Environmental Impact and Sustainability of Calcium Aluminate Cements

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		John Fernando Zapata	+ 4682 word(s)	4682	2022-02-28 04:11:42	\|
2	format	Bruce Ren	-43 word(s)	4639	2022-03-22 04:32:01	\|

This entry is adapted from the peer-reviewed paper 10.3390/su14052751

This investigation presents a critical analysis of calcium aluminate cements (CAC), specifically associated with sustainability and environmental impact, and the potential of these cements to help solve certain worldwide problems. Areas of research include cements as recycling holding materials, sustainability, circular economy, production costs, and energy. Here summarizes the current trends, perspectives, and the main concerns regarding CAC. Detailed information about the materials and processes involved in CAC is also presented. First, a general search was made using the Carrot2 Workbench metasearch engine to identify possible thematic groups correlated with CAC, then a more in-depth and specialized search was done using the Scopus database.

cements materials sustainability construction civil engineering phases

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 Al2O3 (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/m³ (3 and 3.3 g/cm³), and is largely controlled by the iron content. The apparent density is typically between 1.12 and 1.74 g/cm³. 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.

Table 1. Properties of the constituent minerals of CAC.

Mineral	Chemical Composition (wt%)				Tm (°C)	Density (g/cm³)	Crystal Structure	Formation Enthalpy (kJ/mol)
Mineral	CaO	Al2O3	Fe2O3	SiO2	Tm (°C)	Density (g/cm³)	Crystal Structure	Formation Enthalpy (kJ/mol)
C	99.8	-	-	-	2570	3.32	Cubic	-
C12A7	48.6	51.4	-	-	1405–1495	2.69	Cubic	-
CA	35.4	64.6	-	-	1600	2.98	Monoclinic	−2323
CA2	21.7	78.3	-	-	1750–1765	2.91	Monoclinic	−4023
C2S	65.1	-	-	34.9	2066	3.27	Monoclinic	-
C4AF	46.2	20.9	32.9	-	1415	3.77	Orthorhombic	-
C2AS	40.9	37.2	-	21.9	1590	3.04	Tetragonal	-
CA6	8.4	91.6	-	-	1830	3.38	Hexagonal	-
Al2O3	-	99.8	-	-	2051	3.98	Rhombohedral

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]^[6]^[7]^[8]^[9]^[10]^[11]. According to several studies, the initial hydration product that emerges within the temperature range of 0 to 15 °C is predominantly CAH10 (Monocalcium aluminate decahydrate), while between 15 °C and 35 °C the predominant phases are CAH₁₀, C2AH8 (Dicalcium aluminate octahydrate) of hexagonal morphology, and amorphous AH₃ (Gibbsite). Above 35 °C, the major phase formed is C3AH6 (Katoite) ^[4]^[12]^[13]^[14]^[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]^[18]^[19] shows some properties of the hydrated phases (Table 2).

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

Hydrated Phase	Density (g/cm³)	Molar Mass (g/mol)	Formation Enthalpy (kJ/mol)
CAH10	1.72	338.1	-
C2AH8	1.96	358.2	−5433 ^[20]^[21]
AH3	2.44	156.0	−2578 ^[20]
C3AH6	2.52	378.3	-

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]^[22]^[23]^[24]^[25]^[26]. 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.

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.

Figure 1. SEM of hydrated phases of CAC at 20 °C, (a) CAH10, and (b) C2AH8.

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 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.

2. Analysis by Specific Area

2.1. Analysis by Environmental Area

After refining the search and restricting only to CAC + environment, the Scopus database shows 112 articles distributed by cluster as shown in Table 3.

Table 3. Types of documents organized by environmental cluster.

	Article	Conference Paper	Review	Conference Review	Book Chapter	Production Interval (In Years)
Recycling	37	7	1	4	3	1995–2022
Production costs	13	3	1	0	0	1995–2021
Environmental Science	3	2	0	0	0	2001–2020
Additive manufacturing	3	1	1	1	2	2010–2021
CO₂ generation	4	1	0	0	0	2011–2021
Sustainability	10	4	1	1	1	2007–2021
Circular economy	4	0	0	0	0	2017–2021
Life cycle assessment	1	2	0	1	0	2014–2021

2.1.1. CAC + Recycling

A search with the keywords, calcium aluminate cement and recycling (CAC + recycling) in the Scopus database yielded 52 documents only 37 articles were found. The number of documents produced per year is presented in Figure 2a. It can be seen that the highest production was in the last 3 years, with 28 works presented.

