4.3.2. Chloride Ingress

The chloride ions do not cause significant damage to concrete itself, but they contribute to corrosion of reinforcement in structural elements and can negatively affect the serviceability and safety limit states [31,117]. Sim and Park [36] performed chloride ion penetration tests on concrete with recycled concrete sand, whose water absorption was 6.45%. The sum of the principal oxides of the fly ash employed in this study (SiO2 + Al2O3 + Fe2O3) was 30.3%, with high CaO content (61.2%). The water/binder ratio was equal to 0.485, and the replacement contents of natural sand with recycled concrete sand were 30%, 60%, and 100%. The addition of fly ash in replacement of Portland cement CEM I was 15% and 30% by weight. The authors analyzed the depth of chloride ion penetration measured at different cure times compared to the addition of fly ash at the ages of 21 days and 56 days.

Based on the results, it is observed that the penetration of chloride ions at 21 days reduces significantly according to by how much the partial replacement of cement by fly ash is increased. For the concrete, at 56 days, the reduction was not so significant when compared to the reference concrete. According to the authors [36], the concretes with recycled aggregate for applications in structural elements obtained sufficient resistance to chloride ion penetration, and the resistance can be maximized by the addition of fly ash. Similar behaviors were observed by other authors [31,112].

Shaikh [33] used the method proposed by ASTM C1202 31 for the mitigation of chlorides in concretes with ultrafine fly ash and coarse aggregate from construction and demolition waste. The authors analyzed the effects of ultrafine fly ash (UFFA) on the permeability of chloride ions in concrete containing coarse recycled aggregates. The authors observed that for reference concretes (OPC), as the age of the concrete increases, the penetration of chloride ions decreases. By replacing the natural coarse aggregate with recycled coarse aggregate (RCA), there is a significant increase in chloride ion penetration compared to reference concretes (OPC). However, when adding 10% ultrafine fly ash (UFFA) in partial replacement of Portland cement, it is observed that there is a significant improvement in permeability at all ages. It is also observed that the addition of 10% UFFA will, in general, expand the chloride ion resistance of recycled aggregate concretes since, according to Shaikh [33], it serves to promote hydration and block the capillary spaces in the concrete matrix.

Thus, the chloride ions will penetrate the concretes with recycled aggregate more quickly due to a higher permeability rate in function of the capillary pores in the matrix of the cement and recycled aggregate. The addition of fly ash in concretes with recycled aggregate fills the capillary pores of the recycled aggregates making a denser concrete, and consequently improves the resistance of concretes to chloride ion penetration.

## 4.3.3. Carbonation Depth

Carbonation has a significant influence on concrete durability because this reaction reduces the pH of the water in the pores of the cement paste from approximately 12.6 to 8.3 [114]. When the low pH reaches the surface of the reinforcement, the thin passivation layer of oxides that is strongly adhered to the steel in the presence of moisture is destroyed, causing the beginning of the corrosive process [114]. According Khunthongkeaw et al. [118], mortars with fly ash contents lower than 30% have carbonation proposals similar to the reference mortars. Thus, it is crucial to know the resistance to carbonation of concretes with recycled aggregate and fly ash so that these concretes can be used in structural elements.

Geng and Sun [35] used recycled concrete aggregate as sand with a fineness modulus equal to 2.7. The fly ash employed in this study presented a sum of 88% in its principal oxides (SiO2 + Al2O3 + Fe2O3). The w/b ratio was 0.40, and the levels of substitution of natural sand for recycled sand were 20%, 40%, 60%, and 80% by weight, and the levels of substitution of ordinary Portland cement for fly ash were 10%, 20%, and 30% by weight. To perform the carbonation depth tests, the accelerated carbonation test was employed at a temperature of 20 ± 5 ◦C, with a relative humidity of 70 ± 5 and a carbon dioxide (CO2) concentration of 3%. The carbonation depth was measured after 7, 14, and 28 days of exposure to CO2. The authors compared the carbonation depth between the reference concrete (FC0), concrete with 40% replacement content of natural sand by recycled sand (FC14), and concrete with 40% replacement of natural sand by recycled sand combined with 10% (FCF1), 20% (FCF2), and 30% (FCF3) of replacement of Portland cement by fly ash, respectively.

In the first 7 days of curing, all concretes, with the exception of LC14 (4 cm), have practically zero carbonation depth. After 14 days of curing, the LC0 concrete remained with a carbonation depth close to zero. Concretes LCF2, LCF3, LCF1, and LC14 presented carbonation depths of 0.24 cm, 0.26 cm, 0.49 cm, and 0.78 cm, respectively. At 28 days of cure, the authors observed that the LC0 and LCF2 concrete had very similar carbonation depths (0.48 cm). The LCF3 concrete has a carbonation depth greater than the LCF2 concrete, which was 0.98 cm and 0.75 cm, respectively. The LC14 concrete has a carbonation depth of 1.49 cm.

Carbonation depth of the concretes with recycled sand and fly ash is lower than the concretes with only recycled sand. According to Geng and Sun [35], the effect of cement replacement by fly ash on carbonation depth reveals that carbonation initially decreases and then increases with a replacement rate from 10% to 30%, and then reaches the minimum at 20%. However, according to the authors, the amount of cement decreases with increasing fly ash replacement content, which leads to a decrease in the alkalinity of the pore solution, which is unfavorable for the carbonation resistance capacity of the concrete. This behavior was also observed by Limbachiya, Meddah, and Ouchagour [20] and Silva and Andrade [17]. Khunthongkeaw, Tangtermsirikul, and Leelawat [100] and Silva and Andrade [17] suggest that this behavior may be related, in addition to the reduction of calcium hydroxide (CH) due to the reduction of cement content, to the pozzolanic reaction of the fly ash, which predominates in the refinement of the pores, since there is a slowing of the initial hydration process [35]. Limbachiya, Meddah, and Ouchagour [20] also state that the reduction of the initial CaO content in the cement matrix leads to a decrease in the pH of the concrete, which will contribute to accelerating the carbonation rate.

