Direct carbonation of porous materials produced from self-hardened paper mill fly ash

A high-Ca fly ash generated from a paper mill using a fluidized bed boiler was used as the potential medium to sequester CO 2 via the direct carbonation process. In this study, the fly ash samples were exposed to accelerated carbonation to produce hardened pastes. The influence of the porosity controlled by the amount of foam in the matrix for maximizing CO 2 sequestration was determined. The effect of porosity on carbonation was evident. The samples prepared with 40 % foam and a water-to-fly ash ratio of 0.79 recorded porosities of 56.1 % (carbonated) and 66.8 % (naturally carbonated (NC)) with the lowest mechanical strength of 2.1 and 1.1 MPa, respectively. These naturally carbonated (NC) samples have been cured naturally without using high CO 2 pressure. Meanwhile, the samples prepared with 10 % foam and a water-to-fly ash ratio of 0.5 recorded porosities of 41.1 % (carbonated) and 52.9 % (NC) with the highest strength at 10.2 and 4.6 MPa, respectively. The highest carbonation efficiency determined by thermogravimetric analysis was obtained from the carbonated samples with 40 % foam. Scanning electron microscopy – energy-dispersive spectroscopy depicted the areas with high Al/Si content but low Ca – O content (dark areas) and carbonated areas with high Ca – O content (bright areas). Mercury intrusion porosimetry was used to determine the pore size distribution, whereby the samples were dominated by large capillary pores, resulting in their high porosity in this study.