Figure 2. Scopus database search for calcium aluminate cement and recycling (CAC + recycling). (a) CAC + Recycling (b) CAC + Production Costs (c) CAC + Environmental Science (d) CAC + Additive Manufacturing (e) CAC + CO2 Generation (f) CAC + Sustainability.

The use of CAC can effectively mitigate the expansion of the alkali-silica reaction (ASR) of alkali activated cement mortars (AAC) incorporating glass cullet (GC) as fine aggregates, but the mechanism is unclear ^[33]. P. He, et al. ^[33]. present a systematic study on the exploration of the underlying mechanism and to determine the safe use of GC in AAC to maximize the possibility of recycling glass waste in construction materials.

They found that the ASR expansion of the alkali activated glass mortars was more than 1000 με after 1 day of alkaline immersion, when using GC and the incorporation of CAC decreases the expansion to less than 1000 με, even after 14 days of immersion alkaline, decreased by 20%. This result occurs because the aluminium in CAC was incorporated into the AAC matrix with tetrahedral and octahedral units. Increasing the CAC content increases the tetrahedral unit that required more Na ions to balance the excess charge, which caused the alkalinity of the pore solution to decrease as did the expansion of ASR.

B. Zhang, et al. ^[34] feature a way to incorporate recycled glass materials into AAC-based pastes/mortars. The GC waste was used to replace natural aggregates and the glass waste powder (GP) was used to partially replace conventional precursors such as blast furnace slag (GGBS). Using the developed technology, recycled AAC-based mortars can be produced with a 28-day compressive strength of approximately 40 MPa. The AAC mortar mixes showed good resistance to high temperatures, retaining more than 50% of the original resistance after 2 h of exposure at 800 °C. The experimental results also showed that the damaging expansion caused by the alkaline silica (ASR) reaction between alkali and residual GC in AAC mortars could be successfully controlled by using additional sources of aluminum such as CAC.

Chang, et al. ^[34] also investigated the compressive strength, drying shrinkage and ASR expansion of AAC mortars using GC as aggregates and GP to partially replace GGBS as the precursor. They noted that mortars using GC as an aggregate, replacing GGBS with GP decreased compressive strength, and mortars using sand as an aggregate, showed severe drying shrinkage, while replacement of sand by GC could significantly decrease drying shrinkage.

The alkali activated GGBS mortars using GC as an aggregate showed great expansion after alkaline immersion. When GP is used to partially replace GGBS, the expansion was significantly reduced. The replacement of 15% of GGBS by CAC could further decrease the expansion. The optimal proportion of the AAC mix developed was that the mortars incorporated 15% CAC, 10% GGBS, and 75% GP, could meet stipulated mechanical requirements and durability requirements for bulkhead applications.

W. Panpa, et al. ^[35] produced a visible light active compound, Ag2O-Ag/CAC/SiO2, by loading an aqueous solution of silver salt onto the hydrated CAC coating layer in a porous SiO2 sphere. The slight hydration condition of the cement induced the formation of Ag2O precipitates in situ on the cement surface which partially decomposed into metallic Ag after drying and appropriate heat treatment at 200 °C to improve both Ag2O decomposition and crystallinity.

The photo catalytic activity of the compound Ag2O-Ag/CAC/SiO2 (calcined at 200 °C), evaluated under irradiation with UV light and visible light through the photo decomposition of CHP in water, quantified by HPLC, was two times greater than that Ag2O-Ag/CAC/SiO2 compound (dried at 45 °C), and capable of completely decomposing CHP in 5 h. Furthermore, the photo stability of the Ag2O-Ag/CAC/SiO2 photocatalyst remained unchanged during five recycling tests.

