Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry

Cement and Concrete Research 122 (2019) 107–117

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Dynamic microstructural evolution of hardened cement paste during first drying monitored by 1H NMR relaxometry

T

Ippei Maruyamaa, , Takahiro Ohkubob, Tatsuto Hajia, Ryo Kuriharaa ⁎

a b

Graduate School of Environmental Studies, Nagoya University, ES-546, Furo-cho, Chikusa-ku, Nagoya, Japan Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage, Chiba, Japan

ARTICLE INFO

ABSTRACT

Keywords: (a) Drying (b) Calcium-silicate-hydrate (C-S-H) (b) Microstructure (b) Pore size distribution

In this study, the rate of change of microstructural re-organization in hardened cement paste under the first drying, which is caused by the colloidal nature of calcium-silicate-hydrate (C-S-H), is confirmed via 1H nuclear magnetic resonance relaxometry. Under the first drying, water evaporates from larger pores, while the interlayer water begins to increase. During the mass change, interlayer water continues to increase until reaching a peak. After the peak has been reached, interlayer water gradually decreases, while interhydrate water and capillary water appear with little observable change in mass. Based on the data, gel pores and interlayer spaces are considered to share movable C-S-H sheets, and removal of water from such pores causes a change in distance between C-S-H sheets, which transforms gel pores to interlayer spaces.

1. Introduction Moisture transport and shrinkage of cement-based materials has been discussed for more than half a century; this is because calcium (aluminate) silicate hydrate (C-(A)-S-H), which is the main hydrate phase of the Portland cement system, has a colloidal nature, and its microstructure changes under drying, which makes prediction of moisture transport and shrinkage difficult. Nitrogen and water sorption isotherms [1–5] and small-angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) [6–9] have elucidated the microstructure changes of hardened cement paste (hcp) under drying. Atomic-scale changes of calcium-silicate-hydrate (C-S-H) under drying have also been reported in the literature [8,10–12]. In reference to these phenomena, the meso-scale model of C-S-H has been discussed [13–17]. Some relevant reports are introduced here. As discussed in papers regarding the formation of C-S-H [18,19], research suggests that dimers of silicate can attach to a mono‑calcium layer, forming C-S-H with a high Ca/Si atomic ratio. At this stage, dimers are randomly attached to the calcium layer, and, as a result, the CS-H exhibits winding. The randomly attached dimers may also lead to difficulty in keeping the inter-layer space constant, owing to the imbalance of intermolecular forces between layers. During hydration, monolayers of C-S-H (C-S-H sheets) are distributed where the pore water exists, and agglomerated C-S-H sheets form. The final stage of this process is the same as the C-S-H model proposed by Feldman and

⁎

Sereda [13]. The gel-pore water and the interlayer water are isolated from the capillary water by C-S-H sheets. Changes in C-S-H agglomeration during the first desorption were analyzed using ultra-small angle X-ray scattering [8]. Using disc-fractal model fitting, the disc thickness (considered to be C-S-H monolayers) was ~2 nm; however, when the hcp was dried at 50% and 60% RH, the apparent disc thickness became 6.0–7.5 nm. This indicates that the C-SH sheets were stacked as a result of capillary tension or surface energy acting on the C-S-H sheets. When the sample was dried at less than 40% RH, the disc thickness returned to 2 nm. Because the C-S-H atomic-scale layered structure does not change easily, it is concluded that the C-S-H sheets were movable and that the distance between sheets changed during drying. Consequently, the re-organization of the microstructure under drying is attributed to the arrangement of the C-S-H sheets. This is an example of the characterization of the C-S-H solid skeleton during drying. To focus on the behavior of the water, 1H NMR relaxometry has been applied recently. Water-containing pores of different sizes are connected, and an exchange of water molecules occurs for hcp. However, 1H NMR relaxometry methods are based on the premise that these pores are isolated from each other with regard to the pore water exchange rate [20]. Based on the T2–T2 spectrum data of a hcp sample with a storage time of 3 ms, a water-to-cement ratio of 0.4, and an age of 1 day, it is observed that, under these storage conditions, there is water exchange between the interlayer water and the gel-pore water

