Pitfalls in the use and interpretation of TGA and MIP techniques for Ca-leached cementitious materials

Materials and Design 182 (2019) 108041

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Pitfalls in the use and interpretation of TGA and MIP techniques for Ca-leached cementitious materials Quoc Tri Phung ⁎, Norbert Maes, Suresh Seetharam Institute for Environment, Health, and Safety, Belgian Nuclear Research Centre (SCK•CEN), Boeretang 200, B2400, Mol, Belgium

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• TGA is not appropriate to quantify hydrated phases of NH4NO3 leached cementitious material as it leads to overestimation. • Mercury intrusion porosimetry (MIP) overestimates the porosity of leached samples as revealed by poromechanics theory. • Poromechanics theory offers a means to correctly estimate the MIP porosity.

a r t i c l e

i n f o

Article history: Received 21 May 2019 Received in revised form 11 July 2019 Accepted 13 July 2019 Available online 21 July 2019 Keywords: Microstructure Mineralogy Mercury intrusion porosimetry Thermal gravimetric analysis Micromechanics Cement paste

a b s t r a c t The study of Ca-leaching of cement-based materials has necessitated the use of accelerated leaching experiments because of extremely slow leaching kinetics. The microstructural and mineralogical changes resulting from such leaching experiments are typically studied using Mercury intrusion porosimetry (MIP) and Thermal gravimetric analysis (TGA), respectively. This paper closely examines the pitfalls associated with applying these techniques to study the behaviour of leached materials. In this context, accelerated leaching experiments of cement paste samples with two water/cement ratios (0.325 and 0.425) are used as the basis. MIP and TGA results for both leached and intact paste samples are presented in terms of pore size distribution and phase fractions (portlandite, calcium carbonate and C-S-H), respectively. Results suggest that specifically for leached materials, a theoretical correction over and above that suggested by MIP manufacturer is needed to correctly interpret MIP data for original samples. However, TGA should not be used to study the leached materials subjected to accelerated leaching using ammonium nitrate solution. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

⁎ Corresponding author. E-mail addresses: [email protected] (Q.T. Phung), [email protected] (N. Maes), [email protected] (S. Seetharam).

Ca-leaching is a dissolution-diffusion process of Ca ion in the pore solution [1]. Ca-leaching is one of the many detrimental degradation processes affecting plain and reinforced concrete structures for the very long-term (e.g. nuclear waste disposal system) [2] or in hydraulic structures. In the absence of advection, leaching of concrete is an

https://doi.org/10.1016/j.matdes.2019.108041 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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extremely slow process under normal conditions but it could significantly change the properties of the cement matrix by reduction of pH, increase in porosity and transport properties [3] and have detrimental effects on properties related to changes in phase fraction [4]. Considering the slow degradation process and the need to study long-term durability of concrete, accelerated testing is a necessary approach to better understand the effect of leaching on the alteration of microstructure and transport properties on the long-term. A variety of accelerated methods have been proposed such as applying an electrical field [5], using deionized water [6,7], using low pH solutions [8,9]; applying flow-through conditions [10] or using high concentration ammonium nitrate (NH4NO3) solutions [11,12]. Among them, using ammonium nitrate solution to accelerate the leaching kinetics is one of the most popular methods because it results in faster degradation compared to other methods under diffusive-transport conditions. The mechanism of acceleration is not only induced by the lower pH of NH4NO3 solution but primarily because of the increase in the solubility of the leachable phases in cementitious matrix induced by NH4NO3. Many studies focus on the modification of the microstructure after leaching [13,14] and the effects of leaching on mineralogical alteration [15,16] and some commonly used post-analysis techniques for characterizing leached materials are Mercury intrusion porosimetry (MIP) and thermal gravimetric analysis (TGA), which are also the techniques used in this study. Despite some limitations [17] (e.g. underestimation of big pores, inkbottle effect), MIP has been frequently used to characterize the pore structure of cementitious materials because it is simple, fast, reproducible and able to cover a wider range of pore sizes compared to other methods. Mercury as a non-wetting liquid for cementitious materials will not enter into the pore with a certain radius if the intrusion pressure does not reach a corresponding value according to the Washburn Eq. [18]. With this information, the pore sizes can be calculated from the pressures, which force mercury into the pores. MIP provides a variety of pore structure information including total porosity, pore size distribution, bulk density, specific surface area, and especially critical and threshold pore diameters, which are the most important parameters related to transport properties of cement-based materials (e.g. [19,20]). However, one should keep in mind that gel pores which are of nanometer scale are difficult to measure by MIP at normally applied pressure range (200–410 MPa). Furthermore, air voids and cracks are not detected by MIP. It is worth mentioning that other techniques are also used to quantify the porosity and microstructure of cementitious materials including scanning electron microscopy (SEM), N2-adsorption, water porosity, and He pycnometry. However, there is no unique technique that is able to characterize the microstructure at different scales. Therefore, several techniques have to be combined to provide complimentary information of pore structure, for example, Phung et al. [21] proposed a combined method in which N2-adsorption, mercury intrusion porosimetry and scanning electron microscopy were used to quantify the microstructural changes at nano, sub-micro, and micro level, respectively. This approach enables to obtain a lager range of pore size distribution and minimize the limitations of each technique. As proven in the work of Heukamp et al. [22], ductility of leached materials is significantly increased due to the chemical softening by calcium leaching. Young's modulus of degraded materials decreases 5 and 7 times for mortar and cement pastes, respectively. The same order of decrease in Young's modulus has also been observed by other researchers [16,23–28]. This suggests that the samples used for MIP experiments might also be significantly compressed under high mercury intrusion pressure (up to 410 MPa), which may lead to an overestimation of the porosity. This issue raises the question whether MIP is applicable to obtain trustworthy information on the porosity of leached materials. In addition, care must be taken while preparing samples of leached cement-based materials. Due to significant loss in mechanical properties, the samples must be handled in a proper way to preserve the integrity of leached materials [29]. Despite these problems, many

