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TECHNICAL UNIVERSITY OF CIVIL ENGINEERING, BUCHAREST INSTITUT NATIONAL DES SCIENCES APPLIQUEES, TOULOUSE LABORATOIRE MATERIAUX ET DURABILITE DES CONSTRUCTIONS DISSERTATION THESIS Multiscale study of cementitious materials by X-Ray Computed Micro Tomography Glad Calin Licsandru Ioan Alexandru Popa MSc. Students Technology and management of constructions works, U.T.C.B, Romania Department Civil, Industrial and Agricultural Buildings Coordinators: Anaclet Turatsinze, professor L.M.D.C. Ariane Abou Chakra, maître de conférence L.M.D.C. Catherine Noiriel, maître de conférence G.E.T. Gael Blanc, PhD student L.M.D.C. - January June 2015 -

Dissertation thesis - A.Popa

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TECHNICAL UNIVERSITY OF CIVIL ENGINEERING, BUCHAREST

INSTITUT NATIONAL DES SCIENCES APPLIQUEES, TOULOUSE

LABORATOIRE MATERIAUX ET DURABILITE DES CONSTRUCTIONS

DISSERTATION THESIS

Multiscale study of cementitious materials by X-Ray Computed Micro – Tomography

Glad Calin Licsandru

Ioan Alexandru Popa

MSc. Students – Technology and management of constructions works, U.T.C.B, Romania

Department – Civil, Industrial and Agricultural Buildings

Coordinators: Anaclet Turatsinze, professor L.M.D.C.

Ariane Abou Chakra, maître de conférence L.M.D.C.

Catherine Noiriel, maître de conférence G.E.T.

Gael Blanc, PhD student L.M.D.C.

- January – June 2015 -

TABLE OF CONTENTS

GENERAL INTRODUCTION .....................................................................................................1

CHAPTER 1 – MATERIALS, MECHANICAL CHARACTERISTICS .........................................2

1.1 Compression strength ........................................................................................................2

1.2 Elasticity modulus .............................................................................................................3

1.3 Porosity .............................................................................................................................3

1.4 Water saturation in 31 days ..............................................................................................4

CHAPTER 2 - X.C.M.T. ..............................................................................................................6

2.1 Introduction .......................................................................................................................6

2.1.1 Basic description of the method .................................................................................7

2.1.2 Grey scale histograms .................................................................................................8

2.1.3 The basic acquisition process .....................................................................................9

2.2 General presentation of the equipment and its operating mode .....................................11

2.3 Data interpretation ..........................................................................................................12

2.4 Applied procedures .........................................................................................................12

2.4.1 3D rendering and 2D findings ..................................................................................12

2.4.2 Real time experiment monitoring .............................................................................12

2.4.3 Segmenting in components a sample........................................................................13

CHAPTER 3 – CHEMICAL ATTACKS ....................................................................................16

3.1 Acid attack .......................................................................................................................16

3.1.1 Introduction ..............................................................................................................16

3.1.2 The experiment .........................................................................................................16

3.1.2.1 Pretest launch .....................................................................................................17

3.1.2.2 Pretest results .....................................................................................................17

3.1.2.3 Experiment launch .............................................................................................18

3.1.3 Results ......................................................................................................................21

3.1.3.1 Acid penetration evolution over time ................................................................23

3.1.4 Micro – tomography .................................................................................................24

3.1.5 Conclusions ..............................................................................................................29

3.2 Sulfate attack ...................................................................................................................31

3.2.1 Introduction ..............................................................................................................31

3.2.2 The experiment .........................................................................................................31

3.2.3 Results ......................................................................................................................33

3.2.4 Micro – tomography .................................................................................................34

3.2.5 Conclusions ..............................................................................................................37

CHAPTER 4 – EFFLORESCENCE AND CAPILARITY RISE .................................................38

4.1 Internal efflorescence ......................................................................................................38

4.1.1 Introduction ..............................................................................................................38

4.1.2 The experiment .........................................................................................................38

4.1.3 Results ......................................................................................................................40

4.1.4 Micro – tomography .................................................................................................42

4.1.6 Conclusions ..............................................................................................................45

4.2 External efflorescence .....................................................................................................46

