*2.2. Mineral Additions*

Mineral additions are supplementary cementitious materials used together with OPC, with the aim of providing differentiated technological performance to cementitious products. Initially emerged as a need to improve some properties of concrete, it now contributes considerably to environmental aspects [75,76]. The need to replace Portland clinker with low-impact binders is today a common desire, but without losing the properties provided by these binders [77]. This is because ordinary cement is the world's second most used material, second only to water. In addition, OPC production releases great amounts of CO2 mainly due to limestone decarbonation, necessary for clinker production [78]. Another important factor is the high energy demand required to reach a burning temperature of around 1500 ◦C. As a consequence, some authors point out that for every 1 ton of clinker produced, about 0.8–1.0 tons of CO2 are emitted into the atmosphere [76,79,80]. A portion of this emission is mitigated through CO2 uptake by mortar and concrete carbonation [78,81,82]; however, this topic will not be further addressed in this paper.

As a result, some researchers have proposed the replacement of clinker by agroindustrial residues or by-products. This is the case of blast furnace slag, fly ash, silica fume, and other pozzolans such as agricultural ashes. It was observed a reduction in production costs, due to the reduction in the consumption of clinker, which generally has a higher cost than aggregates. For example, Li and Jiang (2020) [83] observed a 21% cost reduction when used 60% slag and 10% limestone in OPC replacement for the same concrete strength class. Zhang et al. (2021) [84] demonstrated that concrete mix design can be optimized simultaneously for environmental, economic, and mechanical objectives with silica fume incorporation. In addition, there is a contribution to sustainable development, and the achievement of concrete with greater mechanical strength, especially with the use of pozzolanic materials [85–87].

Mineral additions can be classified according to their reaction with the clinker as inert. This, in general, only contributes to a physical filling effect improving the packaging. For example, this happens with the application of limestone filler as well as in cement, such as blast furnace slag, and in pozzolanic products, such as metakaolin, fly ash, agroindustrial ash, and silica fume [85,88,89]. Cementing additions are materials, usually based on calcium, that present an accelerated reaction capacity in the presence of alkaline media, as is the case of Ca(OH)2 of portlandite present in OPC [90,91]. They can also be activated with the use of other alkaline hydroxides, such as NaOH and KOH, through alkaline activation, dispensing in this case, the presence of OPC [92,93]. This type of reaction will be described in Section 4.

However, the additions with the greatest potential for the production of HPC/UHPC are the pozzolanic ones that in addition to the physical packing effect, described above, have a chemical effect. Pozzolans are materials based on silica or silica and alumina which, isolated, do not present any binding power, but when finely ground, and in the presence of clinker and water, develop binding powers [94,95]. This happens through the so-called pozzolanic reaction. It is observed that portlandite, formed through the hydration of the silicates present in the OPC, reacts with the amorphous silica and alumina present in the pozzolan and forms C-S-H and C-A-H. As aforementioned, the compound responsible for the strength of hardened OPC is C-S-H. Thus, the conversion of CH to C-S-H provided by pozzolans contributes significantly to the strength of the formed product [94,96].

It is observed that mineral additions have certain requirements for their efficiency in clinker replacement. From a physical point of view, they need to present a high specific surface, measured by the Blaine fineness, for example, to increase the contact area of mineral additions with OPC [96]. Mineralogically, these materials need to be predominantly amorphous, which means that they present structural disorders. Otherwise, the additions are not reactive, as crystalline materials have an organized structure and hardly change their structure under normal conditions of temperature and pressure due to hydration reactions.

From a chemical point of view, there is a wide range of mineral additions used. According to the bibliography [97–99], there are reports of the use of materials rich in calcium, such as furnace slag, limestone filer, and class F ash as well as materials rich in silica or aluminosilicates, such as fly ash, silica fume, and metakaolin, which make up the class of pozzolans.

One of the most used additions is fly ash, a by-product of thermoelectric plants that burn coal to produce energy. Ash is trapped in the combustion gas exhaust system and, when pulverized, it acquires pozzolanic characteristics [95,100]. Another great advantage of fly ash is the spherical shape of the material, which can promote a rolling effect between grains and improve workability, providing a reduction in the amount of water and consequently contributing to mechanical strength. This material is mineralogically amorphous and has a chemical composition of approximately 45–60% SiO2, 30–32% Al2O3, in addition to Fe2O3 and CaO in variable but detectable contents [101–103]. As thermoelectric plants are the main energy sources in the world, with the exception of some countries such as Brazil, the availability of fly ash is high, which justifies the wide application of the material as pozzolans [103].