researchers have used MIP to characterize the microstructure of leached materials, e.g. [30–33]. TGA measures the changes in the mass of a sample when the sample is subjected to a temperature increase (most often a linear temperature increase with time). The changes that occur on heating include thermal events: melting, phase transition, sublimation, and decomposition. TGA is an extremely powerful quantitative thermal technique, but it gives no direct chemical information. Knowledge of reactions/processes occurring at specific temperature range during heating is required to determine the compositions of the sample. TGA measures mass changes in a material as a function of temperature (or time) and has been extensively used to quantify phase compositions in hydrated cement-based materials including portlandite, calcite [34–36], ettringite and gehlenite [37]. The method is thought to yield a higher accuracy compared to other methods (quantitative XRD, SEM). However, in case of overlapping decomposition, TGA alone cannot distinguish different phases. As the case with leached materials, the sample might contain Ca(NO3)2. xH2O, which is the product of reactions between NH4NO3 and hydrated phases in the sample [29], and also residual ammonium nitrate. With portlandite: CaðOHÞ2 þ 2NH4 NO3 →CaðNO3 Þ2 þ 2NH3ðaqÞ þ 2H2 O

ð1Þ

With Calcium silica hydrates (C-S-H): Cx Sy Hz þ 2NH4 NO3 →Cx−1 Sy Hz−1 þ CaðNO3 Þ2 þ 2NH3ðaqÞ þ 2H2 OCx−1 Sy Hz−1 þ 2ðx−1ÞNH4 NO3 →ySiO2 þ ðx−1ÞCaðNO3 Þ2 þ 2ðx−1ÞNH3ðaqÞ þ ðz þ x−2ÞH2 O (2) With Ettringite (AFt): 2C6 AS_ 3 H32 þ 24NH4 NO3 →3CaSO4 þ 9CaðNO3 Þ2 þ Al2 ðSO4 Þ3 þ 2AlðNO3 Þ3 þ 24NH3ðaqÞ þ 76H2 O

ð3Þ

With Monosulfoaluminate (AFm): _ 12 þ 48NH4 NO3 →CaSO4 þ 15CaðNO3 Þ þ Al2 ðSO4 Þ 4C4 ASH 2 3 þ 6AlðNO3 Þ3 þ 48NH3ðaqÞ þ 72H2 O

ð4Þ

Calcium nitrate is a very soluble salt. However, if the sample is not properly washed before TGA measurement, the sample is still contaminated with calcium nitrate and ammonium nitrate, of which the thermal decomposition might overlap with the other phases in the sample. Excessive washing, nevertheless, could induce artificial Caleaching resulting in modification of original state of examined materials. We have detected calcium nitrate in leached samples using XRD techniques [29] and the same observation has been shown by Puertas et al. [38]. This paper examines the issues associated with applying MIP and TGA techniques to study the microstructure and mineralogy of leached materials, respectively. In this context, accelerated leaching experiments of cement paste samples with different water/cement (w/c) ratios are used as the basis. Based on the analysis of MIP and TGA results, a procedure to correct MIP results as well as a proper way to interpret the TGA data are proposed. 2. Materials and methods This section details the materials, a leaching technique, characterization of the material and experiments for examining the thermal overlapping decompositions of calcium nitrate with various cement hydrated phases.

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2.3. MIP measurements

Table 1 Chemical compositions (wt%) of the cement. CaO SiO2 Fe2O3 Al2O3 Sulphate SO3

63.0% 20.0% 3.0% 5.0% 2.9%

3

Chromium(VI) Cl− Na2O eq. Loss on ignition Insoluble residue

b2.10−4% 0.06% 0.85% 1.60% 0.50%

2.1. Materials Leaching experiments were performed on cement pastes made from cement Type I ordinary Portland cement (CEM I 52.5 N) and tap water. Chemical compositions of the cement are shown in Table 1 [4]. Two samples with w/c ratios of 0.325 and 0.425 (S1 and S2, respectively) were investigated. Superplasticizer Glenium 27 was added to the mix with a weight content of 0.5% with respect to mass of cement. Cement pastes were poured and cured in a cylindrical PVC tube and then rotated during 24 h to prevent segregation [39]. The cement pastes were subsequently cured under sealed conditions in a temperature controlled room (21 ± 1 °C) until the age of 28 days. One-dimensional leaching was applied by immersing cured samples in 6 M ammonium nitrate solution chambers for 4 weeks [29]. To impose only onedimensional leaching at the bottom and top of the specimen, the PVC cover in the axial direction was not removed and any gap between the PVC cover and cement paste was filled with epoxy resin. N2 bubbling during the leaching experiment is to prevent the carbonation and to remove the formed NH3 gas (due to reactions (1)–(4)). Intact samples were prepared from the same batch of cement paste and kept under the same conditions but without immersion in ammonium nitrate solution.

