A critical review on CO<sub>2</sub> sequestration using construction and demolition waste: Future scope and perspective

Shaniv Kumar Tiwari; Ki-Hyun Kim; Ram Sharan Singh; Jechan Lee; Taejin Kim; Jurgen Mahlknecht; Balendu Shekher Giri; Manish Kumar

doi:10.4491/eer.2023.256

Environ Eng Res > Volume 29(3); 2024 > Article

Tiwari, Kim, Singh, Lee, Kim, Mahlknecht, Giri, and Kumar: A critical review on CO2 sequestration using construction and demolition waste: Future scope and perspective

Review

Environmental Engineering Research 2024; 29(3): 230256.

Published online: August 11, 2023

DOI: https://doi.org/10.4491/eer.2023.256

A critical review on CO₂ sequestration using construction and demolition waste: Future scope and perspective

Shaniv Kumar Tiwari^1,², Ki-Hyun Kim^3,^†

, Ram Sharan Singh⁴, Jechan Lee⁵, Taejin Kim⁶, Jurgen Mahlknecht⁷, Balendu Shekher Giri^8,^†

, Manish Kumar^7,⁸

¹Indian Institute of Technology Guwahati, Assam 781039, India

²JSW Steel Ltd. Vijayanagar Works, Karnataka, 583275, India

³Department of Civil and Environmental Engineering, Hanyang University, Seoul 04763, Republic of Korea

⁴Chemical Engineering, Indian Institute of Technology (BHU), Varansi 221005, India

⁵Department of Global Smart City & School of Civil, Architectural Engineering, and Landscape Architecture, Sungkyunkwan University, Suwon, 16419, Republic of Korea

⁶Materials Science and Chemical Engineering Department, Stony Brook University, Stony Brook, NY, 11794, U.S.A

⁷Escuela de Ingeniería y Ciencias, Tecnologico ‘de Monterrey, Campus Monterey, Monterey, 64849, Nuevo Leon, Mexico

⁸Sustainability Cluster, University of Petroleum and Energy Studies (UPES), Dehradun, 248007, Uttarakhand, India

^†Corresponding author: E-mail: balendushekher23@gmail.com (B.S.G.), kkim61@nate.com (K.-H.K.), Tel: +91-9450129602 (B.S.G.), +82-2-2220-2325 (K.-H.K.), Fax:

Received April 29, 2023 Revised August 10, 2023 Accepted August 10, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In recent years, the building industry has looked for technological ways to protect the environment and preserve natural resources. Since the COVID-19 epidemic, there has been a shortage of building materials, which has caused construction costs to go up. This has made it more important for sustainable development to be based on the principles of the circular economy. This gives an opportunity to utilise various reliable materials as substitutes, like construction and demolition (C&D) waste. (C&D) wastes are made up of a large chunk of all solid waste, which causes many environmental problems. The most important factor in the struggle against climate change is the reduction of CO₂ emissions from the construction sector. At the same time, globally, climate change caused in part by carbon dioxide (CO₂) emissions is an important problem that requires innovative carbon sequestration strategies. Because C&D waste is alkaline-rich (e.g., calcium hydroxide and calcium-silicate-hydrate (C-S-H)), it can be used to sequester CO₂ by converting it into thermodynamically stable carbonates. Temperature, partial pressure of CO₂, time, process route, humidity, and the water-to-solid ratio (w/s) can affect the CO₂ sequestration over the C&D wastes.

Keywords: Alkaline waste, Carbon capture and storage (CCS), CO₂ sequestration, Mineral carbonation, Waste-to-resources, Waste valorization

Graphical Abstract

Keywords: Alkaline waste, Carbon capture and storage (CCS), CO₂ sequestration, Mineral carbonation, Waste-to-resources, Waste valorization

1. Introduction

The most important factor in the struggle against climate change, which seeks to reduce the average atmosphere temperature by 1.5°C by the end of this century, is the reduction of CO₂ emissions from the construction industry. The cement industry is responsible for roughly 7% of global CO₂ emissions [1]. As of 2019, the construction industry emitted approximately 9.95 Gt/y of CO₂, making it the largest contributor. It is anticipated that the construction industry will reduce its CO₂ emissions by 16% by 2030, leading to net-zero emissions by 2050 [2]. Thus, several measures have been implemented to mitigate the construction industry’s significant CO₂ emissions by capturing and utilizing CO₂. In recent decades, global business and scholars have been more conscious of the issues posed by building and demolition debris. Despite growing efforts by the construction industry to limit the environmental impact of its operations, building sites continue to be a major source of pollution with a detrimental influence on the environment. Physical infrastructure is being constructed, remodelled, and demolished rapidly, with the migration of a fast growing population in urban areas which causes increasing CDW globaly [3]. Central Pollution Control Board (CPCB) estimates the total solid waste generated in India to be 48 million tons per year, of which C&D waste constitutes about 12 million tons per year [4] claimed that the official numbers are grossly underestimated in India. A top-down material flow analysis framework was used to estimate the annual C&D waste generation which is between 112 to 431 million tons in 2016 and it is much higher than the CPCB data. Based on the prevalent practice of landfilling in India, 350 km² of land would have to be turned into landfills to accommodate C&D waste from urban areas alone by 2050 [4]. C&D waste and concrete production are some of the major contributions of emitting carbondioxide constantly into the atmosphere. Till 2012, the global C&D waste gave rise to 3.0 billion tonnes annually including 40 countries worldwide and this trend is increasing rapidly [5].

Even the developed nation like The United States also generates more than 500 million tonnes of C&D waste per year [6]. Canada generated more than 33 million tonnes of C&D waste of which approximately 75.56% is being sent to landfills in 2010 and 74.75% in 2012 [7]. The largest amount of C&D waste is generated by China which exceeded 1.0 and 1.13 billion tonnes in 2012 and 2014, respectively [6]. Countries such as China and India have a huge responsibility to develop a robustsystem to monitor problems and find eco-friendly solutions to their huge C&D waste. Mass awareness is equally important for which government-led initiatives are mandatory. As C&D waste is growing rapidly in India and globally and the burden on rocks and other natural resources as a construction material also increasing. Hence, the Bureau of Indian Standards has called for new construction material with good quality to be made by using waste material [8].

C&D waste includes alkaline metallic oxides such as (CaO, MgO) that are capable of reacting with CO₂ and capturing it permanently as solid carbonate know as mineral carbonation [9]. Mineral carbonation in recycled concrete aggregate and recycled hardened cement powder occurs when CO₂ chemically reacts with calcium hydroxide, calcium silica hydrate, etc., forming thermodynamically stable carbonate minerals to absorb CO₂ and improve fine value, high porosity, and water absorption [10]. Reducing CO₂ emissions from the building sector is the most important aspect in the battle against climate change, which seeks to decrease the average temperature of the atmosphere by 1.5°C by the end of the century [10]. CO₂ sequestration can be a suitable alternative in effective utilization of this humungus waste. CO₂ sequestration (CS) by alkaline C&D waste can be a viable solution and can be used at the industrial level to reduce their carbon footprint. Recycling of C&D waste containing reusable concrete waste can save the landfill space by up to 5% [11]. Major applications of CS are practical at large point sources of CO₂ like thermal power plants, oil refineries and other industrial plants in the manufacturing industry. Landfilling of C&D wastes may be avoided if the residual portion is utilised for CS, with the final result being a new sustainable building material [12]. C&D waste can be modified into a value-added new construction product such as solid carbonate which is formed after mixing the CO₂ into the waste. Implementing this innovative technology will surely benefit from the enormous generation of India’s abundant C&D waste. This process is also beneficial from an economical point of view and produces smaller carbon footprints [13, 14]. Active and Passive CO₂ sequestration using the construction and demolition waste has been presented in Fig. 1.

