The Capture and Transformation of Carbon Dioxide in Concrete: A Review

Abstract

Concrete is one of the most commonly used engineering materials in the world. Carbonation of cement-based materials balances the CO₂ emissions from the cement industry, which means that carbon neutrality in the cement industry can be achieved by the carbon sequestration ability of cement-based materials. Carbon dioxide is a symmetrical molecule and is difficult to separate. This work introduces the important significance of CO₂ absorption by using cement-based materials, and summarizes the basic characteristics of carbonation of concrete, including the affected factors, mathematical modeling carbonization, and the method for detecting carbonation. From the perspective of carbon sequestration, it mainly goes through carbon capture and carbon storage. As the first stage of carbon sequestration, carbon capture is the premise of carbon sequestration and determines the maximum amount of carbon sequestration. Carbon sequestration with carbonization reaction as the main way has been studied a lot, but there is little attention to carbon capture performance. As an effective way to enhance the carbon sequestration capacity of cement-based materials, increasing the total amount of carbon sequestration can become a considerably important research direction.

1. Introduction

Energy, transportation, industry, and residential contribute the most to atmospheric CO₂ emissions (41%, 22%, 20%,10%, and 7%, respectively) [1]. In the cement industry, CO₂ emissions are primarily generated through combustion of fossil fuels and through the conversion of limestone into calcium oxide. The consumption of electricity generated from the burning of fossil fuels contributes indirectly to the emission of CO₂. A large portion of CO₂ emissions originates from the combustion of fuels, and a smaller portion originates from the calcination of limestone [2,3,4,5,6]. The type of fuel used and the amount of CO₂ produced are estimated to be 0.9 to 1.0 tons for a ton of clinker [2,7]. According to Hoenig and colleagues, a cement plant equipped with modern technology and equipment produces 0.65 to 0.92 kg of carbon dioxide per kilogram of cement produced [8]. Cement emits approximately 0.79 tons of CO₂ per ton, according to a study by [6,7]. Approximately 5–9% of the global CO₂ emissions are attributed to the concrete industry [6,7,9,10,11,12,13]. There is a rapid increase in this percentage mostly due to an increase in cement production and a decrease in emission reductions [6,14].

As a result of economic and population growth, anthropogenic greenhouse gas emissions have increased greatly, with unprecedented concentrations of CO₂ in the atmosphere and a dearth of CO₂ sequestration systems [1,15,16,17,18]. Using carbon emission technologies and carbon compensation methods is essential to achieving a zero-carbon footprint on a global scale, and all countries need to work together [9], in spite of the numerous challenges associated with the capture of carbon dioxide, and the numerous political, regulatory, and economic factors that will ultimately influence the time it takes for new CCS schemes to come into operation. In order to address the problem of CO₂ capture, we have an opportunity to play a central role as a scientific community [19]. As governments and industries take action to reduce global greenhouse gas concentrations, the climate crisis is becoming increasingly urgent. The establishment of a number of high-profile collaborative programs has also taken place [19]. As CO₂ is a symmetrical molecule, it is difficult to separate. Sequestering CO₂ emissions can be done in two ways. Using geological sequestration, CO₂ can be sequestered directly from the main sources of CO₂, including cement factories, power plants, and petroleum refineries. Biological sequestration of CO₂ is the second method, in which ants, marine sediments, and soils sequester atmospheric CO₂ [1,18,19]. In geological sequestration, among other technologies, physical separation and chemical absorption, carbon dioxide can be sequestered from the atmosphere through membrane separation and biological fixation [1,20,21].

As shown in Figure 1, the present study was conducted using a systematic literature review methodology. Four main steps were involved in the systematic process: target articles were identified using CO₂ sequestration as a general keyword in the Scopus database between 2009 and 2019. Additional keywords were used for screening, including geological sequestration (n = 4.258), biological sequestration (n = 2421), and concrete carbonation (n = 344). Geological and biological sequestration articles were obtained based on the limitation method [1].

To reduce the time to commercialization, innovative new materials are needed for research, as shown in Figure 2 [18]. CO₂ can be captured from post combustion, precombustion, or oxyfuel processes with a variety of promising new materials, as shown in Figure 3 [18].

Carbon sequestration of materials mainly goes through two stages: carbon capture and carbon sequestration. Carbon sequestration with carbonization reaction as the main mode has been extensively studied, but little attention has been paid to the performance of carbon capture [24]. Most of the previous work was done to enhance the carbon sequestration capacity of concrete by investigating the carbon dioxide carbonation reactions, but using the method that increasing the total amount of carbon sequestration to enhance the carbon sequestration capacity of cement-based materials has been ignored. However, as the first stage of carbon sequestration, carbon capture is the premise of carbon sequestration and determines the maximum amount of carbon sequestration. Therefore, by adding porous biochar into cement-based materials [24], the pore structure can reach the physical state of sudden change in transport property, and the carbon capture performance can be effectively improved, thus enhancing the carbon sequestration capacity of cement-based materials.

In this paper, Section 2 and Section 3 introduce the significance of using cement-based materials to absorb and fix carbon dioxide, as well as the basic characteristics of the carbonization process in cement-based materials, so that readers can identify the most basic research problems in the study of cement-based materials’ carbon fixation. On the basis of cement-based material carbon fixation performance, how to enhance its carbon fixation capacity is the main content of the Section 4. Finally, Section 5 gives the conclusion and prospect through the above introduction.