For MIP measurements, several leached paste pieces were taken within 3 mm depth from the reactive surface (denoted as S1L, S2L), whereas intact samples (S1, S2) were taken after removing the air exposed surface. The mass of each sample was about 3–4 g. The freeze drying method was chosen to prepare the samples for MIP. Samples were directly immersed in liquid nitrogen until the escape of gas bubbles ceased. Subsequently, the samples were transferred to a vacuum chamber where a vacuum pressure of 2.5 × 10−2 mbar was applied for 24 h. Dried samples were kept in closed glass bottles until examination [4]. MIP experiments were performed on the PASCAL 140/440 porosimeter. The pressurization was done in low and high pressure parts. In the low pressure part, after evacuation and mercury filling, the pressure was continuously increased up to 0.2 MPa, while in the high pressure part, the pressure of mercury was continuously increased up to a maximum pressure of 200 MPa. 3. Results and discussion In this section, the thermal overlapping decompositions of calcium nitrate with cement hydrated phases is discussed, which serves as an experimental evidence to explain the issues related to the use of TGA for NH4NO3 leached materials. Next, the TGA results are presented, which shows the complex decompositions of various phases in leached samples making it almost impossible to correctly determine the phase composition of leached samples. In the end, the MIP results are shown and based on poromechanics theory, two correction steps are proposed to overcome the issues induced by compression of leached samples during MIP experiments. 3.1. Overlapping of thermal decompositions of pure phases

2.2. TGA measurements We tried to determine the portlandite and calcite profiles as well as roughly estimate bound water and C-S-H contents of a leached sample by TGA of the powder collected at different depths. A hole with a diameter of 10 mm was longitudinally drilled in the leached sample S2. Drilling was halted every 3 mm to collect the powder before continuing drilling until a depth of 12 mm was reached. With this, three subsamples have been leached (referred to as S2L-d1, S2L-d2, and S2L-d3, respectively, from the reactive surface) in order to observe the profile changes of different phases along the leached depth. To prevent cross contamination, the hole was carefully cleaned by compressed air before continuing drilling. For the intact sample, the surface of the sample that might be carbonated by the air was removed before taking the powder. To prevent further hydration, the powders were immediately vacuum dried for 3 days and kept in sealed conditions before TGA measurements. TGA was performed in a NETZSCH thermal analyzer coupled with mass spectrometry in order to detect released gases during thermal decomposition of the samples. A weighed sample, usually between 30 and 40 mg was heated from room temperature to 1050 °C with a slow heating rate of 5 °C per minute under a constant argon flow rate of 50 ml/min. Note that typically, nitrogen is used, but as in our samples, nitrogen will be released (please see later in reaction (14)), so we used inert gas argon. A slow heating rate helps to achieve good differentiation between different peaks. A blank test (without sample) was also performed to correct the buoyancy effect, which results in apparent mass increases [40]. In order to further examine the overlapping of thermal decompositions of calcium nitrate with other interested phases in the cement pastes, we have performed TGA on pure portlandite, calcium carbonate, calcium nitrate (Ca(NO3)2·4.1H2O), and a mixture consisting of CaCO3:Ca(OH)2:Ca(NO3)2 = 1:5:5 (in mass). Note that the mass ratio is 1:5:7.26 when 4.1 mol of H2O is considered in Ca (NO3)2·4.1H2O.

The decomposition of each pure phase does not seem to overlap as seen in Fig. 1. Ca(NO3)2·4.1H2O decomposes in a temperature range of 53–205 °C releasing H2O and in a range of 560–610 °C releasing NO, NO2, O2 (see later in Section 3.2). While portlandite and calcite decompose in temperature ranges of 390–485 °C and 600–870 °C to release H2O and CO2, respectively, which are outside the temperature ranges of calcium nitrate decomposition. However, when mixing, the decomposition temperature of each pure phase is shifted for both range and values compared to the original one. It can be clearly seen that there are three main decompositions of Ca(NO3)2·4.1H2O corresponding to temperature ranges of 53–172 °C (purple zone), 437–650 °C (blue zone), and 650–792 °C (orange-brown zone). The second temperature range overlaps the ranges of calcite and portlandite, which makes it very difficult to quantify the content of each phase. It means that for leached samples (considered as a mixture) composed of at least portlandite, calcite, calcium nitrate and other phases (e.g. C-S-H, ettringite, ammonium nitrate), the quantification of portlandite and calcite contents are not straightforward if not impossible. 3.2. Issues concerning the application of TGA on leached samples Fig. 2 shows the thermo-gravimetric (TG), derivative thermogravimetric (DTG) and differential scanning calorimetry (DSC) curves of both leached samples at different depths and intact sample S2. Two clear steps of portlandite (above 400 °C) and C-S-H (above 110 °C) decomposition were easy to detect in the intact sample, while a small peak of calcite (above 600 °C) also appeared. All leached samples exhibited a strong peak above 600 °C. This observation raised confusion: is this mass loss due to decarbonation or the result of other decompositions (note with ‘?’ in Fig. 2)? Furthermore, without further analysis, a strong peak at around 370–400 °C of sample S2L-d1 could be wrongly interpreted as the dehydration of portlandite. As shown by XRD measurement [4], portlandite was completely dissolved for the leached

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Fig. 1. Illustration of the overlapping of calcium nitrate decomposition with calcite and portlandite in the “mixture”: 3 main decompositions corresponding to temperature ranges of 53172 °C (purple zone), 437–650 °C (blue zone), and 650–792 °C (orange-brown zone). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

