Comparison of Mercury Intrusion Porosimetry and multi-scale X-ray CT on characterizing the microstructure of heat-treated cement mortar

Journal Pre-proof Comparison of Mercury Intrusion Porosimetry and multi-scale X-ray CT on characterizing the microstructure of heat-treated cement mortar

Shanbin Xue, Peng Zhang, Jiuwen Bao, Linfeng He, Yu Hu, Shidi Yang PII:

S1044-5803(19)32456-8

DOI:

https://doi.org/10.1016/j.matchar.2019.110085

Reference:

MTL 110085

To appear in:

Materials Characterization

Received date:

8 September 2019

Revised date:

18 December 2019

Accepted date:

18 December 2019

Please cite this article as: S. Xue, P. Zhang, J. Bao, et al., Comparison of Mercury Intrusion Porosimetry and multi-scale X-ray CT on characterizing the microstructure of heat-treated cement mortar, Materials Characterization (2019), https://doi.org/10.1016/ j.matchar.2019.110085

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© 2019 Published by Elsevier.

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Comparison of Mercury Intrusion Porosimetry and multi-scale X-ray CT on characterizing the microstructure of heat-treated cement mortar Shanbin Xue a, Peng Zhang a*, Jiuwen Bao a, Linfeng He b, Yu Hu c, Shidi Yangd *Corresponding author at: Fushun Road 11, 266033 Qingdao, P.R. China. E-mail address: [email protected] (P. Zhang). a

Center for Durability & Sustainability Studies of Shandong Province, Qingdao

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University of Technology, Qingdao 266033, P.R. China; b

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Neutron Scattering Laboratory, China Institute of Atomic Energy, Beijing 102413, P.R. China; c

Tianjin Sanying Precision Instruments Co., Ltd., Tianjin 300399, P.R. China;

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d

-p

State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, P.R. China;

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Abstract: In this contribution, the microstructure features of cement mortar exposed

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to various temperatures (105°C, 200°C, 400°C, 600°C, 800°C) was investigated by combining Mercury Intrusion Porosimetry (MIP) and multi-scale X-ray computed

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tomography. The influence of exposure temperature and resolution of X-ray CT on the determination of microstructure parameters of heat-treated mortar was focused. Based on results of MIP test, it was found the porosity and pore size increased slightly when the exposure temperature varied from 105°C to 200°C and significant pore coarsening and micro-damage occurred once the temperature exceeded 400°C. Bimodal pore size distribution (PSD) of the heat-treated mortar specimens was observed when the temperature reached 400°C. To interpret the results of MIP test, the microstructure of heat-damaged mortar specimens was imaged using X-ray CT with a reconstructed voxel size of ~4.0 μm3 and then local volume inside the specimen was focused and 1

Journal Pre-proof scanned with a reconstructed voxel size of 1.5 μm3. A method was proposed to select proper threshold based on the MIP results for segmenting the void space from the X-ray CT images. The fracture aperture, 2D/3D fractal dimension, connectivity and tortuosity of the heat-damaged mortar specimens were further determined at different scale. By analyzing the fracture aperture determined from X-ray CT images, it was

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found the bimodal PSD revealed by MIP test can be associated with the creation of thermal micro-fractures. The fractal dimension increased remarkably when exposure

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temperature was raised from 400°C to 600°C while it varied slightly from 600°C to

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800°C. Linear dependences between the fractal dimension and the volume

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fraction/tortuosity of micro-scale pores and fractures was found. The scale-dependent

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fractal properties of the heat-treated mortar were revealed with the capillary pressure

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data measured by MIP. The fractal dimension of micro-scale pores and fractures measured by MIP exhibited good consistency with that determined based on by X-ray

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CT images with a reconstructed voxel size of 1.5 μm3.

Keywords: Microstructure; Heat-damaged mortar; High-resolution X-ray CT; Fracture aperture; Fractal dimension

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1. Introduction

Fires in the confined space such as tunnels can cause a rapid rise of the ambient temperature and resulted in serious damage to the concrete structure [1,2]. The deterioration of cement-materials exposed to elevated temperatures was normally associated with the drastic changes of its microstructure e.g. pore coarsening and the

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generation of thermal micro-fractures [3–7]. Water re-curing has been recognized as

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an effective method for the repair of heat-damaged cement-based materials [8–11].

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The permeability of heat-damaged cement-based materials can be significantly

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enhanced once the capillary pores were connected by the thermal fractures [12]. Meanwhile, anomalous water absorption in the porous media with the coexistence of

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pores and fractures has been reported [13,14]. Accurate descriptions of the

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microstructure features of the heat-damaged cement-based materials are critical for

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the durability evaluation and recovery of concrete structure subjected to fire [8,9,15]. Cement mortar has been widely used as binders in the concrete. From a microscopic point of view, the cement mortar can be regarded as a multiphase composite comprised by the hydration products, aggregates, un-hydrated clinkers, and void space. When the cement mortar is exposed to elevated temperatures, each phase undergoes a completely different physicochemical transformation [16,17]. The pore size and fracture apertures of heat-treated cement mortar specimens are expected to cover a continuous range from nanometers to hundreds of micrometers [18–20]. Moreover, there are great differences in morphology between capillary pores and heat-induced fractures. Quantitative characterization of the microstructure evolution 3

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of cement mortar specimens exposed to a series of ascending temperature is considered as a challenging task. Mercury Intrusion Porosimetry (MIP) test has been frequently employed to characterize the porosity, pore size and its distribution of the cement-based materials due to its wide coverage [7,20–22]. Nevertheless, the results of MIP test was questioned due to the damage to skeleton caused by high pressure mercury injection

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and the ―ink-bottle‖ effect [19,20,23,24]. Moreover, it has been noted the mechanical

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property of cement-based materials was susceptible to the total void space while the

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transport-related parameters such as permeability and sorptivity were more associated

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with the features of connected pores and fractures [10]. The isolated or connected

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pores and fractures can be well distinguished with imaging methods. With the

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improvement in temporal and spatial resolution, imaging techniques including scanning electron microscope [7,21,25], backscattered electron imaging[26,27], X-ray

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CT [28–30] and neutron radiography [31,32] have been widely utilized to investigate the microstructure features and transport properties of cement-based materials. A trade-off between the spatial resolution and the maximum size of observation area is one of the main considerations when imaging techniques were employed. A higher resolution normally means sacrificing the size of observation area. Although X-ray CT imaging of heat-damaged cement-based materials has been performed [10,33], some microstructure details may be missed by CT images with a voxel size of dozens of microns in the existing researches. To the author's knowledge, the influence of resolution on characterizing the microstructure of heat-damaged cement mortar using 4