S.K.S. Hossain and P.K. Roy ^[36] formulated a nano-lakargiite system (NL) [CaZrO₃] for formless refractories. The NL powder formulation is made from a solution mixture, which is easily capable of recycling by-products. Scrap shells were used as a source of CaO for the preparation of NL. They reached at 1100 °C, the single-phase orthorhombic crystal structure with the Pnma space group of nano CaZrO3 (average crystallite size ~19 nm). The formulation involved replaced CAC with NL was heat treated at 1600 °C, resulting in properties that matched by the different advanced bonding systems of high alumina refractory molds, therefore, achieving good densification, heat resistance, and thermal shock resistance for NL bonded refractory molds.

H. Al-musawi, et al. ^[37] studied the time-dependent transport properties and shrinkage performance of smooth and fiber-reinforced rapid-setting mortars for repair applications, in two single mixes of Steel-Fiber-Reinforced Concrete (SFRC). They found that CSA cement mixes have much lower shrinkage values (about 220 and 365 microspheres) compared to CAC mixes (about 2690 and 2530 microspheres), but most of the shrinkage in these formulations were autogenous. However, the fibers reduced the drying shrinkage of the CAC mixes by approximately 12%.

H. Al-musawi, et al. ^[38] investigated the flexing performance of fast-setting mortar mixes made with two types of commercial cement, calcium sulfonate cement and CAC, for thin concrete repair applications. They performed three-point bending tests on samples of smooth steel-fiber reinforced (FRC) concrete containing 45 kg/m³ of recycled clean steel fibers to characterize the bending performance of notched and notched prisms at different ages, ranging from one hour to one year. They found that recycled fibers improve both the flexural strength and toughness of FRC prisms.

One of the best methods of recycling waste in the construction sector is to use it in the preparation of concrete or mortar. S.T. Yildirim, et al. ^[39] used recycled concrete aggregates and bottom ash as aggregates in the mortar. They used CAC as a binder to improve fire resistance. They investigated flexural and compressive strength, dry unit weight, water absorption, capillary action, thermal conductivity, thermal resistance, and costs, following a Taguchi method for internal cure (IC), the amount of cement, and the aggregate ratio as parameters. They found that IC does not have enough positive effect on mortars. Choosing the appropriate type of cement is believed to be effective. The process of obtaining resistance for CAC is quite fast and water had a negative rather than a positive effect. It is believed that IC water should have increased the amount of water mix. The increase in the amount of CAC has a positive effect because the dosage of cement in the mortars is kept low and the durability with respect to temperature is maintained.

P. Kulu, et al. ^[40] recycled the niobium slag, separating the metallic Nb and the mineral ballast—calcium aluminate. Preliminary tests of cement to use the main product as a binder or substitute for cement confirmed its potential for use in the production of construction materials. P. Ogrodnik, et al. ^[41] investigated sanitary ceramic waste as an addition to concrete. Six concrete mixes on Portland cement and CAC were designed with various aeration mix contents. They found that the compressive strength is adequate for concrete containing ceramic aggregate and CAC at high temperatures.

M. Nematzadeh, et al. ^[42] investigated the compressive behaviour of concrete containing fine recycled refractory brick aggregate, CAC fibers, and polyvinyl alcohol (PVA) in an acidic environment, motivated because earthquake-induced structural debris and other factors cause destruction and problems related to the damaging effects of acidic environments. They found that samples containing CAC along with PVA fibers have an adequate corrosion response against acid attacks, while samples containing fine refractory brick aggregate showed quite unsatisfactory performance in this regard.

M. Nematzadeh and A. Baradaran-Nasiri ^[43] investigated the compression stress-strain behaviour of recycled aggregate concrete. Different levels of replacement of conventional fine aggregate by recycled particles from used refractory bricks were used (0, 25, 50, 75 and 100% volume replacement level) in two groups, one containing PC and the other containing CAC. After exposure to elevated temperatures (110, 200, 400, 600, 800 and 1000 °C), significant degradation occurred for most of the mechanical properties for concrete containing ordinary cement at 400 °C and for concrete containing aluminate cement at 110 °C. Higher fine brick refractory aggregate contents improved concrete compression behaviour at higher temperatures.

A. Baradaran-Nasiri and M. Nematzadeh ^[44] investigated the use of recycled aggregate produced by crushing firebricks. Samples were separated based on CAC and OPC with replacement ratios of 0, 25, 50, 75 and 100% of fine aggregate of refractory brick instead of natural sand. After exposing the samples to temperatures of 110, 200, 400, 600, 800, and 1000 °C, it was found that the addition of refractory brick and the CAC improve the residual strength of the concrete up to twice, at the temperature of 800 °C.