2. Significance of Cement-Based Materials for Carbon Dioxide Absorption

2.1. The Process of Producing Carbon Dioxide and Absorbing Carbon Dioxide

2.1.1. CO₂ Production Mechanism

A concrete example, building materials have been used around the world for thousands of years, and as a result of cement manufacturing, significant amounts of these gases are released. Currently, each person consumes one m³ of water each year [25,26]. In Figure 4, we can see the amount of carbon dioxide produced during concrete production. In Figure 2, the study’s System Boundaries are shown. According to Figure 4, the mining industry is responsible for a certain amount of CO₂ emissions (e.g., energy use), and raw materials are processed and manufactured. In the production of concrete, 1 m³ of concrete is constructed in phases. A summary of the key types of energy expended with each activity is also provided in Figure 4 [26]. In an assessment of each activity’s contribution to CO₂-e, in order to manufacture and construct 1 m³ of concrete, raw materials need to be sourced and manufactured. Figure 5 summarizes the findings. A substantial portion of CO₂-e for OPC concrete comes from Portland cement; it contributes 76.4% of CO₂-e [26].

OPC production is associated with high CO₂ emissions for several reasons. A key ingredient in the process is limestone, which is calcined and releases CO₂ as a result. The manufacturing process consumes a lot of energy. Temperatures exceeding 1400 °C are used for heating raw materials in rotating kilns [25,26]. The calcination chemical process utilizes both energy and chemicals. Calcium oxide (CaO) is formed from limestone (calcium carbonate) according to chemical description, shown as Equation (1). Usually, between 60 and 65% of CaO’s total CO₂ emissions are caused by combustion [10,27].

$${\mathrm{CaCO}}_3=\mathrm{CaO}+{\mathrm{CO}}_2$$

(1)

2.1.2. CO₂ Absorption Mechanism

Concrete undergoes a number of chemical reactions. Calcium oxide is thought to hydrate to calcium hydroxide (Ca(OH)₂) as shown in the equation below [10]: During certain humid conditions, if the cement paste in concrete is exposed to carbon dioxide in the air, the cement paste will become weaker. In the presence of calcium hydroxide, dissociation may occur, described as Equation (2), and CO₂ dissolves in water in the form of hydrogen carbonate (H₂CO₃^*), shown as Equation (3); this leads to calcination, carbonation, and the reverse reaction; as described in Equations (4)–(6), there is evidence that it occurs. The following is a simplified description of the reactions that result in the formation of calcium carbonate (calcite), again described in Equation (7).

$${\mathrm{Ca}\left(\mathrm{OH}\right)}_2={\mathrm{Ca}}^{2+}+2({\mathrm{OH}}^{-})$$

(2)

$${\mathrm{CO}}_2{\left(\mathrm{g}\right)+\mathrm{H}}_2{\mathrm{O}=\mathrm{H}}_2{\mathrm{CO}}_3^{*}$$

(3)

$${\mathrm{H}}_2{\mathrm{CO}}_3^{*}={\mathrm{H}}_2{\mathrm{CO}}_3$$

(4)

$${\mathrm{H}}_2{\mathrm{CO}}_3={\mathrm{H}}^{+}{+\mathrm{HCO}}_3^{-}$$

(5)

$${\mathrm{HCO}}_3^{-}={\mathrm{H}}^{+}+{\mathrm{CO}}_3^{2-}$$

(6)

$${\mathrm{Ca}}^{2+}+{\mathrm{CO}}_3^{2-}={\mathrm{CaCO}}_3(\mathrm{s})$$

(7)

Overall, over the period 1930–2013, carbonate cement materials sequestered an estimated 4.5 GtC. As a result, 43% of the CO₂ emissions from cement production were offset; during cement production, fossil fuel emissions are not included [28]. Furthermore, carbonation mechanism models have been studied by scholars since 2004. The following are examples: Concrete carbonation mechanisms were examined by Bary and Sellier [29]. Relative humidity evolutions in porous materials must be taken into account. Water mass balance equations were used to develop the model; a gaseous phase is characterized by carbon dioxide and a pore solution is characterized by calcium (Ca). This study was expected to provide a complete solution to the problem of cement carbonation due to atmospheric pollution [30].

2.2. Cement Industry’s Role in Balancing the Release of CO₂

There are between 11.4 and 12.6 billion tons of concrete raw materials generated each year [1,12]. Carbon dioxide is shown to contain a number of species; calcite is not the only mineral, The stoichiometric potential to reverse the calcination process within cement paste samples is significant [10]. Alternatively, carbonation can be used. Calcium is formed from the reaction between hydrated cement and carbon dioxide; it has been tested under a wide range of conditions for its ability to sequester carbon dioxide, thereby balancing this release [10]. Therefore, the significance of carbon sequestration by applying cement-based materials is that the carbon neutrality of the cement industry can be achieved by the properties of cement itself.