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X-ray CT technique have not been well addressed. A comparison of the microstructure parameters measured by MIP and high-resolution X-ray CT images was also absent for heat-damaged cement mortars. In addition, it is hard to capture the evolution of multi-scale pore systems in heat-damaged cement mortars using Euclidean geometry. The fractal theory has been

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proved to be a powerful tool for quantitatively describing the complex pore system of natural and artificial porous materials [34]. The fractal property of pores and fractures

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systems in cement-based materials including cement paste, mortar and concrete has

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been reported in the previous researches [35–40]. With the measured fractal

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dimension and established fractal models, the transport-related parameters of the

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porous media including permeability [41,42], sorptivity [43], water characteristic

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curve [44,45] and unsaturated diffusivity [46] can be estimated. Several methods have been proposed to estimate the fractal dimension of porous media while comparisons

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between the results from different methods was seldom [34–38]. Moreover, The scale-dependent fractal properties of the cement-based materials have been noticed i.e. the fractal dimension varies for different ranges of pore size [34,36,47]. The fractal property of heat-damaged cement mortar with coexisting pores and micro-fractures has not been clarified yet. To address aforementioned issues, the manuscript was organized as followings: (I) the porosity, pore size and its distribution of mortar specimens exposed to a series of ascending exposure temperatures (105°C, 200°C, 400°C, 600°C, 800°C) was characterized by MIP test to trace the evolution of microstructure; (II) to interpret the 5

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results of MIP test and select a proper resolution for characterizing the microstructure of cement mortars treated by a temperature ≥ 400°C, the fracture aperture determined by X-ray CT with different resolutions was compared with the PSD measured by MIP. (III) the influence of exposure temperature and resolution on the determination of 2D/3D fractal dimension, connectivity, tortuosity was evaluated and the correction

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between these parameters was explored as well. (IV) the scale-dependent fractal analysis on the microstructure of mortar specimens exposed to various temperatures

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was performed based on capillary pressure data measured by MIP.

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2. Material and Methods

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2.1 The preparation of heat-treated mortar specimens

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Portland cement type 42.5 and river sand with a maximum grain size of 5 mm were mixed with tap water to produce the cubic mortar samples with dimensions of 70.7

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mm×70.7 mm×70.7mm. The cement, sand and water were mixed according to a mass ratio of 500: 1350: 300 resulting in a water-cement ratio of 0.6. The de-molding was finished after 24 hours from casting. The samples were cured in a humid room for 28 days where the temperature was kept at 20±2°C and the relative humidity (RH) was ≥95%. Then the samples were stored in a humid room with RH=50% and a temperature of 20±2°C for four months before the heat treatment. Cuboid specimens with a dimension of 50 mm×25 mm×15 mm were cut from the produced mortar samples for the heat treatment. Smaller specimen was used in order to achieve a relatively uniform heat treatment. Before exposed to the elevated 6

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temperatures, all the mortar specimens were dried in an oven with a temperature of 105°C for 12 hours. A numerical-control heating furnace was used in the test. The target temperatures were set as 200°C, 400°C, 600°C and 800°C. Once the mortar specimens were put in the furnace, the target temperature was achieved in 40 minutes and kept for 120 minutes. Then the mortar specimens were exposed to indoor

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environment (17°C, RH=28%) for 20 minutes. At last, all the mortar specimens were stored in the sealed bags for MIP, X-ray CT imaging tests. The mortar specimens

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dried by 105°C were denoted as M105 which was taken as a reference. The mortar

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specimens exposed to 200°C, 400°C, 600°C and 800°C were denoted as M200, M400,

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M600 and M800, respectively.

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on MIP test

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2.2 Methodology for characterizing the microstructure of heat-damaged mortar based

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When the pore space of porous media is represented by a series of capillary tubes, the pore diameter can be estimated by Younge-Laplace law [48]: P  

4 c os



(1)

where Pc denotes the capillary pressure, σ is the surface tension and θ, λ represent the contact angle and pore diameter respectively. The pore diameter and its distribution can be estimated by measuring the volume of mercury injected in the porous media at different pressure steps based on Equation (1). Here, the MIP test was performed using an Autopore IV 9500 mercury porosimeter with cube specimens (25 mm×15 mm×10 mm) cut from the heat-treated mortar samples with a dimension of 50 mm×25 mm×15 mm. The determined porosity, 7

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average and median pore diameter of the mortar specimens were listed in Table 1. In the present work, the average pore diameter was calculated with 4Vp/Ap because the shape of pores and fractures was assumed as cylinder, where Vp and Ap denoted the cumulative pore volume and cumulative pore area, respectively. Median pore diameter was determined where 50 % of total intrusion volume was reached. Table 1. Pore structure parameters of the mortar specimens exposed to various temperatures measured by MIP. Average

diameter (μm)

diameter(μm) 0.030

0.166

0.065

200

0.183

0.080

0.037

400

0.214

0.117

0.047

600

0.287

0.207

0.079

800

0.313

0.329

0.183

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105

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(°C)

Median

Porosity

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Temperature

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Theories for estimating the fractal dimension of porous media based on the capillary pressure data measured by MIP test have also been established [48,49].

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According to Li et al. [48], the number of pores N(λ) intruded by mercury at a given

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pressure can be estimated as followings: N ( ) 

4  VHg

 2 l

(2)

where VHg denotes the cumulative pore volume intruded by mercury in the specimen under a certain capillary pressure and l represents the length of a capillary tube. When the pore space exhibits fractal characteristics, the correction between number of pores N(λ) and capillary pressure Pc can be described as [48,49]: N ( )  Pc

 (2  D f )

(3)

where Df is the fractal dimension of pore space. The mercury saturation SHg was termed as the ratio between the cumulative pore volume intruded by mercury VHg and the total pore volume VP. By combining 8

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Equation (1)~(3), the relation between mercury saturation and capillary pressure was derived by Li et al. [48] as: SHg  m  Pc

 D f  2

(4)

where m is a constant. Based on Equation (4), a linear relationship between the capillary pressure and the mercury saturation in the double-logarithm coordination is

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expected for a fractal porous media. The fractal dimension Df can be estimated by a linear fitting of the log SHg v.s. log Pc plots as followings [48,49]:

(5)

Df  S  2

(6)

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logS Hg  ( D f  2)  log Pc  log m

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where S represents the slope of the linear fitting function of log SHg v.s. log Pc plots.

