**1. Introduction**

One of the most Sustainable development is now a major development challenge for the entire world and has become the guiding standard for the construction sector [1]. Technological progress in the production of concrete and reinforced concrete requires additional sources of raw materials, in particular, the use of high-quality aggregates [2].

**Citation:** Makul, N.; Fediuk, R.; Amran, M.; Zeyad, A.M.; Murali, G.; Vatin, N.; Klyuev, S.; Ozbakkaloglu, T.; Vasilev, Y. Use of Recycled Concrete Aggregates in Production of Green Cement-Based Concrete Composites: A Review. *Crystals* **2021**, *11*, 232. https://doi.org/10.3390/ cryst11030232

Academic Editor: Cesare Signorini

Received: 9 February 2021 Accepted: 24 February 2021 Published: 26 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The implementation of this problem is impeded by the constantly growing shortage of mineral and energy resources, as well as environmental requirements for environmental protection [3]. Therefore, at present, the task is to comprehensively use the deposits of low-quality raw materials and waste from related industries [4].

Reusing and recycling concrete waste can be a successful strategy for achieving sustainability along the way [5]. Waste concrete is collected and crushed and then used in structural concrete, in which it replaces natural coarse aggregates (NCA) [6]. Many administrations worldwide have introduced various control measures to minimize the use of virgin aggregate and improve the recycling of concrete waste for reuse as materials when environmentally, technically, and economically acceptable [7]. Environmental problems are known to be exacerbated by rising landfill fees and land scarcity. The use of concrete waste in sustainable development can alleviate such problems [8]. However, it has been observed that most concrete plants were reluctant to produce recycled concrete aggregates (RCAs) and make full use of them [9]. Manufacturing plants have not yet mastered RCA's use, not only due to its unclear characteristics for concrete but also due to unexplored manufacturing processes, which, however, have yet to be determined [10]. It has been observed that most concrete batching plants are hesitant to manufacture and use RCA at their optimum [11]. However, it has become necessary to study this problem.

As a result of human-made and natural anomalies occurring on the Earth, there are many destroyed cities, settlements, and houses (see Figure 1 that shows the process of destroyed buildings wastes production as RCAs). The question is how to rebuild these cities and how to use parts of the destroyed buildings and structures [12]. One of the ways is to take everything out to relatively low forms of the earth's surface, store it, cover it with soil, plant a forest on top, produce new building materials and from them rebuild cities and settlements [13].

**Figure 1.** Process of destroyed buildings wastes production as RCAs.

But this is a very expensive undertaking [5]. The second way out of this situation is the use of fragments of destroyed buildings and structures to create building materials, using which to repair and build new buildings and structures in place of the destroyed ones [14]. Demolition of old buildings and construction of new ones is common practice due to natural disasters, expansion of traffic routes, urban redevelopment, structural destruction and change of purpose [15]. In the European Union, about 850 tons of construction waste is generated annually, which is about 30% of the total waste [16]. Debris from demolition alone is about 123 tons annually in the United States [17].

Huge concrete waste is generated during the demolition of old buildings; then, it is most often disposed of in landfills, which poses significant health risks and damages the environment [18]. More than forty years ago, research began on the characteristics of RCA [19]. In the past, most of the research carried out was mainly limited to the production of unstructured concrete due to the deleterious physical properties of RCA, for example, high water absorption, which increases the need for water for a certain workability (Table 1) [20–22].

Insufficient information on the longevity and sustainability of RCA production is a serious issue that requires close attention due to its impact on changing aspects of the sector [23]. It is unknown whether the manufacturing, quality control, and production costs in standard RCA manufacturing industries outweigh the benefits from RCAs purchased as concrete components at a lower cost than natural concrete aggregates [24]. One explanation for the slow acceptability of production for RCA production is the already existing plants for the production of bulk ready-mixed concrete [25].

However, the current models, in which RCA increasingly replaces NCA in various structural designs, have gradually gained in importance for specific reasons [26]. For example, RCA manufacturing provides sustainability for concrete waste and encourages recycling rather than landfill [27]. In addition, it focuses on the lack of natural aggregates, minimizes the need for them, and ultimately allows the preservation of the NCA natural aggregates mined in the open pit [28]. Despite the cost, these and many other benefits have led to increased interest in manufacturing and using RCA in design [29,30]. It is found that insufficient information on the longevity and sustainability of RCA production is a serious issue that requires close attention due to its impact on changing aspects of the sector. Therefore, this study aims to provide a critical overview on the RCAs for the production of high-performances concrete structures. This study reviews the source, originality, types, service life prediction, features and properties of RCA. However, more attention has been paid to explaining the effect of RCA on concrete durability, as well as the properties of fresh and hardened concretes. In addition, this literature review summarizes the research findings to produce complete insights into the potential applications of RCA as raw, renewable and sustainable building materials for producing greener concrete composite towards industrializing ecofriendly buildings today. The paper evaluates high performance concrete structures that favor RCA's manufacturing activities rather than natural materials [31,32]. Therefore, it has also been highlighted the differences in the current state of knowledge between RCAs and NCAs, and offers some suggestions for future research.


