Determination of the CO 2 Uptake of Construction Products Manufactured by Mineral Carbonation

: Mineral carbonation is a technology for capturing and storing CO 2 in solid minerals. When mineral carbonation is used to produce construction materials, the quantiﬁcation of the CO 2 uptake of these products is of the utmost importance, as it is used to calculate the CO 2 footprint of the product and/or carbon offset. The CO 2 uptake is generally determined by measuring the CO 2 content of a material before and after accelerated carbonation. This approach, however, does not take hydration and dehydroxylation reactions into account that may occur during carbonation, and it can therefore under-or overestimate the CO 2 uptake. Thus, a more accurate and practical method to determine CO 2 uptake, which also accounts for hydration and dehydroxylation reactions, is proposed in this paper. This method is based on analytical methods to determine the dry mass and the CO 2 content of the solid products before and after carbonation, and on the calculation of the CO 2 uptake by the following equation: CO 2 uptake (wt.%) = CO 2 carbonated (wt.%) × (weight after carbonation (g)/weight before carbonation (g) − CO 2 initial (wt.%), with CO 2 carbonated being the CO 2 content in g/100 g dried carbonated material, and CO 2 initial being the CO 2 content in g/100 g dried initial material, i.e., before carbonation. The “weight before carbonation” is the dry weight of the initial material, and the “weight after carbonation” is the product’s dry weight after carbonation. In this paper, we show that up to 44% under-or overestimation of CO 2 uptake can occur when hydration and dehydroxylation reactions are not taken into account during mineral carbonation.


Introduction
It is widely recognized that to avoid the worst impacts of climate change, the world needs to urgently reduce CO 2 and greenhouse gas emissions. Mineral carbonation is a technology for capturing and storing CO 2 in solid minerals. Mineral carbonation can be used to make a variety of products (chemicals, minerals, construction products, etc.). The use of mineral carbonation to produce construction materials is of particular interest, since these materials often have a significantly reduced CO 2 footprint compared to traditional construction products. The construction sector accounts for about 13% of global CO 2 emissions, and it needs to achieve complete decarbonization by 2050 [1]. Concrete and concrete-like products that incorporate CO 2 as carbonates may help in reaching complete decarbonization. It can therefore be expected that the construction industry will represent the primary market for products made by mineral carbonation.
Various initiatives and changes in legislation are being drafted to accelerate the reduction of CO 2 emissions. While in the past, the focus was mainly on geological storage, mineralization of CO 2 into products has also recently been recognized as a form of permanent storage. In the latest revision of the European Emissions Trading Scheme ('ETS'), permanent binding of CO 2 into products (through, for example, mineral carbonation) was also included [2]. Another example in Europe is the proposed carbon removal certification scheme, in which processes based on mineral carbonation that store CO 2 in products will be qualified as "carbon removals" as long as biogenic CO 2 or CO 2 captured from the air Minerals 2023, 13, 1079 2 of 12 (DAC) is used [3]. For the enactment of mineral carbonation in the various certification schemes, it is of the utmost importance to be able to measure the CO 2 uptake of these materials accurately. The CO 2 uptake of concrete-or lime-based products during its lifecycle or CO 2 readsorption assessment has also become an environmentally important issue. Many assessment procedures have been proposed for the calculation of CO 2 uptake, and all of these procedures have adopted a pragmatic and/or mathematical approach using various assumptions, such as the degree of cement hydration as a function of water/cement (W/C) ratio and time, the cement type, the exposure conditions during service life and in different recycling scenarios, and so on [4,5]. Typically, validation of the calculations is based on experimental results or published data on CO 2 uptake in construction products as a function of time and exposure conditions [6]. Here, too, the accurate determination of CO 2 uptake is important.