Introduction
In the last century, the increasing concentration of CO 2 in the earth's atmosphere has resulted in the greenhouse effect and climate change.CO 2 emissions from industrial activities, such as cement and steel manufacturing and power plants, were found to be crucial contributors of the greenhouse effect [1].In particular, the high-energy and CO 2 -intensive process of ordinary Portland cement (OPC) production account for nearly 9 % of the anthropogenic CO 2 emissions [2] with the emission of approximately 0.9-T CO 2 for 1-T OPC produced [3].More so, the estimated increase in cement consumption [4] poses a serious necessity to cut down the CO 2 footprint of cement production.Therefore, various industrial solid byproducts have been investigated by researchers as supplementary cementitious materials (SCMs) for the construction industry.In addition to the cementation solutions offered by these industrial wastes to mitigate the high CO 2 emissions from cement manufacturing, their utilization can significantly alleviate landfill disposal costs and large space accumulation.In this regard, the advancement and characterization of SCMs, such as steel slags [5], blast furnace slags, fly ash, silica fumes [6,7], and biomass fly ash, red mud [8] and alum sludge [9], have been progressively studied.Further, geopolymers, alkali-activated materials [10,11], and other cement-free binders [12] have been analyzed.
To achieve significant CO 2 emission reduction by using industrial solid wastes, such as SCMs, a viable option with great potential and flexibility is the use of carbon capture and storage or carbon capture and utilization (CCU) technologies [13].These technologies can be used to sequester CO 2 via mineral carbonation, which refers to the reaction of CO 2 with Ca-and/or Mg-containing materials to form marketable final and stable products, i.e., carbonates [13,14].CO 2 sequestration of waste is a circular economic approach that can be directly performed using available CO 2 point sources, making it an attractive solution [13].Moreover, it is a more permanent and safe method for storing sequestered CO 2 compared to geological storage, which needs monitoring [14].Mineral carbonation is mainly performed with alkaline industrial residues, such as fly ashes from furnaces [8,15], slags from the steel and iron industry [16,17], mining waste [18], red mud [19], concrete wastes from the cement industry, and cement kiln dust [20,21].The amounts of oxides (CaO/MgO) and hydroxides (Ca(OH) 2 /Mg(OH) 2 ) are important factors that determine the CO 2 capture capacity of a material during carbonation [22].
Fly ashes commonly contain reactive alkaline oxides, such as CaO and MgO, which makes it an ideal feedstock for CO 2 sequestration [23].In addition, its pozzolanic and hydraulic properties enhances the performance of fly ash paste compared with OPC pastes [24,25].The slow pozzolanic reaction of fly ashes alone can be valorized utilizing increased temperature [26] in the carbonation process where early strength can be achieved due to carbonation reaction, and late strength due to the pozzolanic reaction, both improving the durability performance [27].
PMFA produced by the paper industry, is a by-product obtained from the incineration of paper and pulp mill sludges.Paper wastes generated annually in Europe is about 11 million tonnes and paper mill sludge is over 4.7 million tonnes [28].PMFA mostly consist of Ca, Al, Si, Mg, and K as oxides, these are generally found in PC and in common SCMs, hence, they significantly have the potential to be used as SCMs.Incineration of paper mill sludge is usually carried out to reduce the volume of landfilled solid waste, but in the process, due to the nature of the fuels during co-incineration and the other types of additives used, the properties of the ashes are significantly affected [29].Hence, there is a significant need for the development of contemporary utilization techniques [30].Therefore, using fly ash in CO 2 capture via mineral carbonation mitigates CO 2 emissions and enhances the stability of fly ash, thereby increasing its application in the production of construction materials.It is important to add that the global warming potential of paste can be reduced by 19.47 % when about 20 wt % paper mill fly ashes is added as a SCMs [25] Fly ash can be carbonated via direct or indirect mineral carbonation.Direct carbonation can be carried out through the dry route (gas-solid reaction), as shown in Eq. ( 1), in which the liquid-to-solid ratio (L/S) is less than 0.2, or the aqueous direct carbonation (gas-liquid-solid multiphase reaction), which is carried out with the L/S of more than 0.2.The aqueous route enables the dissolution of some part of gaseous CO 2 in water, forming carbonic acid, which ionizes further into H + , HCO 3 -, and CO 3 2− (Eqs.2-4) and consequently, enhances the mineral dissolution and carbonation [31].The enhanced carbonation is attributed to the water, which aids in the extraction of Ca or Mg ions from the solid matrix of the fly ash particles [32][33][34].Further, carbonation is carried out in different methods, either by direct CO 2 curing or introduction at a room-or elevated-temperature-cured paste or paste sample.
Porosity and pore size distribution have been attributed to the accelerated carbonation rate of hardened cement paste (Wang et al., 2019).High-porosity concretes undergo simultaneous carbonation that penetrate the surface and inner pores.However, the inhibited CO 2 diffusion into the pores of low-porosity concrete results in the carbonation reaction on the surface only.A lower porosity, which is usually caused by a low L/S ratio, results in a low CO 2 permeability and consequently, an increase in the carbonation resistance, thereby decreasing the depth and carbonation rate [36].At the L/S ratio of 0.4, the porosity increase by 20 % when cement is replaced with 50 % fly ash [37].Moreover, this ratio results in the highest strength of self-hardened fly ash compared to other water-to-fly ash ratios [8].Therefore, an increase in porosity is required to enhance the fly ash carbonation and produce porous lightweight cementitious material.Recently, lightweight cementitious materials have been broadly utilized in concrete infrastructures because of their main advantage of reducing the dead load in structures, along with different properties of compressive strength, heat conservation, density, and noise absorption, which are associated with their pore characteristics [38].However, the effect of the porosity of self-hardened fly ash pastes on their carbonation efficiency remains unknown.
Therefore, this study investigated the effects of porosity on the carbonation efficiency to showcase a valorization pathway for paper mill fly ash (PMFA) to produce lightweight construction materials.An initial water-to-PMFA ratio of 0.4 and superplasticizer were used for all matrixes.A foaming agent was used to produce foam.Different amounts of foam were added to the paste to generate different porosities.Each matrix mixture was cured in ambient temperature for 24 h and carbonated for another 24 h using the same dose of CO 2 , temperature, and pressure.The porosity, compressive strength, and carbonation efficiency of the samples were compared.The microstructural properties, porosity, and pore size distribution were investigated using an He pycnometer and mercury intrusion porosimetry (MIP) to fully establish the effects of porosity on the carbonated samples.
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Materials
PMFA made up of 75 % sludge from the paper-making industry was used in this study.The fuel mixture (mass basis) comprises 75 % paper sludge and 25 % paper rejects.PMFA was collected from the first electrostatic precipitator unit of a power plant of a paper manufacturing company with a 240 MW bubbling fluidized bed boiler.A tumbling ball mill was used to ground the fly ash.Pastes were prepared by mixing ash with deionized water, and a polycarboxylate-based superplasticizer was used to reduce the amount of water.Foam was generated using an ionic surfactant, namely sodium dodecyl sulphate (SDS), in a foam generator (Eabassoc Junior).