As CO₂-emitting businesses often create alkaline waste, sequestering CO₂ through mineral carbonation of alkaline waste is promise for both carbon capture and usage, as well as waste prevention. This paper provided a summary and comparison of CO₂ use in cement-based materials, recycled aggregate, and calcium-rich solid waste through accelerated mineral carbonation also this article reviews the fundamentals of mineral carbonation and examines the mineral carbonation of alkaline waste and byproducts. Thus, in this respect, this review is aimed to discuss (1) the sequestration of CO₂ using C&D wastes, (2) the active and passive forms of CCS and its process routes, (3) the pretreatment of C&D wastes before carbonation, (4) the kinetics of carbonation process, and (5) the factors influencing CCS by C&D wastes. The ecological uses and future applications are also discussed in the present review. Sustainability and efficient management system in the construction industry is a must and this is not just the reduction of carbon footprint and landfill area, but also conservation of natural rocks which are decisive for growth of the coming generations. Thus, carbonation techniques with C&D wastes such as crushed CA and waste cement are discussed. In addition, a comparison and overview of the use of CO₂ by accelerated mineral carbonation in cement-based materials, recycled aggregate, and calcium-rich solid waste are provided. The properties of cementous waste and CA after CO₂ sequestration are also deliberated, along with the unique features of concrete block products as a CO₂ sequestration substrate.

There are still numerous industries, such as cement, steel, energy, and others, that produce both alkaline waste and CO₂, but very few use this technology to sequester CO₂. Mineral carbonation of recycled cementitious materials still faces numerous obstacles and limitations, which must be addressed. Table 1 explain the different process of CO₂ sequestration, their advantages and disadvantages.

2. Carbonation Processes for CO2 Sequestration

2.1. Aqueous Carbonation

The impact of moisture content, relative humidity, and temperature plays a crucial role in the carbonation process, which has a significant impact on the carbon dioxide (CO₂) curing of recycled concrete aggregate (RCA). Typically, concrete that is saturated with water or exposed to high relative humidity will impede the ingress of CO₂, thereby limiting carbonation [10].

Concrete with low relative humidity is not adequate for carbonation. Calcium extraction, CO₂ solubilization, and CaCO₃ precipitation are the three primary chemical stages that occur concurrently during direct aqueous carbonation. It is a single-step CCS reaction wherein CO₂ reacts with cementious waste in a water-based suspension, An example of direct aqueous carbonation using concrete debris was chosen to illustrate the concept. shown in Fig. 2 [15, 16]. The reaction of CO₂ with water results in the production of bicarbonate ion and a proton. The proton released from aqueous solutions reacts with minerals of C&D waste materials and liberates the metal ions which in turn act on bicarbonate ions yielding a solid carbonate-richprecipitate [16]. For instance, reactions shown in Eq. (1–3) represent olivine (Mg₂SiO₄) carbonation [9, 17].

(1)

{CO}_{2} + H_{2} O \to H_{2} {CO}_{3} \to H^{+} + {HCO}_{3}^{-}

(2)

{Mg}_{2} {SiO}_{4} + 4 H^{+} \to 2 {Mg}^{2 +} + {SiO}_{2} + 2 H_{2} O

(3)

{Mg}^{2 +} + {HCO}^{3 -} \to {MgCO}_{3} + H^{+}

When carbon dioxide is dissolved in water, it concurrently forms H₂CO₃, HCO³⁻, CO₃²⁻, and H⁺. Le Chatelier’s principle states that a high pH environment will aid in the dissolving of carbon dioxide. Carbonic acid is created when CO₂ interacts with water. Because it is a weak acid, carbonic acid dissolves into H⁺ and HCO³⁻ ions. When magnesium orthosilicate combines with H⁺ ions, Mg²⁺ ions are produced. Mg²⁺ subsequently reacts with HCO³⁻ to produce solid magnesium carbonate [18]. SEM analysis of before and after carbonation of RCA is presented in Fig. 3.

2.2. HCl Extraction

Extraction of magnesium from serpentine occurs via a four-step reaction given below from Eqs. (4–7) [19]. However, this is an energy-intensive process. In addition, it is not a feasible process when the feedstock contains more than 1% of alkali metals [20]. It also leads to the extraction and solid carbonate precipitation. At the time of carbonation, this could be a possible contaminant [20].

(4)

{Mg}_{3} {Si}_{2} O_{5} {(OH)}_{4} (s) + 6 HCl + H_{2} O \to 3 {MgCl}_{2} \cdot 6 H_{2} O (aq) + 2 {SiO}_{2}

(5)

{MgCl}_{2} \cdot 6 H_{2} O (aq) \to MgCl (OH) (aq) + HCl + 5 H_{2} O

(6)

2 MgCl (OH) (aq) \to {MgCl}_{2} (aq / s) + Mg {(OH)}_{2} (s)

(7)

Mg {(OH)}_{2} (s) + {CO}_{2} \to {MgCO}_{3} (s) + H_{2} O

When reacted with hydrochloric acid, chrysotile [Mg₃Si₂O₅ (OH)₄] becomes magnesium chloride hexahydrate. Magnesium chloride hexahydrate breaks down into water-soluble magnesium chloride and hydrochloric acid. In the third step of the process, magnesium chloride dissolves into magnesium hydroxide, which then combines with carbon dioxide to generate magnesium carbonate [21].

2.3. Molten Salt Method

The process involving the molten salt MgCl₂·3H₂O is an advancement over the HCl extraction process to minimize the energy requirement for extraction. This process involves two reactions: First is a multi-stage reaction shown in Equations (5–7) while the second involves direct carbonation of molten salt, MgCl₃·5H₂O, occuring under 30 bar CO₂ at 300°C [19].

Biochar, molten salt recovery, and CO₂ adsorption were extensively studied. Molten salt biochar has a high specific surface area (324.21–788.81 m²g⁻¹). The air-prepared biochar has a CO₂ adsorption capacity of 72.42 mg g⁻¹, 20% greater than the nitrogen-prepared biochar. Air-induced atmospheres also increased molten salt recovery. Potassium recovered twice as much as nitrogen at 900°C [22].

2.4. Other Acid Extraction

Acids other than HCl can also be used as extracting agents in the process of indirect mineral carbonation. It was reported that a lot of magnesium was leached from serpentine using acetic, sulfuric, nitric, formic and hydrochloric acids [23]. [24] reported that under weak acidic condition, sodium salts of citric acid, oxalic acid, and ethylene diamine tetraacetic acid made good for the dissolving serpentine. It was also found that succinic acid acts as a good leaching agent in extraction of calcium sourced from wollastonite [25].