4.2.1 Introduction ..............................................................................................................46

4.2.2 The experiment .........................................................................................................46

4.2.3 Results ......................................................................................................................46

4.2.4 Micro – tomography .................................................................................................53

4.2.5 Conclusions ..............................................................................................................54

CHAPTER 5 – FINAL CONCLUSIONS ...................................................................................55

BIBLIOGRAPHY.......................................................................................................................57

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

1 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

GENERAL INTRODUCTION

The research for the following these has been carried out at Institut National des Sciences Appliquees de Toulouse (INSA), in Laboratoire des Matériaux et Durabilité des Constructions (L.M.D.C.) through the European Scholarships Program, ERASMUS+.

The these is entitled “Experimental studies of cementitious materials by X-ray micro-tomography” and it is focused on studying the durability of 3 mortar recipes by analyzing their porous network gradually from an exposure period to another, with a technique called X.C.M.T. (X-ray Computed Micro-Tomography). The mortars tested are the property of Menard Company which uses them to implement C.M.C columns (Controlled Modulus Columns) for soil improvement, aggressive soils with toxic infiltrations.

The experimental work has been done in L.M.D.C., the X.C.M.T analysis was done in CIRIMAT laboratory and for data processing the work has been done in Geoscience Environnement Toulouse (GET). The internship lasted 5 months during which there were effected 10 sessions (1 session = 1 day) of X.C.M.T. scans, obtaining the acquisition data for 3 samples/day.

The main part of the present work is outlined by the 2 sets of tests done on the materials. Their durability was tested to chemical attacks (acid attack and sulfate attack) and to the efflorescence phenomenon (internal and external efflorescence). The experiments conducted are based mainly on previous specialty literature references.

The X.C.M.T. analysis was constructed out of 3 basic data processes: rendering, subtraction and segmentation. This was enough to do studies on the evolution of the porous network in the samples during different stages of degradation. Scans on sain samples were also performed in order to have a reference point of the analysis.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

2 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

CHAPTER 1 – MATERIALS, MECHANICAL CHARACTERISTICS

For the current study 3 mortar recipes were used:

1. One reference mortar – composed of cement CEM III, sand, water, CaCO3 powder and super-plasticizer called REF

2. One mortar composed of CEM III, sand, water, CaCO3 powder, super-plasticizer and rubber aggregate, called GC

3. One mortar composed of CEM III, sand, water, CaCO3 powder, super-plasticizer, expanded clay and metallic fibers, called GLF

1.1 Compression strength

The compression strength was measured after 28 days of casting with the hydraulic press (Figure 1.2):

REF – 16.9 MPa

GC – 10.1 MPa

GLF – 9.5 MPa

Figure 1.2 – Hydraulic press

Figure 1.1 - Core sample example

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

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1.2 Elasticity modulus

Using the hydraulic press, the method consists in applying cycles of loads with a maximum 30% of the compression strength (not exceeding the elastic stage), the sample being connected to two sets of sensors which record each time the lateral displacement (Figure 1.2).

The results are:

REF – E = 21.3 GPa

GC – E = 16.3 GPa

GLF – E = 15.6 GPa

1.3 Porosity

The porosity was measured with mercury using the Pascal 140 – 240 apparatus (Figure 1.3). The method involves two phases: the first phase, to determine the macro pores (measuring above 50 nm), the samples are subjected to mercury under a pressure of 140 MPa followed by the second phase, to determine the Nano-pores (measuring less than 50 nm) in which the samples are subjected to mercury under a pressure of 240 MPa.

There were used samples with the size approximately of 1 cm by 1 cm. The method proved to be, in some cases uncertain, the sample shattering during the process of injecting mercury under certain pressure; in that case the process was restarted.

Figure 1.3 – Pascal 140 -240 machine

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

4 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

The results are:

Figure 1.4 – Porosity curves output by Pascal 140 – 240

REF – porosity – 20,1 %

GC – porosity – 21.1 %

GLF – porosity – 30.46 %

1.4 Water saturation in 31 days (in normal pressure and temperature conditions)

Two samples were prepared from each mortar formulation, with a diameter of ~2.1 cm and height of ~ 5-6 cm. which were held into a water filled tub for 31 days to observe the percentage of saturation. The cores were not dried before launching the experiment, they had a normal amount of humidity.