2.3. The Ways to Reduce Emissions in the Cement Industry

Carbon capture is a technique for capturing carbon. The depths of the ocean or underground transport sequester CO₂ as a stream of concentrated gas. This technique may be used to reduce CO₂ emissions using carbon dioxide capture and storage (CCS). There is a possibility of capturing and storing carbon dioxide for a very long time. CCS technologies can reduce CO₂ emissions by 65–70% in cement production, according to Newell and Anderson [31]. Cement is manufactured in the following way: a fuel is burned to provide heat for the process of converting CaCO₃ to CaO, generating CO₂ [32]. Researchers have developed different methods of capturing CO₂ during combustion [33]. Cryogen separation, membrane separation, chemical stripping, and physical absorption are some of the techniques. None of these options has a clear estimate of the implementation cost; there is a direct relationship between fuel type and technical performance [33,34]. In addition to these options, the cement industry also has a number of other possible capture processes, capturing post-combustion emissions, the capture of combustion gases and pre-combustion gases [8,34,35].

Clinker production is an energy intensive process, which emits significant amounts of CO₂ [36]. Carbon dioxide emissions can be reduced most effectively by reducing the amount of clinker in blended cement [6]. Burning waste or biomass as an alternative fuel can reduce carbon dioxide emissions. Because cement kilns have a high process temperature, they are ideal for the combustion of waste [37]. Improved pyro processing will provide the greatest benefits for reducing cement manufacturing’s energy consumption and emissions [6].

3. Carbonation of Concrete: An Investigation of Its Basic Characteristics

3.1. The Factors Affecting Carbonization

Calcite sequestration in concrete in the form of carbonation depends on a variety of factors. The relative humidity of the curing environment is one of these factors, The composition of the atmosphere is determined by temperature and CO₂ concentration [38]. Additionally, alongside the chemical properties of a material, it is also important to consider solid physical characteristics as well as exposure conditions when evaluating carbonation. Figure 6 illustrates this. Additionally, the water–cement ratio (w/c) and concrete density affect concrete carbonation in a significant way by increasing or decreasing carbonation rates—concrete that is aerated, in particular [1].

Crystallization products are affected by temperature [30]. Concrete’s carbonation depth and compressive strength are linearly related to their temperature, according to experimental results [30]. CO₂ and carbonate ions diffuse faster at higher temperatures, and the carbonation reaction runs faster [39]. Increasing the temperature will result in, transmission coefficients and reaction coefficients being high. Consequently, concrete specimens are more affected by carbonation and reaction rates [30].

The high humidity may slow down the carbonation of a filling material or road base. There may be a faster rate of leaching than carbonation in this case. When water is exposed to carbon dioxide, it will carbonate. The carbonation process seems to be accelerated by cyclic wetting and drying. Since concrete without water does not form ions and calcite, it does not carbonate since it lacks the water needed for ion formation and calcite formation. Wet conditions also slow carbohydrate oxidation. In this case, the carbonation rate reaches its maximum at a high relative humidity. RH will determine this maximum. The binder must have a specific porosity and type. In addition, capillary geometry plays a role. The ratio of water to binder determines this in practice, hydrogenation and binder type. Carbon moves fastest when RH (inside the concrete) is between 60 and 80% [39]. RH is related to carbon depth and compressive strength through a quadratic and cubic function, respectively. When the relative humidity reaches 70%, concrete’s carbonation depth reaches its maximum [30].

A surface-to-inner reaction causes carbonation. Due to this, it will be important to measure the surface area exposed. Generally, diffusion will occur in the porosity, but cracks, interfacial zones, and the interstitial zone must also be considered. Carbonation will occur faster in the interfacial zone because it is more porous. A deeper carbonation can be observed in thin slices of paste at the point of contact with stones. The carbon dioxide can reach deeper in surface cracks and react with a larger surface area [40]. Thin sections of cracks with carbonated walls clearly demonstrate this. Hence, durability and deterioration will also affect carbonation. The carbonation of concrete will be enhanced if it is affected by alkali silica or delayed ettringite reactions, for example. Fresh concrete surfaces created by falling concrete pieces will lead to a higher level of carbonation [39].

Generally speaking, with longer carbonation time, higher CO₂ concentration, as the CO₂ pressure increases, the mortar will be able to absorb more CO₂ and have a higher compressive strength [41]. Carbonation and compressive strength are related to carbon dioxide concentration by power and exponential functions, respectively [30]. Even though CO₂ is used to accelerate the process, this technique is still hindered by slow carbonation speeds. Concrete that is of a young age is carbonated at a slower rate due to the diffusion of CO₂ into the matrix. It takes a very long time [42].

3.2. Modeling Carbonization Mathematically

As seen from the perspective of civil engineering, a very important problem is the analysis of its dynamics. As reported in the article, the models used in this study do not provide precise proofs of carbon dioxide transport, but we report some mathematical results regarding it [43].

