**3. Factors That Influence the Properties of Concrete with Recycled Aggregate and Fly Ash**

#### *3.1. Properties of Construction and Demolition Waste That Influence New Concretes*

According to Meddah [4], recycled concrete aggregate is the most abundant waste due to the availability of its origin, which comes from the continuous demolition of old buildings and sidewalks. According to Morales-Martíns et al. [43], recycled concrete coarse aggregates that are composed of original aggregate and adhered mortar contain impurities, such as clay bricks and crushed ceramic materials, and gypsum, which contribute to the existence of contaminants. These adhered products negatively influence the physical and mechanical properties of concrete produced with the recycled coarse aggregate [44,45]. Many studies have announced the contrasts between the characteristics of recycled concrete aggregates compared to natural aggregates [46–49], since the physicochemical and mechanical properties of recycled aggregates influence the properties of concrete [50,51], which are presented in the following.

### 3.1.1. Bulk Density and Water Absorption

Verian et al. [52] observed in their studies that there is a correlation between the water absorption of coarse aggregates and their density because the higher the absorption, the lower the density. The results observed by the authors Verian et al. [52] for recycled coarse aggregates were 14.50%, 12.50%, 12.44%, and 12.10% for water absorption, while for the bulk density, it was 2.05 kg/dm3, 2.18 kg/dm3, 2.21 kg/dm3 and 2.28 kg/dm3, respectively. For natural coarse aggregates, the water absorption was 2.30%, 1.98%, and 1.70%, while the bulk density was 2.69 kg/dm3, 2.74 kg/dm3, and 2.78 kg/dm3, respectively. The same behavior was verified in recycled and natural sands. In natural sand, the absorption was approximately 3.0% and 2.11%, while the specific mass was approximately 2.63 kg/dm3 and 2.68 kg/dm3. Agrela et al. [53] observed the correlation between the concrete content and the dry density of the saturated surface. The results obtained by the authors were: concrete with absorption of 2.48%, 5.1%, 7.49%, 10.2%, and 12.5% have a saturated surface dry density of 2.59 kg/dm3, 2.35 kg/dm3, 2.15 kg/dm3, and 2.08 kg/dm3, respectively.

Kisku et al. [51] also observed, based on their studies, that the presence of the old mortar contained in the recycled aggregate increases the absorption capacity and decreases the specific mass of recycled aggregates compared to natural aggregates. According to da Silva and Andrade [17], the evaluation of water absorption is an essential point that should be considered because the durability performance of concrete is a property of the pervasion qualities of materials, considering the trustworthiness of concrete against aggressive agents.

There are many studies that use different types of waste as aggregate for the production of concretes whose water absorption and bulk density are quite varied. Table 1 presents some comparisons of bulk density and water absorption.

**Table 1.** Bulk density and water absorption of different types of construction and demolition waste.



**Table 1.** *Cont.*

<sup>1</sup> RCA—recycled aggregate concrete, <sup>2</sup> MRA—mixed recycled aggregate.

The glass waste had the lowest percentages of water absorption, which ranged between 0.002 and 0.04% when compared to bricks, rubber, and glass. These percentages were also observed in the studies of Penacho, Brito, and Veigas [67]. However, the recycled concrete and mixed aggregates absorbed the most water, ranging from 4.0 to 8.79%. Cantero et al. [61] observed in their studies that the percentages of absorption of these materials varied between 4.49 and 10%. The bricks presented the most significant variation in bulk density (974 to 2548 kg/m3), and this is due to the characteristics of the clays as well as the preparation and burning temperature. The rubbers also present bulk density varying between 539 and 1050 kg/m3. Agrela et al. [53] recommend a classification into three groups for recycled aggregates from construction and demolition, based on the following proportions:


Based on their studies, the authors concluded that recycled aggregate with many ceramic particles causes an increase in water absorption and a decrease in the density of recycled aggregate. However, Robalo et al. [42] suggest the classification of recycled aggregates by their dry density, resulting in four classes:


The classification was generated from a relative decrease in compressive strength of concrete with recycled aggregate obtained through the study of Robalo et al. [42], and some researchers. According to Robalo et al. [42], this classification allows for the estimation of the minimum compressive strength of the concretes based on the substitution content of recycled aggregate.

All wastes present variations in bulk density and water absorption. This variation is closely linked to the feasibility of the compositions of materials contained in the waste (RCA, RMA), the type and combination of materials for production (glass, rubber), as well as the selection, handling, and firing processes of ceramic materials.