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2.3 X-ray CT imaging of heat-damaged mortar specimens

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To achieve a more accurate description of the microstructure of mortar specimens

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exposed to 400°C, 600°C and 800°C (M400, M600 and M800), X-ray imaging test was performed with a laboratory CT scanning system (nanoVoxel-3502E, SanYing Precision Instruments Co, Ltd., China). The experimental setup was shown in Figure 1(a) and the parameters involved in the test (voltage, current, source specimen distance and source detector distance) were presented in Table 2. Cylindrical specimens with a diameter ~5.0 mm were carefully drilled from the heat-treated cuboid samples (50 mm×25 mm×15 mm) for the X-ray imaging test. First, the drilled specimens were scanned with a reconstructed voxel size of ~4.0 μm3 using a flat panel detector. To achieve a better resolution and minimize the effect of damage caused by

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drilling, a special volume inside the dilled cylindrical specimens was focused and scanned with a reconstructed voxel size of 1.5 μm3 with the help of Region of Interest X-ray Scan technology using a 4× lens CCD detector (16-bits CCD with a pixels matrix of 2048 × 2048). A discussion on the difference between ―resolution‖ and ―voxel size‖ can be found in the references [50,51]. The ―reconstructed voxel size‖

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was called ―voxel size‖ for short in the present work.

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Mortar specimen

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Figure 1. Experimental setups for X-ray imaging of the microstructure of heat-damaged mortar specimens Table 2. Parameters involved in the X-ray imaging test Source detector distance,

SSD

SDD

(mm)

(mm)

17.03

37.15

4X

1.5

40

16.83

36.08

4X

1.5

40

17.53

37.72

4X

1.5

60

10.73

340.16

Flat

4.0

60

8.81

279.69

Flat

4.0

60

9.06

281.01

Flat

4.1

(kV)

(μA)

400-1.5

100

40

600-1.5

100

800-1.5

100

400-4.0

80

600-4.0

80

800-4.0

80

Specimen

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Current

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Source specimen distance,

Voltage

Voxel Detector

size (μm3)

Based on the acquired X-ray CT images, the microstructure of heat-damaged mortar specimens were reconstructed in three dimensions using image analysis software Avizo (Thermo Fisher Scientific & FEI, Avizo User’s Guide, 2013, https://www.fei.com/software/avizo-3d-user-guide.pdf). Then a volume of interest (VOI) was cropped from raw data for further analysis as shown in Figure 2(a) (voxel size of ~4.0 μm3) and (b) (voxel size of 1.5 μm3). A median filtering (3×3 pixels) was 10

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applied to improve the image quality of slices. The size of the VOI for different specimens can be found in Table 3. Then the void space of the heat-damaged mortar specimens was segmented from the raw data by image binarization. When the binarization was applied on gray-scale images, a threshold should be selected properly to distinguish void space from solid matrix. Data volume with a gray value under the

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threshold will be labeled as the void space. Thus, the selection of an optimal threshold is critical for segmenting the void space of the specimens properly. Here, the

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threshold was determined with caution by referring the volume fraction measured by

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MIP. First, the volume fraction was determined with different potential thresholds as

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plotted in Figure 3(a) (voxel size of ~4.0 μm3) and (b) (voxel size of 1.5 μm3). Based

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on the results of MIP test, Horizontal arrow lines were plotted with the determined

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intercept (equal to the cumulative volume fraction of pores with a diameter larger than a voxel size) as shown in Figure 3. At last, the scatter point which was closest to the

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horizontal arrow lines was set as a starting point for drawing a vertical line to cross the horizontal axis. The abscissa of intersection was selected as the optimal threshold. The segmented void space volume with the optimal thresholds listed in Table 3 was presented in Figure 4(a) (voxel size of ~4.0 μm3) and (b) (voxel size of 1.5 μm3). Table 3. Microstructure parameters determined based on segmented void space data from X-ray CT images Specimen

Voxel size 3

(μm )

Threshold

Void volume

Connectivity

fraction

(%)

Tortuosity

Df-L

Df-S

M400

4.0

1950

0.0388

0

4.66

1.89

-

M400

1.5

3700

0.0375

64.11

4.89

-

2.28

M600

4.0

2600

0.0651

90.07

3.23

2.40

-

M600

1.5

3100

0.0874

95.73

2.57

-

2.46

M800

4.1

1950

0.0744

94.53

2.97

2.36

-

M800

1.5

3200

0.1040

97.09

2.26

-

2.49

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M600

M400

M600

M800

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M400

M800

3

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(a) voxel size of ~4.0 μm

M400

M600

M400

M600

M800

M800 3

(b) voxel size of 1.5 μm

Figure 2. Three-dimensional representation of the microstructure of heat-damaged mortar specimens reconstructed based on X-ray images with different voxel sizes. (a) voxel size of ~4.0 μm3; (b) voxel size of 1.5 μm3. 12

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The tortuosity of the segmented void space was determined using the Avizo Centroid Path Tortuosity module [52]. Then the connected void space along specified direction (Axis Z defined in the software) was extracted from the segmented void space. The connectivity was calculated as the ratio between the volume of connected pores and fractures to that of the segmented void space. The determined tortuosity,

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connectivity and fractal dimension of different mortar specimens were summarized in the Table 3. 0.12

M400 voxel size of 1.5 μm3 M600 voxel size of 1.5 μm3 M800 voxel size of 1.5 μm3

0.10

1.592 um

0.06

0.02 2400

1.477 um

2600

2800

3000

4.22 μm 4.21 um

0.06

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0.08

0.04

0.08

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Void space fraction

1.594 μm 0.10

0.04

0.02

3200

3400

(a)

3800

4000

4200

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Threshold

3600

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Void space fraction

0.12

M400 voxel size of 4.0 μm3 M600 voxel size of 4.0 μm3 M800 voxel size of 4.1 μm3

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0.14

0.00 1400

4.02 um

1600

1800

2000

2200

2400

2600

2800

3000

Threshold

(b)

Figure 3. Comparison between void space fraction measured by MIP and that determined based on X-ray CT

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images with different thresholds. Here, the arrow lines were plotted based on MIP data and the scatter points representing data from X-ray CT test. (a) voxel size of ~4.0 μm3 and (b) voxel size of 1.5 μm3.

