# Improvement of the Quality of Recycled Concrete Aggregate Subjected to Chemical Treatments: A Review

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{2}SO

_{4}), and phosphoric acid (H

_{3}PO

_{4}), among others—this paper presents a review of treatments for the removal of adhered paste using acidic solutions on the RCA, and their influence on the mechanical properties and durability of concrete produced with RCA. Pearson’s correlation was used in the statistical analysis to determine the linear relationship of the main factors—for instance, immersion time, acidic solution, and aggregate size—involved in the removal of the paste in the RCA.

## 1. Introduction

_{2}SO

_{4}), and phosphoric acid (H

_{3}PO

_{4}). Then, improvements to concrete in the fresh and hardened state are presented, ending with a Pearson’s correlation statistical analysis of several variables (acid molar concentration, immersion time, aggregate size, and acid solution) to determine its degree of influence on the removal and water absorption of RCA.

## 2. Chemistry of Acid Treatment in RCA

_{4}

^{2−}) in the adhered mortar of the RCA through the addition of HCl, H

_{2}SO

_{4}, and H

_{3}PO

_{4}. The chemical reactions responsible for cement corrosion depend on the type of acidic solution. Cement is an alkaline material with C

_{3}S, C

_{2}S, C

_{3}A and C

_{4}AF as its main components, whose reaction with water form hydration products—calcium silicate hydrate (C-S-H), calcium hydroxide or portlandite (CH), ettringite (C

_{6}AS

_{3}H

_{32}), calcium monosulphoaluminate (C

_{4}AS

_{3}H

_{12}), and hydrogarnet (C

_{3}AH

_{6}), among others; thus, they constitute a cementitious structure that can easily be attacked by strong acids [27,28].

_{4}

^{2−}in the cement structure from the exposure of the RCA to acids and the environment. When external sources of sulfate ions penetrate the cementitious matrix, initiating chemical reactions with the hydrated products such as portlandite (Equation (2)), they form gypsum and calcium aluminate phases, which form ettringite; this is followed by reactions of monosulfate (Equations (3) and (4)), tricalcium aluminate (Equations (5) and (6)), tetracalcium aluminate hydrate (Equation (7)) and hydrogarnet (Equation (8)) [29,30]. The calcium ions are initially supplied by portlandite, and when the ions are no longer available, the calcium silicate hydrate dissociates in the silica gel, providing the ions for the formation of ettringite [30,31].

_{2}due to the action of HCl and nitric acid (HNO

_{3}), results in the formation of CaCl

_{2}and Ca(NO

_{3})

_{2}salts, respectively. These soluble salts can be easily transported to the external parts of mortar using water. In this situation, the continuous reactions increase the porosity of the cement paste, and the increase in pore volume accelerates the reaction rate. In the case of an attack of H

_{2}SO

_{4}, the assessment of deleterious reactions can be divided into two parts. In the first stage, the deterioration of Ca(OH)

_{2}results in expansive plaster formation. Then, the plaster reacts with C-S-H in an aqueous environment and forms a more expansive product called ettringite [20,32].

## 3. RCA Properties

_{3}S content show higher resistance to compression at 28 days, demands a higher w/c ratio, and hydrates faster. The abovementioned results in a higher content of Ca(OH)

_{2}and a larger porous structure that is more vulnerable to an acid attack, increasing the removal of adhered mortar [40]. The effectiveness of removing mortar in RCA from concretes with a partial replacement of cement with pozzolanic admixtures or chemical admixtures varies due to differences in their chemical composition, i.e., the type of cement or chemical admixtures used. Pavlík [41] investigated the influence of the w/c ratio on the mortar corrosion process by acetic acids (CH

_{3}COOH) and HNO

_{3}, showing that the corrosion rate decreases with an increase in the cement content per unit volume of the hardened paste. The author stated that there are two main causes related to this fact: the increased neutralization capacity of the matrix and the increase in diffusion resistance of the corroded layer. The following are the main properties of RCA after being treated with acid solutions.

