Multiple Dimensions of Energy Efficiency of Recycled Concrete: A Systematic Review

Abstract

The focus on building energy efficiency using alternative materials in structures, especially concrete, and the main technical and environmental challenges therein, aligns with Sustainable Development Goals (SDG). This study proposes a review that analyzes structures made with recycled concrete, relating to the energy efficiency of buildings. A classification structure was proposed, addressing the following questions: (i) What are the various dimensions in which research into energy-efficient recycled concrete is concentrated? (ii) What are the themes and classes of research associated with these dimensions? (iii) What are the main shortcomings of current approaches, and what would be a good research agenda for the future development of energy-efficient recycled concrete? A bibliometric analysis was carried out, presenting geographical and cluster maps to understand different research trends and refine future research. This was followed by a bibliographic analysis, reviewing the most relevant studies from the last five years (2019–2024). The results showed some residual alternative materials (around 45 types from five different industries) used in the production of energy-efficient concrete. And, as a negative effect, as substitution rates increase, porosity is the property with the greatest impact on energy efficiency. The greater the number of pores and the greater their interconnection, the lower the material’s thermal insulation.

1. Introduction

Since the Industrial Revolution, the use of concrete has gained momentum as it has great potential and offers flexibility in design due to its structural performance [1]. Growing concerns about sustainability, resilience, and environmental preservation may increase the demand for more economical building materials with lower associated environmental impacts [2]. Increased urbanization and industrialization can lead to the depletion of natural resources [3]. One notable material in this regard is recycled concrete, which incorporates recycled waste as a substitute for aggregates, cement, and additives in concrete production [4].

There is also the issue of energy efficiency, which is a fundamental issue in the building construction sector, accounting for around 40% of total global energy consumption [5] and accounting for 30% of global emissions of CO₂ [6,7]. With climate change and rising living standards, the level of cooling and heating energy required is expected to increase by 83% over 2010–2050 [7,8]. Currently, air conditioning systems for cooling and heating account for approximately 65% of total energy use in buildings [9,10]. Therefore, the invention of energy-efficient structural materials focusing on sustainability has become essential to sustain building’s temperatures [7].

Improving the energy efficiency of buildings over their lifetime and reducing the environmental impact of building designs are the main challenges in sustainable design [11,12]. Gharibi et al. [12] explain that in recent years, the use of recycled aggregates and waste has played an important role in reducing the environmental impact of the construction sector, following advances in concrete technology with increasingly efficient formulations [12].

Globally, approximately 25 billion tons of concrete are produced annually by the construction industry [13]. While traditional concrete emits 328.41 kgCO₂/m³ and needs 2451.32 MJ/m³ of embodied energy (EE), recycled concrete with alternative materials, on average gives off 326.17 kgCO₂/m³ and needs 2406.72 MJ/m³ of EE, equivalent to an almost 2% reduction in kgCO₂/m³ e MJ/m³ [2]. When recycled concrete is applied to concrete blocks for building envelopes, the thermal conductivity of the blocks is reduced by around 20% [14].

The recent literature on recycled concrete highlights the importance of using alternative materials in the three components of concrete (coarse aggregate, fine aggregate, and cement binder) and their ability to replace natural components and influence the physical, chemical, and mechanical properties of the concrete structures produced. When it comes to recycled concrete, recent literature on the subject highlights the increases in compressive strength [2,15,16,17]; reduced manufacturing costs [18,19,20]; increased modulus of elasticity [21,22]; and reduction in environmental impacts through Life Cycle Assessment (LCA) [23,24]. And, as a negative point, the literature highlights the increase in porosity of some material substitutions [2,25,26].

Another issue related to the energy consumption of concrete is the freeze–thaw modification technique [27]. Xia et al. [27] realized that an improved freeze–thaw method shows higher efficiency in treating air-entrained recycled concrete aggregate, allowing better control of repeated temperature alternation. Another point is about optimizing the energy incorporated into concrete mixtures, as analyzed by [28,29]. Wang et al. [28,29] proposed a procedure to optimize the mixture of recycled aggregate concrete (RAC) and concluded that multi-objective optimization of the concrete mix can predict the performance of sustainable concrete in terms of life cycle cost and energy consumption.

Many studies have recognized concrete as one of the most promising energy-efficient building materials [12,30,31] if there is a good selection of efficient constituent materials with high thermal insulation performance [31]. However, there has been little research into the energy efficiency of recycled concrete. Dora et al. [31], based on the idea of maintaining the high thermal insulation performance premise and low density of foamed concrete, found that reducing the amount of aerogel additive has become a key way to improve the economy of and promote the application of high-performance aerogel foamed concrete. This study proposed an equal volume replacement method based on SiO₂ aerogel use, instead of the conventional ultra-light SiO₂ aerogel. Yang et al. [30] focused on the use of solid waste as substitute materials in the preparation of foam concrete.

However, normal density concretes are also relevant in thermal insulation in building studies, such as in Yang et al. [32], which investigated the effects of replacing fine aggregates with coal bottom ash (CBA) and the influence of curing age on the thermal properties of high-strength concretes. They evaluated different CBA contents (25%, 50%, 75%, and 100%), replaced the fine aggregates, and found results of increasingly lower thermal conductivity values as the CBA content increased at the 28-day curing age, with thermal conductivities equal to 6.4%, 11.7%, 14.2%, and 22.5% lower than those of the control concrete mix CBA00 (1.87 W/(m.K)), for the percentages 25%, 50%, 75%, and 100%. Hameed et al. [33] evaluated the incorporation of nanoclay (NC) in concrete and concluded that it could reduce the building’s energy consumption and improve its thermal insulation properties. Hamada et al. [34] evaluated treated desert sand as a substitute for extracted sand in various percentages of 0%, 25%, 50%, 75%, and 100% of the total weight of fine aggregate. It was then concluded that for flexural and tensile strength, the highest values were achieved with the mixture containing 25% and were 11.54 MPa and 3.98 MPa, respectively, and the highest compressive strength was recorded at 56 days with 65.34 MPa. Gencel et al. [35] evaluated the inclusion of recycled concrete powder and found that it had a positive impact (an increase of around 4.5%) on the conductivity value, despite the slightly reduced apparent porosity.

Traditionally, most studies have focused on the technical aspects, evaluating the physical properties and mechanical behavior of structures and how concrete manufacturing processes impact the environment through Life Cycle Assessment (LCA). Given the above, it is not possible to find complete and in-depth review articles on different compositions of recycled concrete and energy efficiency. Therefore, understanding the energy efficiency aspect of recycled concrete requires knowledge that considers technical, financial, infrastructural, and governmental constraints. It is also extremely important to understand how these aspects are interconnected. The novelty of this study lies in the presentation of a classification structure, based on the literature, which covers these aspects—technological, institutional, environmental, and infrastructural—of the energy efficiency of recycled concrete.

To achieve this goal, the literature review presented in this paper aims to address the following critical research questions:

(a)

What are the various dimensions on which research into efficient recycled concrete focuses?

(b)

What are the themes and classes of research associated with these dimensions?

(c)

What are the main shortcomings of current approaches and what would be a good research agenda for the future in the development of energy-efficient recycled concrete?

To answer these guiding questions, the following methodology used Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), which is a support tool for carrying out a systematic review or meta-analysis [36]; similar usage occurred in Stavropoulou et al. [37], Ciccozzi et al. [38], and Marco-Lajara et al. [39]. Using the PRISMA method, combined with VosViewer, Mendeley, and GPS Visualizer software, it was possible to organize, evaluate, analyze, and systematize the most current research from 2019 to 2024 and the most relevant articles on the subject. Therefore, by starting a roadmap for future research, this article will be able to guide academics and professionals to carry out more in-depth and comprehensive studies that promote knowledge, innovation, and sustainable practices in concrete construction.

The study initially considered the analysis of recycled concrete in terms of its energy efficiency. The concrete evaluated was only for civil construction, except for its use in paving, drainage and sewage systems. Only fine and coarse aggregates and cement were considered as alternative materials to be used in energy-efficient concrete. The study of the influence of the thermal capacity of chemical additives and supplementary materials (such as filler) was not considered. It is also important to mention that the PRISMA/PICO classification method has been adapted to the reality of the construction industry.

2. Methods and Systematic Survey

A systematic flowchart of the analysis carried out was proposed, based on bibliometric and bibliographic analysis of research into recycled concrete. VosViewer software was used to create keyword clusters and identify the most influential key patterns in each scenario, presenting a structure that highlights the main themes and classes of research conducted over the last five years. This work was a new systematic review of the energy efficiency of recycled concrete today, and the study data sources used were web databases (Scopus, IEEE Xplore, and Emerald Insight) in the construction domain. The systematic review process suggested by PRISMA, made up of 27 topics, in summary, is as follows: (1) data collection; (2) data screening; and (3) analysis and discussion of the results as illustrated in Figure 1. The full checklist can be found in Table S1 of the Supplementary Material.