Corresponding author. E-mail address: [email protected] (I. Maruyama).

https://doi.org/10.1016/j.cemconres.2019.04.017 Received 11 August 2018; Received in revised form 18 April 2019; Accepted 24 April 2019 Available online 09 May 2019 0008-8846/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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[21,22]. Furthermore, there is no water exchange between the capillary water, gel-pore water, and interlayer water; however, such an exchange was observed in the synthesized C-S-H. It should be noted here that the C-S-H formed in white cement paste was supersaturated with Ca ions; therefore, the C-S-H Ca/Si ratio is probably greater than 1.5 and has a fibrillar shape, while the synthesized C-S-H Ca/Si ratio was 0.75 and has a foil-like shape [23]. This evidence of exchange between only the gel-pore water and the interlayer water in a wet hcp system shows that the surface where the capillary pores are connected to gel-pores and interlayer spaces is small from the view point of moisture transport. Consequently, the C-S-H is considered to have a long-range sheet-type morphology and not the short-range “globule”, in which water molecules are easily exchanged between larger pores and gel pores [15]. The relatively long-term redistribution of water from capillary pores to interlayer and gel pores during water sorption was confirmed by 1H NMR relaxometry [24–26]. In the data reported by Gajewicz et al. [24], a hcp sample dried at 60 °C absorbs capillary water; the absorbed water is then redistributed through interlayer space to the gel pores (Fig. 2 of Ref [24]). During this period, the amount of interlayer water exhibits a peak at 8 h after the commencement of water absorption and remains constant afterwards. All the interlayer water appeared to transfer into gel-pore water from the original capillary water, and the amount of gelpore water gradually increased 22 h after the commencement of water absorption. It is concluded that the redistribution of interlayer water and gel-pore water is a property of C-S-H. Many behaviors of water in hcp have been clarified; however, the role of C-S-H during the drying process of hcp is not yet fully understood. In this paper, based on prior literature, the first desorption process is investigated to confirm how the microstructure changes during the first drying. 1H nuclear magnetic resonance (NMR) relaxometry [21,22,27–42], which enables in situ monitoring of the microstructure by using water molecules as a probe, is applied to understand the dynamic microstructural rearrangement of hcp under drying.

glass cylindrical tube (φ4.5 mm) and a plastic vial (with dimensions of φ30 mm × 125 mm). The former samples were used for 1H NMR relaxometry measurements, and the latter samples were used for thermogravimetric analysis. For 1H NMR relaxometry measurements, three samples were used for each condition. All the samples were sealed immediately after pouring. After 28 days, the hcp samples in glass cylindrical tubes were demolded and immediately used for 1H NMR relaxometry measurements. After that, the samples were set in large vials containing saturated salt solution to dry the samples under different relative humidity (RH) conditions, specifically 80%, 40%, and 11% RH at 20 °C (See Fig. 1.). These conditions were achieved using ammonium chloride, sodium iodide, and lithium chloride, respectively. Due to the impurity of the reagents, actual RH was determined using a mirror dew point meter (AquaLab Series4TE, Meter Group, Inc.). The measured data were 77.8%, 39.4%, and 11.3% at 20 °C, respectively. The samples dried under 80%, 40%, and 11% RH were denoted as W55-RH80, W55-RH40, and W55-RH11, respectively. In addition, the sample mass change was measured with a balance. In the cases of 3, 6, and 9 h of drying, a 1H NMR relaxometry measurement was conducted only for one sample because of time limitations. 2.2. Method TG analyses were conducted on ~20-mg samples from the 28-day sealed samples. Dynamic TG measurements were conducted under N2 gas flow from room temperature to 550 °C, including a 7-h holding period at 105 °C. The mass change from the end of the holding period up to 550 °C, except for the mass change due to decomposition of calcium carbonate, is referred to here as the chemically bound water. The 1H NMR relaxometry measurements were conducted using a Bruker minispec equipped with a digital filter and a φ10 mm probe. The magnet provided a resonance frequency of 20 MHz (0.47 T) for protons. The π/2 pulse length was 2.4 μs, which was calibrated on a solid cement sample. The relaxation delay was 1 s. The quantification of chemically bound water was carried out following the method described in a previous study by Muller et al. [41]; the solid-echo (quadrature) decay signal was acquired and analyzed. The solid echo signal can be expressed by two components: (i) a rapid decay component arising from magnetic dipole–dipole (DD) interaction in solid phases (or chemically bound water) and (ii) a slow decay component resulting from averaged DD interaction by molecular motion attributed to liquid water as shown Fig. 2. The rapid decay due to DD interaction leads to a measurement with a Gaussian line shape centered at an echo time (τ) with a relaxation time T2, solid (on the order of microseconds). On the other hand, the slow decay resulting from liquid water yields a measurement with an exponential shape with T2, liquid (on the order of milliseconds) from the first pulse. Hence, the detected signal intensity I(τ, t) can be fitted as a summation of two functions [43]:

