Experimental study on characterization of lime‑based mineral carbonation reaction and CO 2 sequestration

This paper reports for the first time the use and application of a novel technique in the characterization of mineral carbonation reaction and CO 2 sequestration in soil stabilization using flow meters. Soils based on SiO 2 with two different sizes were tested. Lime (Ca(OH) 2 ) was used as the reactant. Instant CO 2 flow rate (L/min), total CO 2 volume (L), temperature (°C), and absolute pressure (kPa) were monitored and recorded for 1 h by flow meters connected to the mold inlet and outlet. It was determined that the mineral carbonation reaction started in the first seconds and ended before the 5th minute. The mineral carbonation is a short-term and potential reaction, and it is not a time-dependent reaction. It is separated from other car-bonation reactions with these characteristics. The highest CO 2 captured value was obtained in the soil mixed with 5% lime, where fines were not used. The second highest CO 2 captured value was obtained in soil mixed with 1% lime, where fines were not used. CO 2 captured with 1% lime is more than CO 2 captured with 5% lime in the soil containing fines. Accordingly, 1–5% lime can be used in soil carbonation studies. According to the soil properties, the highest CO 2 captured and the CO 2 efficiency was achieved with the use of 6–7% water by weight.


Introduction
As a result of wars, population growth, industrialization, farming, oil and hot water wells, and mining activities, an increasing amount of CO 2 is being released into the atmosphere.It is very important to decrease the CO 2 level for the continuation of life.Political, social, economic, and scientific studies are carried out to reduce C emissions all over the world (Mohammed et al. 2021).Successes and problems have been shared in CO 2 sequestration and CO 2 storage research (Yadav and Mehra 2021;Sargent 2015).How much carbon is sequestered in a sequestration method, and the cost and feasibility of the method are the main research questions.
Minerals based on Ca, Mg, and Fe, their industrial products such as lime (portlandite, brucite), mining and industrial wastes, and underground are commonly used in the carbonation reaction and CO 2 sequestration (Chunyang et al. 2022;Yadav and Mehra 2021;Agrawal and Mehra 2016).As a result, a carbonate mineral is formed.Ca and Mg are preferred because they are widely found in nature and have lower handling costs, relatively.They are good flocculators ions with larger hydrated radii and higher valences (≥ 2 +) (Sumner and Naidu 1998).However, magnesium has a larger capacity for CO 2 sequestration, and it has a greater potential to increase soil strength than calcium (Mohammed et al. 2021;Yi et al. 2016;Sivrikaya et al. 2014).Yadav and Mehra (2021) have drawn the current CO 2 sequestration routes in detail.They have also detailed the mineral carbonation routes that are the subject of the study.The carbonates are thermodynamically more stable than their primary minerals due to their lower energy state (Yadav and Mehra 2021;Saran et al. 2017).Vitale et al. (2021) reported 2 phases in mineral carbonation.They have described the effects of carbonation on the chemo-mechanical behavior of lime-treated soils.However, there is no information about the duration of the reaction phases.Moorehead (1986) defined lime carbonation as a short-term chemical reaction without specifying the duration.Morandeau et al. (2014) explained that the long-term reaction is a pozzolanic reaction (PR).Sargent (2015) listed 5 main reactions between binder and soil to be stabilized and defined carbonation, one of them, as "potential carbonation."Mohammed et al. (2021) have summarized the methods for ground improvement for CO 2 reduction routes in detail.Since the 1960s, lime, cement, and alkali-activated binders have been widely used, depending on the soil type to improve the engineering properties of soils not suitable as construction materials for earthworks (Salehi et al. 2023;Vitale et al. 2021;Hwang et al. 2018;Priyanga et al. 2018;Sherwood 1993).Among them, lime-based mineral carbonation is a soil improvement technique widely used for geotechnical engineering applications (Deneele et al. 2021;Russo 2019).Brucite (Mg(OH) 2 ) is more expensive than portlandite (Ca(OH) 2 ) and cement, and Mg(OH) 2 has higher CO 2 emission than Ca(OH) 2 (Mohammed et al. 2021;Agrawal and Mehra 2020;EPA 2009).
The results of the CO 2 usage calculations expressed so far in mineral carbonation studies are theoretical.It is indisputable that the CO 2 consumed should be measured with a device.It is known that the flow meter is used in concrete curing and CO 2 sequestration studies (Gao et al. 2021).However, there is no information in the literature on the use of flow meters in mineral carbonation and CO 2 sequestration in soil stabilization studies.