To answer the research questions, a systematic literature review was carried out, examining technological, institutional, environmental, and infrastructure considerations, and identifying the various topics that examine the energy efficiency of concrete structures and the influence of sustainable aggregates in the composition of new concretes through citation analysis and keyword grouping. A bibliographic analysis was carried out to qualitatively present the initiatives that were taking place within each specific theme of this research (technical, institutional, environmental, and infrastructure), by reviewing the content of the most relevant articles.

Both types of analysis provide the necessary framework to develop a holistic understanding of the current literature. Carrying out a bibliographic review using a bibliometric evaluation is the most appropriate way of guaranteeing better quality in reference lists, as well as serving as a tool for statistically evaluating the latest research results from published articles, book chapters, and conference proceedings related to the topic [40,41,42]. Figure 2 illustrates the literature review sequence and the steps required to develop this study’s systematic review.

Initially, sets of keywords were searched in the Scopus, IEEE, and Emerald Insight search engines. This was then uploaded to the Mendeley citation manager and VosViewer, allowing for cluster analysis. The next step was to upload it to the GPS Visualizer geolocation manager. This was followed by the other stages: collection of the most relevant articles; descriptive analysis of the literature examined by intended dimensions (technological, institutional, environmental and infrastructural) using PRISMA and Rayyan and the presentation of a classification structure categorizing energy efficiency studies on recycled concrete structures. Finally, the material identified was evaluated on the basis of the classification structure developed in the initial stages.

The research articles were collected from Scopus, IEEE, and Emerald Insight, selected according to PRISMA procedures, and systematically analyzed by categorizing them according to the areas of application of recycled concrete and its methods.

2.1. Information Sources, Eligibility Criteria, Search Strategy, and Data Collection Process

In this phase, the selection criteria, entry keywords, and collection of documents from the research databases were established, and a large and significant collection of literature articles was collected. Before this, it was necessary to develop a set of keywords that would enable the identification of research articles related to the study’s objective of evaluating current energy efficiency in concrete structures. The aim was to ensure a structured and systemic process of searching and filtering published documents that would provide a sufficient framework of intellectual raw material aligned with the purpose of the guiding questions raised in the previous section of this article. To answer PRISMA criterion no. 6 (Information Sources) [36,43], three databases were chosen [44,45]: Scopus from Elsevier, IEEE Xplore, and Emerald eJournals Premier, and each source was searched over 5 years.

According to criteria n° 5 (Eligibility criteria) of PRISMA Research [36,43], it is necessary to specify all components and characteristics of the study, and a way to do this is by using a tool called PICO Strategy for eligible study design and setting. PICO (Problem, Intervention, Comparison, and Outcomes) is a tool that helps to structure and refine the research question by systematically examining the effectiveness and appropriateness of an intervention in a specific study problem, ensuring a comprehensive and accurate exploration of the research question. [46]. This methodological tool is fundamental for developing structured research questions for systematic reviews and focuses on the following aspects, as follows in

Table 1. PICO strategy adopted in the research.

Acronym	Organizing Question	Organizing Response
P (Problem)	What is the study?	Concrete
I (Intervention)	What will be proposed?	Energy Efficiency and Mechanical Proprieties
C (Comparison)	What will the intervention be compared to?	Recycled Concrete
O (Outcomes)	What are the intended results or effects?	Study methodology for achieving thermal transmittance and its value

.

According to criteria No 7 (Search Strategy) of PRISMA Research [36,43], you must provide the complete search strategy as executed in each database with a sophisticated interface and evaluate the search terms. According to criteria No 9 (Data Collection Process) of PRISMA Research [36,43], the researcher must specify the methods used to collect the data from the publications and the details of the automation tools used in the process; in this case, VOSViewer, combined with GPS Visualizer, was used, based on the sequence of terms that were used to search for the interactions of the search keywords in the databases.

Five groups of keywords were developed. Group 1 included the following keywords: “energy efficiency”, “buildings”, and “concrete”. Group 2 focused on concrete aggregates and included the following keywords: “energy efficiency”, “concrete”, and “aggregate”. Group 3 focused on waste from the construction industry that could replace concrete aggregates and included the following keywords: “energy efficiency”, “concrete”, and “waste”. Group 4 focused on the energy efficiency of recycled concrete with the incorporation of alternative materials and included the following keywords: “energy efficiency” and “recycled concrete”. Group 5 focused on the search for recent research that brings the most up-to-date design of experiment methodologies for evaluating the energy efficiency of recycled concrete at the laboratory level and the results of mechanical and physical tests of these efficient sustainable concretes.

2.2. Selection Process, Outcomes’ Data Items, and Study Risk of Bias Assessment

According to criteria No 8 (Study risk of bias assessment) of Research PRISMA [43], it is necessary to create criteria for the inclusion and exclusion of factors from the review and to report all the processes used to obtain or confirm relevant information from the researchers in the study. Criterion 8 also asks for information on how the authors of the review worked on the present work [36]; this research review was carried out with reviewers working independently at each stage of the screening and the abstracts and titles of the articles did not need to be translated into another language, since one of the criteria adopted was the selection of articles in English only.

In addition, another selection process adopted for this research was the adoption of a 5-year timeline, starting in 2019, to reveal more recent theoretical developments, and subsequently evaluate the evolution of studies over the last 5 years. Also considered as a selection requirement was the collection of research articles (including conference articles), review articles (including conference articles), and book chapters as types of documents in the English language. As an exclusion, articles and abstract papers were not considered.

The search was initially carried out using the term “energy efficiency” as the initial point of research and was used in the fields “title”, “abstract”, and “keywords”. In the Scopus database, a total of 123,871 documents were retrieved. In the IEEE Xplore and Emerald databases, 29,454 and 3702 documents were found, respectively, resulting in a total of 157,027 documents.

However, according to the combination of criteria No 5 (Eligibility criteria), No 7 (Search Strategy), and criteria No 8 (Study risk of bias assessment) of Research PRISMA [36,43], it is necessary to select several documents that can be evaluated by the reviewers. Therefore, observing all the pairs of combinations of all the Groups, Group 1 served as the initial point of research interaction to identify the main aspects of this broad subject, and from it, it was possible to size the remaining Groups of words. Then, a second, third, and fourth analysis was performed, looking at all pairs of keywords from Group 2 and matching pairs between the four groups (i.e., at least one keyword from Group 1 and one from Group 2, and so on). The total search resulted in 938, 36, and 1644 documents on Scopus, IEEE Xplore, and Emerald Insight, respectively, totaling 3103 documents in total after applying the filters.

Table 2. Groups of input keywords for the current review.

Keywords
Group 1—G1	Group 2—G2	Group 3—G3	Group 4—G4	Group 5—G5
“energy efficiency” “building” “concrete”	“energy efficiency” “concrete” “aggregate”	“energy efficiency” “concrete” “waste”	“energy efficiency” “recycled concrete”	“energy efficiency” “concrete” “experimental”

summarizes the two groups of input keywords for the current revision.

The published research articles were identified through an extensive literature search in several databases. According to criteria No 10 (Data items) of Research PRISMA [36,43], which requests that all outcomes for which data were collected in each database be listed and defined.

Table 3. Search systems used in this review and the number of documents found by each input keywords group.

Name		Number of Documents
	Before the Filter						After the Filter
	G1	G2	G3	G4	G5	Total	G1	G2	G3	G4	G5	Total
Scopus	1523	220	353	19	445	2560	680	108	149	5	216	1158
IEEE Xplore	78	6	13	1	20	118	31	3	2	0	10	46
Emerald	1000	356	979	12	368	2715	835	215	584	10	255	1899
Total	1602	582	1345	32	833	5393	1546	326	735	15	481	3103

specifies the amount collected in each database before (5393 documents total) and after (3103 documents total) the selection and de-duplication processes. The main reason for choosing these search engines is that they are open access.

The structure proposed in this paper identified the close relationship between energy efficiency, construction, and concrete structures through bibliometric and bibliographic analyses. Once the bibliometric results of the three studies were obtained, we chose to focus future research. This bibliometric analysis was performed using the VosViewer software, version 1.6.20, developed at Leiden University, Leiden, Netherlands, which is integrated with the three databases used. The results of the research were downloaded in (.ris) format and uploaded to the bibliometric software, and then the format (.ris) was converted into (.txt) and (.xlsx) formats to determine the number of publications per year of publication. Once the cluster maps were set to “energy efficiency”, they led to four other types of searches, already enhanced by the results of the first cluster visualizations generated by the VosViewer software.