In calculations of concrete carbonation based on mathematical results, a mathematical model of concrete carbonation was proposed as a free boundary problem for a one-dimensional carbonated front [44]. A solution to this problem was proven to exist and to be unique. The free boundary has been proven to behave in a large time range. In order to analyze the dynamics of the concrete carbonation process, we will construct a mathematical model of this process in three dimensions. This study begins with a review of the literature. Maekawa–Ishida–Kishi and Maekawa–Chaube–Kishi introduced the hysteresis operator into the mathematical model of moisture transport proposed [45,46,47,48]. This model has a unique solution, which has been proven to exist. A solution to this model is unique for this reason [43,49,50,51]. The following balanced law of carbon dioxide transport according to previous research [46,47] was proposed [50,51] as shown in Equations (8) and (9).

$$\frac{\partial }{\partial t}\left\{\varphi (z)[(1-S)v+Su]\right\}-\mathrm{div}(\varphi (z)[H_1(1-S)\nabla v+H_{2}S\nabla u)=-kuw\mathrm{in} Q(T):=(0,T)\times \mathsf{\Omega }$$

(8)

$$w(t)=w(0)e^{-\tilde{k}{\displaystyle {\int }_0^{t}u(r)dr}}u(t), z(t)=1-e^{-\tilde{k}{\displaystyle {\int }_0^{t}u(r)dr}} \mathrm{f}\mathrm{o}\mathrm{r} t>0$$

(9)

$\mathsf{\Omega }$: domain occupied by concrete.

$v=(t,x)$ and $u=u(t,x)$: the concentration of carbon dioxide in air and the concentration of carbon dioxide in water at a time t and a position $x\in \mathsf{\Omega }$, respectively.

$v={\rho }_{0}u$ holds for a positive constant ${\rho }_0$ by Henry’s law.

$\varphi =\varphi (z)$: the porosity, which is the ratio of the volume of the total pore spaces inside the concrete to the volume of the whole concrete.

z: the ratio of the volume of consumed calcium hydroxide to the volume of the total calcium hydroxide.

S: the degree of saturation corresponding to the relative humidity, and the relationship between the relative humidity and the degree of saturation is given as a hysteresis operator in [46,47].

$H_1=H_2$: positive constants.

$k$ is a reaction rate.

$w$: the concentration of calcium ion and this forcing term represents the consumed carbon dioxide in the concrete carbonation process and is given by the reaction rate theory.

The following simplified Equation (10) with the boundary and initial condition was also studied.

$$\begin{gathered} \frac{\partial }{\partial t}[\varphi (1-e^{-{\displaystyle {\int }_0^{t}u(r)dr}})\cdot u]-\mathsf{\Delta }u=-w_{0}u{e}^{-{\displaystyle {\int }_0^{t}u(r)dr}}\mathrm{in} Q(T):=(0,T)\times \mathsf{\Omega } \\ u=u_b \mathrm{on} S(T):=(0,T)\times \partial \mathsf{\Omega } \\ u(0)=u_0 \mathrm{i}\mathrm{n} \mathsf{\Omega } \end{gathered}$$

(10)

where $u_b$ is a given function on $Q(T)$, and $w_0$ and $u_0$ are given functions on $\mathsf{\Omega }$. In this paper, we show the existence and uniqueness of a time global solution of the problem (P) = {(1.5), (1.6), (1.7)} and the large time behavior of that solution.

3.3. Method for Detecting Carbonation

According to what was previously mentioned, concrete carbonation has been studied by many researchers. The majority of studies use thermal gravimetric analyses and focus on calcite or other forms of solid calcium carbonate as a source of carbon dioxide in concrete. Vaterite and aragonite are examples. The TGA process involves heating samples and measuring their mass loss as a function of temperature. There are many processes that can cause mass loss. In some cases, the compounds become gaseous due to evaporation or chemical breakdown. Carbon dioxide and water are examples of these gases [10].

Table 1. Temperature ranges in various studies (°C).

Ca(OH)2 TGA Decomposition	CaCO3 TGA Decomposition	Aragonite–Calcite Conversion	Vaterite–Calcite Conversion	Author
425–550	550–950			Chang and Chen [52]
460				Papadakis et al. [53]
400–500	600–800			Papadakis et al. [54]
300–500	800-1000 unknown/chlorides			Huntzinger [55]
	>500			Huijgen et al. [56]—slag
450–650				Taylor et al. [57]
464(DTA)	850–950 calcite			Ramachandran et al. [58]
	600–750 poorly crystallinedls20 well crystallized			Cole and Kroone [59]
	827–927 calcite	~460	~350–400	Stern [60]

shows the temperature ranges in various studies.

Comparative thermal analysis (DTA) was the method of choice for analyzing compound compositions in earlier studies. A constant heating condition will cause a change in temperature in a sample. It would be an endothermic reaction if the rate of temperature increases decreased, and the reaction would be exothermic if there was an increase. An investigation of the properties of cement was published by Ramachandran et al. [58]. Around 850 °C is the temperature at which calcite decomposes [10].

The compounds in hydrated cement, however, are very diverse; under various conditions, concrete’s chemistry varies, and the chemistry of concrete as it ages is not completely understood. As a consequence, assumptions have to be made regarding the predominant species that emerge at different temperatures. Since these factors are unknown, the process of analyzing thermal decomposition using mass spectroscopy. The temperature-programmed desorption method (TPD) is used in these experiments [10]. After heating the sample again, it is subjected to TPD. The sample is passed over an inert stream of known gases. As a result, gas is released from the sample. Based on amperage for various molecular weights, the composition of the combined released and inert gas streams is determined by passing the stream through a gas analysis system. Once again, it is difficult to distinguish all species, but there is still a great deal of information that can be obtained [10].