Different methods have been used in the literature to quantify CO 2 uptake. The most common method is to determine CO 2 uptake based on the CO 2 content before and after accelerated carbonation. CO 2 uptake is then calculated based on Equation (1), where CO 2 carbonated and CO 2 initial represent the CO 2 content in the dried (105 • C) material after and before carbonation, respectively [7][8][9][10][11]: This equation, however, may under-or overestimate the effective CO 2 uptake as it only takes into account weight changes caused by CO 2 uptake. In addition to CO 2 uptake, other reactions that may occur during carbonation may change the weight of the final product, such as dehydroxylation (loss of lattice hydroxyl ions that are released to form water molecules) and hydration reactions. Therefore, improved methods or equations are needed in order to determine the CO 2 uptake of products manufactured by mineral carbonation.
One method that takes into account dehydroxylation reactions is the mass gain method, which calculates the CO 2 uptake by measuring the mass of the samples before and after carbonation. Water lost during the exothermic carbonation process is collected by absorbent paper, and it is added to the mass after carbonation in Equation (2) [12][13][14]. Since not all vaporized water can be collected, Equation (2)  Another method that takes into account hydration or dehydroxylation reactions is the mass curve method, where the mass of the entire carbonation chamber containing the sample(s) is recorded as a function of time during the carbonation process [12,13]. The generated mass curve represents reaction kinetics. After the sample is placed in the carbonation chamber, the balance is zeroed. The CO 2 is then injected to a specified pressure, and the mass increase is recorded as a function of time. Since the pressure is maintained at a constant level by replenishing the CO 2 gas that is consumed by the sample, the increase in mass is due to the carbon uptake of the sample. At the end of the carbonation process, the pressure is released and the residual mass, M, is recorded. The system is calibrated by repeating the tests using a CO 2 -non-reactive sample of the same volume in order to obtain a second residual mass, m. The mass gain or CO 2 uptake (%) of the sample is given by Equation (3) [13]. Note that the CO 2 uptake measured by this method is not influenced by any carbonates that were already present in the sample before the accelerated mineral carbonation process. The mass gain and mass curve methods work well when studying the carbonation of cement and concrete using small closed reactors fed with pure CO 2 /N 2 mixtures. These methods may not be applicable when using flue gases, since such gases also contain water and other components that may react with the sample being carbonated, or when using flow through reactors. The methods are also difficult to apply in an industrial setting.
Materials that can be used as input for mineral carbonation with their key minerals are listed in Table 1. Since these materials can be rather diverse in chemistry and mineralogy, the quantification of the CO 2 uptake can be challenging. The CO 2 uptake of a material is defined as the amount of CO 2 that is taken up by the material (or gained) in the form of precipitated carbonates during the carbonation process. It is generally expressed in wt. (%), meaning (g CO 2 added during the carbonation process)/(100 g of the initial solid material).  [24] In this paper, we provide an overview of the mass gain and losses that will occur during carbonation of some common minerals due to hydration or dehydroxylation reactions. We subsequently propose a novel calculation method for CO 2 uptake in construction materials based on CO 2 content, which takes into account both hydration and dehydroxylation reactions. Subsequently, an overview is given of the analytical methods and procedures that can be used to determine the CO 2 content in construction materials. Finally, the impact of this novel calculation method on the CO 2 uptake of some key minerals and materials is evaluated, and it is compared to the CO 2 uptake that is calculated by one of the most common methods applied in the literature.

