**1. Introduction**

Calcium aluminate cements (CAC) are classified as hydraulic cements and are mostly used in applications involving extreme environments, which include refractories [1], acid resistant requirements, and fast setting cements. These materials are currently the subject of research on topics like property improvement, durability and more friendly manufacturing processes. CAC are manufactured industrially from mixtures of limestone and materials with a high content of Al2O<sup>3</sup> (bauxites, laterites and alumina obtained via the Bayer process, among other materials) [2]. CAC are considered eco-cements due to the reduced carbon emissions created during their production. They also have diverse applications due to their impressive early age strength and enhanced durability in harsh environments [3].

The main mineral phases present in CACs are calcium mono aluminate (CA), calcium dialuminate (CA2), dodecalcium heptaaluminate (C12A7) and alpha-alumina (α-Al2O3). All CAC properties depend only on their mineralogical phase composition [2,4].

The specific gravity of CACs varies between 3000 and 3250 kg/m<sup>3</sup> (3 and 3.3 g/cm<sup>3</sup> ), and is largely controlled by the iron content. The apparent density is typically between 1.12 and 1.74 g/cm<sup>3</sup> . The specific surface area measured by the British Standard method can vary between 2.500 and 4.000 cm2/gr. The properties of the constituent minerals of CACs as well as their typical chemical composition are shown in Table 1.

**Citation:** Zapata, J.F.; Azevedo, A.; Fontes, C.; Monteiro, S.N.; Colorado, H.A. Environmental Impact and Sustainability of Calcium Aluminate Cements. *Sustainability* **2022**, *14*, 2751. https://doi.org/10.3390/su14052751

Academic Editors: Mazen Alshaaer, Slávka Andrejkoviˇcová and Asterios Bakolas

Received: 14 January 2022 Accepted: 22 February 2022 Published: 26 February 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/).


**Table 1.** Properties of the constituent minerals of CAC.

The main hydraulic phase in all CACs is the CA phase. Of all the influencing factors, temperature is the most important since hydration products depend heavily on the curing temperature [5–11]. According to several studies, the initial hydration product that emerges within the temperature range of 0 to 15 ◦C is predominantly CAH<sup>10</sup> (Monocalcium aluminate decahydrate), while between 15 ◦C and 35 ◦C the predominant phases are CAH10, C2AH<sup>8</sup> (Dicalcium aluminate octahydrate) of hexagonal morphology, and amorphous AH<sup>3</sup> (Gibbsite). Above 35 ◦C, the major phase formed is C3AH6 (Katoite) [4,12–15]. CA hydration is primarily responsible for the early development of resistance, while CA2 hydration occurs after the main CAC hydration reaction has already been exceeded [16]. Refs. [17–19] shows some properties of the hydrated phases (Table 2).

Calcium aluminate cements (CAC) are mainly used in the refractory industry. They are also mixed with other cements such as Portland cement to create products in the ceramic industry, especially in construction [4,14,20–24]. Can recycled materials be used for the manufacture of calcium aluminate cements? How do calcium aluminate cement products fit into a circular economy? What relationship is established between the energy sector and the production of calcium aluminate cements? How are calcium aluminate cements being implemented within additive manufacturing? These are important questions in a world where factors that affect the environment and energy contribute to global warming.


**Table 2.** Densities, molar masses of the hydrated phases, and enthalpy of formation.

Figure 1 shows the SEM images of the main hydrated phases of CAC at 20 ◦C, (a) CAH10, and (b) C2AH8. The laminar structure is very clear, with all thicknesses below 0.5 µm.

Concrete based on Portland cement is the most widely used material in the world and its massive production is responsible for approximately 5 to 8% of global manmade CO<sup>2</sup> emissions [27]. CO<sup>2</sup> emissions are mainly due to energy consumption during the manufacturing process and decarbonization of limestone during clinker manufacturing. These emissions could be reduced by replacing clinker methods with inorganic minerals from industrial wastes such as slag, fly ash, and silica fume, or by using CAC despite its much smaller scale production compared to Portland cement [28]. The study and use of CAC is therefore very valuable to reduce global environmental problems.