2. Experimental method 2.1. Materials White cement provided by Taiheiyo Cement Corporation was used for a binder. Its density is 3.05 g/cm3 and its Blaine value is 3420 cm2/ g. The chemical oxide composition determined by X-ray fluorescence analysis and the mineralogical composition determined by powder Xray diffraction (XRD) and Rietveld analysis are shown in Tables 1 and 2, respectively. As can be seen in Table 1, the Fe2O3 content, which affects the magnetic field during measurement, is relatively small. All the materials were stored in a thermostatic room whose temperature was controlled at 20 ± 1 °C for 1 day prior to mixing. The mixing was performed at room temperature (no control, ca. 17 °C). The water-to-cement ratio of the specimen was 0.55 (hereafter, the sample is designated W55). The paste was mixed using a planetary centrifugal mixer at 1000 r/min for 1.5 min after the water was added to cement. Next, the paste was scraped to detach residual powder from the internal surface of the mixer, and finally an additional 1.5 min of mixing was carried out. After the mixing, the paste was immediately moved to the thermostatic room at 20 ± 1 °C and remixed every 30 min for 8 h to minimize segregation. After obtaining a creamy consistency, the paste was poured into a

I ( , t ) = Is ( ) exp

SiO2

Al2O

Fe2O3

CaO

MgO

SO3

Na2O

K2O

Total

3.18

22.68

4.5

0.19

65.07

1.19

2.75

0.06

0.07

99.69

(t + ) , T2, liquid

(1)

where Is and Il are the contributions from solid and liquid-like water, which are directly related to the molar fractions of solid and liquid water. Decay signals were acquired for τ from 5 to 25 μs in 2-μs intervals, and the signals were decomposed into Gaussian (solid) and exponential (liquid) parts according to Eq. (1). The relaxation intensities should be determined at zero echo time. Therefore, Is as a function of τ was extrapolated to τ = 0, and the accurate intensity for solid water was obtained. Good extrapolation results to τ = 0 were confirmed for all data in the presented cases. When, the target sample is completely sealed, total mass of water can be calculated from the mixture proportion, and the value of Is and Il

Table 1 Chemical composition of white cement by X-ray fluorescence elemental analysis (mass %). LOI

(t )2 + Il ( ) exp (T2, solid ) 2

LOI: Loss of ignition. 108

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Table 2 Mineral composition of white cement determined by a powder XRD/Rietveld analysis (mass %, the value behind ± represents a standard deviation). Alite