Mineral Carbonation
Mineral carbonation proceeds through a reaction of water containing dissolved CO 2 with alkaline materials, such as rocks, slags, concrete, etc., to precipitate solid carbonates. The dissolution of CO 2 in water and the dissociation of H 2 CO 3 is described by Reaction (4): The dissolution of CO 2 is favored at a basic pH, which increases the concentration and availability of CO 3 2− . The dissolution of Ca 2+ or Mg 2+ is described from several key minerals by Reactions (5)- (8): The precipitation of CaCO 3 in the presence of water is described by Reaction (9): The precipitation of anhydrous MgCO 3 generally only occurs at high temperatures (>100 • C) because of the high energy barrier for Mg 2+ dehydration, e.g., [25]. Instead, hydrated Mg-carbonates, such as nesquehonite or hydromagnesite, are generally formed. The precipitation of nesquehonite is described by Reaction (10), and that of hydromagnesite is described by Reaction (11): The overall reaction of the key minerals with CO 2 is then given by Reactions (12)- (16): The exact stoichiometric coefficients of the reactions are not always clearly defined [26], e.g., the amount of water in the (calcium modified) silica gel is unknown. From the overall Reactions (12)- (16), it is clear that the solid material gains mass during carbonation by the binding of CO 2 in solid carbonates. The material to be carbonated may, however, also lose or gain mass as a result of the dehydroxylation of hydroxides (Reaction (12)), or by the conversion of chemically bound water (solid) to free water (liquid) (Reaction (14)), or by the conversion of free water to chemically-bound water (Reactions (13), (15), and (16)).
The CO 2 uptake of a material is generally expressed as weight % (wt.%), meaning (g CO 2 )/(100 g of the initial solid material). In Figure 1, a few examples are provided of the mass changes associated with the carbonation of Ca(OH) 2 , Ca 2 SiO 4 , and Mg 2 SiO 4 , respectively, which are some of the key minerals that are involved in the carbonation of industrial waste streams and natural magnesium silicate rocks (see Table 1). mass changes associated with the carbonation of Ca(OH)2, Ca2SiO4, and Mg2SiO4, respectively, which are some of the key minerals that are involved in the carbonation of industrial waste streams and natural magnesium silicate rocks (see Table 1).

Conversion of the Measured CO2 Contents to a CO2 Uptake
The determination of the CO2 content is generally carried out on a dried sample (dried at 105 °C), and therefore expresses the CO2 content as a mass percentage of the total mass of the dried sample. The CO2 uptake is calculated based on the CO2 content of a material before and after accelerated carbonation. Typically, Equation (1) is used.
To calculate the CO2 uptake more accurately, the equation should also take into account the mass change of the initial solid product that occurs during carbonation. Because processes other than CO2 uptake may lead to weight changes during the carbonation process (e.g., crystal water can be released as free water during carbonation, or free water may become part of the crystal structure), the weight of the products before and after carbonation should be established on a dry weight basis. The conversion of CO2 content to CO2 uptake (in g/100 g dry, initial material) is given by Equation (17): with CO2 carb. being the CO2 content in g/100 g of dried carbonated material, and with CO2 init. being the CO2 content in g/100 g of dried initial material, i.e., before carbonation. The "weight before carbonation" is the dry weight of the initial material, and the "weight after carbonation" is the dry weight of the material after carbonation.
In Table 2

Conversion of the Measured CO 2 Contents to a CO 2 Uptake
The determination of the CO 2 content is generally carried out on a dried sample (dried at 105 • C), and therefore expresses the CO 2 content as a mass percentage of the total mass of the dried sample. The CO 2 uptake is calculated based on the CO 2 content of a material before and after accelerated carbonation. Typically, Equation (1) is used.
To calculate the CO 2 uptake more accurately, the equation should also take into account the mass change of the initial solid product that occurs during carbonation. Because processes other than CO 2 uptake may lead to weight changes during the carbonation process (e.g., crystal water can be released as free water during carbonation, or free water may become part of the crystal structure), the weight of the products before and after carbonation should be established on a dry weight basis. The conversion of CO 2 content to CO 2 uptake (in g/100 g dry, initial material) is given by Equation (17): with CO 2 carb. being the CO 2 content in g/100 g of dried carbonated material, and with CO 2 init . being the CO 2 content in g/100 g of dried initial material, i.e., before carbonation. The "weight before carbonation" is the dry weight of the initial material, and the "weight after carbonation" is the dry weight of the material after carbonation.