**Table 1.** Approval criteria about RCAs.

#### **2. Source and Originality of RCA**

RCA originate from the Portland cement concrete demolition. Aggregate pieces can be expected to vary considerably, given that the original concrete could be weathered or fresh, loose or dense, weak or strong [39]. In the process of crushing, agglomerates of concrete aggregate with adhered mortar are formed [40]. Typically, these agglomerations are less efficient than angles units [41]. Fines are also formed from crushed concrete [42].

The aggregates form a concrete frame. Aggregates often take up about seventy percent of the total volume of concrete [43]. A large percentage of the number of these aggregates is mainly a coarse aggregate [44]. However, in the construction sector, the demand for coarse aggregate is enormous [45]. The growing extraction of raw materials from natural resources is essential to meet these high demands [46]. The ever-increasing use of natural large aggregates creates an ecological imbalance [47]. Therefore, the use of alternative raw materials is essential in the construction industry [48]. The use of RCA derived from demolished concrete buildings is one approach to achieving this goal [49]. The use of reclaimed concrete materials in structures minimizes the need for natural coarse aggregate [50]. In turn, this minimizes the negative environmental impacts due to natural aggregate recovery [51]. Rising landfill costs and NCA shortages have also contributed to the use of RCA in concrete [52]. In addition, the increased distances between the construction site and the NCA quality source have forced contractors to consider replacing the NCA with RCA [53]

The use of RCA in high performance and high strength structural concrete is possible due to proper quality control and mixing, and the addition of pozzolanic additives. It has been noted that poverty imposes specific constraints that lead to delays and sometimes a lack of programs to carry out various engineering processes [54]. Cost issues are limited by the difficulty of implementing environmentally friendly and sufficient, safe solutions such as aggregate recycling. In particular, front-loading and top-hopper industries can be used to reduce additional RCA costs [55]. However, pricing and the supply and demand aspects of recycled materials pose various challenges that are rarely considered.

In addition, it is necessary to carefully study the characteristics of concrete with each type of RCA [56]. In particular, the strength of concrete is influenced by various elements such as the replacement rate, moisture content, RCA type, and water-to-cement

ratio [57]. Unlike natural aggregates, RCA generally has a higher water absorption, which can significantly affect concrete properties, especially workability [58]. In addition, concrete with drier RCA had a higher slump and more rapid slump flow than water-saturated concrete [59]. It is clear that the strengths for completely replacing fine aggregate increase over time, and blends having a higher percentage of the combination get better composite performance [60]. This reflects the characteristics of the presence of reclaimed concrete, which from the available literature review show that the strength of the material increases with the life of the structure [61]. The only reliable way to minimize construction costs is to use available local resources and introduce innovative building materials [62]. The use of RCA as fine aggregates in concrete seems to be the best, especially in areas where demolition waste is freely available [63].

#### **3. Service Life Prediction of RCA Concrete**

The theoretical basis for the design of green composites using the specified raw materials is the transdisciplinary science of geomimetics [64–66], which uses the results of studies of natural processes to create high-strength concretes and building composites of a new generation. The results were tested on raw materials from destroyed buildings and structures in Iraq, which mainly consist of concrete, ceramic bricks, and limestone wall blocks. The performance of RCA concrete is influenced by various key aspects such as air entrainment, cement content, curing conditions, RCA humidity conditions, properties of the original RCA concretes, RCA physical characteristics, RCA size and type, RCA content, and water to cement ratio [67]. The aspects are explained below.

#### *3.1. RCA Features and Percentage*

The physical properties of RCA have a significant effect on the properties of fresh and hardened concrete. For example, fresh concrete with coarse and angular particles becomes tough and therefore difficult to cast [68]. In addition, high RCA absorption can affect the workability of concrete. In addition, large RCA pore volumes can affect the durability characteristics (permeability and water absorption), strength, and porosity of concrete.