samples. These issues will be clarified by gas analysis using mass spectrometry as shown later in Fig. 3. The stepwise method was used to determine the weight percentage of different phases in the intact and leached samples, which is reported in Table 2. Note that the tangential method [41] is sometimes used to quantify the portlandite content in order to take into account the gradual decomposition of C-S-H. The dehydration of C-S-H mainly occurs in a temperature range of 180–300 °C [36]. However, it partially decomposes in a wide temperature ranges from 40 to 600 °C in which portlandite also decomposes (400–500 °C). Typically, the tangential method gives a relatively smaller portlandite content compared to the stepwise method [42]. The estimated portlandite content shows that it drops from 21.38% for the intact sample to a value of about 5.5–6.6% for all leached samples. These results conflict with XRD results reported in [4] where no portlandite was observed in leached samples. It means that other phases exist, which decompose in the temperature range of 400–500 °C. The amount of calcium carbonate doubled after leaching. Note that carbonation was prevented by N2 bubbling during the leaching experiment. Therefore, the double CaCO3 content can be explained by two contributions: (i) the bulk density decrease (38% [4]) due to leaching (portlandite and other phases were leached out) induces an increase in the relative content of CaCO3 (normalized to total solid mass, assuming absolute content of CaCO3 remains constant), and (ii) the decomposition of other phases rather than CaCO3 in the range of 600-800 °C. It is worth mentioning that the leaching of C-S-H results in a lower Ca/Si ratio, which can reach 1 (compared to the initial value of 1.75) as shown in a previous study [4]. A recent study [43] showed that the dehydration of C-S-H with different Ca/Si ratios gives similar mass loss steps. However, the mass loss is larger for C-S-H with lower Ca/Si ratios. Only at high temperature above 800 °C, there are differences in the position and the shape of the DSC events by the exothermal event due to the transformation of C-S-H into β-wollastonite (800–900 °C) and the

endothermal transformation of β-wollastonite into α-wollastonite (1220–1280 °C). As a conventional interpretation, the amount of C-S-H is estimated by considering the total mass loss from 105 to 1000 °C after subtraction of mass loss in the temperature ranges of 400–500 °C and 600-800 °C for Ca(OH)2 and CaCO3. It is worth mentioning that there are also decompositions of other phases (AFt, AFm, gypsum, etc.) in the temperature range of 100–300 °C [42,44]. However, these phases are minor in our case. The quantitative XRD results [4] on intact and leached samples showed that the mass fraction of these phases were always b1%. Hence, these phases are not considered in the estimation of the amount of C-S-H. This is to avoid the complicated quantification and without losing the main purpose of this semi-quantification, which is to compare the amount of major phases (C-S-H, calcite and portlandite) before and after leaching. Note that in order to estimate the C-S-H amount, its stoichiometry is needed. However, C-S-H cannot be described by a single stoichiometry, but a range of stoichiometry. As a first approximation, the stoichiometry of C-S-H is assumed to be (CaO)1.7(SiO2)(H2O)1.80 [45]. As shown in column {4} of Table 2, the C-S-H content is increased due to leaching instead of decreasing as reported in [4] due to the partial decalcification of C-S-H. This overestimation of leached samples is again believed to be a consequence of misinterpretation mentioned above. Importantly, we observed a decreased content of portlandite, calcite and C-S-H as the depth increases. This observation could be interpreted to mean that the sample obtained from near reactive surface are more contaminated with ammonium nitrate and calcium nitrate compared to the one further away from the reactive surface. Fig. 3 shows the ion current curves, which allow us to identify the type of gas released as temperature increases. Note that different scaling factors were used to have a better visualization of thermal events and to obtain a good differentiation between different peaks. The main gas released for the intact sample is water vapour (m/z = 18) with two peaks

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5

Fig. 2. TG (in red), DTG (in purple) and DSC (in blue) curves of leached samples at different depths (S2L-d1, S2L-d2, and S2L-d3) and intact sample (S2): CH = portlandite, CC = calcite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

at 143 °C and 425 °C corresponding to the dehydration of C-S-H and portlandite, respectively. As expected, there is no water peak in the temperature range of 400–500 °C for leached samples S2-d2 and S2-d3 because of a complete portlandite depletion. However, for leached sample S2-d1 (near reactive surface), we observed a strong water peak at 375 °C, which is attributed to the dehydration of NH4NO3 following a series of reactions given below, see Eqs. (5), (6), and (7). It is thought that at 210 °C, NH4NO3 starts dissociating to form HNO3 as an intermediate, which is later decomposed to release O2 and NO2 [46]. At a temperature of 320 °C, NH4NO3 is directly decomposed to form water. The peak of NH3 (m/z = 17) was also observed at 220 °C associated with the dissociation of NH4NO3 at low temperature. NH4 NO3

210o C

→

dissociation

NH3 þ HNO3

HNO3 →NO2 þ H2 O þ 1=2O2 NH4 NO3

320o C

→

decomposition

N2 O þ H2 O

ð5Þ ð6Þ ð7Þ

decomposition (7), but at lower temperature of 320 °C. The appearance of N2O peak partially causes the misleading interpretation of portlandite decomposition. 560o C

NH4 NO3 → 5N2 þ 4NO þ 2NO2

ð8Þ

4NH3 þ 3O2 →2N2 þ 6H2 O

ð9Þ

NH3 þ O2 →N2 O þ 3H2 O

The leached sample near the reactive surface (S2-d1) was more contaminated by calcium nitrate. We observed several water peaks below 250 °C, which is attributed to the dehydration of calcium nitrate following reactions (11), (12), and (13) at different temperatures [48]. CaðNO3 Þ2  4H2 O CaðNO3 Þ2  3H2 O

30−40o C

→

CaðNO3 Þ2  3H2 O þ H2 O

40−140o C

CaðNO3 Þ2  2:5H2 O In all leached samples, there exists a N2 peak (m/z = 28) above 600 °C. The presence of N2 is due to either the dissociation of NH4NO3 and calcium nitrate at high temperature (reactions (8) and (14), respectively) or the oxidation of NH3 (reaction (9)) [47]. This peak results in an overestimation of calcium carbonate content as shown in Table 2. Furthermore, the oxidation of ammonia can also release nitrous oxide (N2O) following the reaction (10). This could explain N2O (m/z = 44) peak at about 500 °C. Note that N2O is also formed during NH4NO3

ð10Þ

→

CaðNO3 Þ2  2:5H2 O þ 0:5H2 O

140−225o C

→

CaðNO3 Þ2 þ 2:5H2 O

ð11Þ ð12Þ ð13Þ

The dissociation of Ca(NO3)2 at high temperature (N560 °C) could release oxygen and either N2, NO, or NO2 (reactions (14), (15), and (16)) depending on a variety of conditions [49]. However, for sample S2-d1, we have seen the peak of O2 (m/z = 32) associated with the N2 peak at a temperature above 600 °C. Therefore, the reaction (14) could have dominated the dissociation of Ca(NO3)2. As a result, the peaks of N2 and O2 could lead to a misinterpretation of CO2 decarbonized from

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Fig. 3. Ion current curves for the gas evolution of leached and intact samples – s is scaling factor.