In this study, mineral carbonation reaction (MCR) and CO 2 sequestration in the soil were followed for the first time using the flow meter.Instant CO 2 consumption, temperature, and absolute pressure (AP) were monitored and recorded with two flow meters, one in front and one behind the reactor.Flow meter recordings were analyzed, and the MCR based on the data was characterized.The true CO 2 captured in the MCR was calculated for the first time instrumentally.The results regarding the Ca efficiency and strength improvement will be discussed in another study.

Materials and methods
In this study, the experimental setup is to monitor the MCR and CO 2 sequestration through flow meters.Instead of real minerals or rocks, lime, which is more reactive, was preferred.Lime (Ca(OH) 2 -calcium hydroxide, portlandite), an industrial product, was used as a mineral.A steel cylindrical mold with four gas inlets and one gas outlet was used as the reactor.All tubings in the setup have an inner diameter of 1/16″.The inside of the mold is 7.3 cm in diameter, and it is 20.7 cm in length.
In the initial experiments in the laboratory, it was determined that as soon as the CO 2 enters the mold, the outer surface of the mold heats up, and after a short time, the outer surface of the mold regresses to room temperature.At the beginning of the experiment, it was noticed that the gas flow from the mold decreased, and there was no gas output for a short time.Subsequently, it was observed that the gas output increased relative to the initial amount.Thereupon, two flow meters were added to the experimental setup after the first carbonation test observations.Standard gas temperature and pressure are 21.11°C and 101.3 kPa, respectively.Carbonation tests were performed at room temperature.
Including water, the weight of each material was calculated according to the usage ratios.And 1, 5, and 10% lime were used in the experiments, respectively.Two types of sand with different grain sizes were used.The finer-grained sand was called fines.Only sand was used in the first group of mixtures.The second group of samples contained 10% fines.Some chemical and physical properties of sand and fines are given in the table below (Table 1).Food-grade lime of 99% purity was used.A scale with a capacity of 810 ± 0.01 g at 10-30 °C was used.
In this study, industrial CO 2 was used.The CO 2 entering the mold has a pressure of 100 kPa.During the test, the gas coming out of the mold was sent to a water container.This is necessary for health (carbon dioxide is soluble in water) and experimental observations (Fig. 1).TSI 5300 (± 0.05) flow meters were used in front and behind the mold.The flow meters were checked, and it was seen that they were running in sync.Instant CO 2 flow rate (L/min), total CO 2 volume (L), temperature (°C), and AP (kPa) were monitored and recorded for 1 h by flow meters connected to the mold inlet and outlet.The flow meter results, which are the basis of this study, are given and evaluated in the next section.

Results and discussion
There are different hypotheses and debates on the reactions in the soil stabilization process, especially on the MCR.Botha et al. (2005) reported after some projects that the process expressed as the carbonation is a water-based reaction in the material.While Moorehead (1986) defines lime carbonation as a short-term chemical reaction, Morandeau et al. (2014) defined the long-term reaction as a PR.Deneele et al. (2016) reported that the carbonates formed disturb soil stabilization mechanisms and the PR, partly because of the calcite (calcium carbonate) formation.Kwon et al. (2011) and Seifritz (1990) reported that the carbonation process is slow under ambient conditions.Dissolution in the water of atmospheric carbon dioxide neutralizes the alkalinity of the pore water and favors the gradual transformation of portlandite to calcium carbonate, whose amount increases over time as a result of the ongoing carbonation reaction (Arandigoyen et al. 2006;Cazalla et al. 2004;Moorehead 1986).Deneele et al. (2021) defined carbonation as a naturally occurring phenomenon that any cementitious or pozzolanic compound undergoes when in contact with air.Unlike previous studies, while explaining the occurrence of carbonation and its effect on the chemo-mechanical behavior of lime-treated soil, Vitale et al. (2020) reported for the first time two phases as short-term (lime carbonation) and long-term (carbonation of secondary reaction products).According to Wang (2002), PR is a secondary process of soil stabilization.Cementitious materials stabilized clayey soils and modified their properties through cation exchange, flocculation and agglomeration, and PRs (Wang 2002).Kurtis (2022) first divided the reactions in soil stabilization into two through the solution and topochemical reactions and then reported that topochemical reactions occur in 5 key phases.Sargent (2015) listed the reactions between binder and soil to be stabilized as cation exchange, agglomeration/agglomeration, hydration, pozzolanic reactions, and potentially carbonation.He described four reactions but not potential carbonation.Carbonation is a complex process, and it has not yet been fully defined.