3. Results and Discussion

3.1. PRISMA Protocol Results Based on Keyword Groups

The total number of RIS files collected for the 1st Group was 1602; however, after organizing them using the Mendeley Reference Manager and applying filters, the number of files was reduced to 1546. Similarly, the 2nd Group was initially made up of 582 RIS files, which were reduced to 326. As well as for the 3rd Group, which went from 1345 to 735; for the 4th Group, which went from 32 to 15; and for the 5th Group, which went from 833 to 481.

When the search was performed with the descriptors of the 1st Group (“energy efficiency” and “building” and “concrete”), there was a strong relevance of a second and third group of words, “concrete” + “aggregate” and “concrete” + “waste”, which were added to this research. Considering that the use of alternative aggregates can turn out to be a good sustainable solution for energy-efficient construction [47], a fourth group consisting of the words “energy efficiency” + “concrete recycled” was created. In addition, as one of the objectives of this work is to evaluate the applicability and evaluation methods of energy-efficient recycled concrete, a fifth group was also created consisting of the words “energy efficiency” + “concrete” + “experimental”.

With the articles obtained through this search by groups of keywords, bibliometric maps were developed, through the correlation of keywords, using three appropriate search systems and through the VosViewer, as shown in Figure 3. When evaluating the bibliometric map, it is possible to observe the relevance of words such as energy performance, thermal comfort, aggregate, replacement, block, project, wall, and sustainable construction in the three pairs of keywords of the 1st Group, showing their relevance in the studies of the researched period.

3.2. Effect Measures and Synthesis Methods

According to criteria No 12 (Effect measures) of Research PRISMA [43] it is necessary to indicate the limits (or ranges) used to interpret the sample size of relevant articles and the effect and justification for these limits, as well as describe the decision rules used to arrive at an overall judgment. From the total number of articles filtered, 61 were selected to be analyzed in full. This number was obtained using the Central Limit Theorem, which asks how large a sample “n” needs to be for the normal approximation to be valid, and the answer depends on the population distribution of the sample data [48].

To define the 61 most relevant articles out of the 3103 (after the filters), some exclusion and inclusion criteria were defined, focusing on the energy efficiency of sustainable concrete. Articles published in journals dedicated to research into energy efficiency in construction, the innovative use of materials in construction, materials technology for construction, or journals dedicated to production issues in civil construction using more sustainable concretes were included. Only publishers whose scope was “construction material” and which were open access were considered. On the other hand, summary articles, summary reports, expert briefings, and the like were excluded. Articles whose titles included the phrases “thermal comfort”, “energy”, or “energy efficiency” or “optimization”, “concrete”, “thermal conductivity”, “concrete strength” and “alternative material” were included. Articles that provided information on the thermal conductivity of the materials used were included, as were articles that replaced natural aggregates with recycled aggregates from various sources (mineral, industrial, and demolition), and the type and origin of the alternative aggregate were also identified.

So, according to criteria No 13 (Synthesis methods) of Research PRISMA [43], The PRISMA protocol states that the processes used to decide which studies are eligible for each analysis should be described. Therefore, by the methods of the PRISMA protocol, a judgment must be made on the adherence of the study survey to the objectives of the work set out in the PICO, with adherence becoming greater as the design and execution of the study minimizes bias and maximizes the validity of the research. [49]. In this sense, and following the objectives of PRISMA 2020, the aim is to assess whether the study has any convergence to produce results and effects based on the criteria and methods designed [50] and whether these results can be replicated and generalized beyond the scenario in which it was developed [36,49]. At this stage, a methodology was developed for classifying and summarizing the most relevant documents. According to

Table 4. Criteria and corresponding scores for assessing the adherence of articles.

Related Research Aim	Criteria	Description/Question	Score
Conventional Concrete X Recycled Concrete	Title of the article	There is the expression “concrete” in the title	5
		There is the expression “aggregate” in the title	4
		There is the expression “waste” in the title	3
		There is the expression “recycled” in the title	2
		There is the expression “experimental” in the title	1
	Year	Publication between 2024 and 2023	3
		Publication between 2022 and 2021	2
		Publication between 2020 and 2019	1
State of the art	Relevance Published in a relevant journal	CiteScore Range 03.00 between 08.13	1
		CiteScore Range 08.14 between 13.28	2
		CiteScore Range 13.29 between 18.50	3

, this methodology is formed with the support of questions based on the aspects covered by the research PICO, and scores are assigned according to the degree of relevance to the research and integers were adopted for the score to preserve the significance of the ranking.

3.3. Quality Article and Bias Assessment and Certainty Assessment—By Year, Country, and Database

According to criteria No 14 (Reporting bias assessment) of Research PRISMA [43] it is necessary to describe the methods used to assess the risk of bias due to missing results in a summarization. However, for this study, this criterion does not apply as there were many documents found in the literature. In addition, there is also criterion n° 15 (Certainty assessment) of Research PRISMA [43] which recommends that reviewers assess all qualitative and qualitative aspects of the study, describing the methods used to assess the certainty, reliability, and distribution of the data, in this case being the articles/documents.

The bibliometric review resulted in graphs to visualize trends in the 3103 documents searched from the 2019–2024 collection using the databases Scopus, IEEE Xplore, and Emerald Insight. In Figure 4, the documents containing the input keywords for this study have been steadily increasing, except for in the year 2024 as the year is still in its first half. This may be related to the scarcity of some natural resources to improve energy efficiency, which has become a key issue in the building construction sector. In addition, the growing demand for more comfortable, energy-efficient environments, combined with the use of alternative materials in construction, has contributed to the search for sustainability in the sector.

Regarding the scientific databases, the number of published documents containing the keywords used in this study is higher in Scopus (with 1158 articles) and Emerald (with 1899 articles), while IEEE had a smaller research reach, perhaps because Emerald and Scopus concentrate the largest publications in construction/materials combined with energy efficiency.

The articles included in this review came from 91 different countries. Using the GPS Visualizer software [51], an online utility created by Adam Schneider in 2002 in Minnesota, USA, and through a Google Maps JavaScript API key, a map of the origins of the publications was created, according to Figure 5, after transforming the citation information (.csv) retrieved from Scopus into location information (latitude, longitude, name, and description) using BibExcel and Excel for tabulation. This information was then entered into the GPS Visualizer software, which geocoded and drew a map with the countries of origin of the Scopus documents. As IEEE Xplore and Emerald Insight do not provide documents with citation location information that can be read in BibExcel, the countries of these articles were added manually. Figure 5, shows that although China accounts for most publications, in terms of geographical distribution, publications are more concentrated in Europe.

Figure 5 also shows the distribution of the articles analyzed in the various origins, based on the top 60 countries. This shows that in the first quartile, with an average output of 55.46 documents, the five largest sources of information on the subject are China, India, Italy, the United States and the Russian Federation, followed by the second and third quartiles with an average output of 14.80 and 7.86 documents, respectively (both with a wide variety of countries from various continents).

In addition, the relevance of having articles published by authors affiliated with one country and using data from another country is that in the international context, the thematic factor of publications from international collaborations tends to have a greater scientific impact than national ones. And, according to Velez-Estevez et al. [52], the thematic factor influences the impact of international research, since the themes of this type of collaboration take the thematic factor into account when designing strategies to improve the competitiveness and collaboration of organizations, countries and communities of countries, designing policies that improve the competitiveness of countries and organizations. This makes it necessary to understand the factors and mechanisms that influence the benefits and impact of international research [52].

3.4. Energy Efficiency—Bibliometric Map and Systematic Analysis

According to criteria No 13 (Synthesis methods) of Research PRISMA [43], as it is the greatest requirement of the PRISMA method [36,50], research needs to be carried out in parts, so the use of bibliometric maps becomes a good analysis solution, allowing the researcher (using large data sets) to recognize the development and state of research in a given field and identify its critical points and limits [53,54]. The bibliometric maps, depict the latest correlations found in research based on the 3103 collections listed in Table 3.

Table 3’s analysis applied a minimum occurrence threshold of 15 for the first and fifth Groups, 10 for the second and third Group, and 3 for the fourth Group, aiming for a minimum occurrence of around 30 and with at least three clusters for analysis and, as a result, according to

Table 5. Number of occurrences of bibliometric maps of keyword groups.