#### 3.1.2. Interfacial Transition Zones (ITZ)

Recycled aggregate is formed of two Interfacial Transition Zones (ITZ), one between the natural aggregate and the old cement matrix and the other between the old cement matrix and the new cement matrix [68–70]. A schematic diagram of the Interfacial Transition Zones (ITZ) in recycled concrete aggregate is presented in Figure 2.

**Figure 2.** Schematic diagram of the old and new ITZ.

The old ITZ makes the microstructure of the concrete more brittle due to higher porosity and cracking; thus acting as the weakest link [49]. The method of crushing the source concrete has been observed in order to reduce the density of cracking in the old Interfacial Transition Zone (ITZ) [49]. According to Xiao et al. [69], the thickness of the old and new ITZ are in the same order of magnitude, with the new ITZ being thicker. It has been found that the increased ratio of mechanical properties, with respect to the modulus of elasticity and strength, of the old ITZ to the cement matrix, results in higher strength but lower ductility [71]. Zhao et al. [26] and Zhang et al. [72] suggest a third ITZ in the recycled aggregate.

The first ITZ is between the new aggregate and the new mortar, the second ITZ is located between the old mortar and the new mortar, and the third ITZ is between the old mortar and the new aggregate. According to the authors, the third ITZ presents stability in its mechanical properties; however, the addition of recycled aggregate in partial replacement of natural aggregate in concrete, Zhao observed in his studies, not only decreases the mechanical properties, such as mechanical strength and modulus of elasticity, but also decreases its durability, including chloride resistance. According to Zhang [72], the ITZ is characterized by its high porosity, high water permeability, and high diffusion coefficient. Consequently, the ITZ allows the ingress of harmful substances from the external environment, and the reaction between SO4 <sup>2</sup>−and C-S-H tends to form first in the transition zone between the paste and the substrate, providing an earlier expansion in this zone than in other phases of the material. This expansion over time may be the key to understanding the macroscopic deterioration of concrete under external sulfate attack.

One of the methods employed to improve the microstructure of recycled concrete aggregate is the coating of aggregates with pozzolanic materials [73], and the Pozzolanic material that will be discussed next is fly ash.

#### 3.1.3. Effect of the Recycled Aggregate Size on Strength and Elastic Modulus Properties

Kang and Weibin [74] developed a study to assess the impact of the recycled aggregate diameter on the mechanical properties of concrete (compressive strength and modulus of elasticity). The authors use three different diameters (5–15 mm, 15–20 mm, and 20–30 mm) of recycled aggregate. In this study, two types of recycled aggregate were used, one being crushed in the laboratory and the second crushed in a large crushing plant. It was observed that the larger the diameter of recycled aggregates, the greater the compressive strength. According to the authors, strength gain is related to the lower mortar content adhered to larger diameter aggregates when compared to smaller diameter aggregates. It was

observed that the control concrete elastic modulus was higher than the concrete elastic modulus with a recycled aggregate. However, the authors noted that concretes' modulus of elasticity with different diameters of recycled aggregate is closely linked to a variation in compressive strength performance. Musa and Saim [75] analyzed the compressive strength of concrete with natural coarse aggregate of different sizes (10 mm and 20 mm). The same behavior was observed by the authors, that the larger the particle size, the greater the compressive strength.

#### *3.2. Fly Ash*

The size and shape of the fly ash particles have a relevant effect on the binder properties (cement-waste ash). The pozzolanic reactivity is directly related to the fineness of the fly ash because the more significant the surface area, the higher the Pozzolanic Index [76,77]. Studies show that the smaller the fly ash particles, the higher the mechanical strength [77,78]. Furthermore, according to Blissett and Rowson [79], the chemical composition of fly ash has traditionally been the basis for evaluating its suitability for use as a cement replacement material. Another inherent factor in the properties of fly ash is, besides the physical and chemical characteristics, the crystalline structure in the hydration process [80].

There is a classification established by Ramachandran [81] for fly ash based on the amount of CaO. Fly ash with CaO contents below 10% is classified as pozzolanic material. Fly ash that has contents equal to or greater than 10% is classified as cementing materials. According to the author, fly ash consists predominantly of silicon oxide (SiO2), calcium oxide (CaO), in addition to aluminum oxide (Al2O3), and iron oxide (Fe2O3). The amount of SiO2 and CaO in the system influences the composition of the hydrate, as the greater the amount of SiO2, the smaller CaO/SiO2 ratio of the hydrates, that is, the lower the alkali– silica reaction due to the lower alkalinity of the pore solution [82]. According to Garzia et al. [83], an alkali–aggregate reaction, more precisely an alkali–silica reaction, can cause damage, such as the appearance of micro-cracks in concrete as well as loss of mechanical integrity properties and durability, which may even compromise the functionality of a structural part.