Characterization of the raw materials
A laser diffraction technique (Beckman Coulter LS 13320) with a Fraunhofer model was used to measure the particle size distribution of the PMFA samples with a median particle size (d 50 ) of 3.5 µm as shown in Fig. 1(a).The reactivity of SCM is improved by its extensive distribution and increased fraction of fine particles, giving rise to high specific surface area, thus, increasing the reactivity [25].The chemical composition of the pressed pellets was determined by X-ray fluorescence (XRF, Omnian Pananalytics Axiosmax 4 kV).The loss on ignition (LOI) was determined with PrepAsh, Precisa.Free lime (CaO) which showed the amount of dissolved CaO within a boiling time of 3 h in a mixture of butanoic acid, 3-oxo-ethyl ester, and butan-2-ol was also measured (Table 1) according to EN 451-1:2017.

Paste preparation, curing, and carbonation conditions
A water-to-PMFA ratio of 0.4 was used for all mixtures.The PMFA pastes were prepared by mixing specific amounts of PMFA with water and the superplasticizer for 2 min at high speed (1200 rpm) using an IKA Eurostar 20 high-shear mixer.The fast-hardening paste was immediately cast in 20 mm × 20 mm × 80 mm prismatic molds, jolted for 60 s, and covered with a plastic film.
For the samples with the SDS foam, the surfactant was mixed with water.Foam was generated using the Eabassoc Junior foammaking machine.Different amounts of foam was added to the paste and mixed with a shear mixer at a lower speed (400 rpm) for 1.5 min before casting.The information on the pastes are shown in Table 2.The samples without foam are identified as 0 %, whereas the other samples were identified with their corresponding foam content, i.e., 10 %, 20 %, and 40 %.
All samples were sealed in a plastic film and cured for 24 h at room temperature.Subsequently, they were weighed and divided into two groups.One of the sample groups was carbonated for 24 h in a carbonation chamber under 20 % CO 2 , relative humidity of 80 %, and temperature of 40 • C, to correlate with reported carbonation efficiencies where at the range of 60-80 % relative humidity, carbonation was reported to be more significant [39], while at the range 2-20 % CO 2 concentration, increase in the depth of carbonation was reported to be high [40], and between 20 and 50 • C, carbonation rate increased [41].Meanwhile, the reference group was kept for 24 h in an oven at the same temperature and humidity condition as those of the carbonation chamber.After 24 h, both groups were removed from their respective chambers and weighed again.Subsequently, they were sealed and stored for the 7-and 28-day strength test.After the strength test, the samples were ground in a paste and dried in an oven at 40 • C for further analysis.
The generated foam comprise a specific amount of water.In particular, the 10 %, 20 %, and 40 % samples contain 17. A. Ezu et al.
70.8 g water, respectively, which are equivalent to the water-to-PMFA ratio of 0.50, 0.60, and 0.79 respectively.

Weight variations
The mass of the samples were obtained immediately after demolding and carbonation.The weight variations were observed using Eq. 5.

Mechanical strength
The compressive strength of the paste samples was tested at 7 and 28 days using a ZwickRoell Z10 universal testing machine with a maximum load capacity of 10 kN.The force speed was 1 mm/min, and an average of four broken halves were used to test the compressive strength.

X-ray diffraction (XRD) analysis
XRD analysis was performed using a diffractometer (Rigaku SmartLab).A quantitative analysis was performed using 10 % rutile (TiO 2 ) as the internal standard for the Rietveld refinement.The analysis was carried out under the following conditions: voltage of 40 kV, current of 135 mA, step width of 0.02 • , scanning speed of 4.1 • /min, scanning range 2Ɵ of 5 • -130 • , and Cu K-beta radiation.Phase identification and quantification were performed using Rigaku PDXL 2 software.