In the research by Mohan et al. (2021) [101], in which the authors used two different types of fly ash as additions for HPC/UHPC, the proportion of additions used was approximately 50% of the OPC mass. The authors obtained a concrete with 69 MPa compressive strength at 28 days. The two fly ashes used, named 1 (59.32% SiO2, 29.95% Al2O3, 4.32% Fe2O3, 1.28% CaO) and 2 (60.56% SiO2, 32.67% Al2O3, 4.44% Fe2O3, 1.41% CaO), presented a fineness by the Blaine method of 5636 and 6210 cm2/g, respectively. Comparing with the OPC used by the authors, with a fineness of 3300 cm2/g, it is observed that their additions were finer than this binder.

Other studies with the application of HPC/UHPC that used fly ash are highlighted below, to justify that this material is one of the most used additions in high-performance concrete: Sujay et al. (2020) [104] studied the effect of the application of steel fibers in high-performance concrete containing mineral addition of fly ash replacing 15% of the OPC mass, obtaining compressive strength at 28 days of approximately 55 MPa. Bahedh and Jaafar (2018) [105] studied the application of fly ash (69.41% SiO2, 28.20% Al2O3, 5.30% Fe2O3, 6.47% CaO) to replace OPC in percentages of 0–40% for the production of UHPC by molding in an autoclave, with the application of pressure in the production stage of the specimens. The authors observed that the use of 40% ash allowed to obtain a compressive strength at 28 days of 120 MPa, while the reference composition presented a strength of 80 MPa at that same aging period. This information is illustrated in Figure 1.

**Figure 1.** Influence of fly ash content on the compressive strength of ultrahigh performance concrete [105].

Zhang et al. (2020) [106] studied the effects of replacing 25% OPC by fly ash (50.35% SiO2, 29.65% Al2O3, 6.61% Fe2O3, 5.85% CaO) in HPC subjected to situations of high temperatures. The authors obtained a compressive strength at 28 days of approximately 55 MPa for the composition containing ash, while the reference composition showed a strength of less than 50 MPa, not being characterized as HPC. At all ages evaluated, the

compositions containing fly ash showed a better mechanical performance, which is directly related to the pozzolanic effect.

Choudhary et al. (2021) [107] evaluated the effect of various mineral additions on the abrasion and mechanical strength properties of high-performance concrete. Although the best results were obtained with the use of silica fume (95.58% SiO2, 0.71% Al2O3, 0.81% Fe2O3, 0.90% CaO), which will be discussed below, the results obtained with fly ash (58.19% SiO2, 26.93% Al2O3, 4.27% Fe2O3, 0.90% CaO) were also positive. For example, obtaining compressive strength at 28 days of 55 MPa. One of the reasons that explain the better performance of silica fume is the fineness of the material, 9550 cm2/g by the Blaine method, while fly ash showed 3530 and cement 2860 cm2/g. Another explanation is the chemical composition since silica fume has a higher SiO2 content than fly ash.

As already emphasized, silica fume is another pozzolanic mineral additive used in HPC/UHPC, with mechanical properties superior to those obtained by using fly ash. In the research by Choudhary et al. (2021) [107], cited in the previous paragraph, the compositions with silica fume obtained compressive strength at 28 days of 75 MPa, while the same compositions containing fly ash obtained 55 MPa. Some relevant information helps to explain this difference in behavior.

Silica fume is a by-product obtained in the production process of plain silicon or in silicon iron alloys. The process is carried out in large metal furnaces, by reducing the quartz in the presence of coal, or iron in the case of alloys, at very high temperatures, around 2000 ◦C. During the heating step, silicon monoxide (SiO) is eliminated as a gas, oxidizing and condensing into extremely small spherical particles of amorphous silica (SiO2) [108,109]. In terms of chemical composition, silicas fume have a SiO2 content above 95%, another characteristic that contributes to their pozzolanic effect [102,107].

This produced SiO2 material is trapped in the furnace exhaust gas filtration systems, being removed and used as a pozzolan. The average diameter of the material is 0.1 mm, about 100 times smaller than the average diameter of the OPC particles, presenting a specific surface by Blaine fineness in the order of 9000 to 10,000 cm2/g [109,110]. This contributes to the greater reactivity of this addition, in addition to contributing to granular packing and helping in pozzolanic reactions in regions that other conventional pozzolans cannot, such as at the paste-aggregate interface. However, it should be noted that silica fume, unlike fly ash, has a higher commercial value, as the production numbers of the silicon industry are much lower than the production of fly ash in thermoelectric plants [110,111].