The fracture apertures (pore diameter) and its distribution of heat-damaged mortar specimens were determined using BoneJ installed as a collection of plug-ins in Fiji (version 1.52n)/ImageJ based on the segmented void space [53–56]. BoneJ was originally developed for the analysis of trabecular geometry and whole bone shape with X-ray CT images in the field of skeletal biology [56]. The fracture apertures were calculated by the ―thickness‖ module working with the maximal sphere algorithm in the BoneJ. Here, the thickness denoted the diameter of the largest sphere that can fill completely the fracture i.e. the aperture of fracture [57–59]. 13

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M400

M800

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ro

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M600

M400

M600

M800

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lP

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(a) voxel size of ~4.0 μm3

M400

M400

M600

M800

M600

M800

(b) voxel size of 1.5 μm3 Figure 4. Two/three-dimensional representation of the void space of the heat-treated mortar specimens segmented from X-ray images with different voxel sizes using the selected thresholds. (a) voxel size of ~4.0 μm3; (b) voxel size of 1.5 μm3. 14

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3. Results and discussion 3.1 Evolution of microstructure parameters with the rise of exposure temperature measured by MIP

The porosity, median/average pore size and its distribution measured by MIP test can provide basic information with respect to the microstructure evolution of

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heat-treated mortar specimens as listed in Table 1. As shown in Figure 5(a), the

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porosity of heat-treated mortar specimens was increased by 10.7 %, 29.5 %, 73.4 %,

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89.3 % respectively when the exposure temperature was raised from 105°C to 200°C,

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400°C, 600°C and 800°C, respectively. The median diameter was enhanced by 24.1 %,

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79.8 %, 218.8 %, 408.0 % in the above-mentioned heating process and the evolution

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Mercury intrusion porosimetry 0.30

0.24

0.21 0.18

0.15 0

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Porosity

0.27

200

400

600

0.20 0.35

Median diameter (μm) Average diameter(μm)

0.30

0.18 0.16 0.14

0.25

0.12 0.20

0.10 0.08

0.15

0.06 0.10 0.04 0.05

800

0.02 0

Temperature (°C)

Average diameter (μm)

0.33

Median diameter (μm)

of average pore diameter was similar as shown in Figure 5(b).

200

400

600

800

Temperature (°C)

(a)

(b)

Figure 5. Evolution of pore structure parameters with different exposure temperatures measured by MIP. (a) porosity; (b) median and average pore diameter.

A slight increase in the porosity and pore size of the mortar specimens was found when the exposure temperature was raised from 105°C to 200°C. It was noticed from Figure 6(a) that the left peaks of PSD curves (0.01~0.5 μm) changed slightly when the exposure temperature varied from 200°C to 400°C. The pore coarsening was normally 15

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attributed to the shrinkage of C-S-H caused by the loss of physically bound water, absorbed water and interlayer water when the cement-based materials was exposed to a temperature of 105~400°C and a loosely packed structure of the cement paste matrix was expected [60]. The microstructure of heat-treated mortar may be damaged by mercury intrusion in the MIP test as well[61].

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The determined median and average pore diameter of M105, M400 were close to the results reported by Chen et al [23] of the mortar specimens with a w/c of 0.5

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exposed to a temperature of 105°C and 400°C for 1 hour. Extensive and connected

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micro-fractures with a width of 2~5 μm has been observed on the heat-treated mortar

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specimens by Chen et al. [23] using SEM and FIB/SEM. Therefore, the right peaks of

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bimodal PSD curve of the mortar specimen exposed to 400°C may be associated with

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the presence of thermal micro-fractures as shown in Figure 6(b). An accelerated growth of the porosity, median and average pore diameters was

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observed when the exposure temperature was raised from 400°C to 600°C as presented in Figure 5 (a) and (b). According to existing literatures [7,20,62], the decomposition of C-S-H was accelerated dramatically in the range of 500~600°C accompanied by the loss of chemically bound water. The dehydration and decomposition of CH mainly occurred above 400°C as well. The generation and propagation of thermal fractures will inevitably be promoted by the decomposition and transformation of the main hydration products as denoted by the visible change of right peaks of PSD curves in Figure 6 (b).

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M400 M600 M800

0.12

dV/dlogD Pore Volume (mL/g)

dV/dlogD Pore Volume (mL/g)

0.12 0.10 0.08 0.06 0.04 0.02

0.10 0.08 0.06 0.04 0.02 0.00

0.00 0.01

0.1

1

10

0.01

100

0.1

1

10

100

Pore diameter (μm)

Pore diameter (μm)

(a)

(b)

Figure 6. Pore size distribution of the heat-treated mortar specimens exposed to different temperatures measured

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by MIP. (a) M105~M400; (b) M400~M800.

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3.2 Characterizing the microstructure of heat-damaged mortars by X-ray CT

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3.2.1 Analysis of the X-ray CT slices of heat-damaged mortar specimens

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As shown in Figure 7, the grey level of void space, sand grains and hydration

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products exhibited obvious differences in the X-ray CT images of heat-damaged

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mortar specimens. The boundary between each phase can be well identified and the denser material appeared brighter in the X-ray CT images. Pores and fractures were

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presented as the darkest areas. Only a few of micro-fractures can be observed in the mortar specimen exposed to a temperature of 400°C while plenty of micro-fractures were captured in the specimens exposed to a temperature of 600°C and 800°C. The trend was consistent with the variation of porosity and pore size measured by MIP. According to Fu et al. [63], three types of thermal fractures can be clarified according to its location: the tangential fractures generated on the boundary between cement paste and aggregates i.e the interface transition zone (ITZ), the radial fractures propagating in the cement paste and the inclusion fractures in aggregates [63]. Except to the inclusion fractures, other two types of fractures can be observed by the X-ray 17

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CT images as denoted in Figure 7(a) and (b). It was noticed the aperture of tangential fractures was generally lager comparing with the radial fractures. It was reasonable considering the ITZ was taken as an area more porous with poor mechanical properties in the cement-based materials. The mismatch of the coefficients of thermal expansion between each phase in the cement mortar accounted for the generation of

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tangential and radial fractures under different exposure temperatures [63–65]. Besides, the main hydration products including C-S-H and CH can be decomposed

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considerably when the exposure temperatures exceeded 600°C [7]. This failure in

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binding capacity of C-S-H and CH will promote the growth of fractures along the

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boundary between hydration products, un-hydrated clinker and sand grains.