Moreover, the composition of carbonized products can be determined with thermogravimetric analysis (TGA) in conjunction with scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The carbonated surface layer is composed of calcium silicate hydrate and CaCO₃ [61].

3.4. Applications and Findings of Carbonation Research

The amount of carbon species that are absorbed in relatively young cement pastes other than calcite does not exceed that in mature cement pastes. There is a slight increase in these absorbed species in samples that are exposed to air and have some additional moisture added. Moreover, calcium hydroxide and calcium carbonate may be present as “hydrated” complexes or absorbed species in cement pastes in addition to their solid phases [10].

A carbonated layer with a thickness of less than 1.6 mm was obtained after the CO₂ treatment, strengthened compressively by a small amount. However, the permeability of the water was significantly reduced. The mortars transmit water vapor and migrate chloride [61]. Additional protection could be provided by the CO₂ surface treatment. To begin with, an enhanced outer layer was created by the carbonated surface layer. Since the carbonated samples do not undergo carbonation, the embedded reinforcement bars are passivated with high alkalinity. In the second place, during surface CO₂ treatment, chloride ions might diffuse slower, while during weather carbonation, they may diffuse more quickly. Then, furthermore, the results of FTIR and TGA indicate that CaCO₃ and silica gel were not completely formed from CSH after CO₂ treatment. Among C-S-H compounds, only Ca/Si decreased. Therefore, there are still some C-S-H gels left on the market. During early hydration, mostly outer CSH is formed, which is more porous and has a higher Ca/Si ratio [43]. It is possible that this C-S-H could react more easily with CO₂ and produce less porous products. This reaction resulted in a decrease in total solid volume; however, several more hydration products were used to fill up the space [61].

4. The Methods Increasing Concrete’s Ability to Adsorb Carbon

Carbon dioxide is mostly captured and stored through physical methods, methods based on chemicals and biology. CO₂ is mainly captured and stored physically by mechanical means, including injection of high-concentration CO₂ into underground rock formations (mined oil and gas wells, coal seams, and deep seas). The process can be used to capture and store large amounts of carbon dioxide at a relatively low cost, and it is suitable for large-scale energy storage [1,62]. If there is movement in the crust, there is a possibility of CO₂ seeping out from deep rock formations that have been stored for a long time; capturing and storing data is more expensive and riskier. CO₂ is captured and stored chemically through chemical reactions with chemical capture reagents (alkaline compounds or catalysts) to form alcohols, carboxylates, carbonates, etc. [63,64,65]. Compounds containing alkalis are usually used to capture CO₂ using hydroxides, alkanolamines, and carbonates. As an example, the hydroxide NaOH can be formed by capturing CO₂ to form Na₂CO₃. NaOH industries consume a large amount of electricity. China and the EU account for 5% and 3%, respectively, of the industrial sector [66]. There are several types of alkanolamines, including monoethanolamine (MEA). As a result, CO₂ emissions are captured and stored. A significant amount of energy is consumed during the regeneration process, however (about 80%) [67]. One ton of carbon dioxide is captured. Water-washing will cause MEA emissions ranging from 0.1 to 0.8 kg [68]. The human body can be harmed by NaOH and MEA [66,68]. CO₂ emissions associated with processes can be reduced through the use of chemicals for CO₂ capture, The carbon-capturing bacteria, however, consume more energy and pollute the environment more than they do. Carbon-capturing bacteria do not produce harmful by-products as urease-producing bacteria do [69]. There is no difference between the cultivation of carbon-capturing bacteria and that of UPB in terms of ease of cultivation. There is usually a cost associated with preparing CO₂ catalysts. Extreme conditions of use, several factors make catalytic storage conversion difficult at large scales, including low efficiency. Catalysts should be used at a limited scale. Methods used in biology, organisms and biological enzymes capture and store CO₂ and convert it into compounds such as glucose, carboxylic acids, and carbonates [9,70,71]. The carbon adsorption of different materials in literatures is summarized in

Table 2. Carbon adsorption of different materials.

Material	Carbon Adsorption	Method	Author
Activated carbons	The superior CO2 adsorption at 1 bar equal to 5.67 and 9.05 mmol/g, for 298 and 273 K, respectively.	The isosteric heat of adsorption calculated on the basis of the Clausius–Clapeyron equation and Sips model	Serafin et al., 2019 [72]
Zeolites	At 30 degrees, zeolite-Y (designated as Z-Y-3, silica to alumina ratio of 2.25) sample exhibited maximum adsorption capacity, and the obtained values were around 114 and 190 mg CO2/g sorbent under atmospheric and 5 bar pressure	CO2 capture capacity of the sorbents was examined at various temperatures and pressures employing a fixed-bed flow reactor and simulated flue gas.	Murge et al., 2019 [73]
Silica gel	The sorbent synthesized with Li2CO3 and the BFS-derived silica gel at the Li/Si molar ratio of 4 (Li2CO3-BFS-4) possesses the best CO2 uptake performance with a sorption capacity of 0.329 g CO2/g, sorbent within 20 min.		Liu et al., 2019 [74]
Metal–organic frameworks	The metal–organic frameworks (MOFs) are proving to be effective adsorbent material for CO2 capture due to their microporous structure.		Younas et al., 2020 [75]
Polyethylenimine	The utility of polyethylenimine (PEI)-implanted MIL-101 for dilute CO2 capture; adsorption capacity of 1.0 mmol/gat 400 ppm.	Temperature Vacuum Swing Adsorption	Darunte et al. [76]

.