#### 3.1. Water Absorption

_{2}SO

_{4}, and H

_{3}PO

_{4}; in addition, RCAs with particle sizes of 20 and 11 mm were soaked for 24 h at 20 °C and the decrease in water absorption was obtained: 12.16% for HCl-treated RCA, 11.0% for H

_{2}SO

_{4}-treated RCA, and 8.36% for H

_{3}PO

_{4}-treated RCA for size 11 mm; the results were similar for a particle size of 20 mm, with a reduction in water absorption of 12.12% for HCl-treated RCA, 10.3% for H

_{2}SO

_{4}-treated RCA, and 7.27% for H

_{3}PO

_{4}-treated RCA. The different acids increase the content of SO

_{4}

^{2−}in the adhered paste of RCA; when reacting with the main mortar compounds of RCA, as described in Section 2, this allows the removal of the adhered mortar, reducing the water absorption capacity of RCA.

_{2}SO

_{4}to remove the bonded mortar in NA at concentrations of 0.1 M, 0.5 M, and 1 M, with a pre-soaking time of 1 and 5 days. The water absorption reductions of RCA were 2.3% for a 0.1 M molar concentration soaked for 24 h and 120 h, and 16.6% and 61.9% for 1 M molarity submerged for 24 and 120 h, respectively. Additionally, greater effectiveness occurred when using higher acid concentrations.

_{2}SO

_{4}in five particle sizes (20.0, 16.0, 12.5, 10.0 and 4.75 mm), at a concentration of 0.1 M and pre-soaked for 24 h, had a reduction in water absorption of 41% and 58%, respectively, for HCl and H

_{2}SO

_{4}, compared to untreated RCA.

_{3}) was used to pre-soak RCA at a 1 M molarity for 24 h, a procedure described by Movassaghi [48]. The results obtained were a 36.7% reduction compared to untreated RCA.

_{2}SO

_{4}, and HNO

_{3}, at a concentration of 0.1 M and after pre-soaking for 24 h; they showed that there was an improvement in water absorption of 10%, 11% and 13% for HCl, HNO

_{3}and H

_{2}SO

_{4}, respectively.

_{2}H

_{4}O

_{2}) on RCA, treated at a concentration of 0.1 M and with a 24 h soaking time. There was a decrease in water absorption of 4.22% for HCl, and of 4.33% for C

_{2}H

_{4}O

_{2}.

_{2}SO

_{4}) in an acidic solution at a 1:4.5 aggregate: acid ratio (1.2 M), submerged for 48 h, and replacing the solution with a new one after 12 hours of pre-soaking. The results showed a reduction of 38.6% for HCl and of 34.9% for Na

_{2}SO

_{4}.

_{2}SO

_{4}by pre-soaking it for 24 h and shaking the RCA occasionally inside the acidic solution. Then, to ensure that the treated RCA has no residue from the acidic solution, the RCA was washed and submerged in water for 24 h. The results after treatment showed a 10% reduction in water absorption compared to untreated RCA.

_{3}COOH) solution at an ambient temperature at three different acid concentrations (1%, 3%, 5%), and three different immersion durations (1, 3, and 5 days). The water absorption of all RCA samples was reduced by 9–19%. The lowest water absorption performance was achieved for RCA treated with 1% acetic acid. The authors mentioned that greater incorporation of acetic acid increases water absorption of the treated RCA. The authors also explained that this occurs mainly because more pores were produced in the treated RCA samples due to the dissolution of more hydration products, and possibly some NA in the acetic acid.

#### 3.2. Determination of Mortar Loss

_{2}SO

_{4}and calcium hydroxide is calcium sulfate which, in turn, causes increased degradation due to a sulfate attack.

_{2}SO

_{4}treatment technique to determine the amount of paste adhered to RCA, and proposed four techniques for total removal of the mortar: the first one was to submerge the RCA in H

_{2}SO

_{4}at concentrations of 1 M to 6 M; the second technique consisted of submerging the RCA in H

_{2}SO

_{4}at concentrations of 1 M to 6 M, renewing it after every 8 h of immersion, after which the RCA was washed before submerging it again in the solution; the third technique was to submerge the RCA at the same concentrations but with continuous agitation of the particles; finally, the fourth technique considered all the procedures previously described in the second and third cases. The results obtained showed that the removal of mortar varied between 12% and 100%, showing that the main factors for this removal were acid concentration and the removal technique (best results: techniques II and IV). In addition, it was observed that the acid concentration lost H

^{+}ions over time, reducing the degree of attack of the mortar. Washing the RCA before each replacement removed a layer of silica and aluminosilicate gels released by C-S-H from the RCA surface [52], increasing the removal efficiency.