Mass readings were performed in order to determine the saturation evolution (Figure 1.5). After 31 days, the samples were pulled out of the water tank and introduced into a 50o oven until their mass reaches a variation of less than 1% between 2 consecutive readings.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

5 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 1.5 – Mass readings in the water saturation test

In Figure 1.5 the amount of water retained by the samples in 31 days, obtained after drying in the oven, was:

REF – 9.92 % of its mass

GC – 11.24 % of its mass

GLF – 15.98% of its mass

Comparing with their porosity, in 31 days the saturation point was of about 50%.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

6 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

CHAPTER 2 - X.C.M.T.

2.1 Introduction

X.C.M.T.=X-ray Computed Micro-Tomography

X.C.M.T. is a non-destructive technique, which allows the user to reconstruct a 3D version of a sample, allowing the study the exterior, as well as the interior structure of a sample. This is done without disturbing the natural bonds found in various materials. This possibility is of high interest in studying various materials, especially composite materials in which one can verify the quality of the interface between binding and strengthening or filling agents.

The first X-ray micro-tomography was realized by Jim Elliott in the 1980’s. The first publication of X-ray micro-tomographic images was in 1982 and they were reconstructed slices of a tropical snail, the pixel size obtained was 50 µm [4].A

Micro-tomography is also known as:

X-ray tomography Industrial Computed Tomography Micro-CT (due to its higher resolution compared to medical CT-scans)

This technique is similar to the medical Computed Tomography, where it is imperative to have a non-destructive method of screening a patient’s inner body. The process is known as a

CT-scan, the basic difference between the two techniques is that for a CT-scan the patient stands still and the X-ray will move around the patient, opposite of the capture screen, while in X.C.M.T. the X-ray source and the capture screen will stand still while the sample will rotate. This is due to the small size of the sample and the mounting and rotating possibilities of such sample.

Resolution is given by these factors:

Detector resolution (capture screen) Sample size Software limitations

The scales of resolutions (precision) usually attained for various CT techniques:

CT-scan - 1mm (medical) High-Resolution CT scan - 100 µm (medical) Micro-CT – between 0.5-10 µm (industrial, research)

The X.C.M.T. technique requires less user interaction by comparison to Scanning Electron Microscopy (SEM) method. The output data is more complex and allows user processing post experiment, opposite of SEM images.

There are other classifications of X-ray micro-tomography:

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

7 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

By imaging system:

Fan beam reconstruction (based on a uni-dimensional X-ray detector) Cone beam reconstruction (based on a two-dimensional X-ray detector)

By type of system enclosure:

Open system;

This set-up presents X-ray leakage risks, so the operations are done under special conditions. The human operator is behind a shield and wears protective clothing, or works from another room. Closed system;

This set-up presents the shielding mounted around the scanner, so that the operator can work without special gear and close to the machine.

The underlined conditions were used in the presented experiments.

2.1.1 Basic description of the method

The X.C.M.T. technique can be split into two phases:

Stage I: Data acquisition process. Stage II: Correction and processing of the acquired data.

Phase I

This part describes the experimental manipulations that the user needs to undergo in order to obtain a proper data acquisition, which can later be processed.

The first step is to establish the sample’s positioning and to set the acquisition parameters

inside the X-ray scanner (synchrotron). These parameters are as follows:

Positioning inside the machine; The X-ray source is the reference point for the Cartesian directions X, Y and Z. The Y axis is vertical and as the instrument considers the XY plane as a referential, when concerning incident irradiation. These parameters are very important as they confer precision to the scan. Distance from the source; the distance from the source to the sample is directly proportional with the pixel size. The precision increases with the closeness of the sample to the source, while the scanned area decreases. This effect is due to the normal projection of an object on a fixed size plane (Fig. 2.1.).

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

8 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 2.1 - Schematics for the acquisition process for different resolutions

Power parameters; these parameters determine the penetration of the X-ray beam. In order to get good acquisition data, the user needs to take in account the absorption rate of the materials constituting the sample. The parameters that must be set are the Voltage and Amperage, their optimization resulting in improved grey-scale histograms.