CaCO3 if one only interprets the TG curve without the ion current curves. As an example from literature, Babaahmedi et al. reported the TG/DTG curves of leached samples obtained from accelerated electrochemical migration experiments using ammonium nitrate as catholytic solution, and from natural leaching experiments using groundwater [15]. Two peaks were clearly observed on DTG curves above 600 °C, which was believed to be due to the contamination of ammonium nitrate or/and calcium nitrate as a product in the accelerated leaching experiments. Whereas, there is no visible peak for naturally leached samples above 600 °C. Other example is the recent work of Cheng et al. [50] in which the authors showed that there was no portlandite peak observed by XRD experiments, but interpretation based on TGA experiments suggested the presence of portlandite because of mass loss in a temperature range of 440–580 °C. These contradicting results should again be attributed to the dehydration of ammonium nitrate and decomposition of calcium nitrate. 560o C

CaðNO3 Þ2 → CaO þ N2 þ 2:5O2 560o C

CaðNO3 Þ2 → CaO þ NO þ 1:5O2

ð14Þ ð15Þ

560o C

CaðNO3 Þ2 → CaO þ NO2 þ 0:5O2

ð16Þ

The question that remains now is whether it is possible to quantify the phase contents of NH4NO3 leached samples by using TGA coupled with mass spectrometry. As seen, the decompositions of ammonium nitrate and calcium nitrate are quite complex following a number of reactions at different temperature ranges. Therefore, it is almost impossible to correctly determine the phase composition of leached samples contaminated with ammonium nitrate and/or calcium nitrate. 3.3. Issues concerning the application of MIP on leached samples As discussed above, significant loss of mechanical properties was observed due to the complete dissolution of portlandite and partial decalcification of C-S-H of cement pastes in ammonium nitrate solution. The deformation at failure of leached samples can increase by a factor of five in tension and two in uniaxial compression compared to intact material [22]. During mercury intrusion process, the sample is under much higher isotropic compression because of very high applied pressures. Therefore, the deformation must be higher than that reported in [22]. Therefore, in the interpretation of MIP results, it is also necessary to

Table 2 Estimation of the decomposition of different phases in different temperature ranges: The first values report mass loss, while values in bracket estimates mass content of indicated phases. Samples

400–500 °C (Portlandite, %) {1}

600–800 °C (CaCO3, %) {2}

105–1000 °C (Bond water + carbonate, %) {3}

{3}-{2}-{1} (C-S-H) {4}

S2 S2L-d1 S2L-d2 S2L-d3

5.20 (21.38) 1.61 (6.62) 1.56 (6.41) 1.35 (5.55)

1.17 (2.66) 2.28 (5.18) 2.06 (4.68) 1.90 (4.32)

16.90 17.37 16.69 15.68

10.53 (60.97) 13.43 (78.05) 13.07 (75.68) 12.43 (65.31)

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account for volumetric deformation of the sample because of isotropic compression. This issue is already recognized by porosimeter manufacturers and hence they provide a provision to include correction in the software supplied by them, for example, SO.L.I.D software of PASCAL series Mercury porosimeter used in this study. However, in carrying out the correction, the porosimeter interprets the sample compression as being directly related to the pores. Thus, the software corrects the measurement by subtracting the volume of sample compression as follows [51]:  vc ¼ vi −

1 1 − ρb ρske

 ð17Þ

where vc is specific compression volume [m3/kg] at the end of the filling process; vi is specific intruded volume [m3/kg]; ρb is bulk density [kg/ m3]; and ρske is skeletal density, which is defined as unit weight of sample excluding accessible pores [kg/m3]. The skeletal density is usually measured externally by He pycnometry. Without this information, the skeletal density is roughly taken as the apparent density ρa [kg/m3] itself without introducing too much error, as the case in this study. The apparent density is defined as unit weight of sample excluding accessible pores at the end of the filling process. The corrected intruded volume at the end of filling process, Vcor [m3] can then be calculated as follows: i V cor i

¼ ðvi −vc Þmsam

difference is larger when the isostatic pressure increases. Therefore, further correction is necessary to retrieve the true intruded volume of an uncompressed sample. In this sense, recourse is made to the well-known poromechanics theory, which provides a relationship between pore, bulk and skeleton compressibility of the sample from which the true intruded volume, Vpcan be retrieved. Note that there is still an open question whether poromechanics theory, which is originally developed for soils, is applicable for cementitious materials. However, in the last few decades a number of researchers in this field have successfully demonstrated that concrete can be treated within the framework of poromechanics (Ulm et al. [52], Heukamp et al. [53] and Ghabezloo et al. [54]). In soil consolidation experiments, a porous material is subjected to different types of consolidation tests under drained, undrained, or unjacketed conditions [54]. In MIP test, the sample is submerged in mercury under isotropic loading, in which equal increments of confining pressure exerted by mercury and pore pressure are simultaneously applied to the sample. These test conditions are very similar to the conditions of unjacketed compression test. Therefore, the pore compressibility, Cpc [Pa−1] (at varying confining pressures) can be defined as [55]: C pc ¼