**Figure 1.** SEM of hydrated phases of CAC at 20 °C, (**a**) CAH10, and (**b**) C2AH8. **Figure 1.** SEM of hydrated phases of CAC at 20 ◦C, (**a**) CAH10, and (**b**) C2AH<sup>8</sup> .

Concrete based on Portland cement is the most widely used material in the world and its massive production is responsible for approximately 5 to 8% of global manmade CO2 emissions [27]. CO2 emissions are mainly due to energy consumption during the manufacturing process and decarbonization of limestone during clinker manufacturing. These emissions could be reduced by replacing clinker methods with inorganic minerals from industrial wastes such as slag, fly ash, and silica fume, or by using CAC despite its much smaller scale production compared to Portland cement [28]. The study and use of CAC is therefore very valuable to reduce global environmental problems. The generation of CO<sup>2</sup> is an important consideration in the manufacture of cement. Thus, it is pertinent to know which mechanisms are involved in the process in order to try to reduce such emissions. 3D printing is a revolutionary new process that impacts many sectors, including medicine and construction. Additive manufacturing (AM) is a process whereby materials are joined to make objects from a 3D model, layer upon layer [29–32], which minimizes the materials wasted. Due to the importance of these latter two areas, they are included in this review to see how much they have been researched already. The value of the other areas included in this review needs no further justification.

The generation of CO2 is an important consideration in the manufacture of cement. Thus, it is pertinent to know which mechanisms are involved in the process in order to try to reduce such emissions. 3D printing is a revolutionary new process that impacts many sectors, including medicine and construction. Additive manufacturing (AM) is a process whereby materials are joined to make objects from a 3D model, layer upon layer [29,30,31,32], which minimizes the materials wasted. Due to the importance of these latter This research shows how investigations into CAC are helping to solve its most significant environmental issues. In order to understand and classify all this research, a generalsearch was first carried out using the meta search engine Carrot2 Workbench. Carrot2Workbench is an open-source search results clustering engine. After that, an exhaustive search was performed using the Scopus database, and the information found there was then analyzed.

#### two areas, they are included in this review to see how much they have been researched already. The value of the other areas included in this review needs no further justification. **2. Materials and Methods**

**3. Results** 

4 [33,34].

This research shows how investigations into CAC are helping to solve its most significant environmental issues. In order to understand and classify all this research, a general search was first carried out using the meta search engine Carrot2 Workbench. Carrot2 Workbench is an open-source search results clustering engine. After that, an exhaustive search was performed using the Scopus database, and the information found A general search was initially carried out using the Carrot2 Workbench metasearch engine and the keywords Calcium Aluminate Cement. The initial search employed the following sources: etools web search and the K-mean algorithm (base line algorithm, bagof-words labels). A second search was done using the etools web search and the Lingo algorithm (well-described flat clusters).

there was then analyzed. **2. Materials and Methods**  A general search was initially carried out using the Carrot2 Workbench metasearch engine and the keywords Calcium Aluminate Cement. The initial search employed the After that, a deeper and more specialized search was conducted in the Scopus database, with keywords provided by Carrot2 Workbench and complemented by others that were more specific to CACs and the environment, hydration, high temperature, refractory, Portland cement. The Scopus database search was performed with the keywords shown in Figure 2a. *Sustainability* **2022**, *14*, x FOR PEER REVIEW 4 of 19

**Figure 2.** Scopus database search for specific areas. (**a**) CACs and the environment, hydration, high temperature, refractory, Portland cement (**b**) CACs and recycling, productions costs, environmental science, additive manufacturing, CO2 generation, sustainability, life cycle assessment. **Figure 2.** Scopus database search for specific areas. (**a**) CACs and the environment, hydration, high temperature, refractory, Portland cement (**b**) CACs and recycling, productions costs, environmental science, additive manufacturing, CO<sup>2</sup> generation, sustainability, life cycle assessment.

