*2.4. Chemical Additives*

Chemical additives are materials used in the production of concrete aiming to improve properties of interest. They are generally used in quantities of up to 5% of the OPC mass [166]. The main types of chemical additives are air-incorporated additives, tack modifying additives, water-reducing additives, and shrinkage-mitigating additives, especially used in UHPC. As the name suggests, air-incorporated additives add air bubbles to concrete voids, improving the plasticity and workability of the material, but potentially compromising compressive strength. They are used in situations where concrete is subjected to ice and thaw, acting as a kind of reservoir where water can migrate, since when freezing the water expands, which could cause the concrete to crack [166–168].

Most used shrinkage mitigating additives are amphyphylic molecules, which have a hydrophilic end and a hydrophobic end. When interacting with the water present in the hydration of OPC, or any other polar solvent, the molecules of these additives are mainly absorbed in the liquid-vapor interface by electrostatic repulsions due to the interaction of hydrogen bonds [169,170]. In this way, there is a reduction in the surface tension of the water in the pores of the concrete, allowing a reduction of up to 50% in concrete shrinkage. The reduction is possible because shrinkage-mitigating additives continue to act in the pore system even with hardened concrete, reducing the effects of water surface tension that contribute to drying shrinkage [170,171]. The main examples of such additives are propylene glycol, general glycol ethers, and polyethylene glycol [169].

Set-modifying additives serve to delay or accelerate the setting time and hardening time of concrete. They do not significantly change the final strength of concrete, however, they do change the strength at early ages [172]. The water-reducing additives, on the other

hand, serve to reduce the w/c ratio without losing the workability of the concrete. They are the main additives used in the production of HPC and UHPC [173,174]. In addition to these additives, there are polyfunctionals that have two or functions simultaneously, generally modifying not only the set but reducing the amount of water.

As mentioned, water-reducing additives are the most used for the production of HPC and UHPC. These additives are subdivided into three generations. Depending on the amount of water they reduce in the concrete, these are: (i) 1st generation of superplasticizers that reduce from 6 to 12% of water; (ii) 2nd generation of superplasticizers that reduce 12 to 20% of water; and (iii) 3rd generation of superplasticizers that can reduce water above 20%, reaching a reduction of up to 45% of the mixing water [174,175]. In HPC and UHPC, 3rd generation superplasticizers are preferably used. The importance of water reduction is related to Abrams' Law, which indicates that the lower the w/c ratio, the greater the mechanical strength, using the same material parameters [176]. Logically, the reduction of the w/c factor impairs the workability of the concrete, which is why chemical additives are used.

The water reduction promoted by 1st and 2nd generations of superplasticizer reducers can be explained through the electrostatic dispersion effect. This effect occurs because the additive involves a system of OPC particle charges of the same sign [177]. Due to the effect of electrostatic repulsion, the superplasticizer will disperse the cement particles, making less water necessary to reach a given workability [178,179].

The 3rd generation of superplasticizers works due to the steric effect or due to the combination of the electrostatic repulsion effect with the steric effect [180]. This effect, which occurs mainly in additives based on polycarboxylate (PCE), the main additive used in HPC and UHPC. PCE features a long main chair, with shorter branches and side chains, increasing floor space in an OPC particulate system, resulting in much greater water reduction than 1st and 2nd generation plasticizers [181–183].

As examples of some additives used in HPC, the following works stand out. Ibragimov and Fediuk (2019) [184] evaluated the influence of different types of superplasticizers on the mechanical properties of concrete. The authors used five different superplasticizer additives: the first is a copolymer based on polyoxyethylene derived from unsaturated carboxylic acids (1st generation); the second is based on sodium salts of polymethylene naphthalenesulfonic acids (2nd generation); the third is a polyfunctional consisting of naphthalenesulfonate and an organic accelerator; the fourth additive used is a superplasticizer based on polyoxyethylene derivatives of polymethacrylic acid (PAA); finally a copolymer based on polyether carboxylates (PCE). The strength results obtained by the authors are shown in Table 4. It is observed that the additives that contributed the most to the compressive strength were the PAA and PCE, which is why they are the most used in the literature.


**Table 4.** Compressive strength results obtained with the use of various water reducing additives. Source: [184].

Benaicha et al. (2019) [185] analyzed the effects of superplasticizer additives on the rheological and strength properties of HPC. They used PCE-type superplasticizers in different percentages, obtaining a compression strength of 73.49 MPa at 28 days with the use of 0.3% of the superplasticizers. Cheah et al. (2020) [186] evaluated the changes in the mechanical and microstructural properties of HPC produced with PCE-type superplasticizers containing a ternary mixture of OPC, blast furnace slag, and silica fume. The authors obtained results compatible with the behavior of HPC, obtaining compressive strength at 28 days of around 80 MPa. Cheah et al. (2019) [132] evaluated the performance of HPC containing fly ash, blast furnace slag, and PCE-type superplasticizers. The compression results obtained were consistent with HPC applications. Finally, the work by Guan et al. (2021) [179] reported the durability effects of HPC sulfates produced with PAA-type superplasticizers. The authors did not perform mechanical tests but emphasize that the applied concrete presents behavior for high-performance applications.

Thus, based on what has been presented, it is observed that the use of superplasticizers is essential to obtain an HPC and UHPC with an adequate behavior. The most used additives are PCE, 3rd generation of superplasticizers that work by the combined principle of electrostatic repulsion with steric effect.