2.1.2 Grey scale histograms

“A histogram is a graphical representation of the distribution of numerical data. It is an estimate of the probability distribution of a continuous variable (quantitative variable) and was first introduced by Karl Pearson.[1]A

”A

“An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image.[1]A It plots the number of pixels for each tonal value. By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance.”

A

“In an image processing context, the histogram of an image normally refers to a histogram of the pixel intensity values. This histogram is a graph showing the number of pixels in an image at each different intensity value found in that image. For an 8-bit grayscale image there are 256 different possible intensities, and so the histogram will graphically display 256 numbers showing the distribution of pixels amongst those grayscale values.”

A “Histograms have many uses. One of the more common is to decide what value of

threshold to use when converting a grayscale image to a binary one by thresholding. If the image is suitable for thresholding then the histogram will be bi-modal --- i.e. the pixel intensities will be clustered around two well-separated values. A suitable threshold for separating these two groups will be found somewhere in between the two peaks in the histogram. If the distribution is not like this then it is unlikely that a good segmentation can be produced by thresholding.”

A

Figure 2.2 - a) Gray scale histogram spread over a large interval, b) Gray scale histogram spread over a small interval

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

9 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

In (Fig. 2.2) a) a well spread histogram (values between 1000-2500), and the segmentation processing can be easily applied.

In (Fig. 2.2) b) a very low spread histogram (values between 0-500) is presented, therefore the segmentation is more difficult to apply.

For the experiments presented the following power intervals were used:

kV=110-120 μA=110-120

Before acquisitions, the value of the atmospheric air is imputed by the used. This is done in order for the instrument to be able to have a known, constant gray-scale value.

After setting these parameters, the acquisition commences.

2.1.3 The basic acquisition process

During the acquisition process the machine will take X-ray images of the sample in sequences established by the user. In this case, exposure times of 500 ms were used for samples of ~1 cm in diameter and of 750 ms for samples with diameters of ~2.2 cm. Each sample was exposed 3 times in each position. The last radiograph of each exposure was kept for the volume’s reconstruction.

For this acquisition method, 1441 exposure points were chosen, for the total 360 degree rotation of the sample. For correction purposes the machine will take the first radiograph in position 1 and another one in the same spot in position 1441. This corrects possible shifts of the sample during the acquisition. (Fig. 2.3)

Figure 2.3 - Rotation example for the reexposure of the same area

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

10 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Phase II This section treats the correction and processing of the acquisition data.

After the acquisition phase, the user will obtain the 1441 image set in Bitmap format. This format is required in the reconstruction process.

Bitmap is a format that attributes to each pixel Cartesian coordinates over two dimensions (2D). This facilitates the process of converting the data from the two-dimensional format, to a three-dimensional (3D) one. Once this procedure is finished, the pixels with three attributed dimensions will become voxels.

The first correction is the automatic axis correction. It refers to the inclination of the samples, vertical axis, and of possible modifications of this axis in accordance with the horizontal plane.

The user will proceed then to overlap the first image (Image 1) and the last (Image 1441). Both images represent the same point, and thus facilitate the comparison. This operation is done manually and it verifies for any possible translation during acquisition in the XZ plane, which then is corrected.

Last of all, the user does manually the correction of unresponsive pixels. This is necessary since the capture screen sometimes presents dead or unresponsive pixels. These form rings in the reconstructed volumes. The method is called a “ring correction” and generally it’s done to a maximum of 1% of the total pixels.

In order to decrease the necessary processing of volumes, the user can cut some parts of the images (i.e. the margins, which represent air and/or the bottom and top parts which are distorted as a consequence of the conical X-ray beam).

In the case of these experiments the vast majority of the images were cut to the size of 2100X2100 pixels, with one exception which was of 2300X2300 pixels.

After these steps, the user will be able to launch the reconstruction of the volumes. Constructing these volumes is the actual computing part of the X.C.M.T. and this procedure transforms the 2D pixels in 3D pixels named voxels.

For the majority of the scans the volumes were 2100X2100X1000 pixels.