Number of Occurrence
Name	Terms	Quant. Clusters	Group Cluster			Links	Items
			n° Clusters	Color	N° items
Group 1	385	5	1	Red	55	7585	178
			2	Green	54
			3	Blue	41
			4	Yellow	26
			5	Purple	2
Group 2	190	5	1	Red	46	2488	104
			2	Green	35
			3	Blue	11
			4	Yellow	7
			5	Purple	5
Group 3	367	4	1	Red	43	5380	139
			2	Green	37
			3	Blue	33
			4	Yellow	26
Group 4	62	5	1	Red	12	431	43
			2	Green	9
			3	Blue	9
			4	Yellow	7
			5	Purple	6
Group 5	217	5	1	Red	32	5044	121
			2	Green	30
			3	Blue	26
			4	Yellow	18
			5	Purple	15
Total	1004		19			15,884	464

, Group 1 had the highest occurrence of terms (385), the lowest occurrence was in Group 4 with 62 terms, and as for the clusters, Group 3 generated 4, and the rest of the Groups generated 5 clusters each.

Each map was created using the create a map function based on text data with read data from “Scopus.ris”, “IEEE.ris” and “Emerald.ris”, and with the option “full counting” and with the options “ignore structured abstract labels” and “ignore copyright statements”, to remove any duplicate documents. After carrying out the first analysis, the VosViewer software identified how many keywords appeared in the title and summary fields of all the documents uploaded. Some general terms, which did not relate to the research question, such as “crisis”, “time”, “allocation”, and “thing”, were excluded before the cluster map was generated. Finally, a relevant score was calculated, the most relevant terms were selected, and the list of keywords was released. Applying the keywords from Group 2, the bibliometric map of clusters in Figure 6 was produced.

From the blue cluster, the keyword links around “waste” (11 links) stand out, showing that it is possible to produce good concrete with resistance (keyword “flexural strength”) and satisfactory mechanical properties (keyword “mechanical property”); this also shows the scarcity of sand (keyword “desert sand”), and that there is a possibility of substitution. In yellow, the intermediate keyword linking cluster highlights “cement” with 70 links, and with links to all the other clusters. It shows that the waste is also used as a partial substitute for cement in the concrete composition, and the keywords “replacement” and “basalt powder” suggesting possible substitutions of aggregates capable of maintaining and improving the thermal performance of concrete. The cluster in purple, with an emphasis on “environmental impact” (72 links), is the cluster with the lowest number of items (5 items), but is with great importance, as it adds relevant keywords to Group 1 and demonstrates the importance of studies related to “alternative material”, “sustainable material”, and “building material”, with 38 links, 30 links, and 79 links, respectively, showing the possible use of alternative substitute materials to achieve energy efficiency in buildings, corroborating the previous clusters that exemplify some materials such as “CDW (construction and demolition waste) generation”, “stone dust”, “basalt powder”, “EPS (expanded polystyrene)”, “fly ash” and “lightweight aggregate”.

The first and second areas of the study to be analyzed are “Alternative Material Efficiency” and “Technical Performance”, respectively, corresponding to the Environmental and Technical dimensions, respectively. Applying the keywords from Group 3, the bibliometric map of clusters in Figure 7 was produced.

Analyzing the Cluster map in Figure 7, you can see that the largest cluster is “sustainability” with 128 links, (color red) and emphasizing the biggest keyword links to “effect” (122 links) and “company” (109 links) with direct links to “SDGs (Sustainable Development Goals)” and “policy” showing a strong importance for environmental policies. In addition, there is great relevance to the keywords “cost” (126 links), “energy” (128 links) and variations “energy saving (77 links), energy performance (64 links), and energy consumption (115 links)”, as well as “emission” (118 links) and “effect” (132 links). These links connect to others (“energy consumption”, “waste”, “policymaker”, “construction” and “climate change”). These are clusters that allow us to check the broader interconnection of all the keywords, allowing us to understand that many keywords are repeated concerning the first group. In purple, the cluster with the third highest number of links, “cement”, with 60 links, shows that cement is the most significant material in concrete and can be partially replaced by other waste. This shows that there is a need to study the main characteristics of concrete with partial substitution of the constituent aggregates and the technical and legislative environmental regulations, thus giving rise to the third area of study to be analyzed, “Environmental Policies, Waste Production, and Cost”, corresponding to the Institutional dimension. Applying the keywords from Group 4, the bibliometric map of clusters in Figure 8 was produced.

Analyzing the Cluster map of Figure 8, it is possible to see that it is the group of keywords with the least interaction between the clusters, the largest cluster being the red one, with the most prominent words being “impact” and “design methodology approach” with 31 links and 39 links, respectively. In this cluster, there is clear evidence of the green building policy and conventional construction practices being discussed, demonstrating the direct relationship between construction companies’ strategies for building sustainability and complying with legal regulations.

Next, there is a tie for the second largest cluster, with nine items, both the green cluster and the blue cluster. The difference between them lies in the fact that the “green” cluster shows the relationship between construction and the use of building materials with the keyword highlighted being “construction industry” (35 links), and the blue cluster highlights the keywords “building” and “research”, both with 33 links, and highlights the research being carried out in the area of sustainable construction practices using recycled concrete with alternative aggregates to achieve energy efficiency.

Then there are the last two clusters in Group 4, in yellow (7 items) and purple (6 items), with the highlighted words being “framework” (26 links), “indicator” (17 links), “analysis” (28 links), and BIM (Building Information Modeling) (18 links)”, respectively. Both clusters show construction control tools and tools for analyzing the energy efficiency in constructions, including BIM tools, assessment tools, indicators, and quality, and linking them to the other clusters shows that there is a construction of energy-efficient recycled concrete parts. This leads to the fourth area of study to be analyzed, “Recycled Concrete Construction Practices with Energy Efficiency”, corresponding to the Infrastructure dimension. Applying the keywords from Group 5, the bibliometric map of clusters in Figure 9 was produced.

Analyzing the Cluster map of Figure 9, it is possible to verify that the largest cluster is the word “concrete” with 69 links, (red color) and highlighting the most relevant keyword links for experiments “strength” (66 links), “compressive strength” (65 links), “mechanical property” (65 links) and “thermal conductivity” (60 links, with direct links to “temperature” and “experimental study” showing strong importance for the design of experiments). These links connect to the others (“thermal performance”, “energy performance”, “CO₂ emission”, “value”, and “design methodology”). These are clusters that allow us to verify the broader interconnection of all the keywords about laboratory tests, allowing us to understand that there is a great relevance of the design of experiments in the evaluation of the energy efficiency of concrete and that the relationship with the keywords of the previous groups is maintained, especially the first group and third group. This shows that there is a need to study the main concrete laboratory tests, both in the mechanical phase and in the energy efficiency phase, and the theoretical application applied to concrete material innovation in the construction sector, which means that the fifth dimension of the studies to be analyzed is “Design of Experiments”, corresponding to the Technical dimension.

The five clusters of keyword groups show that all the terms and links in the bibliometric maps interact with each other. Group 5 interacts with the previous groups and vice versa. This can be explained by the fact that recycled concrete with the use of alternative materials has broad perspectives for study, according to the following studies: “Alternative Material Efficiency”, “Technical Performance”, “Environmental Policies, Waste Production, and Cost”, “Recycled Concrete Construction Practices with Energy Efficiency” and “Design of Experiments”. As justification, and based on criteria n° 15 (Certainty assessment) of Research PRISMA [43], the equality of treatment effects and data collection means must be tested using the analysis of variance (ANOVA) model. Therefore, adopting the factors of total number of articles (before and after filtering), number of articles from 2019 to 2024 (after filtering), number of terms (VOSViewer) (after filtering) and number of items/cluster (VOSViewer) (after filtering) for the five groups of keywords, and using MiniTab 21.1 software, ANOVA was carried out on the articles researched with a 95% reliability level, showing great significance in the data collected after applying the filters, with a p-value of 0.038 (<0.05) and an almost homogeneous residual distribution. In addition, Group 4 had the lowest standard deviation in the number of articles collected (21.61), and Group 5 had the highest standard deviation (252.7).

3.5. Extracting Information from Published Research on the Energy Efficiency of Recycled Concrete

Information was extracted from the latest studies, summarizing the main materials used in each study (type of aggregates, cement, and additives) with the levels of substitution, the analysis tools and tests, the properties assessed in the concrete, and the degree of energy efficiency. This section focused on grouping the selected set of papers, based on the bibliometric analysis carried out. After retrieving more than 157,027 documents from the first type of search, the keyword groups generated by the VosViewer software signaled how to further refine the energy efficiency of sustainable concretes. Consequently, the following four search types were developed using keyword enhancements, resulting in more than 3000 articles.

Adopting the assumptions of criterion 13 (Synthesis methods) of Research PRISMA [43] and adopting the combination of the VOSviewer tool with the artificial intelligence programming of the Rayyan Online Software, which is a free web and mobile collaboration and research tool for organizing, classifying, and managing the application of systematic reviews, which helps to speed up the screening of abstracts and titles using a semi-automated process and artificial intelligence, while incorporating a high level of usability [55], similar to that used in Panakaduwa et al. [56], Xhu et al. [57], and Abouaiana et al. [58]. This allowed for better screening and resulted in the elimination of 1048 duplicates. The degree of similarity adopted was 97%, with 32 articles falling below this degree (ranging from 90% to 95%), so these were evaluated manually, resulting in 242 reports sought for retrieval and 61 relevant articles, according to Figure 10.