Cation exchange capacity, surface area, and moisture content are important in soil stabilization reactions (Bell 1996), and clay is ideal for these properties.Large proportions of most soils comprise clay mineral sand and organic matter, both of which are characterized by negative surface charges, implying that they must attract cation elements of the opposite charge (Terzaghi et al. 1996).Whereas aluminate and/ or silicate clay minerals support PRs and form calcium silicates and aluminate hydrates (Bergado et al. 1996), PRs occur over long time scales (Duxson et al. 2007).Similarly, organic matter also supports PRs.Organic matter decomposes over time, and therefore C becomes active over time.
Temperature, moisture content, pressure, and exposure time of the particles to air and CO 2 are important in the decomposition of organic matter and carbon activity.
The key factor in the degree of carbonation and high Ca efficiency is pH in the MCR, and the pH of limed-soil should have 12.4 (Al-Mukhtar et al. 2010;Eades and Grim 1966).First, it is the decomposition of lime in the MCR that causes an increase in soil pH and a high concentration of calcium ions in the pore water (Rogers and Glendinning 1996).Second, CO 2 dissolves in the water (Mohammed et al. 2021).Third and lastly, CaCO 3 is formed.Mohammed et al. (2021) reported that CaCO 3 formation becomes more difficult when the pH drops below 10.
In this study, the materials given in the chemical and physical properties of Table 1 were used.Clay-free sands with low cation exchange capacity, less cation holding capacity, and low water retention capacity facilitated the monitoring and identification of MCR.CO 2 , temperature, AP, and total CO 2 volume inside and outside the mold were recorded for 1 h by flow meters.Below, the results obtained from the flow meters are summarized in Table 2, and the graphs are given in Figs.2-8.The results regarding carbonation efficiency and strength improvement will be discussed in another study.
Mainly reactions in the mold: CO 2 is dissolved in the water forming a weak acid that can dissolve the available calcium hydroxides (Paige-Green et al. 1990).As the CO 2 enters the mold, H 2 O increases, and according to the first equation, H 2 O increases up to two times.It is clear that mineral carbonation is a water-based reaction.It was noticed that gases, mostly CO 2 , discharge water as they exit the mold.At the end of the 1-h experiment, the amount of discharged water from the mold was measured under 2 g.It was observed that the amount of water discharged is proportional to the amount of lime used and the amount of water used.As the amount of lime and/or water increased, the amount of discharged water increased.Some ions and components are inevitably carried out of the mold with water.However, in this study, no research was made on the possible ions and components in the discharged water and their amounts.
The most important component in mineral carbonation is Ca 2+ , and its removal from the medium causes ( 1) the balance in the third equation to break down against CaCO 3 .As a result, the possible formation of CaCO 3 reduces.A decrease in Ca 2+ causes an increase in carbonic acid (H 2 CO 3 ).Calcite decomposes in an acidic environment and is unstable in water.Increasing H 2 CO 3 will break down CaCO 3 and cause it to decrease in the mold.On the other hand, if more CO 2 is available then: (5)  Lime, a binder material, decomposes and converts to calcite, a filling material in the MCR.The CO 2 sequestration is the first requirement for the MCR, and the CO 2 flow rate is a summary of carbonation for the data in Table 2.There is a chain of reactions in the mold.In the first seconds, with the CO 2 entering the mold, the MCR started between the first 1-6 s, and it started to consume the CO 2 .According to Table 2, it is clear that the MCR takes place mostly in the first seconds.The success of mineral carbonation depends on this first stage.It was observed that when testing started, the CO 2 wasted no time finding ways to get out of the mold.
The time to the second lowest level of the CO 2 flow rate was the second stage.At this stage, the MCR continues.In this study, the periods of the second stage are 35-89 s.After a minute and a half, the CO 2 output started to increase rapidly (Figs. 2a,3a,4a,5a,6a,7a,and 8a).