_{2}SO

_{4}to remove the mortar at concentrations of 0.1, 0.5 and 1 M, reporting mass-losses of 2%, 14%, and 34% after one day of exposure, and mass-losses of 2%, 13% and 34% after 5 days of exposure, respectively. Authors such as Al-Bayati [8] and Saravankumar et al. [14] reported mass-losses of 3.92% and 5% for RCA for one day of soaking at concentrations of 0.1 M and 2.7 M, respectively.

_{2}SO

_{4}. The author obtained mass-losses between 2.66% and 9.09% for RCA sizes from 2.36 mm to 19 mm for Na

_{2}SO

_{4}. The values for HCl were 5.34% to 19.91% for the same aggregate sizes. The biggest removal was for HCl, as it is a more aggressive acid.

#### 3.3. Bulk Density

^{3}. Similarly, an increase in mortar content from 0% to 58% (by mass) for RCA produced from 60 MPa concrete led to a linear decrease in the bulk density of RCA from 2590 to 2340 kg/m

^{3}. The results showed that, as the density of mortar in 60 MPa concrete is higher than that of 30 MPa concrete, a similar mortar content in RCA produced from 60 MPa concrete has a higher bulk density than RCA produced from 30 MPa concrete.

^{3}and 2230 kg/m

^{3}, respectively. These figures were lower than those of 20 mm and 10 mm NA (2600 kg/m

^{3}and 2580 kg/m

^{3}, respectively). However, the physical properties of RCA improved after immersion in acid, with a higher increase for 10 mm aggregates (4.7%) than for 20 mm aggregates (2.2%) owing to the content of adhered mortar tending to be greater in smaller aggregates than in coarser aggregates [50]. Due to the relationship between aggregates’ density and absorption [37,44,54], the increase in RCA density results in the significant decrease in RCA water absorption. The density of RCA increases at varying concentrations of acid treatment.

_{2}H

_{4}O

_{2}and HCl, compared to untreated RCA. Saravanakumar et al. [14] observed that, after RCA treatment using three acids (H

_{2}SO

_{4}, HNO

_{3}and HCl), the bulk density had variations of less than 10%, 13% and 13% for each acid compared to untreated RCA, which showed a variation of 15% in relation to NA.

#### 3.4. Microscopic Analysis of the RCA

^{+}. Figure 2 shows a change in the morphology of RCA before (Figure 2a) and after treatment with HCl at molar concentrations of 0.1 M (Figure 2b), 0.5 M (Figure 2c), and 0.8 M (Figure 2d). A scanning electron microscope (SEM) clearly shows cleaner and more uniform surfaces. Saravanakuar and Manoj [14] also analyzed the microstructural morphology of the effects of treatments with HCl, HNO

_{3}and sulfuric acid on the surface of the RCA, showing the same degradation of the paste observed by Ismail and Ramli [9,10]. Al-Bayati et al. [8] showed that there are differences on the surface when RCA is treated with strong acids than with weak ones, reaching the same conclusion as the previous authors. A summary of the techniques discussed in this section is presented in Table 1.

## 4. Properties of Concrete with Treated RCA

#### 4.1. Fresh-State Properties and Density

_{2}H

_{4}O

_{2}at 0.1 M and a 24-h pre-soaking time. The authors also noted that there is a tendency for density to decrease when the incorporation ratio of treated RCA increases. This trend is also the same as other reported by other authors when using untreated RCA in the manufacture of concrete [55,56].

_{2}SO

_{4}before the production of concrete, and showed that concrete with treated RCA had an improvement in workability of around 23.5% for both acids relative to concrete with untreated RCA. Similar results were recorded by Pandurangan et al. [12] and Butler et al. [57] in concrete made with RCA treated with HNO

_{3}.

#### 4.2. Compressive Strength

_{2}SO

_{4}, and H

_{3}PO

_{4}, and substitution ratios of 5%, 10%, 15%, 20%, 25%, and 30%. The authors showed that the compressive strength after treatment had significant improvement, i.e., increases of 10.1%, 11%, 0,07% 6,67%, 12.8%, and 14.0% for HCl, H

_{2}SO

_{4}, and H

_{3}PO

_{4}, respectively, and incorporation ratios of 20% and 25% of RCA.

^{3}, and a 28-day compressive strength of 50 MPa. The results presented by the authors showed that the maximum replacement level to maintain strength at 28 days is up to 45% for RCA treated with HCl at molarities of 0.1 M and 0.5 M and not 0.8 M; this is because high concentrations are detrimental to the surface of RCA, leaving it more brittle and fragile and interfering with the good connection between cement and the particles. It was also observed that ratios of 15% incorporation of RCA in the mixes result in improvements in compressive strength, relative to mixes that incorporate untreated RCA. On the other hand, the authors showed that the effect of different RCA soaking ages on the compressive strength is insignificant.