The size of the pixels obtained for these scans was between 5 µm and 12.5 µm. A value of 5 µm was obtained for the smallest of samples and the 12.5 µm for the largest (samples sealed resin). With the equipment used, this was the largest resolution possible.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

11 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

2.2 General presentation of the equipment and its operating mode

The basic elements of PHOENIX NANOTOM (Fig. 2.4).

Figure 2.4 - The equipment PHOENIX NANOTOM and its basic elements

Prior to acquisition, there are some mandatory procedures that are done on a daily basis or whenever the instrument is turned off.

The procedures and the order in which the user executes them:

1) Warm up. 2) Alignment. 3) Adjust the filament (automatically). 4) Target check. 5) Align samples. The sample has to be aligned with the X-ray source and the

capture screen. The user can then start increasing the power parameters in increments of 20 (kV and µA) until the desired power is reached. In this case the used power values have been between 110-120kV and 110-120µA

6) Set exposure time (here, 500ms for the small samples and 750ms for the larger ones).

7) Center the source with the sample position. 8) Image calibration, the machine will simulate 100 acquisitions with different

power values. 9) Reference background, establishing the constant gray scale value for air

corresponding to the chosen power values. 10) Start acquisition.

For all acquisitions a copper filter is used so that the low intensity X-rays are eliminated. This is necessary because low intensity X-rays do not have had enough penetration power and would cause distortions on final image (noise).

In order to get the highest quality acquisitions, the sample has to be as close as possible to the X-Ray source. Therefore the mounting system has to be adapted as presented in (Fig. 2.5).

Figure 2.5 – Mounted sample

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

12 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

2.3 Data interpretation

The processed data sets, the 3D volumes, can be analyzed. The analysis is time consuming and the following aspects have to be taken into account.

To be factored in:

Quality of the acquisition, the user needs to take in account power fluctuations and capture screen issues during the acquisition.

Data correction due to sample movement and consequently loss of data Sample’s components and its homogeneity, high homogeneity and less

adsorbent materials in the sample’s composition yield better quality data. Histogram spread and number of peaks. Taking into account artefacts. These are due to the absorbent nature of some

materials and generate white regions on the images. These regions also have shading associated.

Software and hardware errors (i.e. missing few of the 1441 images) Filtering, each applied filter causes an amount of data loss.

2.4 Applied procedures

2.4.1 3D rendering and 2D findings The rendering procedure generates a tridimensional surface of the data set in pre-

established parameters. This offers the user a global perspective over the data set. This also constitutes a very good presentation tool.

In bi-dimensional planes (slice view), a precise view of the interior and the exterior of the sample is revealed. This view is particularly important for follow-ups in order to avoid damaging the sample.

2.4.2 Real time experiment monitoring

One of the strengths of the X.C.M.T. is the fact that it allows real time monitoring of

the samples during a long time frame experiment. This is the case of experiments targeting the influence of acidic conditions over special mortar types over the course of several months. The method is non-invasive and it doesn’t require the interruption of the experiment.

This procedure is done in a series of steps and necessitates a low to medium data processing power.

After obtaining two or more data sets, these are the necessary steps for image processing using the Avizo 8.1 Fire software.

Loading the volumes in the software. Applying median filters for each volume.

These filters get rid of the background noise, by replacement with a mediated gray scale value found on the neighboring pixels. This is an automatic process and it yields better results when applied in 3D.

Synchronizing the volumes in the initial stage manually Synchronizing the volumes in the final stage automatically.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

13 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

The synchronization process offers the possibility to have the volumes in the exact same position so that a comparison can be made. This implies the repositioning of one of the volumes.

Applying the geometric transform to the repositioned volume. The repositioned volume is redefined and recreated by the software

Synchronize port option. Every slice of the first volume will be synchronized with every corresponding slice from the second volume.

Arithmetic option. Is applied as A minus B, A being the subtracted (initial state of sample) and B the subtractor (state of the sample after some exposure time).

Processing the operation. The software will subtract the volume from the other one and reconstruct a third volume from the difference.

Analyzing the third volume. This is done visually or with the measuring tool options of the software (Fig.2.6).