Belite

Aluminate phase

Periclase

Bassanite

Gypsum

Calcite

Total

48.96 ± 2.33

33.98 ± 0.38

4.46 ± 0.24

0.51 ± 0.41

1.01 ± 0.43

3.30 ± 0.80

5.26 ± 0.38

97.48

used. Although the smoothing parameter affects peak widths on T2 distribution, peak integral and position can be independent on the smoothing parameter. As shown in Figs. 5 and 6, CPMG data analyzed with the BRD algorithm confirmed four T2 peaks. The four measured T2 peaks which are consistent during hydration were among the direct evidence that 1H NMR relaxometry can measure the characteristics of hcp porosity and these have been documented by other researchers [24,41,42,45,46]. The different peaks obtained by the ILT algorithm were integrated and the ratio of each peak to the total integrated intensity was calculated to evaluate the amounts of interlayer pore water, gel-pore water, interhydrate pore water, and capillary pore water based on the total movable water obtained by solid echo measurement. 3. Results Table 3 shows the chemically bound water measured by TG and solid-echo signal decay, and the results from both methods are consistent. During drying, the chemically bound water was measured by solid-echo signals, and the results are shown in Fig. 3. There is no consistent change during drying, and chemically bound water did not seem to change under drying conditions. Based on the mass change measurements, the change in evaporable water in the hcp samples was calculated as shown in Fig. 4. This graph indicates that the sample at 11% RH reached equilibrium at around 7 days, and the sample at 80% RH reached equilibrium after14 days of drying, while the sample dried at 40% RH did not reach the equilibrium condition until around 56 days of drying. The obtained data are consistent with the previous results and other data [47,48]. The CPMG echo decays of W55-RH80, W55-RH40, and W55-RH11 are shown in Appendix, respectively (as symbols). The calculated curves obtained from T2 distribution are also shown (as lines). The measured data are in good agreement with the calculated curves. With regards to solid-echo-based chemically bound water, the obtained results showed little difference (1σ = 0.005 g/g-ignited cement) during drying, and it is concluded that the chemically bound water does not change under drying from the viewpoint of solid-echo signal decay. The ILT produced the T2 distributions, as shown in Fig. 5. In this figure, data for one typical sample are shown for each RH condition and drying period. The distribution of intensity is normalized such that the largest peak is 1.0. In some cases, the capillary pore peak cannot be clearly seen, owing to broadening of the peak. Regarding W55-RH80, due to the scattering of the signals, the gel-pore water T2 increased after 14 days. However, the other two samples did not show such a trend; consistent T2 values were observed. Therefore, it is believed that the average values of the three samples give us the representative trends. In the initial wet condition, four peaks can be seen at 2.0–4.5 × 10−4 s, 1 × 10−3, 2–3 × 10−3 s, and ~9 × 10−3 s. Based on the previous studies [24,42], those peaks correspond to the inter-layer water in C-S-H, water in gel pores, water in interhydrate pores, and water in capillary pores, respectively. In all cases, the larger pores empty first, and interlayer water

Fig. 1. Schematic drawing of the setup for RH conditioning of samples.

Liquid signal + solid echo Liquid signal Probe dead

Solid echo Acq.

Fig. 2. The solid-echo pulse sequence and definition of t and τ in Eq. (1). Signal acquisition was immediately carried out after probe dead time following the second pulse.

are used to calculate the chemically bound water and evaporable water content of the sample. After drying, the total intensity (Is + Il) is obtained from the solid echo signal. Next, the ratio of (Is + Il) before and after drying is used to calculate the mass of water in the sample after drying. A Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence experiment was used for all the measurements to quantitatively obtain the distribution of 1H T2 relaxation times based on CPMG echo sequence decay. Two hundred log-spaced echoes were recorded from 0.1 to 2180 ms, and more than 256 averages were recorded to obtain a sufficient signal-to-noise ratio. The total measurement took approximately 1 h to record solid echo and CPMG data. Blank cell correction was also performed [43].The measured CPMG data were analyzed with an inhouse inverse Laplace transform (ILT) algorithm, the Butler–Reeds–Dawson (BRD) algorithm ([44]), to obtain the quasi-continuous T2 distribution. The technique uses regularization for sampling data vector M, and obtains a coefficients vector (F) of T2 through minimization of χ,

= M

KF

2

Table 3 Chemically bound water of W55 (g/g-ignited cement).

F 2, 28 days 91 days

where matrix K is exponential kernel function regarding echo time and discretized T2, α is a smoothing parameter and a value of 1.0 × 10−7 109