To determine the best extraction conditions, time, temperature, acid kinds, and acid concentration were examined. 2M H₂SO₄ @ 40°C for 30 min optimum acid molarity, type, temperature, and extraction duration. H₂SO₄ at 1M removed 85% of calcium from Red Mud (RM) in 2 h at 80°C. The pH swing technique raised the pH from 2.3 (leached solution) to 9.5 to produce high-purity Ca-containing solution. Calcium carbonate (CaCO₃) with a purity of 98% was obtained by carbonating Ca-enriched leachate with sodium carbonate (Na₂CO₃), created by scrubbing CO₂ with NaOH. Estimated carbonation efficiency was 32.71%. Ex-situ mineral carbon sequestration on site may concurrently produce RM and CO₂, which can reduce process costs [26].

2.5. Bioleaching

The process of extracting metals from minerals by using bacteria is called bioleaching. Bioleaching is applied for sequestering carbon by extracting magnesium as well asfrom silicates [9]. Acid-generating chemicals, like sulphides or sulphur (0), can be combined artificially with chemolithotrophs (e.g., Acidithiobacillusthiooxidans, Acidithiobacillusferrooxidans) to weather the silicate minerals [21]. Ferrous moieties and reduced sulphur-based compounds can both be oxidized using A. ferrooxidans whereas only reduced sulphur compounds are oxidized using A. thiooxidans, generating sulphuric acid in both metabolic pathways.

2.6. Ammonia Extraction

For sequestering carbon, ammonium salts are examined for extraction of metals sourced from silicate stones. The procedure to use CO₂, ammonium bisulphate and serpentine for producing magnesium carbonate, silica and iron oxide is dealt in this process, [27].

A study suggests a two-step method for producing vaterite CaCO₃ (rare earth carbonate) by extracting calcium from recycled concrete fines (RCF) using ammonium chloride (NH4Cl) and carbonating the leachate. The study explores the influence of operating conditions on leaching-carbonation of RCF to maximise Ca²⁺ extraction and CO₂ sequestration. Ca²⁺ is leached at a rate of 65.7% under optimum circumstances, and the solid residue is an amorphous gel with a small phase of brownmillerite. Producing Vaterite (1–10 m) with a purity of 97.8% is feasible. A larger aquaammonia dose promotes the production of carbonates with greater stability. The suggested technique covers challenges such as RCF upcycling, vaterite extraction (615 g/kg RCF) and CO2 sequestration (270 g/kg RCF) [21].

3. Pre-Treatment of Construction and Demolition Waste Before Carbonation

An assortment of pre-treatment possibilities exists. The foremost ones are size reduction, thermal treatment and magnetic separation. Their main aim is to increase the rate of reaction by increasing the reactive surface obtainable for carbonation reaction [28].

3.1. Size Reduction

Theoretically, a smaller particle size of recycled concrete aggregate (RCA) typically leads to a greater specific surface area. This, in turn, can enhance the contact area between RCA and carbon dioxide (CO₂), thereby improving the efficiency of the carbonation process [21]. Before being exposed to CO₂ gas, the sediment was pulverized, which accelerated the pH neutralization. These findings indicate that a shorter pH neutralization time can be accomplished by decreasing particle size. Therefore, the effect of particle size on the pH neutralization period of PSAS-treated sediment was also investigated. The results demonstrated that the pH neutralization period decreased as particle size increased. However, it was also observed that as the PSAS addition ratio increased, the difference in the pH neutralization period caused by the change in particle size diminished [29]. To increase the rate of reaction, it is preferred that the alkaline waste is ground well to increases its surface area. In their study, [30] conducted experiments using particle sizes ranging from 5.0–2.0 mm, 0.5–2.0 mm, and 0–0.5 mm. They observed that the small particles exhibited a CO₂ uptake that was approximately 6.4% higher compared to the large particles. In their study, [31] discovered that the carbon dioxide (CO2) absorption of recycled concrete aggregate (RCA) with a particle size smaller than 5 mm was approximately 2.15%. Additionally, they observed that RCA with a particle size between 5–10 mm had a CO2 absorption of 0.81% after being subjected to carbonation for 24 hours under a pressure of 0.5 MPa. The findings of both indicate that recycled fine aggregate exhibits a higher carbonation efficiency compared to recycled coarse aggregate. A size reduction of C&D waste from 150 to 37 μm increased the conversion of CO₂ into solid carbonate from 10 to 90% [32]. High-energy abrasion grinding encourages imperfections into the cost-effective carbonation reaction [32]. To achieve more conversion than size reduction, ‘normal’ grinding is preferred to cost-effective size reduction. More fine grinding, however, is difficult to conduct on a large scale because the grinding process is highly energy-intensive [33].

3.2. Magnetic Separation

The carbonation of serpentine is slowed down by the oxidation of iron due to the formation of hematite on the surface of minerals [34]. Performing the procedure in a non-oxidising atmosphere makes it difficult and increases the total costs significantly. Magnetic segregation of the iron compounds preceding to the carbonation process resolves this difficulty. Furthermore, a commercially-viableiron ore by-product is formed. When a combination of thermal treatment and magnetic separation is used, firstly it is more effective to conduct the step of magnetic treatment before thermal treatment.

3.3. Thermal Treatment

Serpentine holds up to 13 wt% water in chemically-bonded form. When serpentines are heated up to 600 to 650°C the water is evaporated and an open structure is formed. This process significantly increases the reaction kinetics due to the increased reactive surface area. For example, heat-treatment of antigorite increased the surface area from 8.5 to 18.7m²/g [35].

3.4. Gas-Solid Carbonation

The gas–solid carbonation processes employing alkaline sorbents are gaining popularity due to their capacity to capture and mineralize CO₂ via non-catalytic exothermic reactions [36]. This capture method permits the recovery of purified CO₂ through a high-temperature calcination (or de-carbonation) procedure; the temperature is dependent on the nature of the produced carbonate. Alkaline sorbents such as binary oxides (e.g., CaO and MgO), hydroxides (e.g., Ca(OH)₂, Mg(OH)₂ and NaOH, and metastable granular silicates (e.g., Li₂SiO₃, Na₂SiO₃, CaSiO₃, and MgSiO₃) have been proposed to capture and mineralize carbon dioxide (CO₂) via gas–solid carbonation [37, 38]. This process can be used for the production of high temperature steam which can be applied for the generation of electricity along with CO₂ fixation [39]. Before the process of carbonation, the minerals need to be finely ground. Owing to the excessive entropy of CO₂ gas, the temperature and pressure of the reaction need to be elevated in order to improve the reaction kinetics [40].