ð18Þ

where msam is sample mass [kg]. The corrected intruded volumes at the lower pressure filling processes are then possible to calculate by assuming a linear sample compression with pressure increase. Fig. 4 presents the cumulative intrusion curves and differential pore size distributions of intact and leached samples S1 and S2 before and after correcting for the compression effect using set of Eqs. (17) and (18). The more applied mercury pressure, the larger the differences between results with and without compression correction. The deviation starts at lower applied pressure for leached samples compared to intact samples (corresponding to pore diameters of 0.2 μm compared to 0.04 μm, respectively). As a consequence, the error for leached samples is larger compared to intact samples. It can be seen that the pore volume is compressed by 33% and 37% for leached samples S1L and S2L, respectively. Such a high value cannot be neglected. However, it seems that the critical pore diameter (which is the most frequently occurring pore size in interconnected pores) and threshold pore diameter (the largest pore size at which pore volume is significantly increased) are not changed. The above correction is basically just to correct the ‘misinterpretation’ of the raw porosimeter data associated with the sample compression. In other words, the corrected volume Vcor reflects the mercury filled porosity of the compressed sample only (Fig. 5). However, the main goal of the MIP study is to obtain the true intruded volume, Vp of the uncompressed (original) sample. In fact, the pore volume of the uncompressed sample is always higher than the mercury intruded volume due to pore volume compression under isostatic pressure. The

7

  1 ΔV P V p ΔP c Pd

ð19Þ

where Pc is confining pressure [Pa] and Pd = Pc – pf is the effective stress for saturated conditions and remains constant because of equal increments of Pc and pf. By applying Betti's reciprocal theorem and superposition principle, Zimmerman et al. [55] proposed the link between Cpc and the bulk compressibility, Cbp [Pa−1] (at varying pore pressures), and skeleton compressibility Cs [Pa−1], as follows [55]: C pc ¼

  1 1 1 1 1 C bp ¼ ðC bc −C s Þ ¼ − ϕ ϕ ϕ Kb Ks

ð20Þ

where ϕ is porosity [−], Cbc is bulk compressibility at varying confining pressures, Kb and Ks [Pa] are the bulk modulus of the porous matrix and skeleton (solid phase) of the sample, respectively. Substituting Eq. (20) into (19) allows us to compute the pore volume change under mercury pressure (confining pressure): ΔV p ¼

    1 1 1 1 1 − − V p ΔP c ¼ VΔP c ϕ Kb Ks Kb Ks

ð21Þ

where V [m3] is sample volume, V = Vp/ϕ, which is measured during MIP test. Increments of confining pressure, ΔPc is also recorded during the test. Therefore, if the bulk moduli Kb and Ks are known, we can calculate the pore volume change due to compression. However, determination of these moduli requires a highly precise and complex test setup (e.g. ultrasonic pulse velocity, nanoindentation [56]). In this study, we did not measure Kb and Ks but simulated based on an effective media

Fig. 4. Influence of deformation of leached sample on MIP results.

8

Q.T. Phung et al. / Materials and Design 182 (2019) 108041

Vp

Original

Vs

Pores

Solid ΔVs

Compressed

Pores

Solid

Vcor(< Vp) ucor

V

(> Vp)

Fig. 5. A schematic illustration of pore and solid compression of sample during MIP experiment (V: Volume, subscripts p: pores, s: solid, cor: corrected, ucor: uncorrected).

theory [57]. In order to estimate the bulk modulus, the cementitious matrix is represented by different phases including C-S-H, portlandite (CH), clinkers (alite, belite, etc.) and pores, whereas the skeleton modulus is estimated without the pores. The volume fraction of each cement phase is calculated using data reported in the work of Phung [29]. For CS-H, it is widely accepted that it is composed of high density (HD) C-S-H and low density (LD) C-S-H. By using nanoindentation technique, Constantinides et al. [58] concluded that the volumetric proportions of these two types of C-S-H are not affected by the leaching degradation process. The volume fraction of HD C-S-H and LD C-S-H was in the order of 30% and 70%, respectively, for a cement paste of w/c ratio of 0.5. These fractions were used to calculate the volume fraction of LD and HD C-S-H in this study as shown in Table 3. By using the moduli and Poisson's ratio of each phase reported in [58], we are able to obtain the Kb and Ks of both intact and leached samples as shown in Table 4. The ‘real’ intruded volume at the nth pressure increment, Vp, n can then be calculated as (see Fig. 5): V p;n ¼ V ucor n −ΔV s;n

ð22Þ

is the uncorrected cumulative intruded volume at nth preswhere Vucor n sure increment and ΔVs, n is the cumulative volume compression of solid phase, which can be calculated as follows:
cor

ΔV s;n ¼ V ucor n −V n

ð23Þ

−ΔV p;n

Substituting Eq. (23) to Eq. (22) we obtain: V p;n ¼ V cor n þ ΔV p;n

ð24Þ

where ΔVp, n is the cumulative pore compression volume [m3] at nth pressure increment that is computed using Eq. (21) and which forms the second correction; first correction being Vcor n . Fig. 6 presents the cumulative intruded volume of samples without correction, using the first compression correction (basic correction offered by the porosimeter) and with the second (poromechanics based) correction, which is considered as the ‘real’ intruded volume. The pore volume after the second correction falls between the pore volume without correction and first correction for both intact and leached samples. More interesting, for intact samples, the second corrected results are closer to the first corrected results, in contrast to the leached samples. It means that the second correction is not necessary for intact sample, but very important for leached samples. In order to estimate the error induced if only the first correction is made (Error1, %), and error if no correction is made (Error2, %) we use the following simple expressions to compute the relative error at the nth pressure increment. Error1 ¼