In Table 2, a few examples based on the reaction equations of pure minerals are listed to illustrate the difference in the calculation of CO 2 uptake with the new Equation (17) compared to the one that is currently used in the literature (1). Example 1: Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O shows that 1 mol of Ca(OH) 2 (74.095 g/mol) reacts with 1 mol CO 2 (44.009 g/mol) to precipitate 1 mol CaCO 3 (100.089 g/mol) and release 1 mol H 2 O (18.015 g/mol). When starting with 100 g Ca(OH) 2 , this results in 135.1 g CaCO 3 due to the uptake of 59.4 g of CO 2 and the release of 24.3 g of H 2 O. The CO 2 uptake is (59.4 g CO 2 )/(100 g of the initial solid material) or 59.4 wt.%. The CO 2 content of the carbonated material that can be analyzed by an element analyzer or by TGA is, however, only 43.4 wt.%, i.e., the CO 2 content of the newly precipitated CaCO 3 , i.e., 44.009 (molecular weight of CO 2 )/100.089 (molecular weight of CaCO 3 ) or (59.4 g CO 2 )/(135 g of solid product). When calculating the CO 2 content, without taking the water loss into account, the CO 2 uptake would be 78.6% (i.e., 44.009/(100.089·− 44.009). Note that the CO 2 uptake in Table 2 is based on simple reaction equations. These calculations are only made to illustrate that Equation (17) provides more accurate CO 2 uptake values than Equation (1), and it is the latter that is currently used in the literature. In Example 2, the CO 2 uptake of the complete carbonation of Ca 2 SiO 4 (reaction (13)) has been calculated. The water uptake due to hydration of Ca 2 SiO 4 and the formation of a hydrated silica gel or Ca-modified silica gel (when carbonation is incomplete) has been roughly estimated at 2% based on TGA results from carbonated Ca 2 SiO 4 samples, e.g., [27]. Example 3 is based on the formation of nesquehonite, a magnesium hydroxy-carbonate hydrate from the carbonation of olivine (Mg 2 SiO 4 ), i.e., reaction (15). The very high weight gain of 139.4% (62.6% due to CO 2 and 76.8% due to H 2 O uptake) makes Equation (1) underestimate the CO 2 uptake by 43.4%. Nesquehonite is considered a promising solution for CO 2 storage, with potential utilization in "green" building materials [28]. Based on these examples, it is clear that hydration and dehydroxilation reactions play a very important role in the determination of the CO 2 uptake. Input materials suitable for mineral carbonation can be a combination of various minerals (Table 1) for which the impact of hydration and/or dehydroxilation reaction can differ. For metallurgical slags that contain few hydroxides and hydraulic phases, the difference between the two calculation methods will be small (normally less than 5%), especially considering that complete carbonation is very difficult to reach. For materials that mainly consist of Ca(OH) 2 , such as carbide slag or lime waste, the CO 2 -uptake calculated with Equation (1) may overestimate the CO 2 uptake by up to 32.3%. When magnesium hydroxy-carbonate hydrates are formed, such as nesquehonite, the CO 2 uptake determined by Equation (1) may be underestimated by up to 43.4%.
In Table 3, the same calculations were made for waste hydrated cement paste, which contains several minerals that can be carbonated. Note that it was assumed that all of the minerals that can be carbonated are fully carbonated, and that the water content in the C-S-H will be taken up by the amorphous silica that is formed during the carbonation reaction. The difference in CO 2 uptake calculated with the two equations is most likely closer to the difference that can be expected for typical construction materials. Table 3. Examples to illustrate the difference in CO 2 uptake based on the calculation with the new Equation (17) compared to the one currently used in the literature (Equation (1)) for the carbonation of hydrated cement paste (HCP) (for the content of minerals that can be carbonated in HCP, see Table 1).  [32] discuss a large number of methods to analyze carbonate content in sediments. These methods can also be used to determine the CO 2 content. We only focused here on methods that are practical in an industrial context and are thus most relevant for construction materials, and there are three: (1) a method based on the reaction between carbonate and an acid, with determination of the released CO 2 by a volumetric or gravimetric method; (2) a method that measures the inorganic carbon content by an element analyzer; and (3) a method that determines the weight loss of carbonates by decomposition under high temperature conditions by thermogravimetric analysis (TGA).

