**1. Introduction**

The increased rate of industrialization and urbanization due to economic and population growth has made the construction industry one of the segments that most consumes natural resources and generates solid waste that negatively impacts the environment. According to Mehta and Monteiro [1], during the most recent 100 years, the total populace has developed from 1.5 to 6 billion people, with almost 3 billion living around cities. It is estimated that the global world population will reach around 10 billion by 2060, which can be attributed to the technological, medical, and logistical advances that have improved living and health standards since the Industrial Revolution [2,3].

Concrete employed in the construction of cities plays a crucial role in socio-economic development. Still, it also has a rather significant adverse effect on the environment and the depletion of natural resources. Over the years, infrastructures have been basically built with concrete, steel, and wood, as well as glass, which are considered the primary materials used in contemporary construction [4]. However, in volume, the most significant manufactured product in the world today is concrete [1], and it is also considered to be the second most consumed material on Earth after water [4].

The construction industry is responsible for significant environmental impacts due to the extraction of raw materials and a considerable portion of the waste generation that negatively influences the environment [5,6]. It is also responsible for generating large amounts of carbon dioxide (CO2) generated by the cement industries and by the

**Citation:** da Silva, S.R.; Andrade, J.J.d.O. A Review on the Effect of Mechanical Properties and Durability of Concrete with Construction and Demolition Waste (CDW) and Fly Ash in the Production of New Cement Concrete. *Sustainability* **2022**, *14*, 6740. https://doi.org/10.3390/ su14116740

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 8 April 2022 Accepted: 21 May 2022 Published: 31 May 2022

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**Copyright:** © 2022 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/).

burning of fossil fuels used in the equipment employed in the extraction and processing of raw materials.

The extraction of raw materials, the processing of materials for civil construction, the construction of buildings, as well as renovations and demolitions, generate solid waste that, when disposed of improperly, can cause various problems, such as the proliferation of disease-carrying agents, the degradation of springs and permanent protection areas, the obstruction of drainage systems, silting up of rivers and streams, and the occupation of roads that degrade the urban landscape [6]. Aiming to use natural resources in a more sustainable way, several researchers have been seeking alternative uses for the solid waste from construction and demolition as a by-product for reuse in the construction industry as brick waste [7,8], ceramic [9,10], glass [11,12], from rubber [13,14], from concrete [15,16], and mixed waste [17,18].

Studies verifying the feasibility of using construction and demolition waste (CDW) as a partial replacement for natural aggregate for concrete production have presented relevant results. The different compositions of CDW directly influence the mechanical properties and durability of concrete [19]. Limbachiya, Meddah, and Ouchagour [20], Lotfy and Al-Fayez [21], and Poon, Kou, and Lam [22] observed that the replacement of coarse aggregate with recycled coarse aggregate at levels lower than 30% does not have a significant adverse effect on concrete performance when compared to natural concrete. According to the authors, CDW, as a more porous aggregate, has a high-water absorption capacity. This water adhered to the recycled aggregate can be used as an internal curing agent, especially for concretes with fly ash that requires longer wet curing for the pozzolanic reaction.

However, the significant variability of existing waste, with different compositions, and physical and mechanical properties, can present negative results due to increased porosity, roughness, and water absorption, which leads to higher a/c ratios, making the cement paste weaker and more porous [19,23–25]. Other studies evaluate the mechanical bonding in the Interfacial Transition Zone between the mortar and the substrate [19,26], which ultimately decreases the compressive strength of the material [10], which limits the use of recycled aggregate with a percentage higher than 30% in structural concrete [27].

The construction sector is also responsible for the generation and release into the atmosphere of large amounts of carbon dioxide (CO2). Asia alone produces more than 80% of cement in the world and, as a consequence, releases approximately 80% of the CO2 generated by Portland cement production [4]. According to Meyer [28] and Aprianti [29], the reduction of Portland cement production would be one of the alternatives to reduce the environmental impact. Another way would be the use of by-products generated by industrial processes, agricultural waste, and recycled materials. Among the various types of products that can be used, there is fly ash, which is a by-product generated by thermoelectric plants powered by mineral coal. According to Acar and Atalay [30], many thermoelectric plants were built in the world in a period of 80 years due to the growing demand for electricity generation. It is estimated that the annual global production of fly ash varies between 0.75 and 1 billion tons. Other works have already been performed, employing the fly ash and recycled aggregate in concretes simultaneously [5,17,31–37].