4.1. The Physical Method

The economics and costs of process design must be considered when assessing a new material’s potential [19]: whether it is eco-friendly, low-carbon, and has a low production cost, which translates into high economic benefits for the company, resulting in a large adsorption capacity for CO₂. A biochar-based cementitious composite could also improve carbon sequestration performance through the use of biochar. By pore adsorption, biochar stores and captures CO₂ because of its high specific surface area and strength affinity for nonpolar substances [77].

Due to its many advantages, such as low cost, activated carbon (AC) is the best solid adsorbent. With a structure of pores that can be designed easily, an area of great size, it also regenerates easily [78]. There is a growing demand for activated carbon from abundant sources as sorbents in small-scale industries. There is a good case for using low-cost precursors like lignocellulosic biomass to reduce CO₂ emissions. In the production of activated carbon, agricultural waste can serve as a precursor [79]. There have been a number of examples where agricultural biomass has been used as precursor for carbon adsorbents for CO₂ adsorption, including molasses [80], walnut shells [81], corn cobs [82], date seeds [83], rice straw [84], and cotton stalk [85]. However, molasses, walnut shells, corn cobs, and rice stalks have not yet been studied in this manner [18].

Globally, activated carbon is estimated to be produced at 100,000 tons per year [50]. In the production of AC based on biomass, two typical activation methods are utilized: chemical and physical activation. The combination of chemical and physical activation might be a promising technology for improving the porosity and textural properties of ACs [86,87]. Biochar materials and related studies are shown in

Table 3. Biochar materials and related studies.

Biochar Materials		Studies	Author
Agricultural waste	rice straw	In the presence of pure CO2 gas, biochar had 2.6 wt% adsorption capacity	C.K.C. Cabriga et al. [88]
	corn stalks	It has been found that the biochar made from corn straw at 25 °C and 1 bar absorbs 4.51 mmol of CO2 per gram	Y.L. Zhou et al. [89]
	sugarcane bagasse	According to the studies conducted on sugarcane bagasse biochar, it has the capacity to adsorb CO2 at a temperature of 25 °C of 0.75 mmol/g	Christiano et al. [90]
	coffee grounds	With 30 °C adsorption temperature and a 30% CO2 concentration, coffee bagasse biochar could adsorb 2.8 mmol/g of CO2 at 600 °C pyrolysis	Alivia Mukherjee et al. [91]
Wood waste	palm husk cellulose	As a result of maintaining 3.3 mmol/g of carbon dioxide adsorption at 25 °C and 1 bar, it is clear that good carbon capture is possible	Guo et al. [92]
	date palm leaves	When date palm leaves were pyrolyzed to make biochar, they adsorb the maximum CO2 of 0.25 mol/kg	Salemet al. [93]
	palm kernel shells	At room temperature and standard atmospheric pressure, Promraksa et al. obtained a CO2 capture rate of 0.46 mmol/g using biochar produced from palm kernel shells	Promraksa et al. [94]
	bamboo	In their study, Waralee et al. used bamboo as the raw material for producing biochar, which absorbed 2.52 mmol/g of CO2 at 25 °C and a pressure of 1 bar.	Waralee et al. [95]
Domestic garbage waste	pine wood biochar	Biochar produced at 550 °C absorbed CO2 at a rate of 1.66 mmol/g.	Igalavithan et al. [96]
	paper mill sludge biochar	At 25 °C and 1 bar, 0.2 mmol/g of biochar produced from paper mill sludge was measured at 600 °C.	Igalavithan et al. [97]
	biochar made from 70% wood chips and 30% chicken manure	A combination biochar made from 70% wood chips and 30% chicken manure displayed 1.75 mmol/g CO2 adsorption capacity	P.D. Dissanayake et al. [98]
	pure wood chip biochar	Pure wood chip biochar had an adsorption capacity of 1.48 mmol/g at 30 °C and 1 bar of pressure

.

The amount of biochar added to cementitious composites affects their compressive strength. Regardless of the ratio of water to cement, adding biochar to cement mortar increases its early strength [52]. The critical amount of biochar is usually between 4% and 6% and can enhance or reduce the composite’s mechanical properties depending on how much biochar is added [99,100]. An increase of strength was observed at 7 and 28 days following the addition of 2–3% biochar [101,102,103]. A composite’s mechanical properties are adversely affected when excessive amounts of the material are incorporated. By Tan et al., a biochar content of more than 5% was shown to accumulate, which will result in an increase in cracks and macropores and a decrease in hydration products [104,105,106]. Incorporating 2% mixed particle size biochar rather than cement at 3 and 7 days increased the mortar compressive strength compared to using a single particle size biochar mixture in a 1:1 ratio [107].

Strength of concrete: both water-cured concrete and dry-cured concrete could be strengthened by adding biochar from wood waste, A maximum of 5% of wood biochar could be incorporated, with a significant decrease in tensile strength when excessive amounts were used [106]. Biochar introduced pores into cement mortar, which resulted in a decrease in tensile strength. Due to inhomogeneity, the tensile plane is weaker in areas where biochar particle concentration exceeds 6.5% [108,109]. Concrete’s overall strength is reduced due to the aggregation effect of biochar particles, leading to local weak zones [110].