^{3}, a 28-day compressive strength of 50 MPa, and a 60% replacement ratio of NA with treated RCA. The authors determined the compressive strength of concrete at 7, 28, 90, and 180 days. Concrete prepared with treated RCA had better performance than that prepared with untreated RCA. At 7 days, treated RCA concrete had a compressive strength 3% higher than that of control concrete. At 28, 90 and 180 days, the compressive strength of concrete with treated RCA was 96%, 99%, and 98% that of the control concrete, respectively, i.e., almost the same.

_{2}SO

_{4}—at a concentration of 0.1 M, soaked for 24 h, with a w/c ratio of 0.45, containing 380 kg/m

^{3}of cement, and under a 28-day compressive strength of 30 MPa. It was observed that the mixes with untreated RCA reached 80% of the reference mix’s 28-day compressive strength, while the mixes with treated RCA reached 90% and 95% of the same value for HCl and H

_{2}SO

_{4}, respectively.

_{2}SO

_{4}, HNO

_{3}, and HCl designed according to the ACI method [58], and mixing ratios of 1:1.4:2.3 (cement:sand:gravel) were used with a w/c ratio of 0.45. Ordinary Portland cement ASTM type 1, with a specific surface area of 3960 cm

^{2}/g and specific gravity of 3.15, was used. The authors replaced NA with RCA at 100%. The 28-day compressive strength of concrete made with recycled aggregates was 25% lower than that of concrete made with NA aggregates. In the treated aggregates, the loose mortar was removed as much as possible, and the characteristics of the aggregate’s surface were improved. The contact in the ITZ between the treated RA and the new cement paste improved and; thus, the 28-day compressive strength of concrete improved the treated RCA by 8 to 18% compared to concrete made with untreated RAC. The compressive strength development in concretes of RCA treated at later ages, between 28 and 91 days, was considered good. The relative strength development of concrete with recycled aggregate treated with HCl, HNO

_{3}and H

_{2}SO

_{4}was 18%, 18.5%, and 20%, respectively, at the age of 91 days. Among all the treated aggregate mixes, the development of strength was lower for the one treated with HCl.

^{3}, and a w/c ratio of 0.45 for concrete class M35. The replacement ratio was 91.5% according to the design method established by Fathifazl et al. [59]. The authors showed that the compressive strength improves by treating the recycled aggregates and represents more than 95% that of concrete with NA only.

_{2}SO

_{4}and HCl. Unlike the trends of previous authors, they obtained lower results for compressive strength in mortars treated with acidic solutions. This behavior was explained by the production of gypsum in the removal, which, if not correctly eliminated, would come into contact with the calcium alumina of the cement, generating a greater number of voids.

^{2}= 0.70 between the different results can be observed. This variability is due to the fact that the removal is directly linked to the type of cement and the aggregate size. The results also show that there is a positive evolution in the compressive strength when the acid concentration is increased. This evolution in the compressive strength can be affected by the ionization of the acid increasing the content of ions in the RCA [60,61,62,63]. It also presents a confidence interval of 95% (p-Value = 0.0001) for the response in compression with this type of treatment.

#### 4.3. Tensile Strength

_{2}SO

_{4}and 18.58% for 10% RCA treated with H

_{3}PO

_{4}.

^{2}= 0.88.

#### 4.4. Modulus of Elasticity

_{2}SO

_{4}) and 10.82% (30% RCA treated with H

_{3}PO

_{4}) in relation to the reference concrete at 7 days. The improvement was related to the amount of adhered mortar removed using the different acidic solutions, which led to a less porous aggregate with less-fragile particles.

_{2}SO

_{4}, the reductions in E relative to the mixes with natural aggregates were less than 8% and 22%, respectively. This better behaviour was due to the fact that the RCA had an increase in density, a removal of adhered mortar, and an improvement in the ITZ between the RCA and the cementitious paste.