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In the present case, the expansion of micro-fractures can be promoted by cooling

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the heated mortar specimens in the light of the differences in thermodynamic properties between each phase. The existence of air voids may release some thermal

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deformation of cement matrix when the heating temperature was lower. Under a higher temperature, the air void pores can influence the extension of thermal fractures as well as shown in Figure 7(b).

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Hydration products Tangential fractures

Sand grain

Tangential fracture

M600

M800

M400

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M400

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Air void

M600

M800 3

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(a) voxel size of ~4.0 μm

Tangential fracture

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Sand grain

Radial fractures

M400

M600

M400

M600

M800

M800 3

(b) voxel size of 1.5 μm

Figure 7. X-ray CT slices and 2D representations of the fracture aperture of heat-damaged mortar specimens from 3D volume data with different voxel sizes. (a) voxel size of ~4.0 μm3 ; (b) voxel size of 1.5 μm3. 19

Journal Pre-proof 3.2.2 Comparisons between fracture aperture determined by X-ray CT and PSD measured by MIP

The results of MIP test can be examined by a direct visualization of the morphological configuration of pores and fractures of heat-damaged mortar specimens by X-ray CT images. As shown in Figure 8, the PSD curves measured by

of

MIP (blue lines) of mortar specimens exposed to a temperature ≥ 400°C were plotted together with the fracture apertures distribution (FAD) curves determined based on

ro

X-ray CT images with a voxel size of ~4.0 μm3 (red lines) and 1.5 μm3 (green lines).

-p

The FAD curves determined based on X-ray CT images showed coincidence with the

re

right peaks of the PSD curves measured by MIP. It was confirmed that the left peaks

lP

located in the range of 0.01~0.5 μm on the PSD curves measured by MIP

na

corresponded to the fine matrix pores while the right peaks represented the aperture of heat-induced micro-fractures. Therefore, the appearance of bimodal PSD in the mortar

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specimen exposed to a temperature ≥ 400°C can be associated with the presence of heat-induced micro-fractures.

By comparing Figure 7 (a) and (b), it can be found the pores and fractures with a smaller size can be identified more effectively when the voxel size was improved from 4.0 to 1.5 μm3. As shown in Figure 8, the fracture aperture of heat-damaged mortar specimens may be overestimated by X-ray CT images with a voxel size of ~4.0 μm3. FAD determined by X-ray CT images with a voxel size of 1.5 μm3 exhibited more consistency with PSD measured by MIP, especially when the mortar specimen was exposed to a temperature ≥ 600°C. 20

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0.06

8 0.04

6

4 0.02 2

0 1E-3

0.00 0.01

0.1

1

10

100

X-ray CT MIP

15

Voxel counts (1×106)

Voxel counts (1×106)

10

0.12

M400 1.5 μm3/voxel

X-ray CT MIP

dV/dlogD Pore Volume (mL/g)

M400 4.0 μm3/voxel

12

0.08

9

0.06

6

0.04

3

0.02

0

1000

0.10

dV/dlogD Pore Volume (mL/g)

18

12

0.00

1E-3

0.01

Fracture aperture (μm)

0.1

1

10

100

1000

Fracture aperture(μm)

(a)

(b)

0.02

4 0 0.1

1

10

100

(c)

lP

16

M800 4.1μm3/voxel

X-ray CT MIP

14

na

6 4 2 0 1E-3

0.01

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Voxel counts (1×106)

10

0.1

1

10

100

21

0.12 0.10

12

8

of 0.01

re

Fracture aperture (μm)

3

1E-3

1000

0.08 0.06 0.04 0.02

0.06

0.04

0.02

0.00 0.1

1

10

100

1000

Fracture aperture(μm)

(d)

M800 1.5μm3/voxel

X-ray CT MIP

18

Voxel counts (1×106)

0.01

6

0.08

0

0.00

1E-3

9

0.10

0.12 0.10

15 0.08 12 0.06 9 0.04

6

0.02

3

0.00

0

1000

1E-3

Fracture aperture (μm)

dV/dlogD Pore Volume (mL/g)

0.04 8

12

X-ray CT MIP

ro

0.06

12

M600 1.5 μm3/voxel

-p

16

dV/dlogD Pore Volume (mL/g)

Voxel counts (1×106)

0.08

20

15

0.10

Voxel counts (1×106)

X-ray CT MIP

dV/dlogD Pore Volume (mL/g)

M600 4.0 μm3/voxel 24

dV/dlogD Pore Volume (mL/g)

28

0.00 0.01

0.1

1

10

100

1000

Fracture aperture(μm)

(e) (f) Figure 8. Comparison of the pore diameter measured by MIP and fracture aperture determined by X-ray CT images with different voxel sizes. (a) M400 (voxel size of ~4.0 μm3); (b) M400 (voxel size of 1.5 μm3) (c) M600 (voxel size of ~4.0 μm3); (d) M600 (voxel size of 1.5 μm3); (e) M800 (voxel size of ~4.0 μm3); (f) M800 (voxel size of 1.5 μm3). Table 4. Fracture apertures determined based on X-ray CT images Specimen

Voxel size (um3)

Average (um)

Std Dev (um)

Max (um)

M800

1.5

10.25

5.34

174.98

M600

1.5

10.55

6.00

112.61

M400

1.5

5.84

2.91

200.64

M800

4.1

17.44

5.78

730.40

M600

4.0

15.79

5.55

797.52

M400

4.0

12.57

6.10

1066.88

21

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Three-dimensional distribution of fracture aperture was presented in Figure 9(a) (voxel size of ~4.0 μm3) and Figure 9(b) (voxel size of 1.5 μm3). The average, standard deviation and maximum aperture of the fractures (pores) determined by X-ray CT images were summarized in Table 4. The existence of air voids has an important influence on the calculation of average aperture because the size of some

of

air voids is much larger than that of the heat-induced fractures. Therefore, the average aperture and the standard deviation in Table 4 were calculated with the data of the

ro

voids with a size smaller than 30 μm to describe the aperture of heat-induced fractures

-p

more accurately. The maximum aperture was calculated with the origin data. It was

re

noticed that the size of volume of interest can be enhanced from 1.7 mm to 4.7 mm

lP

when the voxel size of X-ray CT images varied from 1.5 to ~4.0 μm3. The features of

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air voids and defects around the larger aggregates can be well captured with a voxel size of ~4.0 μm3. The average aperture determined by X-ray CT images with a voxel

Jo ur

size of 1.5 μm3 was 46~67% of the measured by X-ray CT images with a voxel size of ~4.0 μm3. It was concluded that the X-ray CT images with a voxel size of 1.5 μm3 were more suitable for characterizing the micro-fractures with an aperture of several to tens of micrometers.