When woody biochar was added to cement paste, the maximum flexural strength gain was observed. As the biochar content increased, the flexural strength decreased [108]. Biochar’s porous structure introduced pores in the tensile plane, leading to a decrease in flexural strength. This phenomenon increases the cracking potential of cement by causing more weak interfacial weak zones and porous cement into which biochar has been incorporated instead of cement, at a concentration of 10% biochar. Despite the small decrease in flexural strength, at 15% doses, the strength significantly decreased by 12.9% [106,111].

Additionally, hemp concrete in comparison to Portland cement has been shown to have a lower ecological impact. An essential part of hemp concrete is the use of lime-based binder mixed with hemp fiber. The aggregates in this type of concrete are cellulose matter (hemp, in this case), which suggests that the aggregates are made from cellulose matter. A lot of attention has also been drawn to LHC as a material that emits less carbon dioxide than it absorbs (carbon negative) [112]. Increasing densities of samples result in increasing compressive strength directly proportionately. Material density contributes significantly to the material’s mechanical strength, although the strength of the material is a function of the binding-to-hemp-mass ratio in conjunction with the water-to-binding-mass ratio [112].

The LHC has a negative impact on carbon; LHC absorbs more carbon dioxide during its manufacturing cycle than it produces [112]. By undergoing photosynthesis and carbonation, the slaked lime present in the mixture absorbs carbon dioxide through carbonation. It is possible for lime mortars to sequester carbon dioxide during their service life since they undergo a long process of carbonation [112].

Cement-based material carbon sequestration happens mainly through the two carbon capture and carbon sequestration stages. Carbon sequestration with carbonization reaction as the main way has been studied a lot, but there has been little attention to carbon capture performance. However, as the first stage of carbon sequestration, carbon capture is the premise of carbon sequestration and determines the maximum amount of carbon sequestration. By adding porous biochar into cement-based materials and changing the pore structure to enhance the transmission [24], the carbon capture performance is effectively improved, thus enhancing the carbon fixation capacity of cement-based materials.

To improve the carbon capture capability of building materials by considering the pore structure combination of biochar and building materials, this work is designed to determine an effective pore structure combination. In order to design pore structures for target materials, a model that changes pore structure connectivity is applied. As a result, its mechanical properties are improved through the application of a layered structure. Based on the results of the study, it appears that the pore structure connectivity changing model is applicable. Increasing the ratio of water to cement from 0.25 to 0.4 will result in an increase in the cost of building material from 10% to 148%, and biochar increases the effectiveness of carbon capture by target materials. As biochar’s dosage or size increases, its improvement capacity increases as well. A layered structure with a water-to-cement ratio of 0.25 also increases compressive and three-point flexural strength by, respectively, 44% and 28% when compared with a regular structure [24].

4.2. The Chemical Method

To improve the efficiency and effectiveness of the process, the most important step in the process of capturing and utilizing carbon (CCU) is selecting the absorbent. Ammonia was tested to see if it could intercept and absorb CCU as a liquid during the process [113]. The following are the mechanisms for constructing a recyclable CCU process from gypsum and ammonium sulfate used in fertilizer manufacturing [113].

The basic carbonation mechanism from Equations (11)–(14) illustrates the basic concept of the CCU process. H₂CO₃, ${\mathrm{HCO}}_3^{-}$, and ${\mathrm{CO}}_3^{2-}$ are dissolved carbon dioxide forms that are segregated according to pH. A metal carbonate can be formed from carbon dioxide. For precipitated calcium carbonate (PCC), carbonation reaction can be applied in accordance with the basic carbonation mechanism. Equations (15)–(17) show how ammonia absorbent works. Figure 7 shows a flowchart of the mechanisms presented above [113].

• Basic carbonation mechanism

$${\mathrm{CO}}_2\leftrightarrow {\mathrm{CO}}_2\left(\mathrm{aq}\right)$$

(11)

$${\mathrm{CO}}_2\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{HCO}}_3^{-}$$

(12)

$${\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{CO}}_2\left(\mathrm{aq}\right)\leftrightarrow {\mathrm{H}}_2{\mathrm{CO}}_3\leftrightarrow 2{\mathrm{H}}^{+}{+\mathrm{CO}}_3^{2-}$$

(13)

$${\mathrm{M}}^{2+}+{\mathrm{CO}}_3^{2-}\leftrightarrow {\mathrm{MCO}}_3$$

(14)

2.

Mechanism applied to ammonia absorbent

$${\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{NH}}_4{\mathrm{HCO}}_3\left(\mathrm{aq}\right)$$

(15)

$${\mathrm{NH}}_4{\mathrm{HCO}}_3+{\mathrm{CaSO}}_4\left(\mathrm{imp}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)+{{(\mathrm{NH}}_4)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}$$

(16)

$${{(\mathrm{NH}}_4)}_2{\mathrm{SO}}_4+{\mathrm{Ca}\left(\mathrm{OH}\right)}_2\to {\mathrm{CaSO}}_4\left(\mathrm{p}\right)+2{\mathrm{NH}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(17)

As a liquid absorbent, ammonia was used as part of this research to reduce carbon dioxide usage. It was determined that ammonia has the potential to act as an excellent absorbent after investigating the possibility of each step. Despite precipitated calcium carbonate’s calcite crystal structure, it was formed under norm pressures and temperatures, and vaterite was a major structure of PCC formed in this research. The recycled ammonia solution was used again to absorb gypsum dihydrate, although the absorption performance was slightly lower than the initial state [113].