The highest point of the in-mold temperature and when it started to cool was the limit and end of the third stage (Figs. 2b,3b,4b,5b,6b,7b,and 8b).In this study, the third stage was completed in the first 5 min.After the third stage, the CO 2 flow rate increased rapidly and followed a horizontal course with small fluctuations.At the end of the third stage, the MCR was also completed, and another carbonation phase started in the mold.Contrary to what was reported in some previous studies (Sumner 1998), the MCR was completed in a short-term in all samples, as reported by Moorehead (1986).MCR is not a time-dependent reaction.
The most important stage in the MCR is the first stage where CO 2 is consumed.The short time and high volume of CO 2 captured are the hallmarks of the MCR.If there is a high volume of CO 2 captured in a short time, it can be noticed in the MCR.Contrary to expectations, lime, fines, and water ratios had no effect on the duration of the MCR stages.As can be seen in Table 2, the mineral carbonation reaction was completed in the first 5 min (46-273 s) in this study.
We do not have sufficient data about when the H 2 O increase in the mold starts and ends and how long the CO 2 dissolution, which starts from the first moment, continues.In addition, we do not yet have sufficient instrumental information about their effect on MCR and other reactions.
The MCR, the results, and the questions that occurred during the experiments were explained with the flow meters data for the first time in this study.The average CO 2 volumetric changes according to the CO 2 entering the mold during and after the MCR are given in Table 3.After MCR, it was observed that the CO 2 flow volume increased by 1.5-3 proportionally.With this, although the CO 2 captured after MCR seems to be around 20% when compared to the amount of CO 2 entering the mold, we do not have enough information about the amount of CO 2 captured after MCR and when the reaction(s) ends due to the test duration in this study.
After the MCR is ended, the CO 2 entering and leaving the mold are expected to be equal.However, contrary to expectations, the CO 2 coming out of the mold was not 100% of the CO 2 entering the mold, but an average of 78.3% after the MCR (Table 3).The first reason was that the relatively cold CO 2 from the tube expanded in the mold, and the CO 2 volume increased.Secondly, CaCO 3 grains, which are larger than lime grains and formed in relatively large voids in the soil, were a barrier to the CO 2 transition (Vitale et al. 2020;Moorehead 1986).The third and last important reason was that until the H 2 O was run out in the mold, H 2 O was a barrier to the CO 2 transition (Richardson 1988).In addition to these reasons, there may have been a small amount of CO 2 captured in reaction(s) after the MCR, which was assumed to be relatively less effective in this study.Acceptable errors and sensitivity ranges in the experimental setup and devices may also have been effective in the loss of CO 2 that is leaving the mold.
As can be seen in Table 3, the CO 2 captured increased as lime increased, and the CO 2 captured decreased as water increased in MCR.As the soil grain size decreased, the CO 2 captured decreased.Fines reduced the diffusion of CO 2 .On the other hand, the effect of lime, water, and fines in the reaction(s), which ends and starts after the MCR, was not evident.However, the CO 2 flow rate was almost the same for all mixes after MCR (Table 3).Accordingly, it can be said that the weight of lime, amount of water, and weight of fines that are effective in MCR are not effective in the stage (PR) after the MCR in this study.
After all, what does this research show about the feasibility of the MCR to combat climate change, soil stabilization, etc.? For this reason, we need to know how much CO 2 we use in the carbonation.The following formula (the ideal gas law) might be used for the CO 2 capture: To find the CO 2 volume captured according to Eq. ( 6), the effect of lime (Ca(OH) 2 ) must be known (n).According to the weight of the lime used (Eq.( 7)):  4. To calculate the volume of CO 2 captured, the time when the temperature drop observed in the MCR started was based on (Table 2).The difference between the total volumes of CO 2 entering and leaving the mold corresponding to this time (Figs. 2a,3a,4a,5a,6a,7a,and 8a) was calculated as the true CO 2 captured in MCR (Table 4).
According to the ideal gas law, it was expected that the ratio of CO 2 captured and lime is proportional (Table 4).In this study, it was determined that in addition to lime, water ratio and fines were also effective in MCR and CO 2 captured.The effects of water and fines cannot be ignored in the MCR.Moreover, the difference between the theoretical and true  The CO 2 efficiency (the ratio of true CO 2 captured during MCR) can be seen in Fig. 9.According to Fig. 9, the highest CO 2 efficiency was obtained in the soil with 5% and 10% lime.Considering the emissions in lime production and production costs, it is clear that 5% lime should be preferred to 10% lime.When the samples used in this study whose grain size properties were given in Table 1 are compared, fines reduced CO 2 efficiency.