Figure 2.6 - Subtraction example

2.4.3 Segmenting in components a sample

This procedure is done through a series of steps and necessitates high data processing

power. After obtaining a data set the following steps are done, in the Avizo 8.1 Fire software.

Loading the volume in the software. Applying median filter. Volume conversion, from the 16 bit acquisition form to an 8 bit one, less

processing intensive. This facilitates a faster processing and less random access memory (RAM) usage.

Cropping (cutting out) a parallelepiped from the volume. In this case a 1300X1300X800 pixels dimension was chosen as being the largest possible inscribed value in the cylinder. This is required so that the distorted zones and the air are cut out. Figure 2.7 presents the process.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

14 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 2.7 - a) 3D rendering; b) Sectioned 3D rendering; c) 3D volume crop; d) Sectioned 3D volume crop

Field labeling option. This command allows to manually define sectors of the histogram (this is known as thresholding). It attributes to each sector just one constant value. During processing the software fills the histogram with each of the attributed values and creates a new volume from such values. On this volume the actual segmentation is done (Fig. 2.8)

Figure 2.8 - Example of manual thresholding in Avizo 8.1 Fire

The arithmetic function. It is used to single out the point of interest (i.e. A=1) so that the software identifies the elements and creates a new volume with only that item (Fig. 2.9)

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

15 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 2.9 - a) Multicolored segmented volume; b) Gray segmented volume

Label fields option. This option attributes a unique label to each element found in the newly generated volume.

Label analysis function. The function generates output data in a table, such as volume, area and label of each body (Fig. 2.10).

This data can then be interpreted.

Figure 2.10 - Example Avizo 8.1 Fire segmentation procedures

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

16 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

CHAPTER 3 – CHEMICAL ATTACKS

3.1 Acid attack

3.1.1 Introduction

The subject of the current study is the testing of mortar recipes and their resistance towards acid attacks. The porous network of the material is analyzed together with the evolution of degradation over a range of acid concentrations. The objective is to simulate high acid infiltrations in CMC columned soils in a general fashion. The experiments evaluate the amount of time the columns can undertake soil toxicity before losing tensile strength and becoming unsuited for their purpose. After exposure to the acid attack, the samples are described using the X.C.M.T. technique.

3.1.2 The experiment

Since the conditions for acid attacks on mortars are not stipulated in standards, a literature study was conducted in order to decide on the type and conditions for the experiments.

Nitric acid (HNO3) was the first acid chosen. This acid is a strong acid, meaning that for the same concentration, compared to other acids it should have the most damaging effects on the cement paste. Another reason for choosing this acid is the amount of literature available for it (Fig. 3.1 a).

Figure 3.1 - a) Corrosion efficiency of different acids in time (Pavlik – 1994 -1); b) Aggressively to concentration table for different acids (Pavlik - 1994 – 1)

Out of other studies investigated (Fig. 3.1 b), three initial pretests were chosen to provide initial information. These observations were needed in order to evaluate the degradation speed and its tendencies.

Three nitric acid solutions of the following concentrations were used in the pretests as follows:

0.50 Mol/L resulting pH=0.66 0.10 Mol/L resulting pH=1.1 0.01 Mol/L resulting pH=2.0

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3.1.2.1 Pretest launch

Three plastic tubs were filled with the acid solutions (5L), since this material is not affected by the acid. In each tub a cement sample was placed (Fig. 3.2). These are analyzed after a period of 6 days, without modifying the solutions during this time. In the case of the highest concentration, because of the aggressiveness of the pH=0.66, the sample was pulled out and a disc was cut after 3 days, as well as after 6 days.

Figure 3.2 - Pretest the tubs and there components

Although the pH changes over the course of time, it was monitored throughout the experiments and only unimportant variations were registered.

3.1.2.2 Pretest results

The results of these pretests are presented in Fig. 3.3. The pink color on the sample disks is given by the phenolphthalein. The pH indicator was sprayed on the samples as a contrast liquid. The pink variation is given by a high pH value, such as the natural value of cimentoids, and thus the coloration represents the part of the samples that was not touched by the acid.

Figure 3.3 - a) Nitric acid pH=0.66 b) Nitric acid penetration after 6 days

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After the phenolphthalein application, the damage was measured and the results were as follows (Fig. 3.4).