Limbachiya, Meddah, and Ouchagour [20] and Lima et al. [31], observed that the use of fly ash as a partial replacement for Portland cement in the production of concrete improves its durability as well as contributes to the reduction of CO2 emissions. For a deeper understanding of the influence of fly ash in concrete, Payá et al. [38], Sakai et al. [39], Moon et al. [40], and Shaikh [33] investigated the role of this by-product in cement hydration, as well as the pozzolanic reaction process.

Dabhade, Chaudari, and Gajbhaye [5] verified a slight increase in axial compressive strength in concretes with recycled aggregate and with 10% of fly ash compared to concrete with recycled aggregate only. Lima et al. [31] concluded that the addition of fly ash in concretes with recycled aggregate, in general, improves workability as well as mechanical and durability properties, reducing the harmful effects of recycled aggregate.

The scope of this investigation was to gather and analyze the published information about the effects on the mechanical properties and durability of concretes with construction and demolition waste (CDW) and fly ash (fly ash). Based on this study, it was sought to identify the most suitable composition of concrete mixtures with construction and demolition waste and fly ash to reach a consensus on the most appropriate contents of these wastes to achieve results of mechanical properties and durability closer to the reference concretes. To seek a deeper understanding of the effect of fly ash in concretes with a recycled aggregate, a microstructural analysis was performed in the transition zone of the interface between the aggregate and the paste.

#### **2. Importance of the Study**

Considering the search for a solution for the appropriate disposal of waste generated by the construction industry segment, many studies have been developed with the objective of reusing this waste as by-products in order to reduce the consumption of natural resources and, consequently, reduce the negative impacts that this waste can cause to the environment.

In concrete, Portland cement is responsible for 74–81% of total CO2 emissions, while coarse aggregate is responsible for 13–20% of CO2 emissions. Fine aggregates generate less equivalent CO2 as they are not crushed. The mixing of the conventional concrete process with Portland cement ranged only between 0.29 and 0.32 t CO2-e/m3. It was found that the addition of fly ash is able to reduce between 13 and 15% of CO2 emissions in concrete mixes [41]. Large amounts of waste from construction and demolition are generated every day by the construction industry, which, inappropriately deposited, can bring great harm to biodiversity. The use of these recycled aggregates in partial replacement of natural aggregates can be essential for the concrete eco-efficiency, as well as producing significant economic advantages [42].

There are already many studies focusing on the reuse of waste from construction and demolition (CDW), as well as waste generated by cement industries, such as fly ash (FA), for the production of mortar and concrete. However, there are few studies employing combined wastes, such as CDW and FA for new concrete production.

Analysis of publications focusing on the production of concrete with a combined use of CDW and FA was performed, initially in English, between January 2007 and December 2021. The databases investigated were ANTE (Abstract in New Technologies and Engineering), ASTM International, Aluminium Industry Abstract, and ACS Journals Search. The keywords used for the search were "concrete" + "fly ash" + "construction and demolition waste" or "concrete" + "fly ash" + "waste concrete aggregate" or "concrete" + "fly ash" + "recycled aggregate". A total of 259 published articles were found, and their distribution by year is shown in Figure 1.

**Figure 1.** Number of articles published on the use of construction and demolition waste (CDW) with fly ash in concrete production.

Considering the different types of waste generated in the construction and demolition process, as well as the significant variability of these wastes, it is imperative that studies be conducted for a better understanding of the behavior of concretes with combined wastes. In this regard, this literature review is essential to have a more profound knowledge of the use of these wastes in the production of new cement concretes.

The main objective of this work is to establish an innovative concept in order to minimize the effects caused to the environment, which are: reduction of CO2 emissions caused by cement industries and reduction of natural aggregates extraction from the environment.