A cementitious material rich in calcium absorbs CO₂ well, so it does not need to be augmented with additional absorbents. Figure 7 illustrates how this concept can be implemented using a two-step mixing method. Cementitious materials are prepared by mixing calcium-rich materials with dry ones. A second step produces concrete by adding OPC, aggregates, and admixtures to the slurry and reacting bubbled carbon dioxide with calcium ions. A calcium-rich cementitious material is carbonated with CO₂ in this step of the proposed method. The pre-carbonation method refers to the process of carbonating concrete before it is poured [42]. SEM studies showed that pre-carbonation can produce nanoparticles and sub microparticles of CaCO₃ incorporating heterogeneous nucleation sites for OPC hydration. At the young age of cement manufacturing, isothermal calorimetry tests confirmed significant hydration [42].

4.3. The Microorganism’s Method

There are many types of research being conducted today on biological sequestration. Since this method is cheap and eco-friendly, it is being used to assess the effectiveness of removing CO₂ from the atmosphere [114]. Bio-mineralization is the process of making calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃) from CO₂ by bacteria that produce carbon anhydrase (CA) and urease enzymes [115]. As a result, soil bacteria sequester CO₂ as a result of their actions. Nevertheless, soil chemical properties can negatively affect bio-sequestration [116]. Thus, inoculating bacteria into a system of sequestration might accelerate the process [1].

A calcium and magnesium ion solution can adsorb CO₂ and transform it into carbonates. Energy efficiency is another aspect of the mineralization/carbonization process, in addition to being environmentally friendly. Materials such as cement, steel slag, and limestone can be mineralized/carbonized to provide calcium and magnesium. Carbon-capturing bacteria and CO₂ are used to capture carbon from waste concrete. This microbial process can increase the mechanical properties of recycled concrete aggregates, as well as making cement-based materials more resistant to efflorescence. It is essential to suppress dust on construction sites. By utilizing these potential applications, CO₂ can be used as a reference value in construction materials. When bacteria capture carbon dioxide, they adsorb and convert it into carbonates. A construction material’s properties can be improved through bio-mineralization or carbonization.

The process of biomineralization/carbonization is environmentally friendly. Recent advances in concrete surface treatment have been made by calcium carbonate precipitation owing to its good stability. Mineralization can be induced either by bacterial action or by hydrolysis of dimethyl carbonate (DMC). The bacterial carbonate precipitation technique has been shown to be effective in a number of studies [117,118,119]. Compared to untreated glass, it decreased water absorption by 20–80%, gas permeability by 17–33%, and chloride permeability by 39–51%. Bacterial treatment significantly reduced porosity in mortar, as demonstrated by Achal et al. [61,117].

Construction materials can be converted into cementitious materials by carbon-capturing bacteria. This can improve mechanical properties and aesthetics, while sequestering carbon at the same time. Solid wastes and cement-based materials contain MgO and CaO that can well consolidate CO₂ [9].

5. Conclusions and Prospect

(1)

Carbon dioxide has been sequestered in carbonating cement materials for a cumulative amount of 4.5 GtC since 1930. Over the same period, this project offset 43% of cement’s CO₂ emissions. It does not include the fossil fuel emissions generated during cement production. The reverse process, carbonation in concrete, balances the release of carbon dioxide. Therefore, the significance of carbon sequestration by applying cement-based materials is that carbon neutrality of the cement industry can be achieved by the properties of cement itself.

(2)

Calcite sequestration into concrete is affected by various factors, including carbonation and acceleration. Temperature, relative humidity, and CO₂ concentration are some of these curing conditions. Furthermore, solid physical characteristics as well as the material chemical properties affect carbonation. The aerated concrete carbonation rate is influenced significantly by concrete density and water–cement ratio (w/c), particularly by density and w/c ratio.

(3)

Carbon sequestration of materials mainly goes through two stages: carbon capture and carbon storage. As the first stage of carbon sequestration, carbon capture is the premise of carbon sequestration and determines the maximum amount of carbon sequestration. By optimizing the porous structure of cement-based materials to enhance the transmission to effectively improve the carbon capture performance, it can effectively enhance the carbon sequestration capacity. Carbon sequestration with carbonization reaction as the main way has been studied a lot, but there is little attention to carbon capture performance. Therefore, enhancing the carbon sequestration capacity of cement-based materials by increasing the total amount of carbon sequestration needs more attention, which can become an important research direction in the field of carbon sequestration by cement-based materials.

(4)

The future work in capturing carbon dioxide in concrete can be conducted based on the percolation theory and by changing the pore structure of concrete to improve the carbon sequestration capacity. The research direction includes changing the pore structure of concrete itself and changing the pore structure of concrete by adding porous materials.