According to Mehta [84], the pozzolanic fly ash reaction compared to Portland cement is slower. Fly ash Oxides, when reacting with water and Ca(OH)2, result in a layer of C-S-H around the particle, making it difficult to access the innermost oxides. As a result, the hydration pozzolanic reaction forms more slowly; thus slowing down the resistance development. Concretes with fly ash addition, in partial replacement to Portland cement, may have lower mechanical strength compared to conventional ones at younger ages. However, the fly ash addition in partial replacement to Portland cement tends to reduce the effect of the alkali–aggregate reaction, which occurs between cement alkalis and reactive aggregates in the presence of moisture.

#### 3.2.1. Effect of Fly Ash on the Compression, Tensile and Flexural in Concretes

When fly ash, cement, and water are mixed, silica (SiO2) and alumina (Al2O3) progressively react with calcium hydroxide (Ca(OH)2), which is formed by the cement hydration process; thus producing the calcium silicate hydrate, known as secondary C-S-H. This reaction reduces the calcium hydroxide content and consequently reduces the concrete compressive strength. However, the cement hydration process allows the SiO2 and Al2O3 reaction from the fly ash. The fly ash pozzolanic reaction depends on the CaOH2 concentration, so it can be stated that the higher the Ca(OH)2 concentration, the higher the pozzolanic reaction rate [85]. The production of secondary C-S-H at older ages will depend on the Ca(OH)2 concentration, as the higher the concentration, the longer the pozzolanic reaction time [86]. The flexural strength exhibits similar behavior to the compressive strength. Tensile strength, on the other hand, depends on the shear zone of the interfaces between the paste and the substrate, which, in turn, improves with curing time. This improvement is closely linked to the fly ash pozzolanic reaction [85].

#### 3.2.2. Bulk Density and Water Absorption

The Brazilian Standard NBR 12653/2014 classifies fly ash as class "C", which is produced by burning mineral coal in thermoelectric power plants. However, the international standard ASTM C618:2012 classifies fly ash in "C" and "F". Class "F" is produced by burning anthracite or bituminous coal and presents the exact limits of chemical compounds as the Brazilian standard (Table 2). Class "C" is produced by burning lignite or subbituminous coal that contains a combination of chemicals (SiO2 + Al2O3 + Fe2O3) between 50% and 70%.

**Table 2.** Chemical characteristics required by ABNT NBR 12653: 2014 class "C" and by international standard ASTM C618:2012 class "F".


Studies show that there is a significant variation in the number of oxides within the same class of fly ash. This variation is associated with its origin (characteristic of the coal) as well as the different forms of the process (calcination). Table 3 presents the variations of the oxides present in fly ash according to the literature.


**Table 3.** Range of oxides present in fly ash, as presented in the literature.

Based on the Texas Department of Transportation database, the oxide content variations of approximately 5500 fly ash samples from 36 plants in and outside Texas were analyzed, and authors Du and Lukefahr [95] observed that the oxide contents of ASTM class F fly ash were more variable than those of class C fly ash. The authors observed that the main differences between class C and class F fly ash are CaO and SiO2, and class C fly ash has high CaO content, being mainly compounded with more than 25%. The CaO concentration in class C fly ash is higher than in class F fly ash, which was also observed by Oey et al. [96]. Already, according to the authors, Al2O3 and Fe2O3 showed very close results. According to Aboustait et al. [97], most of the particles of Class C fly ash have the highest contents of CaO + MgO + Na2O+K2O than those of Class F fly ash. On the other hand, most of the particles of Class F fly ash have higher contents of SiO2 + Al2O3, which is mainly due to the higher SiO2 content in the particles of Class F fly ash.

Oey et al. [96] performed an alkali–silica reaction durability index analysis to verify the performance of fly ash in concretes, SiO2, Fe2O3, Al2O3, CaO, and equivalent alkali contents were used for calculation purposes. The alkali–silica reaction is an internal reaction between alkalis, such as Na<sup>+</sup> and K+, and hydroxyl ions (OH-) of the cementitious material and reactive silica in some aggregates, and the product of the reaction is an alkaline silica gel that has a high capacity to absorb water molecules from pore solution as well as from external sources [98]. The durability index of class C fly ash showed an average of 24.6, while that of class F fly ash was 51.3 [95]. According to Du, Lukefahr, and Naranjo [95], the use of fly ash in concrete can be more viable and productive if its durability index is considered.