(8)

CaO (s) + {CO}_{2} (g) \to {CaCO}_{3} (s) + Energy (179 kJ / mol)

(9)

MgO (s) + {CO}_{2} (g) \to {MgCO}_{3} (s) + Energy (118 kJ / mol)

(10)

{CaSiO}_{3} (s) + {CO}_{2} (g) \to {CaCO}_{3} (s) + {SiO}_{2} (s) + Energy (90 kJ / mol)

4. Sequestration of CO2 using Construction and Demolition Wastes

CO₂ has an average lifetime of tens of centuries in the environment, and at the current rate of CO₂ emission into the environment, naturally occurring carbon sinks are unable to sequester it and are less reliable. Concurrently, the use of alkaline construction waste in concrete significantly contributes to the ecological imbalance [2]. Accelerated mineral carbonation has become a popular method for sequestering CO₂ by converting alkaline oxides like MgO and CaO in natural silicate rocks and industrial wastes into stable compounds like MgCO₃ and CaCO₃. In the past few decades, research has been conducted on reducing CO₂ through accelerated mineral carbonation using industrial residues like steel slag, fly ash, municipal incinerator ash, and cement kiln dust. Recently, construction and demolition (C&D) waste (e.g., recycled concrete aggregate and refuse cement) has attracted considerable interest in the sequestration of carbon dioxide (CO₂) due to its high alkalinity (calcium content) and abundance of vast sources [41]. Carbonation is the reaction of carbon dioxide (CO₂) with cement hydration products such as calcium hydroxide [Ca(OH)₂] and calcium silicate hydrates, resulting in solid carbonates. The sequestration of carbon dioxide through natural carbonation is a sluggish process [42]. Recycled aggregates abundant in calcium silicate products are capable of sequestering CO₂ through an accelerated carbonation process, thereby resolving the aggregate scarcity issue. As of 2019, the construction industry released around 9.95 Gt/y of CO₂, making it the largest contributor. It is anticipated that the construction industry will lower its CO₂ emissions by 16% by 2030, leading to net-zero emissions by 2050 [1]. Consequently, numerous steps have been adopted to decrease the building industry’s considerable CO₂ emissions by collecting and using CO₂-C&D wastes and CO₂ gas act as solutions to each other and when applied on a large scale, it can lead to an economically feasible technique [41]. When CO₂ gas interacts with C&D waste under suitable conditions (e.g., temperature between 35 and 50°C, CO₂ partial pressure of 1 L/m, moisture content between 30 and 40%, and relative humidity (RH) between 60 and 80%), the metaloxides of C&D waste (e.g., CaO and MgO) are transformed into solid carbonates (e.g., CaCO₃ and MgCO₃) [42]. In this section, CO₂ sequestration by various C&D waste materials will be discussed, process diagram of this methods is shown in graphical abstract.

4.1. Inactive and Passive forms of Concrete

Carbonation involves reactions of water-dissolved atmospheric CO₂ with hydration-based end products of cement; examples include calcium hydroxide (Ca(OH)₂) and hydrates of calcium-aluminate and calcium-silicate resulting in calcium carbonate [43]. The reactions are represented in Eqs. (11–13), [44].

(11)

Ca {(OH)}_{2} (s) + {CO}_{2} (g) \to {CaCO}_{3} (s) + H_{2} O

(12)

3 CaO \cdot 2 {SiO}_{2} \cdot 3 H_{2} O (s) + 3 {CO}_{2} (g) \to 3 {CaCO}_{3} (s) + 2 {SiO}_{2} (s) + 3 H_{2} O

(13)

4 CaO \cdot {Al}_{2} O_{2} \cdot 13 H_{2} O (s) + 4 {CO}_{2} (g) \to 4 {CaCO}_{3} (s) + 2 Al {(OH)}_{3} (s) + 10 H_{2} O

There are two kinds of carbonation: passive carbonation (or weathering carbonation) and active carbonation (or accelerated carbonation) as shown in Fig. 1. Weathering carbonation takes place at a rate of approximately 10⁻⁸ cm²/s on the surface of hardened concrete [45]. The rate of weathering carbonation is associated with interior factors (e.g., cement content, strength of cement, cement-water proportion, porosity, and water saturation in the cemetouspores) and exterior factors (e.g., RH, ambient temperature, and concentration of CO₂ in the atmosphere) [44]. Hardened concrete continuously absorbs CO₂ which makes reinforcement more liable to corrosion and crack formation. This is because of the decreased alkalinity (pH<9) resulting in deterioration of reinforcement of concrete [46]. For passive carbonation, the strength of concrete could be enhanced by the formation of solid CaCO₃ [46]. Active carbonation involves the reaction of CO₂ with calcium-bearing phases of fresh concrete under an accelerated controlled environment. Active carbonation occurs within 6 to 10 h. It also reduces the time of curing. Active carbonation results in higher early strength leading to enhanced surface hardness, reduced porosity, and increased concrete lifetime [47]. Therefore, carbonation, on an active mode, is beneficial to maximize concrete-related properties [45]. The active carbonation process can be implemented practically for concrete, lacking reinforced steel rods (concrete & paving blocks, etc.). Active carbonation also serves as a substitute for conventional accelerated steam curing processes such as concrete railway sleeper, sewage pipe, and prestressed concrete beam [48].

The carbonation of concrete structure which occurs naturally is generally limited to a few mm of exposed outer surface. Hence, waste CA sourced from demolition wastes (inner content of longstanding concrete) signifies the basic foundation for forming solid carbonate-rich mineral. Recently, for effective CCS as well as proper utilization of C&D wastes, researchers have concentrated on active carbonation. The significant point is that by using active carbonation, recycled concrete aggregate (RCA) quality improves and balances the CO₂ cycle [38].

4.2. Recycled Concrete Aggregate (RCA)

Recycling concrete that has reached the end of its useful life back into the building sector in the form of aggregate has been the preferred method for dealing with the vast volumes of waste concrete. Unfortunately, recycled concrete aggregate (RCA) includes leftover cement paste that has a negative impact on its characteristics and the resultant fresh concrete [18]. Recycling of RCA accounts for approximately 50% of the total C&D waste generated in Australia [49]. Unfortunately, recycling RCA cannot be used directly as a raw aggregate due to high water absorption, low density, and ineffective porosity [50]. CO₂ curing is regarded as an effective way to enhance the quality of RCA. For improving the density of RCA, mortar adherence can be densified by CO₂ curing on RCA [51]. Water absorption and porosity of RCA, which are the main causes of failure of RCA have improved by CO₂ curing. When RCA is cured by CO₂, water absorption is lesser due to the formation of solid carbonate.

High absorption of water by recycled concreate aggregate (RCA) under fresh conditions of concrete is the main problem when RCA is applied. High absorption of water by RCA can substantially affect resistance to the flow of concrete and its setting time due to its high porosity. The RCA has bulk density of 1200–1500 kg/m³ (Katz, 2003), while natural aggregate (NA) lies in 1500–1700 kg/m³ [28]. It is observed that when NA is completely replaced by RA, the porosity of the concrete matrix is increased by 3.5% to 6.5%. Water absorption rate (3–13%) is higher for RCA than NA (0.5–1%) [52].