Subsequently, the documents found for specific areas, recycling, productions costs,

The search was conducted using the Carrot2 Workbench with the keyword calcium aluminate cement**—**Source: etools web search**—**algorithm: Lingo (well-described flat clusters.). The search found 118 documents in general areas, and 33 documents in clusters (see Figure 3). The few articles found are dispersed over 33 clusters, where 40% (48 documents) focus on clusters such as Portland cement and CAC (see Figure 3). This high dispersion of very few articles means that a common thread in the studies cannot be

The search using Carrot2 Workbench with the keyword calcium aluminate cement-Source [33,34]: etools web search**—**algorithm: K-mean (base line algorithm, bag-of-words labels), showed 58 documents in related areas and 19 documents in clusters (see Figure 4). This search found more specific clusters but also showed that there are fewer articles and an increased dispersion. According to the metasearch engine, there are not enough studies to establish a common thread of work in a specific area. No cluster is presented as a developed area. On the contrary, each cluster is open to new research, as seen in Figure

Figure 2b. Finally, the studies found in the last stage were described.

established and practically every cluster is an area of research to explore.

Subsequently, the documents found for specific areas, recycling, productions costs, environmental science, additive manufacturing, CO<sup>2</sup> generation, sustainability and life cycle assessment, related to CACs and the environment were analyzed, as shown in Figure 2b. Finally, the studies found in the last stage were described. clusters.). The search found 118 documents in general areas, and 33 documents in clusters (see Figure 3). The few articles found are dispersed over 33 clusters, where 40% (48 documents) focus on clusters such as Portland cement and CAC (see Figure 3). This high dispersion of very few articles means that a common thread in the studies cannot be

**Figure 2.** Scopus database search for specific areas. (**a**) CACs and the environment, hydration, high temperature, refractory, Portland cement (**b**) CACs and recycling, productions costs, environmental

Subsequently, the documents found for specific areas, recycling, productions costs, environmental science, additive manufacturing, CO2 generation, sustainability and life cycle assessment, related to CACs and the environment were analyzed, as shown in

The search was conducted using the Carrot2 Workbench with the keyword calcium aluminate cement**—**Source: etools web search**—**algorithm: Lingo (well-described flat

science, additive manufacturing, CO2 generation, sustainability, life cycle assessment.

Figure 2b. Finally, the studies found in the last stage were described.

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 4 of 19

#### **3. Results** established and practically every cluster is an area of research to explore. The search using Carrot2 Workbench with the keyword calcium aluminate cement-

**3. Results** 

The search was conducted using the Carrot2 Workbench with the keyword calcium aluminate cement—Source: etools web search—algorithm: Lingo (well-described flat clusters.). The search found 118 documents in general areas, and 33 documents in clusters (see Figure 3). The few articles found are dispersed over 33 clusters, where 40% (48 documents) focus on clusters such as Portland cement and CAC (see Figure 3). This high dispersion of very few articles means that a common thread in the studies cannot be established and practically every cluster is an area of research to explore. Source [33,34]: etools web search**—**algorithm: K-mean (base line algorithm, bag-of-words labels), showed 58 documents in related areas and 19 documents in clusters (see Figure 4). This search found more specific clusters but also showed that there are fewer articles and an increased dispersion. According to the metasearch engine, there are not enough studies to establish a common thread of work in a specific area. No cluster is presented as a developed area. On the contrary, each cluster is open to new research, as seen in Figure 4 [33,34].

**Figure 3.** Carrot2 Workbench database search.

The search using Carrot2 Workbench with the keyword calcium aluminate cement-Source [33,34]: etools web search—algorithm: K-mean (base line algorithm, bag-of-words labels), showed 58 documents in related areas and 19 documents in clusters (see Figure 4). This search found more specific clusters but also showed that there are fewer articles and an increased dispersion. According to the metasearch engine, there are not enough studies to establish a common thread of work in a specific area. No cluster is presented as a developed area. On the contrary, each cluster is open to new research, as seen in Figure 4 [33,34].