22

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600℃ M600

M400

M600

800℃ M800

M800

re

-p

ro

of

400℃ M400

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na

lP

(a) voxel size of ~4.0 μm3

400℃ M400

600℃ M600

M400

M600

800℃ M800

M800 3

(b) voxel size of 1.5 μm

Figure 9. Three-dimensional representation of fracture apertures distribution in the heat-treated mortar specimens determined based on X-ray images. Note that the legend was different for mortar specimens exposed to various temperatures. (a) voxel size of ~4.0 μm3; (b) voxel size of 1.5 μm3. 23

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In this section, the dominant change of the microstructure between pore and micro-damages evolution at different temperatures was analyzed. According to the PSD curves in Figure 8 and the calculated fracture aperture in Figure 9, the volume change of voids with a size larger than 1.5 μm was mainly caused by the propagation of heat-induced fractures. Correspondingly, pore coarsening mainly occurred in voids

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with a size smaller than 1.5 μm. To separately quantify the influence of pore coarsening and fracture propagation, the volume change of pores and fractures was

ro

analyzed individually based on the mercury intrusion data. Contribution made by pore

P 100% 

re

C

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coarsening or fracture propagation to the change of total porosity was calculated as: (7)

lP

where C was called contribution ratio and  denoted the change of total porosity when the exposure temperature was enhanced from a certain value to a higher one.

na

P represented the change in porosity of the voids with a size smaller than 1.5 μm.

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When P was replaced by F which indicated the change in porosity of the voids with a size larger than 1.5 μm, C represented the contribution made by fracture propagation. The calculation results were presented in Figure 10. It suggested that the change of total porosity was mainly caused by pore coarsening in the range of 105~200°C. When the exposure temperature was enhanced from 200°C to 800°C, the contribution made by fracture propagation to the change of total porosity was 2.1~2.9 times of that made by pore coarsening. Therefore, pore coarsening was the dominant change of the microstructure in heat-treated cement mortar in the range of 105~200°C. Fracture propagation contributed more to the change of the microstructure when the 24

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exposure temperature varied from 200°C to 800°C. Pore coarsening Frtacture propagation

Contribution ratio (%)

100

80

60

40

20

0

105~200°C

200~400°C

400~600°C

600~800°C

of

Exposure temperature (°C)

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Figure 10. Contribution made by pore coarsening and fracture propagation to the change of total porosity.

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3.2.3 Evolution of the connectivity and tortuosity of heat-damaged mortar specimens

re

With the initiation and growth of thermal fractures, air voids in the heat-damaged

lP

mortar specimens can be connected and acted as critical channels for heat and mass

na

transfer. The tortuosity and connectivity determined based on the X-ray images with a voxel size of 1.5 and 4.0 μm3 were listed in Table 3. It can be found the connectivity

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increased with the rise of exposure temperatures. For example, the connectivity of pores and fractures with a size larger than 1.5μm can be enhanced from 0.64 to 0.94 when the exposure temperature was increased from 400 °C to 600 °C. However, the change of connectivity was lower than 0.05 when the exposure temperature varied from 600 °C to 800 °C. By contrast, the tortuosity decreased with the rise of exposure temperature. It was considered as reasonable because the length of imaginary path for mass transfer was expected to be shorter with more pores connected by thermal fractures. It was noticed the resolution of X-ray CT images has a significant influence on the 25

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determined connectivity as well. The connectivity of M400 was determined as 0 and 0.64 based on X-ray images with a voxel size of 4.0 and 1.5 μm3, respectively. An increased connectivity of M600 and M800 was found when the voxel size was improved from 4.0 to 1.5 μm3. Meanwhile, the determined tortuosity decreased with the increase of connectivity. It seemed that the connectivity was more sensitive to the

of

change of resolution comparing with the tortuosity for M400 while the tortuosity was more susceptible to the resolution for M600 and M800. Reasons for the influence of

ro

resolution on connectivity and tortuosity can be concluded as followings: first, more

-p

micro-fractures can be identified with the improvement in resolution which will

re

enhance the connectivity and produce a low tortuosity; second, the volume of VOI

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was decreased by 20 times when the voxel size varied from 4.0 to 1.5 μm3 and the

na

pores and fractures were easier to be connected in a smaller space.

CT images

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3.3 Fractal analysis of the microstructure features of heat-damaged mortar with X-ray

3.3.1 3D fractal dimension of the void space determined by X-ray CT images

For a fractal object, the 3D fractal dimension is expected to vary from 2 to 3 and a higher value represents a more complex microstructure [34]. The fractal dimension of the heat-damaged mortar specimens (M400, M600 and M800) determined based on void space segmented from X-ray CT images with a voxel size of ~4.0 and 1.5 μm3 (Figure 4) were denoted as Df-L and Df-S respectively as listed in Table 3. It can be found that Df-L of M400 was lower than 2 (1.89) which meant the detected pores and 26

Journal Pre-proof fractures with a size ≥ 4.0 um showed no fractal property. The Df-L of M600 was determined as 2.40 which was a bit higher than that of M800 (2.36). As discussed above, although more air voids in the heat-damaged mortar specimen can be captured by the X-ray CT images with a voxel size of 4.0 μm3, the microstructure details of heat-induced fractures with a aperture smaller than 4.0 um were missed at current

of

resolution. 3D fractal dimension Df-S of the heat-damaged mortar determined based on X-ray

ro

CT images with a voxel size of 1.5 μm3 was located in the range of 2~3. Df-S was

-p

higher than Df-L for all the specimens. This can be attributed to that more

re

microstructure details can be detected with the improvement of resolution. Df-S

lP

increased with the rise of exposure temperature which can be attributed to the

na

expansion of pores and fractures with the rise of exposure temperatures. Meanwhile, it was noted the Df-S of M600 (2.46) were a bit lower than that of M800 (2.49). It

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implied the microstructure of detected pores and fractures with a size larger than 1.5 um changed slightly when the exposure temperature was raised from 600 °C to 800 °C. By contrast, the Df-S of M400 was determined as 2.28 which was much lower than the Df-S of M600 and M800. It indicated 400 °C was a key temperature with respect to the microstructure evolution of pores and fractures with a size larger than 1.5 um.