Scanning electron microscopy (SEM)
The morphology of the carbonated samples was investigated by SEM-EDS analysis (Zeiss Ultra Plus).The paste samples were carbon-coated, and an accelerating voltage of 15 kV using a backscatter electron detector and working distance of 8.5 mm was used to perform the energy dispersive spectroscopy (EDS) analysis and microstructural observations.

Porosity
The porosities of the samples were studied using an He-gas pycnometer (AccuPyc II 1340, Micrometrics) and mercury intrusion porosimetry.The He-gas pycnometer was used to determine the open and total porosities.The samples were dried in an oven for 24 h at 60 • C, and their geometric density [ρ g (g/cm 3 )] was determined using a caliper and analytical balance.With the He-gas pycnometer, the true densities [ρ t (g/cm 3 )] were determined using the crushed samples (<1 cm), whereas the apparent densities [ρ a (g/cm 3 )] were determined using the pulverized samples.The open and total porosities were calculated as MIP was performed using Micrometrics Autopore IV 9500 with a contact angle of 130 • and surface tension of 0.485 N/m.The applied pressure of 0.00068-414 MPa was used to determine the open porosity for the pore diameter range of 0.006-342 µm.

Effect of carbonation on the physical properties of samples
.

Weight and porosity
The weight of the paste samples decreased after 24 h (Fig. 2).With the increase in the water-to-PMFA ratio as the amount of foam increases, samples with higher foam amount became more porous because excessive water hinders CO 2 diffusion into the pores [42], resulting in a lower density.In the carbonated samples, evaporation of water was inhibited as the paste absorbed CO 2 which fills up the pores resulting in the formation of Ca carbonate from the reaction of free lime, thereby achieving higher density and strength with reduced porosity.While the NC samples (0 foam % -40 foam %) exhibited higher weight loss between − 16.3 wt % to − 32.9 wt % due to the evaporation of water from the samples as they undergo natural carbonation absorbing less CO 2 , resulting in the lower density and higher porosity.These findings are similar to the experimental results in previous studies [8,42,43].
The total porosity of the samples increases as the amount of foam increased (Fig. 3(a)).Compared to the NC samples, the total porosities of the carbonated samples was low because of the CO 2 diffusion, which blocked the pores as it forms dense CaCO 3 , resulting in low porosities and increased density (Fig. 3(b)).This indicates that CO 2 gas hardly penetrates samples with higher density [44].

Mechanical strength 3.3.1. Effect of carbonation on the mechanical strength
The 0 % samples exhibited an increase in strength with carbonation from 3.3 MPa for the noncarbonated sample to 6.4 MPa for the carbonated sample (Fig. 4).This increase in strength by 93.4 % implies that the CO 2 -rich atmosphere enhanced the Ca(OH) 2 conversion into CaCO 3 , thereby reducing the porosity and densification of the samples (Fig. 3).The effect of carbonation significantly showed increase in strength for 10 % carbonated samples recording the highest mechanical strength of 10.2 MPa.Furthermore, this increase in strength is evident from the comparison of porous carbonated and NC samples, suggesting that carbonation enhances the strength in the studied pastes.

Effect of porosity on carbonation
From the porosity and carbonation efficiency results, CO 2 uptake increases as the porosity increases [45].The effect of porosity on the samples is evident in both carbonated and NC samples.The mechanical strength of the 10 % sample increased after carbonation, achieving the highest strength of 10.2 MPa, compared to that of the reference sample with the L/S ratio of 0.4 (6.4 MPa), demonstrating a strength increase by 59.4 %.Despite the higher density of the reference carbonated sample than that of 10 % carbonated sample, the highest strength was still recorded for the latter because of its higher carbonation rate due to the increased porosity.This  effect could be attributed to the increase in the diffusion length of the carbonic acid H 2 CO 3 (Eqs.1-4) owing to the high water content [46].Therefore, the 10 % carbonated samples with the total porosity of 52.7 % and water-to-PMFA ratio of 0.5 is considered the optimum sample with the highest strength.A similar water-to-binder ratio has been previously used in the accelerated carbonation of fly ash concrete, resulting in an enhanced carbonation depth [47].
Comparing the carbonated and NC samples with foam, despite the increase in porosity, carbonation increased the strength of the samples.However, the increase in the foam content to 20 % and 40 % increased the porosity, which weakened the structure.Due to the large number of pores, the amount of precipitated calcite could not fill up all the pores, resulting in further decrease in strength from 5.31 MPa to 2.12 MPa for 20 foam % and 40 foam % carbonated samples, respectively.This indicates that enhancement of strength is dependent on the porosity of the material [48].