The physical as well as mechanical properties of RCA are upgraded when the RCA is pre-treated through carbonation. Carbonation is environmentally beneficial and improves engineering aspects of recycled concrete (e.g., durability & strength) [52]. Carbonation reaction causes numerous chemo-mechanical changes in the mortar, pore size distribution, change in strength, porosity, and chemistry. On the other hand, it causes swelling and shrinkage, breakage of cement matrix. Apart from atmospheric CO₂ reaction with cement paste (passive carbonation), carbonation is to enhance the positive properties of cement-based materials. The sequestration of CO₂ on products of hydration (the formation of CaCO₃) and their precipitants in porous structure results in the dense microstructure of concrete matrix, ultimately leading to enhanced mechanical properties. The compressive strength and elastic modulus of the concrete matrix are enhanced by 20% and 8%, respectively [53]. [54] used RCA which was simultaneous carbonated (40% RH/4.2% CO₂/20°C/48 h before mixing components) and flown-through CO₂ curing method after casting in alkali-activated fly-ash based binders [54]. The compressive strength was found to be enhanced up to nine times when compared with RCA cured at ambient temperature [55]. In the microscopic analysis of mixture, it was shown that the mixture morphology had changed, and more stable polymers formed. Different methods for improving the strength of RCA are shown in Table 2.

From a survey of a cement recycling plant, as investigated by [55], it has been estimated that a ton of recycled-cum-crushed rock can mediate CCS of up to 11 kg. Within a 0.5–2 cm size range, a ton of RCA particles can mediate CCS of up to 7.9 kg. Considering even finer RCA, a kilogram of it could take in as much as 20 g; the corresponding figure would be 110 g for cementitious waste. Upon complete carbonation of a ton of cementitious waste, it can help in CCS of up to 0.27 ton [56]. Post the CO₂-curing process for multiple hours, it is deduced that the costs involved in extracting CaCO₃ from cementitious waste stands at $136/ton via desulphurization while it would be $323/ton, using an ultrahigh-purity procedure. The degrees of mineral carbonation in C&D wastes were investigated via a robust experimental approach to estimate the rapidity and level of Suspensible under programmed outdoor settings. Numerous trials were carried out in a customized rotating batch reactor by sample agitation, in the presence of CO₂, that was infused inside, using water vapor as the carrier medium. When CO₂ was injected at 1 L/min, carbonation was achieved upto25% for C&D wastes after 24 h. Upon elevating the flow of CO₂ to 10 L/min, carbonation of C&D wastes rose up by 24% when operated at 60 rounds per hour/15 h/40 °C. SEM analysis has been conducted for this study as shown in Figs. 3 and 4.The SEM analysis has concluded that after carbonation, surface porosity of the material increased due to formation of solid carbonate on the structure material [15]. The lower S/L ratios result in higher carbonation efficiency. Carbonation tests have been performed using an alkaline industrial waste containing a high CaO content and S/L ratios and found that carbonation efficiency increases due to dilution effect.

4.3. Waste Cement

The sequestration of CO₂ using waste cement has been proposed. For the CCS by waste cement, pressurized CO₂ is used to extract Ca²⁺ ions from wastes followed by CaCO₃ precipitation [46]. When the pressure is reduced, calcium ions are deposited as carbonates, which can either be directly disposed of or applied as a starting material for cement manufacture after it’s recycling [47]. The latter case contributes equally to CCS by reducing the net amount of virgin limestone mined. CCS in the form of carbonates provide a long-term solution to the problem because of the fact that they last very stably in nature. However, CCS as carbonates needs massive proportions of magnesium and calcium [48]. Naturally existing rocks with these elements (e.g., serpentine, wollastonite, etc.) were examined for the formation of carbonate species. CSH (3CaO· 2SiO₂·3H₂O) and Ca(OH)₂ are contained in waste cement. Calcium present in waste cement is relatively more reactive in comparison to that of calcium-containing rocks due to which it forms a long-term solution to the problem of CCS.

Ionization of dissolved CO₂ in aqueous media is represented by Eqs. (14–20). Removal of Ca²⁺ from cementitious waste using carbonic acid is represented by Eqs. (16 and 17). Precipitation of calcium carbonate is represented by Eq. (19). The reactions are exothermic in nature and are conducted with the aid of an external heat source (e.g., electric heater and electric bulb) [56]. CO₂ can react most readily with Ca(OH)₂ Eq. (20), thus precipitating the thinly-soluble CaCO₃ within the pores [57].

(14)

{CO}_{2} + H_{2} O \to H_{2} {CO}_{3}

(15)

H_{2} {CO}_{3} \to H^{+} + {HCO}_{3}^{-}

(16)

{HCO}_{3}^{-} \to H^{+} + {CO}_{3}^{2 -}

(17)

3 CaO \cdot 2 {SiO}_{2} \cdot 3 H_{2} O (s) + 6 H^{+} \to 3 {Ca}^{2 +} + 2 {SiO}_{2} (s) + 6 H_{2} O

(18)

Ca {(OH)}_{2} (s) + 2 H^{+} \to {Ca}^{2 +} + 2 H_{2} O

(19)

{Ca}^{2 +} + {CO}_{3}^{2 -} \to {CaCO}_{3}

(20)

Ca {(OH)}_{2} + {CO}_{2} \to {CaCO}_{3} + H_{2} O

Cement paste (mortar) is well known to convert CO₂ into solid carbonate [57]. However, it can create erosion problems in constructed buildings (a reason for reinforcement corrosion) [57]. Rolling carbonation was used to improve properties of recycled sand for CCS [58]. Mixing and attrition in the rolling carbonation process facilitates access of CO₂ which results in accelerated CCS. In the case of static carbonation, the bottom portion of the alkaline material is unavailable for the carbonation reaction, whereas in the case of rotating carbonation, all of the material is available for the carbonation reaction, which increases the surface area and efficacy of the carbonation reaction. An increase in mass can be achieved up to five times the gain in mass in static carbonation process. When pressure is increased by 1.4 times, the gain in mass is doubled. Rolling carbonation results in increased density, reduced water absorption and anear-uniform surface sand particles (which results in high workability).

4.4. Fly Ash

Worldwide, 0.6 billion metric tons of fly ash (FA) and 12,000 million metric tons of CO₂ are emitted from thermal power stations annually. Cement and concrete manufactures consume 16% of the FA produced, while the rest is buried in landfills [54]. FA promises to be a highly reactive starting material for CCS and can be used without any pre-treatment. Some disadvantages of CCS using FA are low rate of carbonation and low efficiency. The FA produced from Shenfu Coals (China) was taken into consideration for the study of mechanism and efficiency of carbonation as well as to analyse the problem faced by the carbonation reaction. It is observed that due to huge availability of FA, it can be advantageous to use FA for CCS as the efficiency of carbonation is less (4–5% by wt) [59].

ACCS capability of 2.6 g CO₂ per 100 g ash was achieved over 18 h in the water at 30°C with an initial CO₂ pressure of 10 bar. 7.85 g CO₂ per 100 g ash was obtained over 2 d in water at ambient temperature with the same initial CO₂ pressure of 10 bar, noting that the nature of FA impacted the CCS load [59].