TG

Solid echo

0.134 0.137

0.130 0.145

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in the sample after drying. Then the ratio Il/(Is + Il) is used to evaluate the evaporable water in the sample. The CPMG data allocates interlayer space water, gel pore water, interhydrate pore water, and capillary pore water. It should be noted that Fig. 5 shows the data of one specimen for each condition. The average values for three samples are applied for the calculation in cases when the drying period is more than 1 day. The data for cases with drying for less than 1 day were taken for only one sample for each drying RH condition because of the required recording time. Samples dried under 80% RH (Fig. 6(a)) show an increase in interlayer water, which reach a peak after 14 days of drying. Water in gel pores decreased gradually. At the same time, water in interhydrate pores and capillary pores decreased quickly. Because the T2 value for interlayer water does not change dramatically, it can be concluded that the saturated interlayer spaces have common intrinsic dimensions, and they increased during drying. These interlayer spaces originate from gel pores because gradual water redistribution from gel pores to interlayer spaces was confirmed. In the case of drying at 40% RH (Fig. 6(b)), the number of saturated interlayer spaces increased until 9 h of drying and then decreased gradually until 56 days of drying. The water in gel pores decreased continuously; after 1 day of drying, almost no water was observed in gel pores. The water in capillary pores and interhydrate pores decreased dramatically until 9 h of drying, and only water in interhydrate pores remained. Interestingly, the total amount of water in interlayer spaces and water components, as well as the total amount of evaporable water at 9 h of drying under 40% RH were identical to sample W55-RH80 observed 14 days of drying, when the amount of water in interlayer spaces showed the peak. Regarding the sources of evaporable water, similar trends were reproduced for samples dried at 80% and 40% RH; they showed an increase in the amount of water in interlayer spaces, which is confirmed until 6 h of drying. Simultaneously, after 9 h of drying, the amount of water in gel pores decrease until no water was observed in the gel pores. The water in interhydrate and capillary pores decreased dramatically, and although there was no measurable capillary water from 3 to 9 h of drying. In summary, the history of evaporable water sources distribution under drying was similar and reproducible.

Fig. 3. Chemically bound water dried under 80%, 40%, and 11% RH conditions as a function of drying period measured by 1H NMR relaxometry (the starting points were set at 0.01 days).

4. Discussion Fig. 4. Evaporable water content change of W55 under 80%, 40%, and 11% RH obtained by mass difference (the starting points were set as 0.01 days).

In this section, the increase of water in C-S-H interlayers during drying is discussed. The data shown in Fig. 6 are interpreted from a microstructural perspective. At the beginning of drying, water exists in capillary pores, interhydrate pores, gel pores, and interlayer spaces. When the sample was exposed to drying conditions, water evaporated from capillary pores. During this process, as is suggested by the Kelvin equation, the pore water remained in smaller spaces. At the same time, capillary tension of pore water played an important role in contracting the distance between dispersed C-S-H sheets. Based on the observed data for 0 to 3 h of drying shown in Fig. 6(a), it is possible that gel pores changes into interlayer spaces. Actually, when the amount of interlayer water (Mint, volume of water), the corresponding T2 relaxation time (T2−int) (volume/surface ratio), the amount of gel water (Mgel), and the corresponding T2 relaxation time (T2−gel) are considered, Mint/T2−int+Mgel/T2−gel should produce an approximate/representative surface area (for a more realistic value, we need a calibration between T2 and corresponding pore size, with larger pores being neglected due to their small contribution). The calculated surface area, in units of g/g-ignited cement/s, for 0 h and 14 days are 553.5 and 581.9, respectively. They are very similar; this slight increase in the surface area (5.1%) may be explained by the precipitation of hydrates from the pore solution. Therefore, based on this quantitative evaluation, it is considered that the gel spaces

remained. In the 80% RH drying case, the water in gel pores remained, but in the cases of 40% RH and 11% RH the water in gel pores was removed. Interestingly, the T2 value for interlayer water in the 40% RH and 11% RH cases became smaller (from 4 to 4.5 × 10−4 s to 2.3 × 10−4 s for 40% RH and 1.7 × 10−4 s for 11% RH) after drying for 14 days and 7 days, respectively. Since the T2 value corresponds to the volume-to-surface ratio, these results support the concept that interlayer space decreases when drying in an environment with less than 40% RH. These data agree with the previous data shown by Gajewicz et al. (Ref. [49], p.74, Fig. 5.11), in which the interlayer spacing (representative of the T2 relaxation time) starts to decrease from around 40% RH. This is consistent with previously reported data for synthesized C-S-H analyzed via XRD [10], and C-S-H in white Portland cement analyzed via wide-angle X-ray scattering [8]. Quantitative sources of evaporable water in each hcp sample are shown in Fig. 6. There are two methods to obtain the evaporable water. One is using change in mass assuming that all the change in mass is associated with the evaporable water content. The other is solid-echo measurement. The total intensities (Is and Il) before and after drying were obtained and the ratio between them gives the total mass of water