In this study, the water ratio was randomly chosen as 6%.It is expected that the water ratio increases as the soil grain size decreases.Sand 74% + lime 10% + fines 10% + water 6% sample exposed to MCR test failed, and mixtures containing water 7-10% were prepared without changing lime and fines ratios.However, test results containing water 6%, 8%, and 9% are not given in Fig. 9.It is necessary to determine the ideal water ratio for maximum CO 2 efficiency.It was observed that when the amount of water is low or high, it reduced CO 2 efficiency.The ideal water ratio was determined as 7% for lime 10% + fines 10% samples in this study (Fig. 9).
It should be noted that CO 2 efficiency, Ca efficiency, and strength improvement may not be relative to each other.In this context, more detailed laboratory studies on MCR are needed.The results regarding the Ca efficiency and strength improvement will be discussed another study.
Another proof that the MCR starts as soon as the CO 2 enters the mold is the temperature rise (Figs. 2b,3b,4b,5b,6b,7b,and 8b).The MCR is an exothermic reaction that starts with CO 2 and ends with the end of CO 2 captured.The temperature in the first second was 6-7 °C higher than room temperature.After 5-6 s, the temperature read on the flow meter screen dropped by 3-4 °C and started to rise again after a few seconds.This increase continued until 28-30 °C.The in-mold temperature is estimated to be several degrees higher.In this study, the temperature, which determines the end of the MCR, the third and final stage, was determined to be 28-30 °C.One of the results obtained is that the outside temperature is not important in the MCR, since the temperature in the mold changes depending on the CO 2 captured.Room temperature did not affect the reaction.Three stages required for MCR were not observed in Figs. 2b,3b,4b,5b,6b,7b,and 8b after the 5th min.Initial temperature, temperature drop, maximum temperature, cooling of the mold interior, and their durations are decisive for the MCR.Contrary to expectations, the lime ratio did not affect the reaction and in-mold temperature.After the end of the MCR, the in-mold temperature dropped from 28 to 30 °C to slightly above room temperature, and the temperature continued regularly until the end of the 1-h experiment, with little ups and downs.However, the fact that the temperature in the mold does not fall below room temperature despite the cold CO 2 transition indicates the presence of a reaction or more reactions in the mold.Undoubtedly, AP is important for the mineral carbonation reaction.Duration of in-mold and after-mold minimum and maximum AP is compatible with CO 2 flow and reaction stages (Figs. 2c,3c,4c,5c,6c,7c,and 8c).The pressure dropped in the first seconds and then increased rapidly until the 5th minute in the mold.Except for the sample of lime 10% + fines 10% + 70% sand + water 10%, it was observed that the pressure in the mold increased very slowly from the 5th minute to the end of the 1-h experiment.A decreasing trend in AP was observed towards the end of 1 h in samples containing lime 10% + fines 10% + 70% sand + water 10% (Fig. 8c).As seen in Fig. 8c, the AP of sample including lime 10% + fines 10% + 70% sand + water 10% in the mold increased from 100 to 101 kPa to 150 kPa at the end of 1 h.The AP of the sample that used the most water was over 149 kPa.The APs of other samples varied between 123 and 130 kPa.The AP increased up to about 50% in the mold and then started to decrease.
The continuous increase in AP after the temperature in the mold drops indicates that there is a dynamic chain of reactions in the mold.However, considering that the experiment time was 1 h, we do not know how long the AP increased in the mold and how the AP follows in the mold and after the mold.The increase in water (Eqs.( 1)-( 4)) was effective in the regular and continuous increase of AP and in the reaction chain.The H 2 O required for the MCR caused the expected reactions to be disrupted and new reactions started.H 2 O and its effects are important to the MCR and its results.As well as the reactions in the mold, the formation of CaCO 3 was also effective on the AP.As Moorehead (1986) and Vitale et al. (2020) reported, the CaCO 3 grains expanded into larger pores, resulting in a reduced frequency of larger pores.The reduced permeability caused an increase in AP in the mold.CaCO 3 is a good filling material used in many industries.It was determined that the lime ratio did not affect AP in this study.H 2 O is more effective on AP than lime (Figs. 2c,3c,4c,5c,6c,7c,and 8c).