Volumetric Determination of CO 2 (Reference Method for EN 459-2:2021)
There are several methods that are based on the dissolution of carbonates and the determination of the released CO 2 [29,31]. Reference [31] stipulates a method for the determination of carbonate content of building lime and then soils and soft rock, respectively. The CO 2 contained in these materials in the form of carbonates is released as CO 2 gas during the reaction between the sample and an acid (e.g., hydrochloric acid). In [29], quite concentrated HCl is used, and it is also heated to boiling point to assure complete reaction with the carbonates. The volume of the air/CO 2 mixture is measured as well as the air without CO 2 . The latter is accomplished by adsorbing all of the CO 2 in an absorption vessel filled with potassium hydroxide solution. The difference in volume gives the volume of CO 2 in the sample, and it is used to calculate the CO 2 content of the sample. In [29], the sample mass used depends on the expected CO 2 content in the sample, and it varies between 0.1 g (expected CO 2 content > 40%) and 2 g (expected CO 2 content < 2%). In [31], 5 g of sample is used. The CO 2 content is deduced from the pressure induced by the released CO 2 , and the CO 2 content is determined through a pre-calibrated curve.

Gravimetric Determination of CO 2
In [29], a gravimetric method is provided as an alternative to determine the CO 2 content. In this method, the sample is treated with phosphoric acid to decompose the carbonates present in the sample. The liberated CO 2 is passed through a series of absorption tubes by a CO 2 free carrier gas. The first two tubes are used to remove H 2 S and H 2 O. The following tubes absorb the CO 2 , and their weight gain is measured to determine the released CO 2 content of the sample.
An automated carbon (TC analyzer) or element analyzer can also be used to determine the inorganic and organic CO 2 content of a sample by the combustion gravimetric method. Two representative samples (generally 0.5 to 1.0 g) are put into two individual small containers after being weighed: one for total carbon (TC) determination and the other for TOC determination. In the one for TOC analysis, carbonates are removed either by reaction with HCl or phosphoric acid, e.g., [33]. Then, both samples are combusted under O 2 at 900 • C to 1150 • C in the presence of a catalyst, which oxidizes the organic carbon and decomposes the inorganic carbon. The CO 2 in the evolved gases is then collected in a suitable absorbent, and its mass is determined. If no organic carbon is present, the total carbon (TC) content represents the total inorganic carbon (TIC) content, and the CO 2 content in wt.% is given by Equation (18): CO 2 wt.% = TC × 44.01 (molar mass of CO 2 )/12.01 (atomic mass of C) (18) If organic carbon is present, an indirect method can be used, in which the TC of the sample (TC 1 ) is determined, and in a second sample, the TC (TC 2 ) content is determined after removing the carbonates. In this way, the TC and TOC are determined and the total inorganic carbon is calculated as TIC = TC − TOC, and the TIC can be recalculated to CO 2 by Equation (19): TIC = (TC 1 − TC 2 ) × 44.01 (molar mass of CO 2 )/12.01 (atomic mass of C) TC 1 = TC is determined after the drying of the sample, and TC 2 = TC is determined after the drying of the sample and the dissolution of the carbonates with acid. Some acid treatments may not be effective in removing all carbonates, and they may therefore lead to erroneous results. This may be the case when cold dilute 10% HCl is used and the sample contains dolomite, ankerite, and/or siderite. These minerals are less soluble than calcite, and they may need a stronger acid and/or slight heating to react and release all inorganic carbon (TIC).