R. Stonys, et al. ^[45] studied the waste from mineral wool (dome dust—CD) and its possible reuse in the production of CAC-based refractory concrete, replacing the silica addition with CD. It was found that CD can be used for the production of refractory concrete. Í. Navarro-Blasco, et al. ^[46] studied waste foundry sand (WFS) in CAC mortars with a replacement level of 50%. Compared to OPC mortars, the use of CAC showed several advantages, improving compressive strength and retention of toxic metals.

L.J. Fernández, et al. ^[47] studied the recycling of crystalline solar cells incorporated into cement matrices. The hydration process of a mixture of CAC and photovoltaic solar cell waste was analyzed, founding that the presence of up to 5% solar cell residue in cement matrices does not result in new hydration products that are different from those derived from normal CAC hydration. Furthermore, the developed material can be considered an expansive cement mix because it releases H2 in the early stages. The presence of residues causes a decrease in mechanical resistance and an increase in the total porosity of this material, although, it could be used for applications such as thermal insulation.

Chen et al. ^[48], showed that CAC had an excellent immobilization efficiency of potentially toxic elements in fly ash from municipal solid waste incineration.

2.1.2. CAC + Production Costs

A search with the keywords, calcium aluminate cement, and production costs (CAC + production costs) in the Scopus database yielded 17 results. The number of documents produced per year is presented in Figure 2b. The low production in this topic can be observed in the last 25 years.

M. Erans, et al. ^[49] oriented their research to the production of improved sorbents, motivated by the fact that the calcium loop (CaL) is a CO2 capture technology and a fundamental problem for the commercialization of these technologies is to maintain a high level of sorbent reactivity during the long-term cycle, to mitigate the decrease in carrying capacity. A great strength of CaL compared to other carbon capture technologies is the synergy with the cement industry, due to the use of spent sorbent as Clinker raw material.

M. Erans, et al. ^[49] investigated limestone doped with HBr through a particle surface impregnation technique and granules prepared from limestone and CAC. These were tested in a 25 kWth dual fluidized bed pilot scale reactor to understand their capture performance and mechanical stability under realistic CaL conditions and the spent sorbent was subsequently used as a raw material to make cement. It was found that HBr-doped limestone showed better performance in terms of mechanical strength and stability of CO2 absorption compared to that of granules prepared from limestone and CAC, and that the cement produced has characteristics and performance similar to that of commercial CEM I cement. This demonstrates the advantages of using spent sorbent as a raw material for cement manufacturing and shows the benefits of synthetic sorbents in CaL and the end-use suitability of spent sorbents for the cement industry. It can be concluded from the work of M. Erans, et al. ^[49], the feasibility of using various practical techniques to improve the performance of CaL on a pilot scale, and more importantly, it shows that commercial grade cement can be manufactured at from the lime product of this technology.

Abolhasani et al. ^[3] discusses the results of a comprehensive study incorporating rice husk ash (RHA) in CAC concrete to limit the phase transition of CAC hydration product and stabilize its long-term properties. The findings indicate that, at 90 days, the mechanical strengths of the mixes containing RHA were higher than those of the control mix, with the maximum improvement occurring at the substitution percentage of 5%.

2.1.3. CAC + Environmental Science

A search with the keywords, calcium aluminate cement and Environmental science (CAC + Environmental science) in the Scopus database yielded 5 results. The number of documents produced per year is presented in Figure 2c. This reveals almost no production in the last 20 years.

L. Xu, et al. ^[50] investigated setting times, mechanical strength and drying shrinkage ratio of mixed OPC and CAC systems (less than 25%). They found that with CAC, the set times of combined systems are shortened; the compressive strength first increases slightly (peaks with 6% calcium aluminate cement) and then decreases significantly. T.T. Akiti Jr, et al. ^[51]^[52] developed a calcium-based sorbent to desulfurize the hot carbon gas, this led to the development of a spherical granule-like material, which has a limestone-based core enclosed in a strong support layer. Strong granules are prepared by incorporating some CAC into the core and a larger amount into the shell, along with the limestone.

