In this study, the results of reported treatments are obtained, specifically the percentage decrease in the water absorption of RCAs and the increase in the compressive strength of RC at 28 days, substituting 100% treated and untreated RCAs. The relationship between the water absorption of RCAs and the compressive strength of RC is shown in Figure 2, where coarse RCA particles are between 4.75 and 20 mm, as well as in Figure 3, where fine RCA particles are less than 4.75 mm. All treatments that show a standard deviation have been extensively studied.

Compared to fine RCAs, the improvement in coarse RCAs quality has been more relevant in terms of number of proposed treatments, especially treatments that weaken the cement paste that adheres to the RCA surface (see Table 1). This result is explained by the fact that an increase in the particle size eases cement paste removal. In contrast, the results of the strengthening treatments for coarse and fine RCAs are significant, where an efficiency in compressive strength is approximately two and six times higher for coarse and fine RCAs, respectively, compared to the weakening treatments (see Figures 2 and 3).

For coarse RCAs treated with weakening methods (Figure 2), acetic acid soaking is preferred to improve absorption by only 20%, which also increases compressive strength by approximately 20%. This result is mostly caused by the reaction of acetic acid with the hydration products of the cement bonded to the RCA surface. This reaction weakens the bonded mortar, allowing it to be removed by subsequent mechanical rubbing (51, 52). However, the degree of adsorption in RCAs reportedly increases with the acetic acid concentration because it dissolves more hydration products; thus, increased porosity may be generated (34, 51). Meanwhile, to decrease absorption by 20% to 40% or more, scrubbing/crouching treatment is employed, which gives the greatest improvement in compressive strength (approximately 30%). According to Pandurangan et al. (35), in this case, the RCAs are treated using a Los Angeles abrasion machine, and the amount of mortar still adhered to the coarse RCAs after treatment is around 5%. The specific gravity is also improved (39).

For coarse RCAs treated with strengthening methods (Figure 2), although rehydration using fly ash (FA) significantly improves strength, it is not directly attributed to the improvement in the RCA quality but to the effect of adding the concrete mix. Therefore, the absorption value decreases, which is not highly significant. In this sense, soaking in diammonium hydrogen phosphate (DAP) favorably increases the concrete strength of treated RCA (approximately 30%), which results in a decrease in water absorption (approximately 20%). This result is due to the reaction between DAP and hydration products rich in free calcium (portlandite) on the RCA surface, which produces hydroxyapatite complexes responsible for achieving a pore-refining effect (90). Conversely, to improve absorption by 20% to 40% or more, FA or silica fume (SF) coating is the optimal treatment, which can increase compressive strength by over 50%. In this case, coating the RCA surface with a pozzolanic material, which is one of the most common methods, has been shown to improve the bond between RCA and new mortar (79). The process involves creating a pozzolanic powder slurry using FA or SF and subsequently incorporating RCA into the slurry. Its main effect is due to the filler effect of the pores on the RCA surface, which develops a stronger ITZ due to the pozzolanic reaction over time (81).

Finally, some treatments have reportedly shown no significant effect on compressive strength. Other studies with rehydration (FA) and coating (silane) have reported adverse effects in which compressive strength decreases. Thus, the study conditions should be investigated in detail to establish the feasibility of their application.

Figure 3 shows the data for fine RCA treated with weakening methods. Although there is a significant improvement in the percentage water absorption using acetic acid and hydrochloric acid (HCl) soaking, the increase in compressive strength is not more than 10%. These results, reported at a low significance level, verify that particle size influences the treatment effect because fine RCAs have a greater specific surface area than coarse RCAs. This implies an increase in the reaction kinetics, which generates an opposite effect. Furthermore, according to Tam et al. (41), focusing on the concentration level and time of treatments with acids, such as HCl and sulfuric (H₂SO₄) acids, is important due to the possible increase in the chloride and sulfate content, which affects the durability of concrete made from treated RCA and presents a health risk.