The AP increase is important for the explanation and results of the MCR.The observations made during the experiments were finalized with numerical data.Other reactions took place in the mold with and after the MCR.The pressure increase is a distinguishing feature in the mineral carbonation reaction.
Unlike most of the previous studies, in this study, the potential carbonation mentioned in Sargent's (2015) list was observed as an MCR.After Table 3 ,Figs. 2a,3a,4a,5a,6a,7a,and 8a, and the explanations above, it is clear that in the MCR, a relatively high volume of CO 2 is consumed within seconds and ends in 5 min.The MCR is a short-term reaction.If there is a strong exposure to CO 2 and the CO 2 captured is a relatively high amount, then mineral carbonation can be mentioned.Otherwise, time-dependent and slow CO 2 consumption is a PR.In this study, the reaction observed after the 5th minute was the PR.
The MCR can be confused with the PR.The MCR is a potential reaction and cannot be expected to occur all the time.It is not possible to predict a sequence between mineral carbonation and pozzolanic reactions.MCR can also occur during the PR, although there is a high probability that the MCR will occur before the PR.However, as the PR progresses, the probability of MCR decreases.On the other hand, the MCR limits the PR as it uses lime grains.In the long term, if the hydrated phases formed due to PRs are exposed to CO 2 , hydrated compounds can become carbonated (Morandeau et al. 2014).It should be known CaCO 3 that occurs after the decomposition of the PR products cannot be defined as an MCR.It is a decay reaction.
As with other carbonation reactions, the PR is an exothermic reaction.The decrease in the temperature in the mold after MCR and the regular course of the temperature slightly above room temperature are the indicators of the PR.The PR is a time-dependent reaction ruled by the amount of unconsumed portlandite (Ca(OH) 2 ) in the system, the curing time, the temperature, and the pressure (Yadav and Mehra 2021;Vitale et al. 2020;Al-Mukhtar et al. 2010;Rao and Shivananda 2005).The most important difference between MCR and PR is that PR is time-dependent, whereas MCR is not time-dependent.Contrary to the MCR, the weak bond of Ca and SiO 2 makes the ambient conditions important in the PR.
As can be seen in Eq. ( 8), there is no entry of H 2 O and CO 2 in the PR.Ca 2+ and CO 3 = ions released in the MCR stage continue to precipitate in the PR, and CaCO 3 formation continues.CO 2 that continues to enter the mold and H 2 O in the mold may continue to be effective in the decomposition of the lime, and limited MCR may occur in the PR stage.In the evaluation of the PR and its results, the long setting time of the lime and the fact that the reaction is exothermic should be considered.However, in this study, MCR was not observed in the PR stage according to the flow meters data.No significant changes in CO 2 flow, temperature, and AP were observed during the PR stage.They were regular after the MCR until the end of the 1-h test.The reason for the possible increase in CaCO 3 in the PR stage is the Ca 2+ and CO 3 = ions released in the MCR stage and present in the ambient.As seen in Table 3 and Figs . 2a, 3a, 4a, 5a, 6a, 7a, and 8a, the main reason for the relative CO 2 captured after MCR is the CO 2 decomposition in H 2 O, and H 2 O delaying CO 2 ( 8) There was no CO 2 captured during the PR stage in this study or it was so little that it can be ignored.When the H 2 O in the ambient is finished, the CO 2 entering and exiting the mold will be equalized.It should be noted that the reactions in soils containing clay and/or organic material will develop differently.Compared to the MCR, the CO 2 captured in the PR is relatively less, and it is regular.The difference and regularity between CO 2 captured are distinctive features in characterizing the MCR and PR stages.The effect of lime, water, and fines ratios on CO 2 captured in the MCR stage was not observed in the PR stage in this study.
During the experiments, it was noticed that the outer temperature of the mold increased from top to bottom and cooled from top to bottom.The temperature change on the mold surface shows that CO 2 is effective from top to bottom and that mineral carbonation is a short-term and potential reaction.This is a result of the CO 2 captured and reaction levels required for the MCR.The MCR started in the upper grains that first encountered CO 2 , and the reaction went forward as CO 2 progressed.On the other hand, water is needed initially for the MCR (Mohammed et al. 2021;Cai and Liu 2017;Song-yu et al. 2017;Yi et al. 2013), and according to Eq. ( 1), with the dissolution of CO 2 , the amount of water between soil grains doubles.The diffusion of CO 2 in water is approximately 10,000 times lower than in air (Richardson 1988).CO 2 and other gases prefer the easiest route.CO 2 moves through the mold by siphoning from top to bottom.Mold inner edges for CO 2 are an easy way.The reaction proceeds like a wave or front through the sample from the outer surface inward (Moorehead 1986).In an MCR, it is very difficult for CO 2 to be homogeneously dispersed, propagated, and effective on all grains.As Moorehead (1986) explained, the rate at which the carbonation reaction occurs in a lime compact is controlled by the diffusion of CO 2 to the reaction site.For these reasons, soil improvement with mineral carbonation requires engineering planning, and there will always be well and pump costs.