In addition, the Rayyan tool allows the integration of PRISMA’s PICO questions, speeding up and increasing the reliability of the survey [55]. In addition, for the selection of relevant articles, only articles published in journals dealing with building materials and construction innovation were adopted, following the adherence requirements of Table 4. In addition, the relevant articles were based on the set of papers retrieved, considering the greater adherence of the clusters to the dimensions calculated.

3.6. Study Perspectives and List of the Most Relevant Scientific Articles

In view of the bibliometric analysis developed, the related articles were selected and expanded in the Supplementary Materials: Table S2. These tables also present, in an objective manner, the methodology and results of each of the selected documents, based on the associated cluster. It presents a summary of each of the selected documents and consolidates the information from the review papers and published studies for recycled concrete and efficient energy. It consists of the publication objective, methodology, results, and five different scores per keyword. The “CP” means Conference Proceedings, “JA” means Journal Article, and “RP” means Review Paper in the Type of Document (TD) column.

From Table S2, it is possible to conclude some propositions. It can be seen that studies into sustainable, energy-efficient concretes cover a range of concretes: normal concretes (with a density of between 2300 and 2500 kg/m³) [12,32,34,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74], including high performance concretes with strengths greater than 100 MPa [75,76]; lightweight concrete (with a density between 1000 and 1200 kg/m³) [30,31,35,60,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93]; heavy concrete (with a density between 3300 and 4200 kg/m³) [94,95]; and self-compacting concrete [76], in addition to presenting various concepts regarding Phase Change Materials (PCMs) [7,11,35,96], which are materials that can store and release large amounts of latent heat during the phase change process, making them highly thermally inert and allowing them to be used in combination with alternative materials in the building envelope. [11].

Other research involves more specific studies of the application of computational tools, such as mathematical prediction algorithms, machine learning languages, BIM and the Internet of Things (IoT) being applied to recycled concrete structures [97,98], and applications in masonry, luminance, thermal comfort of users, on a scale, as well as in the general construction of concrete structures, evaluation of the climatic influence on the thermal conductivity of concrete [47,99,100,101,102,103], and studies on waste management, general sustainable concrete management, and various alternative binders in a broad manner [104,105,106,107,108,109,110].

This systematic search resulted in various percentage representatives, with 34% of the relevant articles dealing with normal-density sustainable concrete, as well as light-density concrete (34%), and heavy concrete (3%); for PCMs used in combination with sustainable concrete, the representativeness was 7%, and for the more generic subjects, on the use of waste in concrete and application in construction in a practical way and construction management of concrete structures, it was 26%.

3.7. Study Perspective—Alternative Material Efficient

From the perspective of “Alternative Material Efficients” based on the systematization of this work, many materials are used to make sustainable concrete. It was possible to identify almost forty-five waste materials that could be used as alternative materials for making sustainable, energy-efficient concrete; and, these were from five different classes of industry, encompassing waste and purges from the chemical, construction, mining, and steel industries, and even material of biological origin, according to Figure 11.

Alternative materials are applied at various levels of substitution, replacing cement [63,68,84], the fine aggregate [64,65,68,79], the coarse aggregate [12,66,70] and with Supplementary Materials to improve concrete properties such as workability [77,78,94]. Some of the alternative materials used include:

(a)

biological materials such as coral sand [60], powdered eggshell waste [62], corn stalks [81], rice husk ash [82], palm oil ash [82], hemp [83], expanded vermiculite [31], crushed peanut shells [88] and sugarcane bagasse ash [74];

(b)

chemical waste materials such as polymeric [59], sugar sediment waste [94], polypropylene fibers [78], molybdenum tailings (MTs) [79], fly ash [68,82], active silica [82,84], silica aerogel [85,86], capric acid [31], recycled motor oil [104], polyethylene powder [92], and TiO₂ photocatalytic [73];

(c)

materials from the construction industry, mostly waste, such as ceramic electrical insulation waste [12], recycled plastics [77], waste concrete powder [63], construction and demolition waste [66,68], recycled aggregate [7], expanded clay [84], expanded glass [85], plastic bubbles [85], lime-rich calcined clay [69], powders from the recycling of concrete [70], powders from the recycling of clay bricks [70], desert sand [34], and red mud [72];

(d)

mining waste materials such as quartz sandstone mine waste [61], sodium silicate waste [64], stone dust [65], self-ignited coal gangue power [65], phosphogypsum [65], limestone tailings [30], gold tailings [30], and coal bottom ash [32];

(e)

steel mills’ waste materials such as granulated blast furnace slag [65].

3.8. Study Perspective—Technical Performance

From the perspective of “Technical Performance”, recycled concretes, in general, had good performance compared to reference conventional concretes [65,66,68], other concrete models did not perform better than the reference but were higher than the minimum normative requirements [34,70]. As an important factor in thermal performance, the thermal transmittance value (U-value) depends on the different types of cement, recycled coarse aggregate, and water–cement ratio. The U-value is also defined as the inverse of the thermal conductivity value, which expresses the resistance to heat flow through a given thickness of material [102], and the smaller it is, the better the thermal insulation is. Still, thermal conductivity was reduced by almost 48% when there was a complete replacement of the aggregates by waste from crushed ceramic, and there was a reduction of about 21% and 37%, respectively, in the case of replacement of coarse and fine natural aggregates separately by ceramic waste [12].

Figure 12 shows the thermal conductivity of various concretes with alternative materials, such as the thermal conductivity (λ) of concrete with agricultural tailings (corn stalks) gradually increasing with the decrease in the size of the corn stalk, from large sizes to corn stalk powder, from the largest grains (8.25 mm) to the smallest (0.055 mm), went from 0.060 to 0.197 W/(m.K) [81]. With the use of expanded clay and silica fume in the properties of lightweight aggregate foamed concrete, the thermal conductivity, the volumetric specific thermal capacity, and the thermal diffusivity were in the range of 0.23–0.45 W/(m.K), 1136–1631 kJ/(m³.K), and 0.20–0.275 mm²/s, respectively [84]. With the use of expanded glass aggregate, silica aerogel, and prefabricated plastic bubbles, the thermal conductivity of the aerogel concrete varied between 0.356 W/(m⋅K) and 0.342 W/(m⋅K), and with prefabricated plastic bubbles presented 0.302 W/(m⋅K), being the lowest coefficient value [85]. Using Perlite Light-Weight Concrete (PLWC) provided an optimized mix, with bulk density and compressive strength values of 1761 kg/m³ and 21.3 MPa, respectively, and presented a higher energy dissipation capacity of 0.1118 J [87]. Some concretes with vegetables showed that the coefficients of thermal conductivity, volumetric specific thermal capacity, and thermal diffusivity were in the range of 0.114–0.192 W/(m.K), 0.81–1.321 MJ/(m³. K), and 0.109–0.145 mm²/s [72]. The use of recycled plastic aggregates has resulted in a significant reduction in annual operating energy, carbon emissions, and overall energy consumption of up to 15%, 13%, and 21%, respectively [77]. Unlike the use of coral sand concrete under different conditions of relative humidity (30%, 50%, 70%, 85%, and 100%,) and temperature (25 °C and 35 °C) the average daily energy consumption increased by 2.23% and the thermal conductivity by 89.35% [60].

Specific heat capacity and volumetric heat capacity characterize a material’s ability to store heat, with specific heat capacity being the amount of heat per unit mass required to increase temperature by 1 °C, while volumetric heat capacity is related to a unit of volume [78]. According to Table S3 of Supplementary Materials, several alternative materials can be used to make sustainable and energy-efficient concrete, and act as replacements for cement, fine aggregate, and coarse aggregate; among the examples in the table, we can mention ground granulated blast furnace slag; pulverized fly ash; fume silica; recycled coarse aggregate; coal bottom ash; peanut shell; calcined clay soil (calcined cao-based clay); expanded clay lightweight aggregate; fly ash lightweight aggregate; expanded slate lightweight aggregate; sodium silicate waste; polypropylene fiber; and steel fiber.

Still, about Table S3, Gomes et al. [80] applied various thermal analysis methods to the samples, in the quasi-steady state using QSM and using dynamic/transient state using MTPM and HTM, as the results of thermal properties are affected by the type of test and test conditions. The tests can be categorized into stationary and transient methods, and for concrete, the λ of the transient impulse method tends to be lower than that of the stationary method [80]. In stationary methods, a thermal gradient is established between two points and a constant unidirectional heat flow is measured; the most used example of this type of test is the heat flow meter (EN 12667:2001 [111]) and the hot plate protected (EN 12667:2001 [111]) test. Although accurate, the method is laborious and time-consuming and is best suited for homogeneous materials [112]. In transient methods, the response to a thermal impulse is measured, which creates a heat flow, and is more suitable for heterogeneous materials and different moisture contents [113]. The most common examples are the transient flat source, the transient linear source, and the modified transient flat source. These methods are faster and provide results in a short period, but with less accuracy than stationary methods [112,113].