The thermogravimetric analysis (TGA) method determines the CO 2 content by measuring the mass loss of a sample during heating of that sample up to about 1000 • C using a TG analyzer. The heating of a sample over a temperature range allows for mass loss differentiation based on mineral form (for example, CaCO 3 , MgCO 3 , Ca(OH) 2 , etc.). The decomposition of carbonates generally results in the release of CO 2 over the temperature range of 500-900 • C. Note that the exact temperature range depends on the nature of the carbonate minerals, with poorly crystalline and amorphous CaCO 3 decomposing between 300 and 630 • C, aragonite and vaterite decomposing between 600 and 700 • C, and crystalline calcite decomposing between 700 and 850 • C [34]. Magnesium carbonate, nesquehonite and hydromagnesite release their CO 2 between 400 and 500 • C [35,36]. Note that the decomposition of carbonates during TG analysis may be as low as 300 • C [19,27,37].
In order to verify that the mass loss is due to the decomposition of carbonates, the TG analyzer can be coupled to a mass spectrometer (MS), which enables the simultaneous analysis of the evolved gases that are emitted during the heating of the sample. The CO 2 quantity can be directly determined by the mass loss of the sample over the appropriate temperature range (e.g., 500-900 • C) as long as there is no overlap with the release of other gases, such as water from hydration products. In the case of cement samples, possible overlap with water loss from hydrated phases and materials containing monoand hemicarbonate (where the release of CO 2 is around 400 • C (see Zajac et al. [38]) or amorphous carbonates (where the release of CO 2 is around 300 to 600 • C) makes an accurate determination of the CO 2 content of these samples difficult. A simplified method is the Loss-On-Ignition (LOI) method, which measures the weight loss between two predetermined temperatures. For carbonates, the LOI method generally uses Equation (20), e.g., [39]: CO 2 wt.% = ((wt.% at 500 • C − wt.% at 900 • C)/(wt.% at 105 • C)) × 100 (20) However, the results of this method have been shown to depend on factors such as sample size, exposure time, and the position of the samples in the furnace [40], and, as has already been pointed out, some carbonates may release CO 2 at lower temperatures and are thus not counted in the LOI method, which only considers losses between 500 • C and 900 • C.

Comparison of Different CO 2 Determination Methods
A comparison of different methods should be conducted on samples from diverse origins, as some of the methods may work well for some samples but be less accurate when certain constituents are present in the samples. Most studies comparing methods have been carried out on natural sediments and rocks or soil samples (e.g., [32,39,41]). Li et al. [39] concluded that the automatic carbon analyzer method provided more accurate values with the lowest uncertainties in comparison to chemical methods and the LOI method. In addition, the automatic carbon analyzer can produce results in the shortest time (about 10 min). This allows for the analysis of more replicates, which will increase the precision of the results. Moreover, since the measurement is automatic, the constant presence of operators is not required.

Drying of Samples before Determination of the CO 2 Content
Complete drying of the samples is required to provide accurate measures of the CO 2 content of a sample. In the literature, the CO 2 content is generally measured as a weight percentage relative to the total weight of the sample dried at 105 • C [8,10,11]. This is the most commonly used method for determining the dry mass of a sample in various standards [42][43][44][45].
The dry weight (W d ) of a sample may, however, depend on the drying method, with different drying methods giving different results. The difficulty lies in the removal of all free water without affecting the chemically-bound water in the sample. This is visualized in Figure 2: ls 2023, 13, x FOR PEER REVIEW 9 of 12 are thus not counted in the LOI method, which only considers losses between 500 °C and 900 °C.