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Fig. 5. T2 distributions calculated by an ILT of the CPMG echo signal decays of W55-RH80, W55-RH40, and W55-RH11. The Y-axis is normalized such that the largest peak is 1.0. Data for one typical sample at each measurement time and each RH condition are shown.

developed into interlayer spaces. The C-S-H sheets are movable and the distance between the C-S-H sheets is easy to change. The capillary tension (on the order of ~30 MPa at 80% RH and 293.15 K) caused by pore water in hcp can change the microstructure of the C-S-H sheets under such conditions. With regards to gel pore water, it has been proven that it needs more than 100 MPa of negative pressure to cavitate the gel water when the gel pores are confined by narrow interlayer spaces under the rapid drying [50]. Therefore, C-S-H sheets enwrapping gel pores become closer according to the negative pressure of gel pore waters. After 3 h of drying at 80% RH, the gel pores gradually transform into interlayer spaces with little mass change. This behavior appears to be a delayed response to the drying/mass-change that occurs mainly during drying from 0 to 1 days. The transformation from gel pores to interlayer spaces peaks at 14 days,

and then the interlayer water decreases while the gel-pore water and capillary water increase; this has been confirmed even though the total mass does not change. This might be the creeping effect of C-S-H due to capillary tension, disjoining pressure, and surface tension acting on C-SH sheets. The microstructural reorganization due to drying does not seem to reach the equilibrium state until 56 days of drying. These movable C-S-H sheets and the transformation from gel pore to interlayer space are supported by other recent data. Wyrzykowski et al. [51] applied 1H NMR relaxometry to hcp samples stored at different temperatures. In the experiment, rapid changes in moisture attribution between the gel-pore water and interlayer water was confirmed. Water in interlayer spaces was expelled to gel pores when the temperature was increased and this behavior was reversible when the temperature decreased. This evidence also supported the theory that gel pores and

111

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Fig. 6. The evolution of water components of a sample dried under different RH conditions. The total bar length shows the amount of evaporable water in the sample, which is determined by the solid-echo signal decays. The red symbol indicates evaporable water calculated by mass change. For 3-, 6-, and 9-hour measurements, which are indicated by green symbols, only one sample was used for measurement. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

interlayer spaces commonly share the C-S-H sheets and that water easily transports between gel pores and interlayer spaces. During the hydration, the bulk density (including gel-pore water) of C-S-H increases [46]. In the case of sealed hcp samples with W/C = 0.40, the amount of gel-pore water decreased by 0.07 (as an NMR signal fraction) between 60 and 300 days after casting, while the interlayer water increased by 0.06 (Fig. 2 of Ref [46]). This fact supports that, if additional C-S-H sheets are formed/inserted in gel pores, then gel pores become interlayer spaces. Imbalance between gel-pore water consumption and interlayer water production contributes to form portlandite or other hydrates. This evidence shows that the gel pores are convertible to interlayer spaces when the distance between C-S-H sheets is smaller. Hence, the C-S-H sheets were commonly shared by the gel pores and the interlayer spaces, and it is possible that the gel-pore water consists of water molecules existing in a space between C-S-H sheets with a larger distance between them, rather than in an interlayer space. In the cases of 40% and 11% RH drying, both samples showed similar trends of increasing interlayer water and decreasing capillary water, interhydrate water, and gel-pore water. After 9 h of drying, the samples showed a peak in interlayer water, followed by a gradual