#### 4.5. Shrinkage

#### 4.6. Chloride Ion Penetrability and Carbonation Resistance

_{2}SO

_{4}, the penetration was 11% and 14% higher, and those with NA observed an improvement. The carbonation depth of the mixes followed similar trends to those in the chloride penetration tests. The carbonation depth of cast concrete increased by 9% compared to the concrete made with NA. This clearly shows the reduced durability characteristics of this mix when subjected to severe exposure conditions. According Kim et al. [11] the greater depth of carbonation is attributed to the limited formation of C-S-H gel during hydration, resulting in greater porosity and a less dense matrix.

#### 4.7. Interfacial Zone between Cement Paste and RCA

_{2}SO

_{4}and H

_{3}PO

_{4}, the ITZ was reduced (as seen in Figure 5b–d), resulting in a stronger bond between the aggregate and the cementitious paste; this was reflected in the improved mechanical properties of the concrete, as explained above.

## 5. Statistical Analysis

_{2}SO

_{4}), because they are the ones most used by researchers. The analysis established the correlations that exist between the input parameters and the improvement after processing.

#### Statistical Analysis of RCA Properties

^{2}= 0.76; this indicates that the variation in removal is explained by the molarity in 76.3% of cases.

_{2}SO

_{4}treatment between molarity and water absorption (r = 0.055, not linearly associated between them—p-value = 0.00), time and molarity (r = 0.022, p-value = 0.022 linearly related), and time and mass-loss (r = −0.170, p-value = 0.015 linearly related) have a very low degrees of correlation (Table 4). The two factors that have a reasonable level of correlation are the loss of mass and molarity (r = 0.563), and the negative correlation between loss of mass and water absorption (r = −0.992). Neither of these is linearly related. This means that just like in HCl treatment, the variable that is most relevant for mass-loss is molarity, with a degree of variation of 56%; moreover, in this case, the water absorption is 99% correlated with the loss of mass. Figure 7 shows a weak linear correlation between molar concentration and mass-loss where the coefficient is R

^{2}= 0.31.

## 6. Conclusions

_{4}

^{2−}) are consumed in the reaction in the first steps. The total removal of the paste present in the RCA occurs when the acidic concentrations are increased to values greater than 3 M [6].

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 2.**(

**a**) Normal (untreated RCA); (

**b**) treated RCA at 0.1 M; (

**c**) treated RCA at 0.5 M; and (

**d**) treated RCA at 0.8 M (Ismail and Ramli [9]).

**Figure 4.**Drying shrinkage of concrete mixes versus drying time (Ismail and Ramli [10]).

**Figure 5.**(

**a**) ITZ for concrete with RCA without pre-soaking treatments; (

**b**) interfacial zone for RCA treatment with HCl; (

**c**) interfacial zone for RCA treatment with H

_{2}SO

_{4}; and (

**d**) interfacial zone for RCA treatment with H

_{3}PO

_{4}(Tam et al. [15]).

**Table 1.**Summary of physical and mechanical properties measured in concrete and mortar mixes, and techniques reported for RCA.

Measured Parameters | Technique | References |
---|---|---|

Water absorption | Immersion of RCA in acidic HCl, H_{2}SO_{4}, and H_{3}PO_{4} | Tam et al. [15] |

Water absorption, mortar content, and bulk density | Immersion of RCA in acidic H_{2}SO_{4} | Akbarnezhad et al. [6,7] |

Water absorption, mortar content, bulk density, and microscopic analysis of the RCA | Immersion of RCA in acidic HCl | Ismail and Ramli [9,10] |

Water absorption | Immersion of RCA in acidic HCl and H_{2}SO_{4} | Purushothaman et al. [13] |

Water absorption | Immersion of RCA in acidic HNO_{3} | Pandurangan [6,12] |

Water absorption, mortar content, and bulk density | Immersion of RCA in acidic HCl, H_{2}SO_{4} and HNO_{3} | Saravankumar et al. [14] |

Water absorption, mortar content, and microscopic analysis of the RCA | Immersion of RCA in acidic HCl and C_{2}H_{4}O_{2} | Al-Bayati et al. [8] |

Water absorption and mortar content | Immersion of RCA in acidic HCl and Na_{2}SO_{4} | Kim et al. [11] |

Mortar content | Immersion of RCA in acidic HCl | Juan and Gutierrez [50] |

Mortar content | Immersion of RCA in sodium sulfate (Na_{2}SO_{4}), magnesium sulfate (MgSO_{4}), and magnesium chloride (MgCl_{2}) | Abbas et al. [5,51] |

Water absorption, bulk density, and specific gravity | Immersion of RCA in acidic H_{2}SO_{4} | Tang et al. [49] |