X-ray diffraction (XRD) analysis
The XRD pattern of the carbonated and NC samples reflects the significant changes during carbonation (Fig. 5(a) and (b)).The reference sample, which is the starting material of the carbonated samples, has multiple peaks specific to calcite phases with few amorphous phases.The XRD results in Fig. 5(a) indicates the complete reaction of portlandite to promote the formation of calcite, as shown in the dominant calcite peak at 2θ of ~34 • .Although an obvious calcite peak is provided, portlandite (Ca(OH) 2 ) peaks are still     visible in all the samples, as shown in Fig. 5(b), indicating partial reaction of CaO.The appearance of hydrocalumite, as obtained in similar studies, indicates a reaction between chloride and Ca aluminate hydrate [49,50], which inhibited the mobilization of chloride ions and discharged the hydroxyl ions [51], thereby forming unreacted Ca hydroxide (portlandite) due to the incomplete carbonation [52] (Fig. 5(b)).While decomposing during the accelerated carbonation [53], hydrocalumite filled the pores of the particles in the hardened paste, resulting in further compaction and strength increase [54], as observed in the carbonated samples.These phases suggest that the samples require a slightly longer exposure for effective carbonation.
The quantification of the calcite phase showed the significant effect of porosity on the accelerated carbonation during calcite formation, compared to the natural carbonation in the NC samples.Compared to the amount of calcite in the raw material (16.2 %), the increase in calcite formation is evident in the 10 % samples, whereby the highest increases of 214 % for the carbonated samples and 57 % for the NC samples as shown in the quantitative XRD (Table 3).This correlates with the strength results, whereby the 10 % sample significantly increased in the calcite content, thereby achieving the highest densification of the carbonated samples and consequently, the highest strength gain.

Thermogravimetric analysis
TGA provides additional information on the reaction products and XRD data.Similar to previous studies [32,55], peaks attributed to the water loss in the hydration products (C-S-H) or Ca aluminate hydrate (C-A-H) are noted in the temperature range of 100 • C-130 • C. Within this range, the effect of carbonation is evident owing to the more pronounced peak of the NC samples, which could be attributed inhibited Aft, CSH, and CAH production or their phase transition under the CO 2 -rich environment for the carbonated samples [8].The carbonated samples showed no further mass drop until 500 • C-850 • C owing to the decomposition of the carbonates [8,56] and release of CO 2 .At temperatures below 160 • C, the weight losses of C-S-H and hydrocalumite in the NC samples overlapped with each other.The weight loss at 237 • C-360 • C can be attributed to the hydrocalumite dihydroxylation [57].The original PMFA (Ref) as shown in both Figs.6(a) and 6(b), carbonated naturally to some extent, but the carbonation degree in both figures was significantly higher.The result is a reflection of the free CaO content of the original PMFA (Table 1), indicating that the higher the free CaO content, the higher the degree of carbonation.These results confirm the identified phases in the XRD analysis.