Gel pore water

Interlayer water

decrease. It should be noted that, while the interlayer water was decreasing, the mass change was not significant. Among the samples measured 56 days after drying, W55-RH80 showed the largest amount of interlayer water, followed by W55-RH40 and W55-RH11. It is reasonably deduced that the interlayer spaces govern the surface area (water vapor Brunauer–Emmett–Teller (BET) surface area, SH2O), so this order would be expected based on the previous results of water vapor sorption isotherms of hcp after long-term drying in different RH conditions (Fig. 8 of Ref. [5], Fig. 9 of Ref. [51], and Fig. 12 of Ref. [8]). A schematic depicting the re-organization of the C-S-H microstructure and related water molecules is shown in Fig. 7. Above 95%RH (left side), water is abundant and, between the wound C-S-H sheets originating from their formation process, both interlayer water and gel pore water can be found. After drying (middle), due to the release of water and negative pressure, the C-S-H sheets become closer. Some of the gel pore water becomes interlayer water because it is confined by CS-H sheets whose spacing becomes smaller. Therefore, an increase in interlayer water is observed via 1H NMR relaxometry. The C-S-H sheets are stacked more neatly. This behavior was confirmed by SAXS

Interlayer space becomes narrower. Tightly stacked C-S-H

Gel poreà Interlayer space

Re-created gel pore

C-S-H sheet (mono C-S-H layer)

>95% RH

95~40% RH

40~11% RH

Fig. 7. Re-arrangement of C-S-H microstructure and related water conditions. Greenlines represents sheets/monolayers of C-S-H, dark blue circles represent water molecules in interlayer space, and light blue circles represent water molecules in gel space. In this figure, calcium ions in interlayer spaces are not shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 112

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C-S-H sheet

Interhydrate pore becomes large and connected each others ( a part of decrease in interlayer space contributes to expand the internal pores)

Interhydrated pore

Water molecules in C-S-H sheets are not shown (b) Dried (~11% RH)

(a) Saturated

Fig. 8. Schematic of deformation of C-S-H layers and resultant change of larger pores.

measurements showing an apparent increased thickness of building blocks of C-S-H sheets [8] as well as by water vapor sorption isotherms showing smaller incremental sorption amount between p/p0 = 0.40 and 0.95 [51]. Some of the C-S-H sheets are tightly stacked and due to this, there is reduction of water vapor BET surface area and production of inaccessible pores by nitrogen [3–5]. In the case of severe drying conditions (less than 40% RH, right side of Fig. 7), additional removal of water molecules between C-S-H sheets, some of the C-S-H sheets become tightly stacked and some of the C-S-H sheets become detached from each other. In the case of tightly stacked C-S-H sheets, the interlayer distance becomes small, this is confirmed by the reduction of T2 of the interlayer water and by SAXS [8] and SANS [12] measurements. At the same time, the detaching of C-S-H sheets was indicated by the increase in incremental water vapor sorption amount between p/ p0 = 0.40 and 0.95 from drying conditions at 40% RH to 11% RH [5,51]. The mechanism of the shrinkage of hcp is also addressed here. In the previous experiment, long-term first drying shrinkage and subsequent re-humidifying expansion (ranging from saturation to 11% RH) were found to be a function of the incremental statistical thickness of adsorption (where the origin was the saturated state) (Ref. [52], Fig. 6). If the shrinkage of well-hydrated cement paste is determined by an average distance between the movable C-S-H monolayers and these distances are determined by the number of water molecules adsorbed on the surfaces of the movable C-S-H monolayers, the current interpretation are well in line with the previous shrinkage data. However, it should be noted that the total deformation of C-S-H sheets (change in average interlayer space, approximately 25% decrease in distance based on the T2 value) will not be of the order of the shrinkage of hcp (~1%). This is most likely due to the presence of cement hydrates, which are stable under the drying, hinder the shrinking behavior of C-SH sheets, and expand the larger pores (as shown in Fig. 8). The reverse, the creation of larger pores on water uptake, are confirmed by 1H NMR relaxometry measurements [24]. To evaluate the shrinkage of hcp, the sole capillary tension theory based on a rigid porous body will not work, particularly in the re-humidifying process. Therefore, for the shrinkage mechanisms, alteration of the pore system and disjoining (or hydration) pressure should be taken into account. The roles of unhydrated cement particles, portlandite, or other cement hydrates on the shrinkage of hcp are issues to be solved. The water flow through the narrower spaces tends to be slower because the effect of interaction between the pore surface and water