V cor V cor −V p;n n −V p;n 100%Error 2 ¼ n 100% V p;n V p;n

ð25Þ

Table 3 Input parameters for estimating the bulk and skeleton moduli. Parameter phase

Phase modulus Ksi, GPa

Poisson's ratio

Volume fraction

Phase modulus Ksi, GPa

0.193 (0.171) 0.451 (0.398) 0.112 (0.197) 0.149 (0.16) 0.095 (0.074)

Intact materials S1 (S2) 29.4

0.24

LD C-S-H

21.7

0.24

Pores

–

–

38.0

0.31

135

0.30

Clinkers a

a

Volume fraction

12.0

0.24

3.0

0.24

–

–

–

–

–

–

0.217 (0.185) 0.505 (0.431) 0.278 (0.384) 0 (0) 0 (0)

Leached materials S1L (S2L)

HD C-S-H

Portlandite

Poisson's ratio

Modulus of clinkers is the average value of unhydrated cements summarized in [58].

Q.T. Phung et al. / Materials and Design 182 (2019) 108041 Table 4 Estimation of the bulk moduli using the effective media theory. Sample

Bulk modulus Kb, GPa

S1 (w/c = 0.325) S2 (w/c = 0.425)

Bulk modulus Ks, GPa

Intact

Leached

Intact

Leached

24.6 19.3

2.7 2.0

31.2 29.9

5.2 5.2

Note that Vucor [m3] is the pore volume without any correction at the n n pressure increment, which is reported by the MIP porosimeter. The absolute porosity deviation induced by compression can also be calculated as follows: th

V cor n −V p;n 100% V sam V ucor n −V p;n 100% V sam

If only the first correction is made: ð26Þ If no correction is made:

S1 S1_1st correction S1_2nd correction S2 S2_1st correction S2_2nd correction

70 60

degraded samples (Fig. 6) is much larger than intact sample, and with the definition of the error expressed in Eq. (25), we might observe opposite trends when comparing the error and absolute porosity deviation. At a conventional applied pressure of 200 MPa, the error and absolute porosity deviation after the first correction of intact samples is below 5% and 0.5%, respectively, which is acceptable when considering the uncertainty of MIP measurement. Nevertheless, the error and absolute porosity deviation for leached sample could be up to 25% and 6%, respectively, which is very high and induces a significant underestimation of the porosity of leached materials. Therefore, an additional correction is needed for leached materials as described in Eq. (24). In any case, in order to limit the error below 10%, the applied pressure should not be higher than 50 MPa, otherwise, corrections should be made. This applied pressure corresponds to a pore diameter of the order of 30 nm, which is in the range of conventional critical pore diameter of cementitious materials (20–40 nm). Therefore, MIP should be combined with other techniques, for example, N2-adsorption [21] in order to quantify lower pore size range, especially for leached material. Finally, the error is shown to be significantly higher at low pressures especially for intact S1 and S2 samples and less pronounced for other samples. However, this is attributable to the sensitivity of the MIP porosimeter at low pressures and the way to define the error expressed by Eq. (25). Therefore, the absolute porosity deviation is also shown (Fig. 7 and Eq. (26)) in order to provide a comprehensive view of compression effect. The correction procedure of MIP results may also be applicable for other chemically degraded materials. In general, the correction steps are not necessary if the solid phase is only slightly compressed (e.g. carbonation) under testing pressure of MIP experiments (200 MPa in this study). For other degradation processes such as acid attack that may result in a significant increase in solid compressibility, the corrections are necessary. 4. Conclusions and suggestions This work presents a detailed analysis of TGA and MIP results for examination of ammonium nitrate leached cementitious materials with a view to highlight possible pitfalls in the use and interpretation of these techniques. Our comprehensive experimental data set combined with a detailed analysis using poromechanics theory and thermal reactions/ processes allow us to draw a number of key conclusions and suggestions as follows: • TGA analysis (even coupled with mass spectrometry) could not be used to precisely quantify phases (e.g. portlandite, calcite, C-S-H) of

Cumulateve intruded vol. /mm3/g

Cumulateve intruded vol. /mm3/g

Fig. 7 presents the errors and absolute porosity deviations (with and without the first correction) associated with MIP measurements for the intact and leached samples S1 and S2. Note that if the second correction is made, then the error is deemed zero. It is generally seen that the higher the applied pressure, the larger the error (and deviation). This is because the effect on sample compression becomes more pronounced at higher applied pressure. Without any correction, the pore volume is overestimated (positive value) for both intact and leached samples. Whereas, the pore volume is underestimated (negative value) if only the first correction is made for both intact and leached samples. Intact samples with higher w/c ratios display larger errors (and porosity deviation) either with the first correction or without any correction. This is because of relatively higher compressibility of higher w/c samples. However, this observation is only true for degraded samples with the first correction. In case of no correction, MIP porosimeter overestimates the pore volume of leached samples (Δv = Vucor – Vcor), which is compensated by the compression of pore volume (ΔVp). In fact the absolute value of Δv is always larger than ΔVp because it includes the compression of both solid and pores as illustrated in Fig. 5. Therefore, leached samples with higher compressibility will give larger ΔVp resulting in smaller error. Furthermore, with the same w/c ratio, the absolute porosity deviation with the first correction of degraded sample is around 7–8 times larger than non-degraded samples, regardless of applied pressure. However, without any correction, the deviation of degraded samples only doubles the one of intact samples (Fig. 7b). Note that the error of intact samples is still larger in this case for S2L and similar for S1L (Fig. 7a). This is because of the fact that the pore volume of

50 40 30

9

300

S1L S1L_1st correction S1L_2nd correction S2L S2L_1st correction S2L_2nd correction

250

200

150

100

20

Non-degraded

10 0

Degraded

50

0

0

50

100

150

200 Pressure /MPa

0

50

100

150

200 Pressure /MPa

Fig. 6. Comparison of the cumulative intruded volume of samples without correction, with the basic compression correction and with the second poroelastic correction.