Direct solid-gas carbonation of FA was studied using the thermos gravimetric analyser and fixed bed reactor systems by varying operating parameters (temperature, sequestration time as well as the contents of CO₂ and steam. It was found that the capability of circulatory fluidized bed FA to sequester CO₂ increased with an increase in temperature, CO₂ pressure, and moisture content [60]. CO₂ content did not affect sequestration efficiency and was not as significant as steam content and temperature [59].

At the highest point, the CCS capacity was 60 g of CO₂ for every kg of FA, absorbing with 28.74% efficiency at 600ºC in the presence of 20% water [40]. Carbonation resulted in pore blockage and forming a solid protective carbonate layer. The formation of carbonate layer increased average pore size from 0.006 to 175 μm and decreased pore volume from 0.386 to 0.375m³/mg. The specific surface area and the size of pores increased with the addition of steam by converting CaO into CaCO₃. At 45ºC, a CO₂ sequestrating capacity of coal fly-ash stood at 18.2 wt% which corresponds to 74% carbonation efficiency, at maximum, for gas-solid carbonation treatment at a pressure range in a 10–15 bar window [61]. At ambient temperature and 10 bar the coal FA could sequester to maximum of 26.3 g of CO₂/kg.h [62].

Direct mineralization is a benign and long-term solution to the problem of CCS and concerns geological CO₂ storage. It can be used to sequester CO₂ in the area where geological CO₂ aggregation is impractical and by the small and moderate scale CO₂ emitters to reduce their CO₂ footprint. Use of industrial byproducts such as FA for CCS serves as a better feedstock over natural minerals due to its easy accessibility near the CO₂ emitting source, high reactivity, low material cost, and no requirement of pre-treatment. The high reactivity of FA owes to alkaline oxides (e.g., MgO, CaO) [63].

Direct mineralization of FA has been done to thoroughly study the influence of solid-liquid ratio, temperature and gas flow rate of gas on its carbonation efficacy. The simplest form of direct mineral carbonation involves reaction between CO₂ and metal oxide particulates at defined pressures and temperatures. The high-temperature steam produced in direct mineral carbonation can be used in electricity generation as well. Reaction kinetics can be improved by fine grinding of the minerals and elevating the temperature and pressure [64]. However, at higher temperatures, the equilibrium of the equation shifts towards free CO₂ due to increasing entropy of CO₂. This results in an upper limit of carbonation kinetics. The upper temperature limit ranges from 170°C to 410°C for natural silicate-based minerals.

Calcium sourced from lime and Portlander have comparative higher carbonation rate than calcium present in other crystalline or amorphous compounds [40]. The efficiency of fly-ash carbonation does not improve merely with the increase in temperature, rather it is the coaction of elevated pressure and temperature in the presence of sodium carbonate leading to a drastic improvement in carbonation efficiency as demonstrated by [61] using experiments in a batch reactor. Fly-ash was carbonated under temperatures of 140, 180 and 220ºC and pressures of 10 and 20 bar with 0.5mol/L of Na₂CO₃, leading to 29% and 35% carbonation reaction.

4.5. Steelmaking Slag

The iron and steel industry makes up 10–15% of the total industrial energy demand. This amounted to an estimated 1942 Mt of CO₂ emissions in 2015. The steelmaking industries, can sequester gaseous CO₂ using alkaline steel slag with a high content of CaO and MgO (about 30% to 40%) [64]. The different forms of slag are typically calcium- and magnesium-rich, although their chemical compositions vary widely. Calcium is mostly found as amorphous calcium silicates; but, depending on the kind of slag, calcium hydroxide, calcium oxide, and gypsum may also be present and this is beneficial to the carbon sequestration [65]. Annually, steel slag-mediated CCS is estimated at 171 Mt, worldwide which would be 0.6% of annual CO₂ discharge, sourced from burning fossil fuels. [65] studied the direct aqueous carbonation of steel slag. Successful carbonation was accomplished under both ambient and elevated temperatures and pressures. It was possible to sequester nearly 25 g CO₂ for every 100 g of slag and similarly, carbonate transformation was feasible up to 74%. Alkaline oxides, in presence of CO₂, undergo mineral carbonation to form stable solid carbonates. However, this technique has several bottlenecks such as high cost and slow carbonation kinetics, thereby having a limited practice in real fields. Much of the research work deals with the optimization of the parameters that affect the carbonation reaction of alkaline wastes (e.g., FA, steel slag, ash from municipal solid waste incinerator). This has implications in enhancing the rate of carbonation and reducing the cost, making it more economical and practical [66].

5. Factor Influencing CO2 Sequestration by Construction and Demolition Waste

5.1. Temperature, Moisture and Relative Humidity

Water and water vapour are obtained as a result of exothermic reaction of hydration and carbonation, respectively. As monitoring carbonation efficiency is quite important, the internal temperature and relative humidity (RH) are recorded using chamber sensors and data loggers. Process were tracking every minute variation in moisture content and RH, occurring during the CO₂ curing process [67]. Temperature and humidity measurement type sensor data analysis software and real time transmission software are attached as accessories. Water greatly influences the carbonation by CO₂ curing of RCA, and CO₂ entry for carbonation is restricted for high RH or water-saturated concrete. RCA was cured under two conditions: (i) drying and (ii) alternative wetting followed by drying. With 15% moisture at 20°C and RH of 65%, the alternatively wetted and dried concrete particles were CO₂-cured, and the rate of carbonation was faster than normally dried concrete particles. The carbonation rate of solid phase Ca(OH)₂ dissolution increases significantly when alternatively wetted and dried condition creates a similar environment [12].

Different conditions for the pre-treatment of RCA before carbonation are used to optimize the moisture content by (1) oven-drying (2 h/105°C) and (2) stockpiling RCA devoid of treatment and (3) water-soaking (2 h) [69]. Pre-treatment helps to increase the mass gain ratio and carbonation rate as moisture content is controlled as 0.6% [51]. For RCA, ahigh porosity accommodated by more water can block CO₂ penetration into RCA, reducing the carbonation efficiency. For allowing carbonation reaction in an ideal environmental condition, the RH must range from about 40 to 70% [51]. In a report by [70], the drying chamber temperature was maintained at 25°C along with RH 50% before carbonation for pre-treatment. After the pre-treatment, carbonation stipulates an optimal RH of 40–70%. A study reported RCA curing by keeping the temperature and RH at 20°C and 60%, respectively, in the carbonation chamber using CO₂ without pre-treatment.

5.2. Dimensions of RCA Particles

The rate at which carbonation occurs is affected by RCA size [71]. The CO₂ uptake increased with decreasing particle size, as a result of more specific surface are, being open to CO₂ [72]. The particles less than 5.0mm (e.g., recycled fine aggregate) exhibited a higher rate of carbonation than particles in the range of 10 to 20mm [68]. RCA with particle size 0.5–1.0 cm led to 56% CO₂ consumption and 3.88% gain of solid carbonate mass after carbonation reaction [51]. Two RCAs with different particle sizes were introduced (1) new type RCAs derived from ready mix plant and (2) old type RCAs derived from C&D waste. Both types of RCAs with 0.5–1.0 cm size particles sequestered higher CO₂ when compared to aggregates with 1–2 cm-sized particles [31].