From the values of Table S3, there is a variation in density influencing thermal conductivity. This is because the higher the density value, the higher the coefficient of thermal conductivity is, which can vary up to 25%; concretes with high density are associated with high thermal conductivity, requiring less energy to raise their temperature [80]. Porosity also influences thermal conductivity [64,69]; according to Gomes et al. [80] there is an average reduction of 0.6% in thermal conductivity for every 1% increment in porosity. In addition, in the transient pulse method (measured in cylindrical specimens), the specific thermal capacity decreases with the density of the concrete, as well as in the heat transfer tests, although with greater dispersion this is due to the influence of the porosity of the aggregate, for which the higher it is, the greater the thermal capacity will be [69,80].

Still, according to Table S3, it is possible to use organically sourced material for energy-efficient concrete production, such as peanut shells. Horma et al. [88] concluded that groundnut shells crushed in concrete decrease density by up to 17.5% for coarse aggregate. The addition of up to 6% by weight of peanut shells reduces thermal conductivity by more than 55%, regardless of morphology, and the compressive strength varies with the size of the peanut shell, decreasing from 18 to 51% with 6% content by weight.

Finally, in Table S3, through the study of Yang et al. [32], the industrial waste produced at the bottom of a coal furnace in coal-fired power plants affected the texture of the concrete, which presented more irregular and porous particles than the control concrete, and lower workability. In addition to decreasing the thermal conductivity of the concrete as the residue content increased, it soon proved that there is a lot of dependence on the pore structure of the concrete and, subsequently, that the density of the concrete on the thermal properties being the pore structure is one of the key elements that affects the thermal conductivity [32,80].

3.9. Study Perspective—Environmental Policies, Waste Production and Cost

From the perspective of “Environmental Policies, Waste Production, and Cost”, according to a systematic review of Algahtani et al. [77], global consumption of non-biodegradable plastic waste (including linear low-density polyethylene bags (LLDPE), polyethylene terephthalate bottles (PET), and expanded polystyrene (EPS) foam) has peaked at nearly 150 million tons [77]. In the mining scene, granite, gravel, and limestone are the most popular and widespread sources of gravel, which are expensive, and their deposits are not available in all countries [61]. Cement production increased dramatically in the 2000s, reaching 4 billion tons in the previous decade, and cement production and its use are responsible for 8% of global carbon emissions [62]. Eggshells as a possible substitute for cement is a product that has seen global egg production from 2000 to 2020 increase from 55 million tons to 85 million tons, and eggshell ash (PES) has been used as a source of calcium carbonate (CaCO₃) to accelerate cement hydration [62]. As for the construction industry, the global generation of CWD waste has exceeded 3 billion tons per year, with concrete waste accounting for 20–30% and approximately 35% of CWD waste being disposed of in landfills [63].

Recycling waste for construction is not a new idea; Europe, along with other developing countries, in the middle of war, chose to recycle materials obtained from damaged buildings after the Second World War and used them to rebuild buildings [99]. However, the implementation of a systemic approach to waste management has yet to be implemented and is mandatory to achieve sustainable development [107].

One of the impeding factors is the implementation of waste integration strategies linked to production, directly to the industrial plants, as the preferred form of waste management, which is hampered by economic and social barriers [105]. The economic aspect is related to the cost-effectiveness of waste control [105,108]; the cost of waste management and reduction is often higher than the cost of storage and production [108]. The social barrier is linked to the fact that social acceptability has a great influence, as it affects both clients and engineers in the construction sector, who often still do not trust the characterization of alternative concretes [105], despite the market’s acceptance of new types of concrete components, sometimes due to a lack of natural resources.

Moreover, at the same time, there is a need to reduce waste, since more than half of the waste generated in the construction industry comes from end-of-life activities and operations, especially demolitions, and only around 30% of these materials are reused or recycled [107]. Figure 13 shows the main results for the Study Perspective—Environmental Policies, Waste Production and Cost; almost 50% to 60% of a project’s cost is accounted for by materials, so any reduction in their waste has a major effect on a project’s cost reduction [114]. On average, there is no single material that contributes less than 20% to project costs [107,108]; however, Shahid et al. [108], after quantifying waste from 40 projects, concluded that wood (36.2%) and sand (28.8%) are the most wasted items, while concrete (14.5%), bricks (13.7%), ceiling tiles (13.6%), tiles (13.5%) and aggregates (12%) are wasted in moderate quantities, and cement (5.4%) and steel (4.5%) in smaller quantities, with a level of contribution to combating waste of 21.02, 3.79, 8.18, 19.34, 4.84, 8.88, 4.03, 12.53, and 17.39 for wood, sand, concrete, bricks, ceiling tiles, tiles, aggregates, cement, and steel, respectively.

As far as political agreements are concerned, it is possible to note some global efforts to combat waste and waste management, such as the European Union’s European Green Deal, which calls for an increase in the renovation rate of buildings, making them more energy efficient [109], the Circular Economy Policy Strategy in the United States, with average recovery practice rates (these are systemic actions to restore the environment through reuse, repurposing, and recycling) at around 70%, while China’s recovery rate remains limited to less than 5%, while in the UK it is 89.9%, in France it is 47.5%, in Spain it is 37.9%, and in Germany it is 34% [107]. Regarding mining activities, it is important to mention that the European Union’s Mining and Extractive Activities Zone (MEAZ) (a region around Romania, Finland, Sweden, and Bulgaria) was responsible for generating 178.6 million tons of mining waste in 2018 [109]. The Raw Materials Initiative (RMI) launched in 2008 is the “Mining Industry Strategy developed for the period 2017–2035” which has as one of its strategic objectives the insertion of innovative mineral resource technologies from the cradle to the exploitation, processing, or recovery of waste [109].

Other initiatives for extractive industrial waste have been defined and classified by Directive 75/442/CEE [115] and later by Directive 2014/955/EU [116] into four major groups: tailings; waste rock; sewage sludge; and topsoil. In Austria, there is a prevailing policy trend to carry out a substantial part of construction activities precast to minimize costs and waste, and to improve the overall cost-effectiveness of the project [107]. In New Zealand, designing components with a focus on reuse and recycling not only leads to waste reduction but also actively promotes Circular Economy strategies [107,109]. Socially, the adoption of Circular Economy strategies can create job opportunities and contribute to sustainable development by minimizing waste generation and promoting responsible construction practices [105].

Regarding financial and economic issues, Kanagaraj et al. [64] state that the production cost of waste is zero because waste is widely available and that an initial substitution of 25% provides a marginal reduction in production cost. As for the cost, Kanagaraj et al. [59] found that replacing 70% of SSW with coarse aggregate was more cost-effective than the control, showing $43.61 against $51.87, and in another study [64], also with SSW/aggregate charged $71.13 against $76.04. Tolstoy et al. [61] evaluated the economic efficiency of reducing the cost of aggregates using mine waste, and found that in addition to being costless, this waste can also generate income for those who use it. The total cost of the material, for 1 m³ of concrete, can be reduced by 14.26% for an MT replacement rate of 100% of the natural sand, and considering the cost of the material and the compressive strength, the samples with MT replacement rates of 40% and 60% not only reduced the cost but also met the highest compressive strength [79].

3.10. Study Perspective—Recycled Concrete Construction Practices with Energy Efficiency

From the perspective of “Recycled Concrete Construction Practices with Energy Efficiency”, Alqahtani et al. [77] used Robot Structural Analysis software to carry out the structural analysis. Alqahtani et al. [77] carried out a Life Cycle Analysis (LCA) study and used the indicators potential for global warming (GWP), ozone depletion (ODP), acidification potential (AP), eutrophication (EP), and photochemical ozone formation (POCP), as well as cumulative energy demand (CED) and freshwater availability (FW), and the Ecoinvent database, finding that there were average reductions of 17. 91%, 14.90%, 15.09%, 15.19%, 15.00%, 15.36%, 15.23%, and 15.24%, respectively, when natural aggregates were replaced by sustainable recycled plastic aggregates. Quan et al. [79] used GWP, AP, nitrification potential (NP), POCP renewable energy (RE), and primary energy (PE) as indicators, based on the online LCA assessment software eFootprint and the Chinese Reference Life Cycle Database (CLCD), and found that the reduction benefits of AP and NP were irrelevant, and PE and GWP had the greatest impact.