10

Q.T. Phung et al. / Materials and Design 182 (2019) 108041

Error 2, %

35 25

Error without any correction

5

Leached

Intact

15

Error 1, %

-5 -15 -25

Error with the first correction

-35 0

50

100

150 200 Pressure /MPa

8

Error without any correction 2 0

Leached

4

Intact

Absolute porosity deviation, %

6

-2 -4 -6

Error with the first correction

-8 0

50

100

150 200 Pressure /MPa

Fig. 7. Comparison of relative errors (a) and absolute porosity deviation (b) induced by compression of intact and leached samples with the first correction and without any correction.

NH4NO3 leached cementitious samples. The error may reduce if a proper washing step is performed (to remove remaining ammonium nitrate and calcium nitrate) before TGA measurement, but this washing can invoke dissolution of the solid phases. • TGA overestimates the portlandite, calcite and C-S-H content of NH4NO3 leached cement samples. It is therefore advised to use complementary post-analysis techniques such as quantitative XRD,

SEM-EDX to determine various phases of leached materials instead of TGA. • The correction procedure of MIP results involves two steps: (i) the 1st correction to correct the ‘misinterpretation’ of the porosimeter results due to sample compression (made available by porosimeter manufacturer); (ii) the 2nd correction to retrieve the true intruded volume of an uncompressed sample via a poromechanics approach that takes

Q.T. Phung et al. / Materials and Design 182 (2019) 108041

into account the compressibility of pores and solid phases. For intact samples, the second correction is not necessary, but very important for leached samples because the error could be up to 25%. • MIP porosimeter usually overestimates the porosity of leached samples, which is compensated by the compression of pores and solid phases (underestimation). Therefore, by coincidence, without any correction, the error of leached samples is smaller than intact samples. • It seems that the critical and threshold pore diameter are not changed even without applying any correction. However, the cumulative pore volume changes induce misinterpretation of other microstructural properties (e.g. specific surface area, average pore diameter). Therefore, if the second correction is not performed due to complex procedure and requirements of knowledge on compressibility and elastic modulus of individual phase, MIP should be combined with other techniques (e.g. N2-adsorption) to limit the error and allow quantification of lower pore size range, especially for leached material. CRediT authorship contribution statement Quoc Tri Phung: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing - original draft. Norbert Maes: Funding acquisition, Resources, Supervision, Writing - review & editing. Suresh Seetharam: Conceptualization, Validation, Writing - review & editing. Declaration of Competing Interest None. Acknowledgements This work is supported by SCK•CEN as part of their internal RD&D program on concrete durability. The authors are thankful to Saeid Babaei from SCK•CEN for providing theoretical estimates of the effective bulk modulus of the materials used. Data availability statement All data reported in this paper is contained within the manuscript. References [1] F.H. Heukamp, Chemomechanics of Calcium Leaching of Cement-Based Materials at Different Scales: The Role of CH-Dissolution and C-S-H Degradation on Strength and Durability Performance of Materials and Structures, PhD thesis Dept. of Civil and Environmental Engineering, Massachusetts Institute of Technology, 2003. [2] D. Jacques, N. Maes, J. Perko, S.C. Seetharam, Q.T. Phung, R. Patel, A. Soto, S. Liu, L. Wang, G.D. Schutter, G. Ye, K.v. Breugel, Concrete in engineered barriers for radioactive waste disposal facilities - phenomenological study and assessment of long term performance, 15th International Conference on Environmental Remediation and Radioactive Waste Management - ICEM2013, Brussels, Belgium 2013, pp. 1–10. [3] Q.T. Phung, N. Maes, D. Jacques, G.D. Schutter, G. Ye, Microstructural and permeability changes due to accelerated ca leaching in ammonium nitrate solution, in: M. Grantham, P.A.M. Basheer, B. Magee, M. Soutsos (Eds.), Concrete Solutions - 5th International Conference on Concrete Repair, CRC Press 2014, pp. 431–438. [4] Q.T. Phung, N. Maes, D. Jacques, G. De Schutter, G. Ye, Investigation of the changes in microstructure and transport properties of leached cement pastes accounting for mix composition, Cem. Concr. Res. 79 (2015) 217–234. [5] H. Saito, A. Deguchi, Leaching tests on different mortars using accelerated electrochemical method, Cem. Concr. Res. 30 (11) (2000) 1815–1825. [6] K. Haga, S. Sutou, M. Hironaga, S. Tanaka, S. Nagasaki, Effects of porosity on leaching of ca from hardened ordinary Portland cement paste, Cem. Concr. Res. 35 (9) (2005) 1764–1775. [7] J. Jain, N. Neithalath, Analysis of calcium leaching behavior of plain and modified cement pastes in pure water, Cem. Concr. Compos. 31 (3) (2009) 176–185. [8] A. Bertron, J. Duchesne, G. Escadeillas, Accelerated tests of hardened cement pastes alteration by organic acids: analysis of the pH effect, Cem. Concr. Res. 35 (1) (2005) 155–166. [9] L. De Windt, P. Devillers, Modeling the degradation of Portland cement pastes by biogenic organic acids, Cem. Concr. Res. 40 (8) (2010) 1165–1174. [10] E.J. Butcher, J. Borwick, N. Collier, S.J. Williams, Long term leachate evolution during flow-through leaching of a vault backfill (NRVB), Mineral. Mag. 76 (8) (2012) 3023–3031.

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