molecules on water molecule mobility becomes larger, and the apparent viscosity of water is increased [53–56]. During drying of wet hcp the distance between mobile C-S-H sheets becomes smaller, and the rate of moisture flow through mobile C-S-H sheets becomes slower. That is why the wet sample dried in the mid-RH range (approximately 40–60% RH) required longer times to reach mass equilibrium. If the hcp is dried under severe drying conditions (less than 40% RH), then the surface parts of the sample dry more dramatically and create a large number of capillary pores. In addition, the water seeped out from the central portion goes through the capillary pores of the surface parts (the water will probably emerge from the defects of the C-S-H sheets structure to the capillary pores as a vapor phase) easily pass these parts, and evaporate from the sample surface. Therefore, the moisture transport distance becomes short when the sample is dried at less than 40% RH, therefore, the apparent rate of drying becomes rapid. The final microstructure state is most important for defining the mechanism of moisture transport. In the future, the delayed response and long-term drying results should be analyzed. 5. Conclusion The 1H nuclear magnetic resonance (NMR) relaxometry technique was applied to observe the dynamic microstructural evolution of white Portland cement paste during the first desorption. Under drying conditions, water was evaporated from the larger pores, and the microstructure reorganized as soon as the hardened cement paste (hcp) was dried. Initially, interlayer water increased, and capillary water, interhydrate water, and gel-pore water decreased. However, after the interlayer water reached its peak, it decreased. After the peak of the interlayer water, the change in mass was not significant. Based on previous research on the calcium-silicate-hydrate (C-S-H) structure, the microstructural change during drying is explained by the presence of movable C-S-H sheets, which are shared by gel pores and interlayer spaces. The C-S-H sheets easily change their spacing during drying and can mutually transform from gel pores to interlayer pores. Acknowledgements These experiments were partly sponsored by Japan Society for the Promotion of Science KAKENHI Grant Number 18H03804, and the research was completed in collaboration with Chubu Electrical Power Co., Inc. White cement was provided by Taiheiyo Cement Corporation.

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Appendix A A.1. CPMG echo signals The obtained CPMG echo signals are shown in Fig. A1.

Fig. A1. CPMG echo decays of each drying sample (denoted with “-exp”) and curve-fitting results from the BRD algorithm (denoted with “-sim”).

A.2. Calibration between the NMR signal and the gravimetric mass change There is a possibility that the time domain of the Carr–Purcell–Meiboom–Gill (CPMG) analysis used herein did not resolve the short decay component. Therefore, the sample mass loss (mass at the starting point – mass after drying) was plotted as a function of the difference of I0, which is the y-intercept value of the CPMG decay signal (I0 at the starting point – I0 after drying). The results are shown in Fig. A2. In this figure, the samples dried at 11% RH, 40% RH, and 80% RH are shown and all the results are linearly correlated. This relationship can be used to evaluate the movable water in the samples. The movable water content calculated from CPMG measurements, calibrated according to Fig. A2, was compared with the movable water content calculated from solid echo measurements. The results are shown in Fig. A3, and they display good agreement. Therefore, the applied CPMG measurement is considered to reflect the total movable water content.

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Fig. A2. ΔI0 (y-intercept value) versus ΔW (mass loss of sample).

Fig. A3. Comparison of evaporable water contents calculated using CPMG calibration and using solid echo measurement.

A.3. Impact of sample size Because the sample size is relatively large (~4.5 mm in diameter), theoretically, there exists a distribution of movable water inside the sample. This might influence the measurement data. To confirm this, the sample was crushed to ~1 mm in diameter and the particles were dried at 40% RH. The same measurement protocol was applied at 3 and 6 h after drying. The results are shown in Fig. A4. The increase in interlayer water under drying (mass loss) can be clearly confirmed. Therefore, the results were not affected by the sample size.

Fig. A4. Measurement results for crushed samples with diameter ~1 mm under 40% RH: (a) movable water composition and (b) T2 times for the components.

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Appendix B. Supplementary data The supplementary data are provided in the following link: https://doi.org/10.1016/j.cemconres.2019.04.017. In the document, followings are discussed. 1. Reliability of the inverse Laplace transform calculation 2. Solid echo measurement affected by Fe2O3 content

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