The granules are prepared by a two-step granulation method, followed by steam curing and heat treatment. It was found that the granules are capable of absorbing relatively large amounts of hydrogen sulfidic at high temperature (e.g., 1150–1200 °K) that can be regenerated by a cyclical oxidation and reduction process.

Secondary aluminum dross (SAD) is a dangerous pollutant as well as a valuable resource. About 95% SAD is disposed by stockpiling on the spot due to its complex composition and technical limitations, causing severe ecological damage and public health threat ^[53]. Mingzhuang et al. ^[53] developed a new green process for the preparation of ultrafine and high-whiteness Al(OH)3 from SAD. This phase can then be used as a high-strength cementitious material by mixing it with CAC.

2.1.4. CAC + Additive Manufacturing

A search with the keywords, calcium aluminate cement and Additive manufacturing (CAC + Additive manufacturing) in the Scopus database returned 10 results. The number of documents produced per year is presented in Figure 2d. This reveals almost zero production in the last 10 years.

P. Shakor, et al. ^[54] They discuss a methodology for replacing the typical powders currently used in 3D printing to make it possible to use printed samples in construction applications. They found that the highest compressive strength (14.68 MPa) is obtained for samples that were first cured in water and then oven dried for one hour at 40 °C, compared to samples that were cured without drying at 40 °C (4.81 MPa). Therefore, the post-processing technique has a significant and effective impact on the resistance of printed samples. P. Shakor, et al. ^[55] developed a mixture of CAC through a 150 µm sieve and OPC for the Z-Corporation 3D Printing Process (3DP). This cement mix was mixed, and the resulting compound powders were printed with a water-based binder using a Z-Corporation 3D printer. In addition, lithium carbonate was added to some samples to reduce the setting time of the cement mix. The maximum compressive strength of the cubic samples for cementitious 3DP was 8.26 MPa at the 170% saturation level for both the shell and the core. The minimum porosity obtained was 49.28% at the saturation level of 170% and 340% for the cover and the core, respectively.

V. Antonovich, et al. ^[56] carry out a review on the use of nano technologies in the manufacture of refractory concretes and some other cementitious materials, examining the influence of nanostructure formation on the bonding material on the properties of refractory concretes. In one case, investigations were carried out using two-component bonding materials (sodium silicate solution mixed with dicalcium silicate) and three-components (sodium silicate solution mixed with dicalcium silicate plus calcium aluminate cement). Cement refractory concretes with mullite aggregates, micro silica, and hybrid and simple deflocculant additives (Castament FS20 polycarboxylate ether and sodium tripolyphosphate) was studied, finding that the three-component bond material hardens as opposed to the two-component material, since one of the bonding components (combination of sodium silicate and dicalcium silicate solution) hardens very quickly and affects the hydration process of the other component, CAC. This has a powerful impact on the entire structure of the material. The application of nanotechnology in the manufacture of refractory concretes has increased the compressive strength three times, from 55 MPa to 165 MPa. CACs can be effectively used as an accelerator for PC hydration for the purpose of layered extrusion. Das, et al. ^[57] established that CAC mixed with a source of calcium sulfate could be a better alternative than using CAC alone.

2.1.5. CAC + CO2 Generation

The search with the keywords, calcium aluminate cement and CO2 generation (CAC + CO2 generation) in the Scopus database returned five results. The number of documents produced per year is presented in Figure 2e. This reveals almost no production in the last 9 years.

S. Bharati, et al. ^[58] developed, with a sustainable approach, a low cost mouldable for low temperature applications using steelmaking slag as raw materials. The ladle slag generated after the secondary steelmaking process was used in the 50–70% range as a complete source of CaO to develop a CAC. The slag was used as a 100% replacement for the limestone and partial replacement of the Al2O3 source in the cement. In this way, it was possible to eliminate the CO2 emission associated with the dissociation of the limestone during the clinker manufacturing process. The castable was prepared using primary steel slag as an aggregate and slag cement was developed as a binder. The CA and CA2 phases were detected as primary phases in the slag cement and gehlenite and mayenite as secondary phases.