Silva and Andrade [17] evaluated the resistance to CO2 of concretes with fly ash and mixed recycled coarse aggregate subjected to accelerated carbonation with a CO2 concentration of 3%, moisture content between 65 and 75%, and a total exposure period of 23 weeks. The first measurement of carbonation depth was at 15 days of exposure, and the other measures were taken every 30 days. They observed that the carbonation coefficient (adjusted according to Fick's second law) tends to be higher in concretes with recycled coarse aggregate and fly ash compared to reference concretes. This analysis was performed with reference concrete (R0F0), concrete with a 25% replacement of natural coarse aggregate by recycled coarse aggregate (R25F0), concrete with a 25% replacement of natural coarse aggregate by the recycled coarse aggregate, and 30% addition of fly ash in partial replacement of Portland cement, for a w/b ratio of 0.50.

The relationship between K (mm/month0.5) and t (months) was analyzed in all concretes and based on the authors' results. All concretes presented in the carbonation coefficient with increasing time of exposure to CO2. This behavior was observed in all concretes. In the first 15 days of exposure to CO2, the reference concrete was a carbonation coefficient of 4.9, while the R25F0 and R25F30 concretes were a carbonation coefficient of 5.9 and R25F30. At 145 days of exposure to CO2, the reference concrete was a carbonation coefficient of 3.97, while the concretes R25F0 and R25F30 were a carbonation coefficient of 3.98 and R25F30. According to the authors, the carbonation process is straightforwardly associated with the exposure time of the specimens to CO2, and the carbonation coefficient will, in general, balance out after some time. Similar behaviors were observed by Meddah and Ouchagour [20], Khunthongkeaw, Tangtermsirikul, Leelawat [113], and Ati¸s [119].

In general, it is found that with a partial replacement of Portland cement with fly ash, the CH decreases and makes the concrete more vulnerable to CO2 penetration. However, the pozzolanic reaction of the fly ash in the pore refinement, which occurs in concretes with higher ages, contributes to the carbonation resistance. Thus, it is possible to observe in the studies that initially, the carbonation resistance is lower at early ages, and as the age increases, the carbonation resistance is improved.

### 4.3.4. Microstructural Analyses

Microstructural analyses are essential to verify the products formed due to the chemical reactions between the components of the cement matrix and the aggregates, especially on the ITZ. Li, Xiao, and Zhou [120] observed the ITZ employing scanning electron microscopy (SEM). The authors investigated the properties of two groups of concretes: in the first (A) part of the mixing, water was mixed with a pozzolanic powder consisting of fly ash, silica fume, and blast furnace slag for 60 s in order to produce a paste with a relatively low water/binder ratio; then recycled concrete aggregate (RAC) was added to the paste and mixed for another 60 s to coat the surfaces of the recycled aggregate; finally, the remaining water, sand, and Portland cement were added to the mixture and mixed for another 120 s. The second group (B) mixtures were performed in a conventional way and without the pre-mixing of the recycled aggregate.

Microstructural analyses were performed on the concretes RC04A and RC04B to verify the influence of different types of the mixture on the ITZ of the two concretes that present the same proportions of materials. For concrete production, recycled aggregate from an old cement sidewalk was used, whose apparent density was 2.497 g/cm<sup>3</sup> and water absorption of 4.6%. The amount of binder and water was established at 500 kg/m3 and 220 kg/m3, respectively. A superplasticizer was added in a fixed amount of 0.8% to the binder in all mixtures. The density (in g/cm3) of the fly ash, silica fume, and blast furnace slag were 2.38, 2.20, and 2.75, and specific surface area values (in m2/kg) were 410, 20,000, and 240, respectively.

According to the authors, the RC04A concrete has a denser ITZ, and the hydrates are mainly composed of uniform CSH gel. With the new mixing technique, the pozzolanic coating layer forms a barrier that prevents water penetration. Workability is improved, and the ITZ is strengthened. On the other hand, concrete RC04B showed a crack with a length of 30–40 μm (perpendicular to the ITZ), and a large amount of CH crystals was observed in the ITZ. According to the authors, the occurrence of the cracks may be due to the water absorption from the paste by the recycled aggregate. After the water evaporation, it was verified the occurrence of voids that correspond to the cracks in the ITZ. Thus, it can be observed that different mixing techniques can contribute to making the material denser and with fewer cracks. The use of recycled aggregate without pre-mixing, resulting in a weaker ITZ, was also verified by Poon, Shui, and Lam [68] and Sidorova et al. [121].

To evaluate the ITZ bond, Juan-Valdéz et al. [122] produced concretes with 50% replacement of natural aggregate with mixed recycled aggregate whose composition was 44.11% stone, 33.56% bricks, tiles, sanitary ware, 17.51% stone with mortar, 0.44% asphalt, 0.75% glass, 3.48% gypsum, and 0.16% other materials. The authors used a Hitachi S-4800 scanning electron microscope with tungsten as the X-ray source, a Si/Li detector, and a Brucker XFlash 5030 EDS analyzer to verify the EDX elemental mappings. It was found that aggregates (natural and recycled) developed ITZ with satisfactory properties [122]. According to the authors, this result was due to the wetting of the recycled aggregate before its addition to the concrete mixture. Thus, it can be seen that saturation of recycled aggregates results in beneficial effects regarding the improvement of concrete microstructure, making a denser paste that improves ITZ properties.