Comparison of Different CO2 Determination Methods
A comparison of different methods should be conducted on samples from diverse origins, as some of the methods may work well for some samples but be less accurate when certain constituents are present in the samples. Most studies comparing methods have been carried out on natural sediments and rocks or soil samples (e.g., [32,39,41]). Li et al. [39] concluded that the automatic carbon analyzer method provided more accurate values with the lowest uncertainties in comparison to chemical methods and the LOI method. In addition, the automatic carbon analyzer can produce results in the shortest time (about 10 min). This allows for the analysis of more replicates, which will increase the precision of the results. Moreover, since the measurement is automatic, the constant presence of operators is not required.

Drying of Samples before Determination of the CO2 Content
Complete drying of the samples is required to provide accurate measures of the CO2 content of a sample. In the literature, the CO2 content is generally measured as a weight percentage relative to the total weight of the sample dried at 105 °C [8,10,11]. This is the most commonly used method for determining the dry mass of a sample in various standards [42][43][44][45].
The dry weight (Wd) of a sample may, however, depend on the drying method, with different drying methods giving different results. The difficulty lies in the removal of all free water without affecting the chemically-bound water in the sample. This is visualized in Figure 2:  [46]). Wd, mild = Dry weight after mild drying; Wd = dry weight (including bound water); Wd, harsh = dry weight after hard drying, which may include part of the bound water.
Free water is defined as water that can be removed by drying (evaporable water). It is water that may still be present in the pores of the initial material to be carbonated, or water that is added to the material, as the carbonation reaction can only proceed in the presence of water.
Chemically-bound water is water that is chemically bound in a crystal structure (e.g., the hydration water of ettringite (CaAl2(SO4)3(OH)12·26H2O), or C-S-H). The removal of this water leads to changes in the crystal structure of the solid phase. This water is considered part of the solid phases contained in the sample, and it is part of the dry mass of the sample.  Figure 2. Effect of the drying efficiency on the estimated amount of free (unbound chemically) and chemically-bound water (Figure adapted from [46]). W d, mild = Dry weight after mild drying; W d = dry weight (including bound water); W d, harsh = dry weight after hard drying, which may include part of the bound water.
Free water is defined as water that can be removed by drying (evaporable water). It is water that may still be present in the pores of the initial material to be carbonated, or water that is added to the material, as the carbonation reaction can only proceed in the presence of water.
Chemically-bound water is water that is chemically bound in a crystal structure (e.g., the hydration water of ettringite (CaAl 2 (SO 4 ) 3 (OH) 12 ·26H 2 O), or C-S-H). The removal of this water leads to changes in the crystal structure of the solid phase. This water is considered part of the solid phases contained in the sample, and it is part of the dry mass of the sample. Dry mass, in many cases, is defined as the mass after drying at 103-105 • C until constant mass, but for some materials (e.g., hydrated cement paste), however, this may be too harsh, as hydrated cement phases may start to lose bound water at around 50 • C [47,48].
Korpa et al. [49] presented a thorough study on the effect of drying procedures on cement paste. The authors compared four drying methods: drying at 105 • C, drying over magnesium-perchlorate-hydrate, drying over dry ice (−79 • C), and freeze drying (−10 • C). In addition, drying time also influences the results. Korpa et al. [49] assumed that the correct dry mass is obtained when the mass loss during 1 day does not exceed 0.001 g per gram of the initial material. Of the methods tested, drying at 105 • C was the most harsh, leading to the lowest dry mass.

Conclusions
The techniques that are used to dry and determine the CO 2 content of a sample and especially the way in which the CO 2 content of a sample is translated into a CO 2 uptake play an important role in the accurate determination of the CO 2 uptake of a material. Calculations based on the carbonation of pure minerals showed that not taking dehydroxylation and hydration reactions during carbonation into account may over-or underestimate the CO 2 uptake by up to 32.3% and 43.4%, respectively. Construction materials or waste products rich in Ca(OH) 2 or Mg 2 SiO 4 are most prone to over-or underestimation of CO 2 uptake. Taking into account the policy incentives to encourage and monetarize CO 2 storage through mineralization, it is of utmost importance to determine CO 2 uptake during mineral carbonation in a correct way.