3.3.2 Correlations between 3D fractal dimension and other microstructure parameters

In this section, the relations between 3D fractal dimension Df-S and volume fraction, 27

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tortuosity of the detected pores and fractures with a size larger than 1.5 um was focused. Based on above analysis, the microstructure of heat-damaged mortar became more complicated with the generation and extension of thermal fractures. As shown in Table 3, the volume fraction and 3D fractal dimension Df-S of pores and fractures increased with the rise of exposure temperatures. However, the tortuosity decreased

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with the rise of exposure temperatures due to the enhancement of connectivity. As shown in Figure 11, a positive linear dependence between the volume fraction and 3D

ro

fractal dimension was found while tortuosity showed a negative linear correlation

-p

with the 3D fractal dimension Df-S. y= 0.303x-0.654 R2=0.976

y= -12.651x+33.729 R2=0.999

5.0

re

0.10

0.06

0.04

2.4

Jo ur

2.3

4.0

Tortuosity

lP

0.08

na

Volume fraction

4.5

3.5

3.0

2.5

2.0

2.5

2.3

Fractal dimension

2.4

2.5

Fractal dimension

(a)

(b)

Figure 11. Linear analysis of the relation between 3D fractal dimension Df-S and other microstructure parameters. (a) volume fraction; (b) tortuosity.

3.3.3 Variation of 2D fractal dimension with exposure temperature

The 2D fractal dimension was a useful parameter for describing the anisotropy of the material in two-dimensional with a value in the range of 1~2. Here, the variations of 2D fractal dimension for different slices and its correction with the void area fraction and average fracture apertures was analyzed. The 2D microstructure parameters for different slice were determined based on the segmented void space 28

Journal Pre-proof data from X-ray CT images with a voxel size of 1.5 μm3. The variations of 2D fractal dimension, void area fraction and average fracture aperture with the change of slice number were represented by the black, red and blue lines respectively as presented in Figure 12. As shown in Figure 12(a), 2D microstructure parameters of M400 showed considerable fluctuations with the change

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of slices number. This indicated strong heterogeneity of pores and fractures in M400. The variation trend of 2D fractal dimension, void area fraction and average fracture

ro

aperture with the change of slices in M400 was basically consistent. Obvious

-p

difference was noticed when the air void was absent in the slice. For M600, the

re

relevance between the three parameters was not strong as shown in Figure 12(b). It

lP

can be found from Figure 12(c) that the 2D fractal dimension of M800 varied from

na

1.35 to 1.37 with an average of 1.44 while the void area fraction changed from 0.083 to 0.138. It was noticed that the evolution of fractal dimension showed basic

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consistence with void area fraction with the change of slice numbers for the specimen M800. Nevertheless, the fluctuation of average fracture apertures in M800 was much more obvious which was caused by the presence of air voids in the mortar specimen. When the average 2D fractal dimension determined based on the X-ray CT slices was denoted as D2d- s, it was found the differences between Df-S and the average value of D2d-S were 1.003, 1.055, 1.054 for M400, M600 and M800. It was considered as reasonable because the difference between 2D and 3D fractal dimension of the same object has been concluded as 1.0 [34].

29

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M400 1.5 μm3/voxel

Void area fraction Fractal dimension Fracture aperture

0.07 0.06

1.36

80

1.34

70

1.32

60

1.30

50

1.28

40

1.26

30

1.24

20

1.22

10

1.20

0

0.05 0.04 0.03 0.02 0.01 0

200

400

600

800

0.12

Void area fraction Fractal dimension Fracture aperture

ro

M600 1.5 μm3/voxel 1.48

0.10

0.07 200

400

600

na

0

lP

0.08

800

20

1.44 1.42

re

0.09

22

1.46

-p

0.11

of

Slice number (a)

1000

18 16

1.40

14

1.38

12

1.36

10

1.34

8

1000

Slice number (b)

0.12

0.11

Void area fraction Fractal dimension Fracture aperture

M800 1.5 μm3/voxel 1.47

26

1.46

24

1.45

22

Jo ur

0.13

1.44

20 18

1.43 16

0.10

1.42 14 1.41

0.09

0.08 0

200

400

600

800

12

1.40

10

1.39

8

1000

Slice number (c) Figure 12. Evolution of two-dimensional microstructure parameters determined based on X-ray CT images (voxel size of 1.5 μm3) with slice numbers. The variation of void area fraction, 2D fractal dimension and average fracture apertures with the change of slice number were represented by the black, red and blue lines respectively. (a) M400; (b) M600; (c) M800. 30

Journal Pre-proof 3.4 Scale-dependent fractal characterization of heat-damaged mortar with MIP data

As shown in Figure 13, the linear functions were employed to fit the data of Log SHg v.s. Log Pc determined by MIP test based on Equation (5). The fractal dimensions were calculated by Equation (6) with the determined slopes of fitting lines as listed in Table 5. The scale-dependent fractal properties of heat-treated mortar specimens were

of

witnessed i.e. fractal dimension varied for pores and fractures with different sizes.

ro

According to Equation (1), a higher capillary pressure corresponded to a smaller

-p

pore/fracture size. It can be found from Table 5 that five different ranges with respect

re

to the size of pores and fractures can be identified based on the change of fractal dimensions.

lP

The fractal dimensions of smallest pores i.e. range #1 varied from 2.02 to 2.08 for

na

mortar specimen exposed to different temperatures. It should be noted the structure of

Jo ur

nano-scale pores in the heat-treated cement-based materials may not be well characterized by MIP test comparing with nitrogen adsorption and SEM techniques [19,20]. The structure can be destroyed under elevated temperatures which may account for the limited fluctuation of the fractal dimension as well. It was noticed that the pores and fractures in the range #2 and #4 shows a stronger fractal property for each mortar specimen. The fractal dimension of range #4 for specimen M400, M600 and M800 were close to that determined by X-ray CT images with a voxel size of 1.5 μm3 (Df-S). It was not surprising because the pores and fractures in range #4 of M400, M600 and M800 can be identified effectively by X-ray CT images with a voxel size of 1.5 μm3. The boundary of range #4 increased with the 31

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rise of expose temperatures which can be attributed to the improved connectivity by the growth and expansion of thermal fractures. The fractal dimension of range #5 showed a regular increase with the growth of exposure temperatures (≤ 400°C). The pores in range #5 can be sorted as air voids. The fractal dimension of range #5 for M400 was higher than that determined by X-ray

of

CT images. When the mortar specimen was exposed to a temperature higher than 400°C, the skeleton of the material become fragile and may influence the results of