A deeper and more specialized search was performed in the Scopus database, with the keywords given in Figure 2a. The search in Scopus found that the most commonly investigated factors associated with CAC are the hydration process, refractoriness, high temperature behavior, and the mixture of CAC + Portland cement. A total of 2832 documents were found. It can be established from this review that the factors associated with the most investigated CACs are the hydration process, high temperature behavior, refractoriness, and mixing with Portland cements. A quantification of the type and number of studies carried out in relation to these factors is presented in Figure 5 and in Table 3.

**Figure 3.** Carrot2 Workbench database search.

**Figure 4.** Carrot2 Workbench database search for specific areas. **Figure 4.** Carrot2 Workbench database search for specific areas.

A deeper and more specialized search was performed in the Scopus database, with

**Figure 5.** Documents organized by research factor in the Scopus database. **Figure 5.** Documents organized by research factor in the Scopus database.



The bibliographic search was performed using the Scopus database until November 2022, in which 112 documents were found for the environmental areas shown in Figure 6. The search in the specialized Scopus database shows very few studies. It can be divided into 7 clusters, which indicates a high dispersion, and a common thread or an area of It is clear that the hydration process is the most widely investigated topic in CAC, followed by topics related to refractories, behavior at high temperature, and the CAC + Portland Cement mixtures (see Figure 5).

research cannot be defined. Therefore, each cluster shown in Figure 3 and Table 4

Table 4 shows the type of document organized according to environmental clusters, like the years the articles were published. As shown in Table 4, from 112 documents, only 70 articles were found in all areas. In Circular Economy, four articles were registered, and in Life Cycle Assessment, only one was found. In Environmental Science and Additive Manufacturing, three articles per area were found. In Sustainability, 10 articles were found, and in CO2 Generation, four articles were found. The most widely investigated

**Figure 6.** Scopus database search for environmental area.

areas are Production Costs and Recycling.

The bibliographic search was performed using the Scopus database until November 2022, in which 112 documents were found for the environmental areas shown in Figure 6. The search in the specialized Scopus database shows very few studies. It can be divided into 7 clusters, which indicates a high dispersion, and a common thread or an area of research cannot be defined. Therefore, each cluster shown in Figure 3 and Table 4 represent an area of investigation to explore. The bibliographic search was performed using the Scopus database until November 2022, in which 112 documents were found for the environmental areas shown in Figure 6. The search in the specialized Scopus database shows very few studies. It can be divided into 7 clusters, which indicates a high dispersion, and a common thread or an area of research cannot be defined. Therefore, each cluster shown in Figure 3 and Table 4 represent an area of investigation to explore.

CAC 579 137 16 10 18 1975**–**<sup>2022</sup>

**Paper Review Conference** 

Hydration 939 189 27 10 2 1957**–**2022 High Temperature 412 78 11 8 5 1946**–**2022 Refractory 298 69 16 5 3 1946**–**2022

**Review**

**Book Chapter** **Production Interval (In Years)** 

**Figure 5.** Documents organized by research factor in the Scopus database.

**Table 3.** Types of documents organized by CAC study factor.

**Article Conference** 

Portland Cement +

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 6 of 19

**Figure 6.** Scopus database search for environmental area. **Figure 6.** Scopus database search for environmental area.



Table 4 shows the type of document organized according to environmental clusters, like the years the articles were published. As shown in Table 4, from 112 documents, only 70 articles were found in all areas. In Circular Economy, four articles were registered, and in Life Cycle Assessment, only one was found. In Environmental Science and Additive Manufacturing, three articles per area were found. In Sustainability, 10 articles were found, and in CO<sup>2</sup> Generation, four articles were found. The most widely investigated areas are Production Costs and Recycling.