Water absorption and apparent density | Immersion of RCA in acidic CH_{3}COOH | Want et al. [20] |

Parameter | Concrete Mix | References |
---|---|---|

Density | Concrete with RCA treated with HCl and Na_{2}SO_{4} | Al-Bayati et al. [8] |

Workability of concrete, density compressive strength, tensile strength, UPV, modulus of elasticity (E), and shrinkage | Concrete with RCA treated with HCl | Ismail and Ramli [9,10] |

Workability of concrete and compressive strength | Concrete with RCA treated with HNO_{3} | Pandurangan et al. [12] |

Compressive strength and modulus of elasticity (E) | Concrete with RCA treated with HCl, H_{2}SO_{4}, and H_{3}PO_{4} | Purushothaman et al. [13] |

Compressive strength | Concrete with RCA treated with HCl, H_{2}SO_{4}, and HNO_{3} | Saravanakumar et al. [14] |

Compressive strength, tensile strength, and modulus of elasticity (E) | Concrete with RCA treated with HCl, H_{2}SO_{4}, and H_{3}PO_{4} | Tam et al. [15] |

Compressive strength, tensile strength, and modulus of elasticity (E) | Concrete with RCA treated with HCl and H_{2}SO_{4} | Wang et al. [20] |

Compressive strength | Concrete with RCA treated with HNO_{3} | Pandurangan et al. [12] |

Compressive strength, chloride ion penetrability, and carbonation resistance | Concrete with RCA treated with HCl and Na_{2}SO_{4} | Kim et al. [11] |

Compressive strength, flexural strength, and modulus of elasticity (E) | Mortar with RCA treated with HCl and H_{2}SO_{4} | Kim et al. [60] |

Workability of concrete | Concrete with RCA treated with HCl and HNO_{3} | Butler et al. [57] |

Molarity (M) | Time (days) | Size of Aggregate (mm) | Water Absorption (%) | |
---|---|---|---|---|

Molarity (M) | 1 | - | - | - |

p-Value | 1 | - | - | - |

Time | −0.094 | 1 | - | - |

p-Value | 0.633 | 1 | - | - |

Size of aggregate (mm) | 0.077 | −0.007 | 1 | - |

p-Value | 0.708 | 0.975 | 1 | - |

Water absorption (%) | −0.140 | 0.235 | −0.211 | 1 |

p-Value | 0.478 | 0.230 | 0.300 | 1 |

Mortar loss (%) | 0.400 | −0.170 | −0.368 | −0.632 |

p-Value | 0.050 | 0.415 | 0.077 | 0.000 |

Molarity (M) | Time (days) | Water Absorption (%) | |
---|---|---|---|

Molarity (M) | 1 | - | - |

p-Value | 1 | - | - |

Time | −0.300 | 1 | - |

p-Value | 0.022 | 1 | - |

Water Absorption (%) | 0.055 | 0.135 | 1 |

p-Value | 0.889 | 0.730 | 1 |

Mortar loss (%) | 0.563 | −0.170 | −0.992 |

p-Value | 0.000 | 0.015 | 0.000 |

**Table 5.**Physical property requirements for the proposed classes [44].

Aggregate Class | A | B | C | D | ||||||
---|---|---|---|---|---|---|---|---|---|---|

I | II | III | I | II | III | I | II | III | ||

Maximum water absorption (%) | 1.5 | 2.5 | 3.5 | 5 | 6.5 | 8.5 | 10.5 | 13 | 15 | No limit |

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**MDPI and ACS Style**

Forero, J.A.; Brito, J.d.; Evangelista, L.; Pereira, C.
Improvement of the Quality of Recycled Concrete Aggregate Subjected to Chemical Treatments: A Review. *Materials* **2022**, *15*, 2740.
https://doi.org/10.3390/ma15082740

**AMA Style**

Forero JA, Brito Jd, Evangelista L, Pereira C.
Improvement of the Quality of Recycled Concrete Aggregate Subjected to Chemical Treatments: A Review. *Materials*. 2022; 15(8):2740.
https://doi.org/10.3390/ma15082740

**Chicago/Turabian Style**

Forero, Javier A., Jorge de Brito, Luís Evangelista, and Cláudio Pereira.
2022. "Improvement of the Quality of Recycled Concrete Aggregate Subjected to Chemical Treatments: A Review" *Materials* 15, no. 8: 2740.
https://doi.org/10.3390/ma15082740