Carbonation efficiency and CO 2 uptake
The CO 2 uptake and carbonation efficiency (Table 4) were calculated by evaluating the mass loss corresponding to the Ca carbonate decomposition in the temperature range of 500 • C-850 • C using TGA/DTG.Assuming that all Ca-containing minerals in the sample carbonates eventually, the maximum theoretical capacities for CO 2 sequestration were quantified from the TGA and XRF results (Table 1).As the porosity increases, the CO 2 uptake (Fig. 7) and carbonation efficiency (Fig. 8) increase.Eq. ( 9) [58], (10) [59], and (11) [60] were used to determine the values in Table 4. A. Ezu et al.   where: Δm CO2 is the mass loss under the decomposition of CaCO 3 (500 • C is the dry weight of the sample at 105 • C; MCaO is the molecular weight of CaO (56 g/mol); MCO 2 is the molecular weight of CO 2 (44 g/mol); CaO total is the weight fraction of CaO in the fresh fly ash (55.46), as provided in Table 1.sample achieved the highest sequestration capacity of 207.1 kg CO 2 per t PMFA and carbonation efficiency of 74.4 %.This demonstrates that the increased porosity in the 40 % samples achieved the effective diffusion of CO 2 that penetrated both the surface and inner pores, which resulted in the formation of samples with low density, thereby a higher CO 2 uptake with lower strength is achieved due to the inability of the calcite precipitates to fill up all available pores.

SEM-EDS
The effect of porosity on the carbonation of the samples is evident in their microstructure, as shown in the SEM-EDS images in Fig. 9 (a)-(f).The dark areas represent high Al/Si and low Ca-O content, and the bright areas area carbonated areas with a high Ca-O content.In addition to the higher content of heavier elements, such as Ca, in the bright areas, the brightness can also be ascribed to its high density (Fig. 9(b) and (e)) for the 10 % NC and carbonated samples, respectively.These images, particularly Fig. 9(e), exhibits the highest brightness among all samples, depicting an evenly distributed Ca carbonate, thereby confirming the highest mechanical strength of the 10 % carbonated sample.The images also correlate with the porosity results for the 40 % samples, as shown Figs.9(c) and 8(f), which have higher porosity and a larger number of black pores filled with epoxy resin.The 0 % (Fig. 9(a) and (d)) and 10 % (Fig. 9(b) and (e)) samples have less pores.Fig. 9(a) depicts a developed network of wider cracks upon carbonation, as shown in Fig. 9 (d), which could be the resultant effect of the C-S-H gel shrinkage during decalcification and successive silicate condensation [61].
The effect of porosity is well depicted in the foam samples in Fig. 9, which have more pores that facilitate the high precipitation of Ca carbonate, thereby achieving the highest CO 2 uptake and carbonation efficiency.Moreover, the effect of this porosity reduced the density and strength owing to the pores left unfilled by the precipitated Ca carbonate.

Mercury intrusion porosimetry (MIP)
Fig. 10(a) and (b) show the relationship between the cumulative pore volume and pore diameter in the range of 0.001-1000 µm for the carbonated and NC samples respectively, to yield the pore size distribution measured by MIP.According to Zhang and Islam [62], pores of the size of 10-0.05 µm are classified as large capillary pores and those of 0.05-0.01µm are classified as medium capillary pores, whereas pores sized less than 0.01 µm are gel pores.Based on this classification, the samples are dominated by large capillary pores, resulting in their high porosity (Fig. 3).Generally, macropores, which are known to be pores larger than 0.05 µm, play a crucial role in the mechanical properties and mass transfer resistance of concrete [63], resulting in the reduced mechanical strength (Fig. 4).This is also evident in the large capillary pores shown in the SEM images (Fig. 9(c) and (f)), resulting in their notable low mechanical strength compared to the other samples.The trend of the pore size is consistent with that of the porosity of the samples.In particular, the total cumulative intrusion increase for the 10 % and 40 % samples indicates the increase in porosity with similar tendencies for both the carbonated and NC samples.
As shown in Fig. 10(a), all samples exhibit a significant decrease in the total intrusion due to carbonation, which caused a notable reduction in the pore concentration, indicating that CaCO 3 is advantageous for pore modification [64].This result is consistent with the increased mechanical strength shown in Fig. 4. Fig. 10(a) and (b) demonstrate the highest CO 2 uptake and carbonation efficiency in the samples with 40 % foam due to their high number of pores, which facilitated the precipitation of Ca carbonate.However, the uneven distribution of these pores suggest their excessive content for the precipitated Ca carbonate, thereby reducing the density and consequently, strength.