According to Wright, Shafaatiann, and Rajabipour [93], for a reduction of the alkali– silica reaction expansion to occur, for class C fly ash with 27.3% CaO, it was necessary to replace 31% Portland cement, while for class F fly ash with 13.5% CaO, 18% replacement content was required. The reduction of the alkali–silica reaction occurs due to the decrease in alkalinity ([OH-]) of the pore solution, significantly decreasing the ionic diffusion coefficient of mortars, which is due, in part, to the reduction of porosity when Portland cement is replaced by fly ash of a lower density, and, in part, due to the pozzolanic reaction promoted by the high temperature and alkalinity of the system [94].

3.2.3. Physical and Mineralogical Properties of Fly Ash

According to the Brazilian Standard NBR 12653/2014 and the international standard ASTM C618:2012, the physical properties should be in accordance with the requirements established according to Table 4.

**Table 4.** Physical requirements for fly ash established in Brazilian (NBR 12653/2014) and international standards (ASTM C618:2012).


A high amount of coarse particles (Ø > 1 μm) causes an irregular distribution of the material and leads to high macroporosity [99]. However, finer particles tend to reduce water absorption due to the refinement of the capillary pores of the concretes [17].

Due to the employment of more advanced characterization techniques, such as scanning electron microscopy (SEM) imaging as well as energy-dispersive X-ray spectroscopy (EDS), it is possible to identify the morphology and the chemical hydration products formed. These techniques are widely employed for performing visual analysis to observe numbers and ranges of chemical compositions [17,92,99,100]. Through these applied techniques, it was observed that the fly ash used in the study of da Silva and Andrade [17] presented spherical and flat shapes while the Portland cement presented irregular and rough shapes.

For higher levels of fly ash incorporation, the workability of concrete increases due to the spherical and smooth shape of the particles that influence the rheological properties of the cement paste, causing a reduction in the water requirement [101]. By scanning electron microscopy (SEM), Tosun-Celikoglu et al. [92] noted that the particle size of class F fly ash is finer than that of class C fly ash particles.

The automated scanning electron microscopy (ASEM) technique employed by Aboustait et al. [97], allowed us to verify that the fly ash particles larger than 5.0 μm were more spherical than the smaller particles, and the particles with sizes between 0.1 and 1.0 μm were the least spherical. The authors also noted that class C fly ash seems to show a wider range of size distribution than those of class F. Furthermore, the pozzolanic reactivity is directly proportional to the fineness of the fly ash, as the finer the fly ash, the higher the Pozzolanic Index [76,102]. Tkaczewska [102] observe in his study that the finer fly ash (0–16 μm) increases the degree of depolymerization of SiO4, which is responsible for the increase of pozzolanic reactivity.

The X-ray diffraction (XRD) technique is fundamental for knowing the crystalline structure and microstructure of a material to understand its properties. Silva and Andrade [30] used XRD on a fly ash particle for sample analysis. The authors observed a high concentration of quartz, calcite, and muscovite/illite as crystalline phases and amorphous phase content throughout the fly ash particles. Ma, Hu, and Ye [103] also used XRD in their studies, where they observed that the main crystalline phases of fly ash were quartz (SiO2) and mullite (3Al2O3, 2SiO2).

Durdzinski et al. [104] observed that the fly ash in studies was made of glassy material of amorphous nature, and because of that, the constituent materials largely include chemical reactions. According to the authors, fly ash with elevated amorphous substance is more viable in increasing the pozzolanic reaction.

#### **4. Influence of Fly Ash Replacement in Concretes with Construction and Demolition Waste (CDW) in Concrete Properties**

Feasibility studies of the use of construction and demolition waste (CDW) as a substitute for natural aggregate for the production of concrete in small quantities show promising results. However, the significant variability of existing waste, with different compositions, and physical and mechanical properties, can present adverse effects due to increased porosity, roughness, and water absorption that leads to higher w/c ratios, making the cement paste weaker and more porous [19,23–25]. To minimize the adverse effects regarding the significant variability of CDW, many studies have been adding fly ash as a partial replacement for Portland cement and will be presented as follows.
