5.1.1. 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 8a. It can be seen that the highest production was in the last 3 years, with 28 works presented.

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 [47]. P. He, et al. [47]. 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. [48] feature a way to incorporate recycled glass materials into AACbased 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.

guiding to future durability tests for these promising materials.

year, of an effort by the U.S. Department of Transportation.

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 [74] 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

It is well-known that cement production represents 1.65 billion metric tons of annual global CO2 emissions [75], 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. [76] present promising results after the first

**Figure 8.** 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 + CO<sup>2</sup> Generation (**f**) CAC + Sustainability.

Chang, et al. [48] 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 in this study 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. [49] 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 SiO<sup>2</sup> 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/SiO<sup>2</sup> (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/SiO<sup>2</sup> compound (dried at 45 ◦C), and capable of completely decomposing CHP in 5 h. Furthermore, the photo stability of the Ag2O-Ag/CAC/SiO<sup>2</sup> photocatalyst remained unchanged during five recycling tests.

S.K.S. Hossain and P.K. Roy [50] 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 CaZrO<sup>3</sup> (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. [51] 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. [52] 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/m<sup>3</sup> 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. [53] 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. [54] 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. [55] 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. [56] 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 [57] investigated the compression stressstrain 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 [58] 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. [59] 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. [60] 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. [61] 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. [62], showed that CAC had an excellent immobilization efficiency of potentially toxic elements in fly ash from municipal solid waste incineration.