Figure 3.4 - Penetration depth data

The X.C.M.T. analysis was done on two of the samples that were exposed to pH= 0.66 (after 3 days and after 6 days, as presented in Fig. 3.5). The damage sustained is quite severe, as parts of the sample were entirely dissolved during the testing period.

Figure 3.5 - a) Disc out of GC after 3 days, b) Disk out of ref after 6 days

These experiments established that nitric acid has a high corrosion efficiency on the tested mortar recipes. For the following tests two solutions of nitric acid were made, one of pH=3 and one of pH=4.5. The values were chosen so that the results could be compared to other literature experiments, as well as to monitor conditions closer to those found naturally in toxic soils.

3.1.2.3 Experiment launch

For each experiment, 6 samples were prepared of each cement recipe (Fig. 3.6). They were examined at certain intervals of time. The mass evolution was registered at the following intervals of 1, 2, 3, 6, 10, and 15 weeks.

In the acid solution of pH=3.0, a supplementary 4 samples of smaller diameter were inserted in order to be able to characterize them using the X.C.M.T. method.

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Figure 3.1.6 - The experiment setup

Due to the large number of samples in each tub the pH has the tendency to rise. When an increase of 1 pH unit was observed, the solutions were refreshed to the initial pH.

After the first week it was concluded that the pH cannot be maintained in the desired parameters with the initial solution volumes, so they were increased to 10L for each tub. With the new volumes, the pH=3.0 solution showed a stabilizing tendency (Fig. 3.7). This stabilization tendency was not observed for the pH=4.5 solution.

Figure 3.7 - a) pH=3.0 evolution over time, b) pH=4.5 evolution over time

For the pH=4.5 solution, it was established that this value is not achievable in a stable manner using nitric acid. As can be seen from its titration curve, (Fig. 3.8 a). The 4.5 value is placed in the middle of the pH jump. The experiment was closed after 16 days for this reason, and during this time disks were cut out of each sample weekly.

The same experiment was started with acetic acid. This acid was chosen due to its high stability on the pH 4-6 interval (Fig. 3.8 b). Acetic acid is a week organic acid, providing other experimental possibilities on account of corrosion.

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Figure 3.8 - a) Titration curve nitric acid, b) Titration curve acetic acid

The two experiments were made with 5L of acetic acid solution, with pH values of 3.0 and respectively 4.5. Six samples of each recipe were introduced in each tub. Out of these 2 samples served for mass monitoring, while the other 4 will be cut into disks, once every week for acid penetration measurements (Fig. 3.9).

Figure 3.9 - Acid acetic experiment tubs for pH=3.0 and pH=4.5

The pH levels stabilized quickly (Fig. 3.10 a and b) and the solutions were changed when the pH went over the one unit limit previously mentioned. The “maximum” dotted line presented in Fig. 3.10 represents this pH limit.

Figure 3.10 - a) pH=3.0 evolution over time, b) pH=4.5 evolution over time

The solutions presents a fairly stable pH evolution. In the case of the pH=3.0 solution over a period of 28 days, the solution had to be changed once. For pH=4.5 solution, it was necessary to change the solution twice over 72 days.

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3.1.3 Results

The mass evolution is measured over time. These results expressed in grams (Fig.3.11) and Fig.3.12 a) or as percentages (Fig.3.11 b and Fig.3.12 b) are presented lower for the experimental conditions.

Figure 3.11 - a) Mass variation for pH=3.0 in grams, b) Mass variation for pH=3.0 in percentages

Figure 3.12 - a) Mass variation for pH=4.5 in grams, b) Mass variation for pH=4.5 in percentages

All the samples, in the beginning of the experiment, gain weight until they reach a saturation point. This is due to water intake in the samples. During this time the samples also lose some weight due to the acid. Because of these two parallel effects, the first point to take in account is the point where the mass reaches a maximum. The second important point is the one the mass is lower than the initial mass of the dry samples. These are marked on the charts with doted arrows.

For pH=3.0 conditions, the least absorbent recipe is REF, which also suffers the fastest mass loss. After 21 days of exposure the sample’s mass is lower than the dry sample’s. For the other mortar recipes, GLF and GC reach this point after 31 and 33 days respectively. After this point the GC suffers great mechanical propriety loss. After 69 days and a 14.14% mass loss, it crumbled when weighed.