A study of properties with particle sizes of 1 and 2 cm of CO₂-saturated recycled mortar aggregate (RMA) reported that the carbonated aggregate having particle size of 10 mm portrays superb properties with respect to density and water absorption as compared to a particle size of 2.0 cm [73]. RCA with particle size <0.5 cm exhibited up to 2.15% CO₂ uptake, while that with particles of 0.5–1.0 cm exhibited only 0.81% CO₂ uptake [74]. When the particle size of RCA was below 2.5mm, the CCS rate was faster than that of the RCA (>0.5 cm) [74].

5.3. Effect of Partial Pressure of CO₂ and Carbonation Time

When CO₂ is at 0.1 bar, the increase in carbonation percentage made higher gains at the beginning (i.e., 0.5 and 1 h) than in comparison to 2, 3, and 4 h, when the rate of rise in carbonation percentage became less obvious [51]. CO₂ curing of RCA resulted in significantly lower water absorption-cum-porosity. Rasing CO₂ pressure from 0.1 up to 5 bars for 24 h, the traits of RCA like crushing value, carbonation percentage, density, 10% fine value rose for 100% CO₂. However, water absorption for 5–10 mm and 10–20mm size aggregate reduced by 1.2 and 1.1% after 24 h [70]. Carbonate particles of 1–2 cm size at 100 millibar pressure portrayed lesser uptake of CO₂. The two RCAs (i.e., ordinary recycled concrete aggregate (ORCA) and natural recycled concrete aggregate (NRCA)) under 5 bar pressure exhibited uptake of 0.54 and 0.81% for particle size 5–10mm, respectively. Investigation of coarse RMA at 0.1 bar CO₂ with particle sizes of 10 and 20mm, when cured for 6, 12, 24, 48, and 72 h with 99% CO₂ resulted in an increase in density of the cementitious aggregate surface and a decrease in water absorption with the time of curing [73]. The physical properties made a marginal rise of CO₂-cured RMA after 24 h [76]. A study was carried out on RCA by altering CO₂ pressure as well as curing duration, concluding that high CO₂ pressure results in the breakdown of soft RCA. However, the properties of RCA improved at lesser pressure and prolonged curing time. In another report, experiments using RCA with particle size of 0.5–1.0 cm and <0.5 cm were conducted at 100% CO₂ and 5 bar for 24 h [74], showing an increase in particle density and decrease in water absorption on the aggregate surface.

5.4. Effect of CO₂ Curing

Recycled fine aggregates (RFAs) obtained from demolition concrete possess a poor content of carbon sequestration capacity due to a long duration of storage. Thus, regular carbonation is not sufficient to improve its performance. In a previous study, demolition RFAs were soaked in Ca(OH)₂ prior to carbonation to have elevated strength, lower crush value and powder content, and reduced water absorption capacity as compared to RFAs carbonated without pre-treatment [67]. The optimization of various carbonation parameters that affect the CO₂ curing was done through experiments. As an example of the process of CO₂ curing, RFAs were placed in a carbonation chamber under a temperature of 20ºC and RH of 60% after pre-soaking of RFAs in Ca(OH)₂ followed by drying and commercially available CO₂ with 99.5% purity was supplied. The influence of various parameters such as the concentration of CO₂, Ca(OH)₂ content and moisture content present in RFAs were varied and studied to examine the impact on CO₂ curing process and has been detailed in [51].

As suggested by experimental results curing RFAs under optimal condition ORFAs (ordinary recycled fine aggregate) has been found to have considerably lowered magnitudes of crush value, powder content and water absorptivity. X-ray diffraction and scanning electron microscopy were used to study the enhancement mechanism, as shown in Figs. 3 and 4 [73].

5.5. Effect of SO_x and NO_x Removal from Construction and Demolitation Waste

SO_x, NO_x, and CO₂ are acidic gases while C&D waste is alkaline and hence, it has the capacity to adsorb these gases in their pores. Gaseous SO_x and NO_x critically influence CCS process by negatively impacting the lifespan of the adsorbent. As the SO_x and NO_x are more acidic gases in comparison to CO₂, it will react fast with alkaline waste and reduce the available number of active sites and theoretically occlude transport through the porous network [77].

C&D waste has the capacity to sequester NO_x gases. A possible mechanism where in, Ca(OH)₂ present in concrete pores acts as a base to sequester NO₂ [78].

(21)

2 Ca {(OH)}_{2} (s) + 4 {NO}_{2} \to Ca {({NO}_{3})}_{2} + 2 H_{2} O

(22)

3 Ca {({NO}_{2})}_{2} (s) + 2 H_{2} O \to Ca {({NO}_{3})}_{2} (s) + 4 NO + 2 Ca {(OH)}_{2}

This reaction Eqs. (21 and 22) has a Gibbs Energy of −437 kJ/mol. Experiments were conducted on pulverized concrete samples of 28 days and 12 years, in the presence of humidified synthetic air and NO₂ and absence of CO₂ [79]. Despite reduced alkalinity observed in aged concrete, NO₂ absorption rate was 60% of the rate observed in fresh concrete. This phenomenon was attributed to increased porosity in aged concrete due to carbonation in pores. Though sequestration rates of CO₂, NO_x, and SO_x are higher under basic conditions, sequestration rates of NO_x and SO_x are less sensitive to decrease in pH as compared to CO₂ as observed in the treatment of flue gas [80].

6. Utilisation of Mineral Carbonation to Produce Construction Materials

The mineral carbonation process increases the use of discarded or wasted CO₂ in the production of building materials. Numerous factors make the mineral carbonation process pertinent for CO₂ sequestration. First, solid carbonate is the standard-bearer for construction materials in this industry, and it can be produced readily through mineral carbonation reactions [81]. The formation of carbonates from calcium and magnesium-based chemical reactions is simple to comprehend. This method demonstrates that carbonate formation will be stable and long-lasting. Fourthly, the thermodynamics of the reaction between CO₂ and alkaline solids are favorable; thus, very little extrinsic or external energy is required [82].

Mineralization and utilization of CO₂ in recycled cementitious materials could make a substantial contribution to the decarbonization of the cement industry in the following ways: (1) enormous potential for CO₂ utilization and sequestration; (2) performance enhancement for recycled concrete aggregate and recycled hardened cement powder; and (3) substitution of a portion of high CO₂-emitting Portland clinker and aggregate [83]. There are still numerous industries, such as cement, steel, energy, and others, that produce both alkaline waste and CO₂, but very few use this technology to sequester CO₂. Mineral carbonation of recycled cementitious materials still faces numerous obstacles and limitations, which must be addressed [84].