GWP is the best tool for assessing the impact of different gases on global warming [64]; composites mixed with SSW have 24.1 CO₂-eq./m³ of concrete production, reducing approximately 12% of CO₂-eq./m³ more than conventional concrete [64]. In terms of energy evaluation, Kanagaraj et al. [59] showed a reduction of 4.65% (1.2713 GJ/m³) of the energy requirement compared to the that of the reference (1.3398 GJ/m³) with the use of SSW by the coarse aggregate. Alqahtani et al. [77] showed an average reduction of 14.28% (from 83,444.87 kBtu/h to 97,354.70 kBtu/h). Kanagaraj et al. [64] informs that by-products of industrial waste and the energy demand for its production can be considered as zero because it is waste, so an increase in the content of industrial waste reduces the energy demand to produce concrete, and the demand for SSW is, on average, 2.8 GJ/m³, and, for CO₂ emissions, is 0.35 t/CO₂ -e/m³ [64]. Alqahtani et al. [77] used the Integrated Environmental Solutions-Virtual Environment (IES-VE) program to simulate the building’s environmental performance based on operational energy estimates [77].

Regarding the assessment of CO₂ emissions, Alqahtani et al. [77] found an average reduction of 12.12% (from 12,545.50 kgCO₂/h to 11,024.30 kgCO₂/h) with the use of SSW to replace coarse aggregate. Another important factor for the energy efficiency of concrete is humidity. Studies have shown that as the moisture content of concrete increases, the dry air in the pores is replaced by water and humid air and, for this reason, the thermal conductivity of concrete tends to increase with increasing moisture content [60].

3.11. Study Perspective—Design of Experiments

From the perspective of “Design of Experiments”, according to the systematic review of this work, the evaluation of the temperature between the high and low values in thermal conductivity is very significant for the durability analysis of building materials [60]. Several international standards have been adopted for some designs of experiments in terms of energy efficiency: ISO 8302:1991 [117] (heat flow meter), ASTM C177:1990 [118] (thermal conductivity), ASTM C1784:2014 [119] (specific volumetric heat). Since the effect of temperature on the thermal conductivity of materials in the dry state is being measured, it is necessary to keep the temperature of the external environment constant [60] and use a steady-state heat flow meter [78]. Various pieces of equipment were used, such as a dual test sample steady-state thermal conductivity tester (GHP456) obtained from NETZSCH GmbH of Germany [60].

Most of the articles adopted a temperature and relative humidity test plan to evaluate the thermal conductivities of the materials determined at 25 °C and 35 °C under relative humidity values of 0%, 30%, 50%, 70%, 85%, and 100% [60]. Within the temperature range of 20–50 °C, the thermal conductivity of concrete with coral sand increased by 0.47%, while that of conventional concrete increased by 5.16% [60].

Gomes et al. [80] used various testing methods to evaluate the test properties of concrete modified by various lightweight aggregates; the most common methods involving ASTM D5334:2008 [120] are the transient flat source, the transient linear source, and the modified transient flat source, which are suitable for heterogeneous materials and different moisture contents [113], although they are less accurate than stationary methods [80].

Some studies have also evaluated the microstructure of sustainable concrete [61,63,79], using a scanning electron microscope (SEM) (e.g., model: Quanta 200; maximum resolution 3.5 nm) and a mercury microparticle porosimeter (e.g., model: Auto Pore IV 9500 V1.09; working pressure 414 MPa) [60] and TESCAN MIRA 3 LMU [61]. The study of pores is justified by the fact that the large porosity and size of the pores allow for an increase in liquid water content, as humidity has a great influence on thermal conductivity, and the greater porosity increases conductivity [60]. Porosity is one of the main factors affecting the thermal conduction properties of concrete and closed, non-interconnected pores reduce conductivity due to the low thermal conductivity of air [78,79].

The response surface methodology (RSM) [63] has been used in various situations to vary the water/cement ratio and carbonation time [63], and a strength activity index has been used to assess whether the residual material is suitable for use as supplementary cementitious materials (SCM) in mortar/concrete [63]; analysis of variance (ANOVA) was used for the interaction effect of the competent [63]. Kanagaraj et al. [75] used the Taguchi Method, which is a series of design tests based on the Design of Experiment (DOE) concept to minimize data dispersion and inspect countless variable parameters with minimal testing and minimize the number of trials [121].

To complement the thermal analysis of alternative materials, some articles have used different computer software, some with machine learning programming, to recognize patterns in a set of data and make predictions regarding the thermal conductivity of various sustainable concretes, including [97,103], followed by BIM software to evaluate thermal comfort management [77,98], BEM (Building Energy Modeling) [101], Circular Economy and Life Circle Analysis techniques (LCA) [24,77,79,107] and SWOT tool [109].

3.12. Dimensions and Themes of Research into the Energy Efficiency of Recycled Concrete

The organization of the retrieved documents based on a series of research dimensions is important for achieving the structure proposed in this study, which is the evaluation of the energy efficiency of recycled/sustainable concretes [32,34,72,91,103,106]. To be relevant, these dimensions can then be subdivided into themes that relate to and define the areas of research verified in recycled/sustainable concrete. Looking at Figure 6, Figure 7, Figure 8 and Figure 9, four main dimensions are derived: Technical, Institutional, Environment, and Infrastructure, as seen in

Table 6. The four main dimensions of the “concrete recycled” concept and commonly seen subjects regarding each one of them.

Dimensions	Scope/Perspective of Study	Frequent Subjects	References
Technical	Technical Performance Design of Experiments	Mechanical performance Thermal performance Energy consumption	[15,78] [33,100] [9,70]
Institutional	Environmental Policies, Waste Production and Cost	Modernization of the technological flow Government policies Financial subsidies Waste management plans	[107] [44,107] [20,77] [24,107,122]
Environment	Alternative Material Efficiency	Industrial waste materials Replacing aggregates and cement Reuse	[30,99] [25,62,123] [24,74]
Infrastructure	Recycled Concrete Construction Practices with Energy Efficiency	Energy saving Material reuse CO2 emissions Embodied and emitted energy	[8,70] [13,104,109] [63,66] [7,33,87]

. The Technical dimension covers studies of concrete properties that examine the use of recycled aggregates as CDW [66,68], mineral waste [30,61,99,109], waste agricultural products [14,65], and replacement cement [18,62,82] as essential to making building concrete sustainable. For the Institutional dimension, issues such as global governance are discussed [44,107,108,122], as well as standards and technical manuals [80,98,117,120]. Finally, the Infrastructure dimension covers recycled concrete infrastructure studies that examine the use of energy-efficient alternative materials [47,105] and tools for analyzing construction efficiency using recycled concrete [5,10,23,98] to improve the user experience and thermal comfort of buildings.

Restricting the dimensions and scope/perspective of study even further, a categorization of the total number of relevant publications was defined according to the main theme and, after retrieving the themes that appeared most frequently in the results presented by VosViewer (version 1.6.20) (identified from the bibliometric analysis), they were as follows: mechanical properties, performance, thermal proprieties, thermal performance, replacement, economy, analysis, recycling, sustainability, energy consumption, CO₂ emission, environmental impact, thermal comfort, LCA, temperature, construction, machine learning, applications, BIM, and IoT.

A list of 20 themes was created and a classification mechanism was highlighted that was most appropriate for the intended studies. The proposed classification structure for studies of energy-efficient recycled concrete is shown in Figure 14, generated based on a sunburst graphic in spreadsheet software.

3.12.1. What Are the Various Dimensions on Which Research into Efficient Recycled Concrete Focuses?

There are four main focal points when it comes to research dimensions in energy-efficient recycled concrete: Environmental, with a representation of 35.71% of the entire study, followed by Technical (34.29%), Infrastructure (15.71%), and Institutional (12.86%) dimensions. What can be seen is that research papers tend to focus on one or two of these dimensions, without taking all four dimensions into account. This means that most of the documents are very technical [12,15,33,67,70,78,80] (focusing on the technological aspect, the mechanical performance of the concrete samples and the physical performance of the natural and alternative materials, and comparing them) or very environmental [7,105,124] (focusing on the environmental impacts caused by the use of conventional concretes with natural materials compared to sustainable concretes) or technical–environmental [20,77]. When it comes to review articles, this problem occurs less frequently, increasing to up to three dimensions [1,47,82,105]. The most frequent issues reported in the literature for each dimension are presented in the table below Table 6.