The scenario for fine RCA is promising with treatments that strengthen the cement layer attached to their surface (Figure 3). To improve absorption by approximately 20%, biodeposition- and nanosilica (nSi)-coating treatments are available, which increase the compressive strength by approximately 40%. The biodeposition method is based on the ability of bacteria, specifically Bacillus pseudofirmus, to produce calcium carbonate on the cell surface in the presence of suitable calcium sources. Chun-Ran Wu et al. (68) clarified that during biodeposition treatment, calcium carbonate formation was influenced by external factors, such as particle shape, size, and bacteria concentration. Similarly, Singh et al. (94) found that due to its high pozzolanic reactivity, nano-SiO₂ effectively improves RCAs quality. They concluded that the treatment effectiveness depends on the type, particle size, and reactivity of the employed pozzolan, as well as the calcium-hydroxide content that remains in the bonded cement paste for the pozzolanic reaction. In addition, Caijun Shi et al. (18) found that nSi coating treatment was effective in improving the quality of old and new ITZs of RC made with treated RCAs. However, a slight reduction in the mixture fluidity occurs because of the high specific surface area of the nanoparticles.

Figure 3 also shows a couple of treatments that reduce the water absorption by more than 20% (SF coating and CO₂ curing). Although the CO₂ curing of fine RCAs achieves a larger reduction in water absorption than SF coating, the improvement in the concrete strength of RCAs treated with SF coating is higher than that of CO₂ curing. This improvement is mainly attributed to the accelerated carbonation of portlandite (CH), which forms calcium carbonates with a large solid volume that fill the pore spaces and thus densify the microstructure (59). Chinzorigt et al. (95) found that the effect of carbonation on RCAs occurs mainly on the RCA surface and that the strength of the cement paste bonded on the RCA surface does not seem to show significant improvement because no carbonation occurs toward the interior. As demonstrated in several studies, CO₂ curing treatment is seemingly effective in increasing RCA quality and improving the hardening properties of RC, especially its mechanical properties. Conversely, research on the durability properties of concrete obtained with carbonated RCA is still limited and does not adequately optimize the concrete mix design for effective environmental benefits (65).

In RC, an increase in the volume of cement paste adhered to the RCA surface and the number of cracks or fissures increases the porosity, promoting the transport of external agents and thus reducing mechanical and durability performances (101, 103, 104). A negative effect is more evident for RC with fine RCAs than with coarse RCAs due to the increased specific surface area (105). Thus, the reduction in RC permeability is related to the RCA particle size.

According to the results reported by D. Pedro et al. (2017) and D. Brito (2016), fine RCAs affect RC permeability more than coarse RCAs due to the increased number of capillary channels in the concrete system. Conversely, the superior size of coarse RCAs results in a reduced surface area, thus reducing the amount of water required and increasing RC permeability (88, 106). For example, in Figures 2 and 3, regardless of the enhancement treatment used, coarse RCAs tend to show higher percentages of water absorption reduction (about twice as much) than fine RCAs. This behavior is associated with the fact that thin RCAs have a higher content of cement paste adhered to their surface.

In contrast, related to the effect of fine RCA content on RC permeability, Basheer et al. (107) found that an increase in fine RCA content decreased air permeability, concluding that increased flow path tortuosity contributed to this effect. According to their results, RCAs with an inferior average size and an efficient improvement treatment based on particle size should be used to improve the air permeability of RC.

The resistance of RC to carbonation is observed to be poorer than that of conventional concrete due to its porosity (101). In particular, the carbonation depth of RC can reach five times that of conventional concrete (108, 109), which can increase with an increase in the RCA content (10) and w/c ratio (110, 111). Furthermore, according to the literature, the carbonation thickness ratio varies from 1.0 to 2.5 for coarse RCA substitution and 1.0 to 8.7 for a fine RCA (11).

Figure 4 shows the effect on carbonation depth of some treatments applied to RCAs (treatments presented in Table 2), where a correlation between carbonation depth and compressive strength of RC after 28 days of curing, made from treated fine or coarse RCA, is observed.

