Regarding mineral additives, the most used are pozzolanic ones, such as fly ash and silica fume, although there are HPC and UHPC applications using blast furnace slag. Silica fume is the ideal pozzolan due to its high specific surface, high amorphism content, and high presence of SiO2 (>95%). This promotes the occurrence of pozzolanic reactions in a more intense way. Regarding aggregates, greater care must be taken when applying them in HPC and UHPC than with aggregates used for CC. These precautions are related to the degree of packaging, chemical composition, and shape of the grains.

For the production of HPC and UHPC, it is essential to use chemical additives, especially shrinkage mitigators and superplasticizers that allow the reduction of the w/c factor without loss of workability. The 3rd generation of superplasticizers, which work due to the electrical repulsion effect and the steric effect, are the most used for these applications.

Due to the low tensile strength and lack of ductility of HPC and UHPC, some authors proposed the inclusion of fibers for the formation of composites. The main type used is steel, followed by carbon, glass, polymeric and natural, such as sisal. The fiber improves mechanical properties but reduces workability properties due to the densification of the cement matrix. As a result, fiber contents must be studied to avoid harming the behavior of HPC and UHPC.

Another new perspective on the application of HPC and UHPC emerged due to the environmental problems generated by the production of OPC clinker. Thus, the use of alkali-activated cement, with the possibility of using waste and by-products as a binder, became a reality. The alkali-activated cement is produced through a precursor, rich in calcium (usually with >20% CaO) such as blast furnace slag, or rich in aluminosilicates, giving rise to geopolymers. The main precursors of this last group are metakaolin and lowcalcium fly ash. The main difference between the high-calcium and low-calcium precursors is that the former presents a higher reaction rate due to the higher solubility of calcium in comparison with silica and alumina, generally leading to higher mechanical strength at early ages compared with the latter. In addition, high-calcium precursors form calcium aluminosilicate hydrate (C-A-S-H) as the binding phase, while low-calcium precursors form 3D sodium/potassium aluminosilicate hydrate (N/K-A-S-H) frameworks. The last component used is the alkaline solution, generally based on sodium and/or potassium hydroxides or silicates. The results obtained, both in the fresh and hardened state, are compatible with HPC and UHPC applications. Besides, alkali-activated HPC and UHPC composites tend to present higher durability when compared with OPC-based composites.

In general, HPC and UHPC produced with OPC (and mineral admixtures) are easier to produce in practice because they do not require the manipulation of highly alkaline materials. In turn, those produced with alkali-activated materials generally have a lower environmental impact.

Finally, some perspectives for future work are highlighted:


**Author Contributions:** Conceptualization, A.R.G.d.A. and M.T.M.; methodology, A.R.G.d.A., C.M.F.V., M.T.M. and P.R.d.M.; validation, S.N.M. and P.R.d.M.; formal analysis, M.T.M.; investigation, A.R.G.d.A. and M.T.M.; resources, M.T.M.; data curation, S.N.M. and C.M.F.V.; writing—original draft preparation, M.T.M., A.R.G.d.A. and P.R.d.M.; writing—review and editing, M.T.M., A.R.G.d.A. and P.R.d.M.; supervision, S.N.M. and C.M.F.V.; funding acquisition, S.N.M. and C.M.F.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** CNPq: an FAPERJ, Proc. No. E-26/010.001953/2019 and E-26/210.150/2019.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data is available within the manuscript.

**Acknowledgments:** The authors thank CAPES, CNPq and FAPER for the financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.