Some researches that studied the application of silica fume are highlighted below. Wu et al. (2019) [112] studied the changes in rheological and mechanical properties with the use of silica fume (95.2% SiO2) in ultra-high performance concrete. The authors obtained strength of 120 MPa at 28 days of cure, with the use of 20% silica fume (94.8% SiO2) to replace OPC. Smarzewski et al. (2019) [113] used silica fume to evaluate the mechanical properties of UHPC, obtaining a strength of 95 MPa for the reference composition and 110 MPa for the composition containing 20% of the additive. Pedro et al. (2017) [114] and Pedro et al. (2018) [115] evaluated the mechanical properties and durability of HPC produced with recycled aggregate and silica fume (94.2% SiO2). The authors obtained compressive strength of 76.70 MPa after 28 days with the compositions containing silica fume in the first study and observed that the performance in the durability tests was superior for the compositions containing 20% silica fume, in all evaluated conditions (strength to carbonation, strength to chloride attack, water absorption by immersion, and by capillarity) in the second study.

Chen et al. (2018) [116] studied the application of fly ash to replace OPC in percentages of 0–30% for the production of UHPC by molding in an autoclave, with an application of pressure in the production stage of the specimens. The authors observed that the use of 20% ash allowed to obtain a compressive strength at 28 days of 125 MPa, while the reference composition presented a strength of 105 MPa at the same age, as shown in Figure 2. This change in strength was attributed by the authors to the greater formation of C-S-H, which is directly attributed to the pozzolanic reaction promoted by the material, as well as to the likely improvement in granular packing, related to the physical effect. In addition to the aforementioned research, several other studies used silica fume as a mineral addition [68,117–121].

**Figure 2.** Compressive strength of Ultra-High Performance (UHPC) containing silica fume [116].

Other incorporations are also used in the production of HPC and UHPC, although in much smaller proportions than fly ash and silica fume. Several countries, such as China, Japan, Brazil, the USA, India, and Germany, have a great potential for using blast furnace slag, obtained through the steel industries, a strong industrial sector in these countries. Blast furnace slag is generated as a by-product of the production of pig iron with a high calcium content [122,123]. The same can be highlighted in the ceramic industry, a strong sector in countries such as China, Brazil, Italy, and Spain, responsible for the production of ceramic waste and metakaolin, two pozzolans with a high content of silica and alumina [124–126]. Several countries, such as China, the USA, Argentina, and Brazil, also present the possibility of using agro-industrial ash, such as rice husk ash and sugarcane bagasse ash, creating alternatives for the application of renewable forms of pozzolans [127–129].

On the use of slag, the following works stand out. Shen et al. (2020) [130] and Shen et al. (2020) [131] studied the use of blast furnace slag in HPC obtaining a strength of 52 and 66.9 MPa, respectively. Cheah et al. (2019) [132] and Ma et al. (2018) [133] studied the use of blast furnace slag together with fly ash for the production of high-performance concrete, obtaining strength at 28 days of approximately 50 MPa. On the application of ceramic waste, it is worth mentioning the following research works. Kannan et al. (2017) [134] obtained a concrete of 51.5 MPa at 28 days with the replacement of 10% OPC by ceramic waste. Xu et al. (2021) [135] obtained a UHPC of 120 MPa at 28 days using 15% ceramic waste as pozzolan replacing OPC. Salami et al. (2020) [136] studied the application of metakaolin as a mineral additive in HPC obtaining a compressive strength of 60 MPa. Song et al. (2019) [137], Shehab et al. (2017) [138], and Tafraoui et al. (2016) [139] studied the use of metakaolin as an additive in UHPC containing different types of fibers. All authors obtained compressive strength at 28 days above 100 MPa.

Regarding the use of agro-industrial ash, the research by Le and Ludwing (2016) [140] stands out, which evaluated the use of fly ash, silica fume, and rice husk ash together for the production of UHPC, obtaining a strength of 110 MPa at 28 days. Le et al. (2015) [141] evaluated the durability of HPC containing rice husk ash as a pozzolan. Shaaban et al. (2021) [142] evaluated the rheological and hardened state properties of HPC containing this same additive, obtaining a strength of 60 MPa at 28 days. Finally, the research by Gar et al. (2017) [143] studied the use of sugarcane bagasse ash as pozzolan in HPC, obtaining strength at 28 days of 52 MPa.