These results and discussions are extremely important for large-scale carbonation expenditures and applicability.Undoubtedly, the most important issue is the environmental conditions of the place where soil improvement will be made.The groundwater level, surface water, and groundwater discharge are important.The CO 2 density at 21.1 °C, which is the standard gas temperature in this study, is 1.83 kg/m 3 .The weight of CO 2 relative to dry air is 1.53, and the current concentration is about 0.0416% by volume.The H 2 O ratio in dry air can reach up to 4%.CO 2 is in balance in the atmosphere, and contrary to expectations, carbon dioxide, which is heavier than air, does not completely settle on the soil in this balance.Even ignoring moisture, rain, and the dissolution of CO 2 in water, considering that the soil must be strongly exposed to CO 2 in the MCR, MCR cannot be expected at depths or even just the surface.
An important issue to consider when using lime as a binder in soil improvement works is lime production emissions.A 1 Mt lime generates about 0.75-0.86Mt of CO 2 emission (Latifi et al. 2017;EPA 2009).Emissions from the lime industry were estimated to be 25.4 million metric tons of CO 2 equivalents in the USA in 2004.A 14.3 Mt is process-related emissions, and 11.1 Mt is on-site stationary combustion emissions (EPA 2009).However, Hwang et al. report that 1 kg of MgO-based binder could sequester up to 0.507 kg of CO 2 (Hwang et al. 2018).Undoubtedly, energy consumption and the total cost of lime production are also important.
According to EPA (2009) and Latifi et al. (2017), there is 0.41-0.47cm 3 CO 2 emission for 1 g of lime production at these study conditions.The table below shows the differences between the CO 2 emission at the lime (Ca(OH) 2 ) production and the CO 2 captured in this study (Table 5).
Table 5 is a summary of the results of this study.Except for sand 70% + lime 10% + fines 10% + water 10%, in which unit emission is more than the CO 2 captured, the other recipes can be used for CO 2 sequestration.The highest CO 2 captured value was obtained in the soil mixed with 5% lime, where fines were not used.The second highest CO 2 captured value was obtained in soil mixed with 1% lime, where fines were not used.In soils containing fines, it is important that the CO 2 captured values for 1% and 5% lime are close to each other; even more, CO 2 captured in 1% lime is more than CO 2 captured in 5% lime.Accordingly, 1-5% lime can be used in soil carbonation studies.As the soil grain size decreases, a relatively higher rate of CO 2 captured can be obtained with less lime.Less lime can be used as the soil grain size decreases.On the other hand, the presence of more water than the amount of water needed in the environment may cause less CO 2 sequestration than the CO 2 emission in lime production.The MCR and the CO 2 sequestration are under the control of water.

Conclusion
In this study, flow meters were used for the first time in front and behind the reactor, and the MCR was characterized.According to this, the mineral carbonation reaction is a short-term and potential reaction, and it is not a timedependent reaction.Depending on the CO 2 consumption, 3 stages were observed in the MCR.It was determined that the MCR started in the first seconds and ended in the 5th minute.At the end of the third stage, the MCR ended, and then a new carbonation reaction started; the CO 2 followed a horizontal course with small fluctuations.The MCR is separated from other carbonation reactions with these characteristics.

Table 1
Chemical and physical properties of sand and fines (%) *Loss on ignition

Table 2
Results of the flow meters

Table 3
Changes of CO 2 volumes at and after MCR relative to CO 2 entering the mold (%) CO 2 captured in Table4is significant, and the difference between them is striking.Comparing the theoretical and true CO 2 captured, it is clear that the ideal gas law does not apply to the MCR.

Table 5
Comparison of CO 2 emission at the lime production and the CO 2 captured in this study