7. Conclusions and Future Prospects

Global warming, the acidification of the oceans, and the melting of glaciers are just a few of the major environmental problems that high CO₂ levels cause. CCS from alkaline C&D waste aids in the resolution of these issues. The mineralization and utilization of CO₂ in recycled cementitious materials have the potential to make a significant contribution to the decarbonization of the cement industry. This can be achieved through several means:

CO₂ utilization and sequestration: There is a vast potential for utilizing and storing CO₂ in recycled cementitious materials. This process helps to reduce the overall carbon footprint of the cement industry.
Performance enhancement for recycled concrete aggregate and recycled hardened cement powder: By incorporating CO₂ mineralization and utilization techniques, the performance of recycled concrete aggregate and recycled hardened cement powder can be improved. This leads to more sustainable and durable construction materials.
Substitution of high CO₂-emissions Portland clinker and aggregate: The use of recycled cementitious materials allows for the substitution of high CO₂-emissions Portland clinker and aggregate. This substitution helps to reduce greenhouse gas emissions associated with cement production.

In summary, the mineralization and utilization of CO₂ in recycled cementitious materials offer significant benefits for the decarbonization of the cement industry. These benefits include CO₂ utilization and sequestration, performance enhancement for recycled materials, and the substitution of high CO₂-emissions components. However, there are still numerous challenges and limitations that need to be addressed in order to effectively implement mineral carbonation for recycled cementitious materials.

A thorough study of the research literature leads to the following findings:

Mineral carbonation is one of the most promising technologies for reducing CO₂ emissions. The process has the double advantage of reducing CO₂ emissions and utilizing solid wastes to produce certain products with added value.
Using the mineral carbonation process, concrete debris can be used to make new solid carbonate.
There are many ways to get CO₂ out of the air, and mineral carbonation is a great one.
For CO₂ sequestration, indirect carbonation utilizing the extracting agent NH₄Cl is superior to extracting agents such as HCl, HNO₃, and CH₃COOH. During the precipitation of calcium carbonate, it is advantageous to use NH₄Cl because it is readily regenerable and does not require basic reagents.
Most likely, the temperature, pH, pressure, amount of moisture, speed of rotation of the reactor, S/L ratio, flow rate, shape, and geometry of the input material will affect the carbonation reaction.
The high porosity of recycled aggregate makes it hard to work with, but CO₂ gas can make it easier to work with. CO₂ conditioning can induce morphological changes in RA, hence increasing the material’s compressive strength.
The carbonation of RA has drastically decreased its water absorption, porosity, and pulverizing value, while increasing its superficial density.
The compressive strength and elastic modulus of concrete made with CRA are superior to those of concrete made with untreated RA, while the solid phase volume is increased by 11.8%. Between 5 and 20 mm in diameter, RA sequesters nearly 7.9 kg CO₂ per ton.
Carbonation is greatest in solid residues with a high CaO content (but the phase of the mineral in which calcium is bonded is also significant), a large surface area, and relative humidity between 50 and 70%. The pH of solid waste was decreased below the threshold value (11.5) by carbonation, making it less hazardous.
Rolling carbonation is significantly more effective than static carbonation.
Since this carbonation process is good from a thermodynamic point of view, it can be used to make stable carbonate at temperatures close to room temperature while using less energy.
FA can also be used in conjunction with CO₂ gas to create solid carbonate. Calcium-rich fly ash yields superior carbonate synthesis outputs in comparison to calcium-deficient FA.
Synthetic carbonates can be produced with the use of CO₂ conditioning and “cement waste.”
The mining of natural rocks for new construction materials does not appear environmentally responsible.

Recycling C&D trash can get us out of this mess and change the future of the building industry.

A CO₂ imbalance in nature causes global warming; it can be intentionally sequestered by combining it with any form of carbonate substance to make fresh solid carbonate, thus creating wealth from waste.
Instead of using newly mined materials, deteriorating C&D waste could be used to make new, more sustainable building materials.

The CCS process requires the use of a reaction chamber to process the reaction and the pretreatment of C&D waste; thus, this process is extremely energy intensive. To make this process more environmentally friendly, additional research must be conducted to reduce energy usage throughout the pretreatment and reaction phases.

Acknowledgements

The author (BSG and SK) are highly thankful to Director, Indian Institute of Technology, Guwahati, Assam 781039, India for his valuable encouragement. Also, BSG and MK is grateful to VC, UPES Dehraduns, India for his valuable encouragement. KHK recognizes the grant from National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning (Grant No: 2016R1E1A1A01940995).

Notes

Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Authors’ Contributions

S.K.T. (PhD Student & Environment Scientist) conducted all the experiments and prepared first draft manuscript. K.H.K. (Professor) supervised and critically reviewed several times the paper and given the critical comments to improve the manuscript. R.S.S. (Professor) supervised and given the comments to improve the manuscript and project investigator. J.L. (Professor), T.K. (Professor), J.M. (Profesor) and M.K. (Professor) have given the several comments to improve the manuscript for the various aspects and supervised. B.S.G. (Assistant Profesor) conducted experiments since the beginning of the studies and supervised.

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Fig. 1

Active and Passive CO₂ sequestration using the construction and demolition waste.

Fig. 2

SEM analysin of before (a) and after (b) carbonation of Cementious waste treated with CO₂

Fig. 3

SEM analysis of before and after carbonation of RCA

Fig. 4

Process of CO₂ sequestration by Construction & Demolition waste.

Table 1

Different process of CO₂ sequestration, their advantages and disadvantages.

CO₂ Sequestration Process	Advantages	Disadvantages
Mineral CO₂ sequestration	Permanent CO₂ disposal Single step simplest method of carbonation Storage can be more effective	Additional CO₂ emissions Risk of potential leaks Safety and handling hazards
Indirect carbonation	High purity products which have high value High rate of carbonation efficiency by Ph swing method	Additional use of chemicals It is a two-step process which requires Pre-treatment
Coal & fly ash Sequestration	Available in large quantities Acceptable mineral carbonation efficiency	Low CO₂ sequestration capacity
CO₂ Sequestration in Ocean	Large CO₂ sequestration capacity	Ocean water acidification which causes danger to sea creatures and structures
CO₂ Sequestration by C&D waste	Available in huge amounts Results into value added carbonated products It improves properties of C&D waste
CO₂ Sequestration in Gas Reservoir	Availability of Large capacity reservoirs in future	Large distances of CO₂ transportation
CO₂ Sequestration Red mud	Carbonation at relatively low pressure and at ambient temperature Reaction time is less than one hour Good sequestration capacity equals to 50gm/kg mud.	Available in relatively low quantities

Table 2

To Improve the mechanical strength of RCA different suggested solutions.

Concrete Property	Mechanical Strength	Shrinkage
Suggested Solutions	Replacement of NA with RA must be limited up to 50% [usually 25 – 40% by weight] [79] Use little more quantity of cement about 5% in cementitious composites [81] Giving attention at strength, quality of RA and composition of alkaline solution which will result n improvement at interfacial transition zone. [82] Pre-saturate RA and use more water while mixing to adjust the water demand of RA [81]	Use RA having size less than 2 mm which leads to internal curing [53]
Suggested Solutions	Use thermal curing conditions at temperature range of 600°C to 700°C [82] Use RA not more than 30% [83] Treat RA using CO2 sequestration [53]	Increase the molarity of NaOH [84]

A critical review on CO2 sequestration using construction and demolition waste: Future scope and perspective