Environmental aspect and economic advantage
Fluidized bed combustion (FBC) fly ashes with CO 2 sequestration potential contain 10 %-42 % CaO, which are similar to recorded value in this study.Theoretically, 1-T fly ash with 55.46 % CaO could sequester 440 kg CO 2 .This indicates only 3-T high-Ca fly ash is needed to sequester 1-T CO 2 , if the carbonation efficiency is 100 %.From the experimental results in this study, the highest CO 2 sequestration was 207.1 kg CO 2 per t fly ash.In other words, 4.8-T fly ash is needed to sequester 1-T CO 2 .Similarly, Ohenoja et al., 2020 [8] noted that approximately 150 kg CO 2 per t fly ash could be sequestered successfully, which is equivalent to 6.7-T fly ash to sequester 1-T CO 2 , during fly ash self-hardening.
The FBC fly ash generated annually amounts to half a million tons in Finland, several million tons in Europe, and 14 and 15 million tons in USA and China, respectively [65,66].Therefore, the elimination of landfill disposal of these ashes can be largely achieved by utilizing them in CCU and as construction materials, considering that the requirements of the construction products for earth construction, such as a compressive strength of 7 MPa, can be achieved.Moreover, through carbon sequestration, the price of the European emission trading, which is currently at 100.34 €/t CO 2 [67], could be greatly reduced to save millions of euros.
Although this study is in line with other studies [35][36][37], whereby the increase in porosity decreased the mechanical strength and increased the CO 2 uptake and carbonation efficiency for both the carbonated and NC samples, NC samples with higher porosities than those of their corresponding carbonated samples could achieve higher CO 2 uptake and carbonation efficiency.However, carbonation became eminent as the carbonated samples display a wide margin of increase in these parameters compared to the NC samples, as shown in Table 4.This indicates the high carbonation potential for industrial wastes with high Ca contents.
Overall, this study provides a pathway for producing porous samples to enhance carbonation efficiency while producing highstrength lightweight cementitious materials that can be broadly utilized in concrete infrastructures.Therefore, as technology advances, the use of a high-resolution 3D tomographic image from X-ray computed tomography could be adopted to achieve more explicit analytical results in the characterization of porous cementitious materials.Additionally, the relationship between porous cementitious materials and their unique characteristics, such as heat conversion, water drainage, and noise absorption, is a useful topic that should be considered.Further research is also necessary to fully evaluate the utilization of this method in the carbonization of large-scale alkali cementitious materials.

Conclusion and recommendation
In this study, based on the carbonation efficiency, the porosity of the samples has increasing effect on CO 2 uptake.The increase in the amount of foam from 0 % to 40 % increased the total porosity from 28.84 % in the 0 % carbonated samples to 41.08 %, 48.15 %, and 56.10 % for the 10 %, 20 %, and 40 % carbonated samples, respectively, whereas NC samples exhibited similar tendencies of increased porosity of 42.93 % in the 0 % NC sample to 52.86 %, 58.41 %, and 66.84 % for the 10 %, 20 %, and 40 % NC samples, respectively.The maximum carbonation efficiency (74.4 %) was obtained in the 40 % carbonated samples with a porosity of 56.10 %, demonstrating that an increase in porosity is essential for effective CO 2 sequestration.Therefore, the optimum porosity of 56.10 % was noted for the effective carbonation in this study.Moreover, these results showed that carbonation reduced porosity, thereby enhancing the mechanical strength.The 10 % carbonated samples achieved the highest mechanical strength with a porosity of 41.08 % and water-to-PMFA ratio of 0.5, which are considered as the optimum conditions for strength enhancement.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 2 .
Fig. 2. Weight change of the carbonated and NC samples.

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Fig. 3 .
Fig. 3. Effect of carbonation on the (a) total porosity and (b) density.

Fig. 4 .
Fig. 4. Compressive strength of the carbonated and NC samples.

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Table 2
Composition of the PMFA paste mixtures.
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Table 4
Carbonation efficiency and CO 2 uptake of the pastes with different porosities.