The slag was used as a 100% replacement for the limestone and a partial replacement for the Al2O3 source in the cement. Therefore, it was possible to eliminate the CO2 emission associated with the dissociation of limestone during the clinker manufacturing process. The castable was prepared using primary steel slag as an aggregate, and slag cement was developed as a binder. CA and CA2 were formed as the primary phases in the slag cement, with the Gehlenite phases. Slag cement with a slag content of 60% exhibited superior strength compared to commercial medium purity CAC. In addition, the crush strengths of 110 °C and 1000 °C were found to be better for casting slag cement than commercial cement. Therefore, it was possible to develop a green mold using by-products from the steel manufacturing process that would not only help reduce the carbon footprint, but also reduce manufacturing costs.

2.1.6. CAC + Sustainability

A search with the keywords, calcium aluminate cement and Sustainability (CAC + Sustainability) in the Scopus database returned 17 results. The number of documents produced per year is presented in Figure 2f. This reveals almost zero production in the last 8 years.

M. Giroudon, et al. ^[59] analyze the deterioration mechanisms of different binders focusing on the impact of the binding nature on the medium (biochemical composition) during digestion. Binders with a favorable composition for chemically aggressive media were tested: slag cement (CEM III/B), CAC and alkaline metakaolin-based material (MKAA), and a reference binder: OPC (CEM I). Under the explored conditions, biodeterioration mainly led to carbonation of the cement pastes.

Alternative cementitious materials (MCAs) are receiving increasing attention worldwide, but there is a lack of knowledge about the resistance of these materials against the intrusion of harmful ions. Furthermore, current accelerated test methods that measure ion diffusion under an electric field are not reliable when comparing binders with very different pore solution chemistry. M.K. Moradllo and M.T. Law ^[60] solve these problems by using laboratory transmission X-ray microscopy (TXM) and micro-X-ray fluorescence imaging (μXRF) to perform real-time measurements of ion diffusion in paste samples and mortar for five commercially available ACMs and a OPC. They compared the apparent rate of ion diffusion, quantified the change in the rate of ion diffusion over time, and gave an idea of ionic binding. It was found after 42 days of ion exposure that samples made with calcium aluminate cement had the lowest rate of ion penetration, while samples with activated alkali cement and calcium sulfoaluminate had the highest rate of ion penetration. Portland cement had an ion penetration level that was between these two. Furthermore, both alkali activated and calcium sulfoaluminate samples showed a decrease in penetration rate over time. These measurements are important to quantify the sustainability of MCAs, better justifying their uses where durability is a concern, and guiding to future durability tests for these promising materials.

It is well-known that cement production represents 1.65 billion metric tons of annual global CO2 emissions ^[61], making it one of the largest contributors to global CO2 emissions. One way to reduce CO₂ emissions associated with concrete construction is using alternative cementitious materials and binders (ACMs), such as calcium sulfoaluminate, calcium aluminate, and alkali activated binders. These materials often require lower production temperatures than OPCs and have lower calcium contents, reducing the emissions associated with the CO2 released by calcium carbonate during calcination. Most ACMs are not new materials, but the past uses have been primarily limited to small-scale applications, such as pavement repairs, and there is little field experience regarding their long-term durability in heavily travelled structures, such as pavements and bridge decks. L.E. Burris, et al. ^[62] present promising results after the first year, of an effort by the U.S. Department of Transportation.

2.1.7. CAC + Life Cycle Assessment

The search with the keywords, calcium aluminate cement and Life cycle assessment (CAC + Life cycle assessment) in the Scopus database returned four results.

Henry-Lanier, et al. ^[63] evaluated the environmental footprint for the CAC, calculated according to the guidelines defined in the ISO 14,040 and 14,044 standards for Life Cycle Analysis (LCA). After focusing on the methodology and data used, summarizes the intrinsic carbon footprint and energy consumption of CAC production from the cradle to the factory gate. Other data drawn from the LCA literature and databases are provided in this document to give a comparative view of the data for CAC. This document presents results to compare different refractory systems (monolithic and shaped) used for a selected application, showing the intrinsic opportunity to reduce the carbon footprint of specific refractory applications by using monolithic refractories containing calcium aluminate. The conclusions suggest that the use of LCA can be a powerful tool to optimize the types of refractory products and reduce the overall environmental impact of refractories throughout the life cycle of use.

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