-p

fractures in range #5 for M600 and M800.

ro

MIP test. Thus the MIP test failed to measure the fractal dimension of pores and

M200

M400

M600

M800

#2

0.003~0.014μm 2.077

lP

M105

#1

0.017~0.040μm

#3

#4

#5

0.05~0.18μm

0.2~5.2μm

6.0~363.6μm

2.288

3.666

2.189

2.075

0.017~0.040μm

0.05~1.22μm

1.5~32.9μm

45.1~180.2μm

2.057

2.267

2.868

2.175

2.345

0.003~0.026μm

0.029~1.48μm

1.9~9.0μm

10.0~50.6μm

60.3~180.9μm

2.067

2.386

3.052

2.231

2.512

0.004~0.021μm

0.026~5.194μm

7.2~25.8μm

30.2~145.2μm

180.2~353.6μm

2.030

2.281

3.118

2.648

3.982

0.014~0.040μm

0.05~5.37μm

7.2~15.7μm

17.2~145.1μm

179.9~331.9

2.022

2.312

3.798

2.553

3.011

0.003~0.014μm

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Specimen

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Range

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Table 5. Fractal dimension of heat-treated mortar specimens estimated by the results of MIP

32

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1.0

M200 45.1~180.2 μm 1.5~32.9 μm 0.05~1.22μm 0.017~0.040 μm 0.003~0.014 μm y=0.345x+0.992 R2=0.944 y=0.175x+0.693 R2=0.972 y=0.868x+0.632 R2=0.991 y=0.267x+1.457 R2=0.981 y=0.057x+1.854 R2=0.971

1.8 1.5

Log SHg (%)

6.0~120.1 μm 0.2~5.2 μm 0.05~0.18 μm 0.017~0.040 μm 0.003~0.014 μm y=0.075x+0.660 y=0.189x+0.728 y=1.666x-0.481 y=0.288x+1.405 y=0.074x+1.810

1.5

Log SHg (%)

2.1

M105

2.0

R2=0.948 R2=0.990 R2=0.978 R2=0.981 R2=0.976

1.2 0.9 0.6

0.5

0.3 -3

-2

-1

0

1

2

3

-3

-2

-1

Log PC (MPa)

(a)

-3

-2

-1

0

1

2

Log PC (MPa)

M800

2.0

3

of

Log SHg (%)

-0.5

3

-3

-2

-1

0

1

R2=0.961 R2=0.997 R2=0.990 R2=0.993 R2=0.752 2

3

Log PC (MPa)

(d)

na 179.9~331.9 μm 17.2~145.1 μm 7.2~15.7 μm 0.05~5.37 μm 0.014~0.040 μm y=1.011x+2.116 y=0.553x+1.205 y=1.798x+2.639 y=0.312x+1.545 y=0.022x+1.960

1.0

0.5

0.0

-0.5 -3

-2

Jo ur

Log SHg (%)

1.5

0.0

lP

(c)

0.5

180.2~353.6 μm 30.2~145.2 μm 7.2~25.8 μm 0.026~5.194 μm 0.004~0.021 μm y=1.982x+4.369 y=0.648x+1.442 y=1.118x+2.079 y=0.281x+1.497 y=0.030x+1.928

-p

0.0

R2=0.951 R2=0.959 R2=0.981 R2=0.987 R2=0.917

1.0

re

Log SHg (%)

60.3~180.9 μm 10.0~50.6 μm 1.9~9.0 μm 0.029~1.48 μm 0.003~0.026 μm y=0.512x+1.102 y=0.231x+0.602 y=1.052x+1.281 y=0.386x+1.295 y=0.067x+1.834

ro

1.5

1.5

0.5

2

M600

2.0

1.0

1

(b)

M400

2.0

0

Log PC (MPa)

-1

0

1

R2=0.892 R2=0.993 R2=0.994 R2=0.992 R2=0.695 2

3

Log PC (MPa)

(e) Figure 13. Estimation of the fractal dimensions of heat-treated mortar specimens based on capillary pressure data measured by MIP. (a) M105; (b) M200; (c) M400; (d) M600; (e) M800.

4. Conclusions

In the present work, the microstructure evolution of mortar specimens exposed different temperatures was characterized with MIP and multi-scale X-ray CT tests. 33

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Fractal theory was introduced to characterize the microstructure feature of heat-damaged mortar specimens as well. The main conclusions can be drawn as followings: (I) The porosity and pore diameter of the heat-treated mortar specimens increased with the rise of exposure temperatures. Bimodal pore size distribution was witnessed

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when the mortar was exposed a temperature ≥ 400 °C. The appearance of right peaks of the PSD curves measured by MIP can be related to the generation of thermal

ro

micro-fractures due to incompatibility in thermal behavior between each phase as

-p

revealed by X-ray CT images. The features of air voids can be efficiently captured by

re

X-ray CT images with a voxel size of 4.0 μm3.

lP

(II) When the voxel size of X-ray CT images was enhanced from 4.0 to 1.5 μm3,

na

more microstructure details of the thermal fractures can be captured. The fractal dimension increased with the rise of exposure temperatures. 400°C was critical with

Jo ur

respect to the evolution of fractal dimension, tortuosity and connectivity of heat-damaged mortar specimens. The air voids have a significant influence on the determination of 2D microstructure parameters. (III) Positive linear dependence between the volume fraction of detected pores and fractures with a size lager than 1.5μm and 3D fractal dimension was found. The tortuosity decreased linearly with the increase of 3D fractal dimension. (IV) The scale-dependent fractal properties of the heat-damaged mortar specimens were revealed with the capillary pressure data measured by MIP test. The fractal dimension of micro-scale pores and fractures measured by MIP data was close to that 34

Journal Pre-proof determined based on X-ray CT images with a voxel size of 1.5 μm3.

Acknowledgements

The financial support for ongoing projects by the National Natural Science Foundation of China (U1706222, 51778309, 51420105015) and Natural Science Foundation of Shandong Province (ZR2018JL018, ZR2019PEE001) is greatly

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acknowledged.

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Data Availability

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The raw/processed data required to reproduce these findings cannot be shared at

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this time as the data also forms part of an ongoing study.

[1]

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Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or

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the review of, the manuscript entitled.

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Highlights: 3D fracture aperture of heat-damaged mortar was presented at multi-scale; Microstructure parameters measured by MIP and X-ray CT images with different voxel sizes were compared;

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The connectivity, tortuosity and fractal features of micro-fractures were analyzed.

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