For coarse RCAs, the applied treatments showed two tendencies (Figure 4 (a)). The first shows that the treatments reduce the carbonation depth, but the compressive strength tends to decrease, indicating a somewhat insignificant improvement of approximately 10%. The second allows identification of treatments where, in addition to exhibiting a significant reduction in the carbonation depth (between 25% and 40%), a linear trend appears with increasing compressive strength (approximately 25%). In this case, the treatment with maximum improvement is the CO₂ curing treatment. These results are consistent with those of previous research. Levy et al. (112) found that the carbonation depth is lower when recycled aggregates (100% substitution) are used compared to concrete made with NAs. They associated this result with the alkaline reserve of RCAs (which is superior and acts as a protective barrier). Thus, this positive effect of the alkaline reserve on carbonation provides a beneficial relationship with the service life of RC, which can delay the possible corrosion of steel. Further, CO₂ curing treatment reportedly provides a major beneficial effect on the durability properties of RC, showing a strong correlation between water absorption and permeability and suggesting that the water absorption value of RCAs can be used as a criterion for RC durability (57).

Regarding fine RCAs, which are widely used in practice, a reduction in mortar and concrete strength has been reported; therefore, replacing fine RCAs by not more than 30% has been proposed (113, 114). Considering this, treatment of fine RCAs is necessary. Figure 4 (b) shows that, according to the reported studies, a trend line can be drawn using four treatments (SF coating, nSi coating, CO₂ curing, and FA coating), which shows that a proportional improvement in carbonation (decrease in carbonation depth) is correlated with compressive strength (largely significant up to 55% for SF coating). In this case, the use of pozzolan is more effective than other treatments due to the pore refining effect on the structure, especially in old and new ITZs. Additionally, a reduction in the flowability of mixtures when SF and nSi are used has been reported, which is due to the large specific surface area of these materials (18, 40). This result suggests combining the carbonation methods with fine RCA and rehydrating the mixtures with pozzolanic materials.

Chloride ion migration has been shown to be a determining factor in concrete durability, especially because it promotes the corrosion of steel in reinforced concrete (48, 81). The penetration of chloride ions into concrete is also related to carbonation: when carbonation (reduction in porosity at the concrete surface) occurs, the free chloride content in the carbonated region decreases (60, 115). Thus, replacing NA with RCA can enhance the formation and propagation of cracks, which increases the risk of corrosion (116, 117). However, studies on this subject to identify the effect of treated RCA on the long-term performance of RC are few.

Figure 5 shows the effect of various treatments on the penetration of chloride ions into the RC with treated coarse RCAs (Figure 5 (a)), where four regions of enhancement are defined. In all cases, chloride ion penetration is reduced. When correlated with compressive strength, some regions (Region 1) do not show a significant effect (less than 10% improvement). In improving the compressive strength by approximately 10%, the combined effect of FA mixing/rehydrating reduces the chloride ion penetration due to the filling of pores and cracks in the cement paste bonded to the RCA surface (which decreases the RC porosity) and the high alumina content of FA (which can bind more chloride ions and thus increase the electrical resistivity of concrete) (86).

The last two regions show an improvement in both chloride ion penetrability and compressive strength, where we observe that the SF mixing/rehydrating, SF coating, and nanomaterial coating treatments produce a mostly significant improvement. The improved reduction in chloride permeability for the SF and nanomaterial treatments is mainly due to the pore-filling effect of SF, owing to its small particle size compared with other supplementary materials. Additionally, Faysal et al. (86) found that the effectiveness of supplementary materials against chloride migration depends on their pore-refining effect, which tends to increase the electrical resistivity of concrete. Conversely, the chemical reaction between chloride ions and tricalcium aluminate decreases electrical resistivity due to the decrease in free chloride ions.

For fine RCAs (Figure 5(b)), the results are organized as a linear response that reflects the increase in compressive strength and decrease in chloride ion penetration. These results are consistent with those of Zhang et al. (75), which showed that a nano-SiO₂-treated RCA enhances a new ITZ in RC and contributes to preventing chloride ion migration over time. In summary, it is possible to identify that carbonation treatments are the most reported method to improve the quality of RCAs. As mentioned above, this result is attributed to the formation of calcium carbonate during carbonation, which contributes significantly to the blocking of the pore network, and the transport of external agents (liquids and gases) in RC being majorly controlled by the pore network properties and the degree of water saturation (59). Figure 5

(a) RC with coarse RCAs. (b) RM with fine RCAs.