The fresh and hardened properties of concrete are strongly influenced by the RCA percentage used as a complete or partial replacement for NCA. Using seven independent variables, [69] created a model of aggregate quantity and type to predict concrete performance when replacing 0.0–100% NCA with RCA. The researchers reported that concretes created using RCA had a lower modulus and compressive strength than NCA concretes. Higher RCA content also increases water absorption but decreases density, resulting in increased concrete porosity. The use of coarse RCA reduces concrete density by 50–100% and increases water absorption by about 2.11% to 3.50% and from 0.14% to 0.38%, respectively. In addition, it is noted that as the content of grounded RCA increases, the resistance to chloride ion penetration decreases, while the tensile splitting and compressive strength of concrete decreases [70]. In addition, the researchers noted that concrete drying shrinkage improved with increasing RCA content. It can be controlled by lowering the water to cement ratio.

#### *3.2. RCA Sizes and Original Concrete Quality*

It is used three different aggregate sizes to evaluate the effect of RCA size on concrete properties [71]. A more significant reduction in modulus of elasticity was obtained for concretes prepared with smaller RCA dimensions. On the other hand, they reported that the strength increases with the size of the RCA. In addition, they found that the water absorption of concrete decreases with increasing RCA dimensions. This is due to the relatively low content of weak solutions adhered to the coarse aggregate. It is investigated the effect of base concrete quality on RCA concrete performance [72]. Scientists reported that RCA water absorption increases with the strength of the base concrete. This is due to the fact that for concretes with higher strength, higher cement content is required in principle; therefore, the amount of mortar adhering to the aggregate increases. Thus, adjustments to mix water content are necessary for newer concretes, including RCA made from older, harder concretes, to obtain the preferred workability. Porous RCAs affect the strength of RCA concrete. The proportional loss of tensile or compressive strength of new concretes due to the use of RCA is more significant when it is obtained from weaker old concretes than from strong old concretes [45].

#### *3.3. Influence of Cement Content and Water to Cement Ratio*

RCA concretes with high cement content are reported to be highly resistant to carbonation. It is found that higher cement content in RCA concrete results in a preferred compressive strength [73]. It is also found that the RCA tensile strength of concrete increases with increasing cement content in concrete [74]. The degradation of RCA concrete is related to the water to cement ratio used in the mix design. It is reported that compared to the original RCA concrete, RCA concretes require a higher cement content and a lower water-to-cement ratio to achieve a certain compressive strength [75]. The resistance of the NCA to melting and freezing at water to cement ratio of 0.290 was exceptionally high. But for RCA concrete, the same water-to-cement ratio does not provide suitable freeze-thaw resistance.

#### *3.4. Hydration of Cement in the Original Concrete*

It has been established that the composition of the crushing concrete scrap contains about 30% of non-hydrated Portland cement, which makes it possible to use it as an active microfiller in the production of multicomponent highly active binders [34]. Aggregate from the concrete scrap has a partial or solid shell on the surface of its grains from the cement paste of crushed concrete, actively influences the process of forming both the structural characteristics of the cement paste and the dense interfacial transition zone between them [76]. The structure of concrete composites is characterized by lower water absorption (up to 3–7%) and the presence of rather small and uniform pores in size [77].

### *3.5. Sources or Types of RCAs*

Several studies have been carried out to investigate the effect of sources or types of RCA on concrete performance [78–80]. With the exception of concretes made from recycled aggregates made from masonry ceramics, which reported an increase in compressive strength, concretes made with coarse RCA had lower compressive strengths [81]. The modulus of elasticity of concrete has been reduced for all RCA types. However, RCAs made from red ceramics had a greater effect on reducing the modulus of concrete due to the lower density.

#### *3.6. Curing Conditions, RCA Moisture Conditions, and Air Entrainment*

External curing in the environment is more detrimental to RCA concretes than NCA ones. It is shown that the differences in splitting tensile strength among NCA and RCA concretes are large when they are hardened in the external environment [79]. In addition, It is found that the carbonization depth of RCA concrete water-cured is almost twice that of RCA air-cured concrete [73]. The decrease in carbonization depth caused by curing in water may be partly due to the higher internal moisture content of the concrete. The moisture conditions of the aggregate affect the workability of the concrete. The initial slump of concrete (workability measurements) is highly dependent on the initial free water content of the concrete mix. It is shown that although air-dry and saturated surface-dry RCA exhibit typical initial slump and slump loss, kiln-dried RCA results in faster slump losses and higher initial slump [79]. Thanks to the appropriate air entrainment, durable concrete can be obtained from RCA [82]. Air entrainment for NCA concretes is as successful as for RCA concretes [83]. In addition, the use of entrained air is more effective than reducing the water-to-cement ratio in increasing freeze- resistance of RCA concrete [84].