For the Environment dimension, the most prominent issues seem to be LCA and environmental impact assessment, more concentrated on GWP, AP, NP, POCP, RE, and PE [77]. And in the energy aspect of materials, there is a strong focus on embodied energy and CO₂ emissions [64]. It is important to note that the issues of environmental impact analysis require analysis of the quality of the data collected and the uncertainties in the data, using a more qualitative method such as Monte Carlo or Regionalization. As far as the Technical aspect is concerned, there is a broader universe, just as much as in the Environment aspect, with subjects more directed towards studies of laboratory tests of resistance to compression and traction; with the remainder evaluating shear resistance; porosity, and water absorption, sometimes the articles do not mention these two properties combined, leading to incomplete analyses; more common tests almost all the publications mention are modulus of elasticity and density [12,15,20,34,78]. As for the technical analysis of thermal performance, some publications deal with thermal conductivity but do not evaluate other properties such as thermal capacity, thermal diffusivity, and specific heat [32,69].

For the Infrastructure dimension, there is little application in the construction industry, although there are efforts to apply it in practice. Few studies have evaluated using simulation tools [5,77,98]. Many studies focus only on laboratory testing rather than numerical computation. Some studies that used computational information focused on formulating thermal conductivity prediction models. The Institutional dimension was the least studied and had the least impact since many researchers did not focus on the governmental aspects of social legislation and the social impacts of the Institutional dimension [107,114,122].

3.12.2. What Are the Themes and Classes of Research Associated with These Dimensions?

The main aim of this research question was to further filter the dimensions identified in recycled concrete research, based on a set of interrelated themes and classes of research topics. One of the most widely published research themes is associated with the interrelationship of mechanical properties and sustainability. It is also important to note that many of the themes—for example, performance, economy, recycling, CO₂ emissions, and thermal comfort—have papers that deal largely with the technological aspect, as follows in

Table 7. Classification of literature based on themes and associated classes.

Dimensions		Detailed Topic (Themes)	Occurrences
Technical	22	Mechanical properties	8
		Performance	6
		Thermal proprieties	4
		Thermal performance	2
		Replacement	2
Institutional	6	Economy	3
		Analysis	1
		Recycling	2
Environment	23	Sustainability	7
		Energy consumption	5
		CO2 emission	3
		Environmental impact	2
		Thermal comfort	2
		LCA	3
		Temperature	1
Infrastructure	10	Construction	4
		Machine learning	2
		Applications	2
		BIM	1
		IoT	1

which shows the references associated with each theme and the further refinement of studies based on research classes. The 20 classification themes for studies on efficient recycled concrete include energy, mechanical performance, energy conductivity, BIM, economics, government, and life cycle analysis.

3.12.3. What Are the Main Shortcomings of Current Approaches and What Would Be a Good Research Agenda for the Future in the Development of Energy-Efficient Recycled Concrete?

A comprehensive view (involving all four dimensions of energy-efficient recycled concrete) is fundamental for a deeper understanding of the impacts and benefits of recycled concrete. Some studies continue to focus on just one or two of the dimensions identified, as already mentioned in the previous topic, while other publications do not present a clear roadmap for implementing the analysis methodology applied that guarantees the involvement and consideration of the other two or three dimensions. This review of studies on energy-efficient recycled concrete indicates that there is still a possible gap in the standardization of technical thermal conductivity tests due to the wide variety of materials and regulations, and that in the integration of LCA and BIM studies for recycled concrete structures there is a lack of studies regarding the systematic survey of the application of recycled concrete in construction in a more effective way and its implementation policies.

In light of this bibliometric and systematic review, based on the PRISMA methodology, the main deficiencies identified in the current literature are as follows:

(a)

A more comprehensive understanding of the impacts of socio-technological practices integrated with LCA on the future roadmap of sustainable concrete development is needed, in addition to an understanding of the deficiency of integrating LCA with BIM for energy-efficient sustainable concrete structures to assess thermal comfort in the building envelope;

(b)

Most studies tend to focus on how to transform, improve, or create sustainable concrete structures and do not necessarily sufficiently address the question of how to apply them;

(c)

A few topics qualitatively examine data on the impacts of waste generation and reuse on energy-efficient sustainable concrete structures. There is an urgent need for studies that assess the reliability and quality of LCA databases and the realization of these aspects through comparisons or in-depth analyses of uncertainties;

(d)

Studies combining the reuse of waste with the economic growth and politics of the region are also rare. There is a need for studies that assess the realization of these aspects through comprehensive analyses of cases.

4. Conclusions

This article presents a review of the use of various alternative industrial waste materials as partial substitutes for the constituents of concrete, to produce energy-efficient concrete. Based on the bibliometric and systematic review, and aligned with the guiding questions of the research, the following conclusions can be drawn:

i

In terms of dimensions—There are four main focal points when it comes to the dimensions of research into energy-efficient recycled concrete: Technical, Institutional, Environment, and Infrastructure, under five study perspectives: “Alternative Material Efficient”, “Technical Performance”, “Environmental Policies, Waste Production, and Cost”, “Recycled Concrete Construction Practices with Energy Efficiency” and “Design of Experiments”;

ii

Frequent themes—The main objective of this guiding research question was to filter and further refine the various dimensions identified in the investigation of energy-efficient recycled concrete, based on a set of twenty of the most frequently searched and interrelated themes and classes (mechanical properties, performance, thermal properties, thermal performance, replacement, economy, analysis, recycling, sustainability, energy consumption, CO₂ emission, environmental impact, thermal comfort, LCA, temperature, construction, machine learning, applications, BIM, and IoT). One of the fastest-growing research topics is associated with mechanical properties, and sustainability, because concrete produced with alternative materials should be developed with minimal impact on its mechanical properties, and the result of the recycling process should be beneficial to the environment, providing technical and environmental compensation;

iii

As for future suggestions—A complete and holistic view involving all four dimensions of making energy-efficient recycled concrete is fundamental for a deeper investigation and understanding of recycled concrete in terms of energy efficiency. Some studies continue to focus on just one or two of the dimensions identified, while others do not present a clear roadmap for implementing manufacturing processes. In addition, there have been no comprehensive studies on the political and regulatory effects of reusing alternative materials in recycled concrete, and there is a study gap in conjunction with the economic growth of the region and the government policies implemented. There has been a lack of BIM computer modeling studies focused strictly on concrete structures, and few practical application studies are using this comparative technology in the world of recycled concrete.

The results of the study therefore highlight the main areas in which future research into the energy efficiency, performance and functionality of recycled concrete should focus. In recent literature, various methods to approaching recycled concrete from the perspective of energy efficiency have been examined, and notable advances have been made in the field of research. Analysis of the literature has revealed that the use of alternative waste materials from various industries in the manufacturing of concrete offers an advantageous technical, environmental, institutional, and infrastructural approach when compared to traditional concrete manufacturing methods. However, it is imperative to carry out a full and comprehensive study to validate or make any adjustments to the conclusions presented in this review. Once the shortcomings have been recognized, recommendations for a future research agenda could be as follows:

Firstly, methods of producing highly energy-efficient concrete for application in building envelopes should be developed. At the same time, efforts should be directed towards formulating strategies for incorporating waste from industries (chemical, mining, construction, and agricultural) into the industrial process itself, and the remainder into the concrete production process.

Secondly, the production of recycled concrete should be conceptualized as an emerging socio-technological issue that will only be fully understood, in all four dimensions, if a comprehensive vision is studied and well contextualized so that there is an assessment of the social and manufacturing impacts of incorporating waste into technological processes.

Recommendations for future studies should focus on the transformation, integration, and computer simulation of recycled concrete applied to building structures. Many current studies are restricted to physical and mechanical tests in laboratories. There is a need for research into the practical application of the material in the building envelope to investigate in a computational environment what is needed to conserve energy, temperature, and thermal comfort.

Important to solid evaluation, studies are needed on the impact of the infrastructure of cities on economic growth and politics, combined with the reuse of waste with the economic growth and politics of the region. However, there is a need for more comprehensive studies of global policies on the reuse of waste in concrete structures, which look at technical and production constraints, since specific choices about infrastructure have consequences for the distribution of natural (and recycled) raw materials and energy conservation and CO₂ emissions in the city.

The fifth recommendation concerns previous studies that have documented a lack of sufficient evidence regarding embodied energy, CO₂ emissions, and LCA data from input recycling processes, resulting in inconsistencies between the findings of various researchers. In forthcoming systematic investigations, it is vital to give due consideration to these characteristics.

And finally, it is recommended that more research is needed to carry out further studies into the reliability and degrees of uncertainty of data from LCA databases to produce recycled concrete, about both primary and secondary processes. It is important to evaluate various recycling, production, and landfill scenarios in these studies, since they deal with processes that differ from each other depending on the location of the study. In addition, current LCA studies (about recycled concrete) have failed to incorporate the technical and economic dimensions of using recycled inputs in concrete production in an integrated way with the LCA and BIM studies. The main concern relates to the establishment of an individual cost analysis without integration with the LCA being carried out.