For pH=4.5 conditions, the only recipe to reach the point of weighing less than initially, is REF after a 30 day exposure. Globally it can be said that the samples kept at

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pH=4.5 lose about 50% less mass in the same amount of time as the pH=3.0 kept ones, with the exception of GC that crumbled. In (Fig. 3.13) the effectiveness of acetic acid can be seen.

Figure 3.14 - pH=3.0 degradation in acetic acid

Figure 3.15: a) Mass variation for pH=3.0 in grams b) Mass variation for pH=3.0 in percentages

For the nitric acid experiment, the samples show less to none damage. A very long saturation period and a first weight loss can be seen after a 20 day exposure for REF, followed by GC at 32 days and GLF at 40 days. In this experiment only REF lost enough weight so that it passed the initial dry state mass after 91 days (Fig. 3.15).

Figure 3.16 - Centralized mass loss graph for all experiments

From the chart above (Fig. 3.16) the following observations can be made: acetic acid of pH=3.0 had the strongest corrosion effect and that the largest amount of mass loss was

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found on REF. The GC tensile strength loss was also an important observation paired with a very fast mass loss curve.

3.1.3.1 Acid penetration evolution over time

At certain time intervals disks were cut out of the samples and phenolphthalein was applied to them. The coloration (Fig.3.17) symbolizes the surfaces of the samples not affected by the acid, offering easy to interpret visual results. The acid penetration was measured in three points and a mediated value of these was taken into account.

Figure3.17 - a) Sample disks nitric acid pH=3.0; b) Sample disks acetic acid pH=3.0; c) Sample disks acetic acid pH=4.5

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These measurements were manually taken using a micrometer. For the nitric acid the measured penetration after a 42 day exposure period was 0.2 mm for all mortar compositions. For the acetic acid we can see after 6 days (Fig. 3.18), a penetration of about 6 mm for pH=3.0 and of about 4mm for pH=4.5. For the acetic acid of pH=3.0, full penetration has been reached for GC and GLF in a 9 week period of exposure.

Figure 3.18 - a)Acetic acid penetration depth pH=3.0 b) Acetic acid penetration depth pH=4.5

3.1.4 Micro – tomography

For the nitric acid test, scans were performed for the smallest diameter core samples of about 1cm in diameter and a height of 2-3 cm for each of the compositions.

The following scans were performed:

On the dry form (used as a comparison basis) (Fig. 3.19) During the experiment (in wet form wrapped in plastic film) at the 3, 8 and 15

week marker

Figure 3.19 - Dry sample mounted in for X.C.M.T. analysis

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The results of the scans over the compositions are presented in the following figures:

REF compositions:

Figure 3.20: a) 3D rendering; b) Sectioned 3D rendering

After the first wet state analysis at the 3 week marker, there was little to no corrosion. By processing the scans a difference of about 0.09mm was measured (Fig. 3.21 a, b, c).

Figure 3.21: a) Slice of the dry sample; b) Slice after 3 weeks of exposure; c) New volume resulted after the subtraction procedure of (a-b); d) Slice of the dry sample; e) Slice after 8 weeks of exposure; f) New volume resulted

after the subtraction procedure of (d-e);

At the 8 week marker the measured corrosion was of about 0.2 mm (Fig. 3.21 d, e, f). Another observation was the finding of a pore in the mortar as seen in (Fig 3.22)

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Figure 3.22 - a) Acid infiltration in pores after 8 weeks; b) Zoom on filled pore

In the pore a liquid is present, due to its meniscal form. This shape is specific to a small spherical pore based on kinetics.

GC composition:

Figure 3.23 - a) 3D rendering; b) 3d rendering sectioned

After the 3 week marker the corrosion was of about 0.1 mm (Fig. 3.24 a, b, c).

After the 8weeks marker the corrosion was of about 0.3 mm (Fig. 3.24 d, e, f).

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Figure 3.24 - a) Slice of the dry sample; b) Slice after 3 weeks of exposure; c) New volume resulted after the subtraction procedure of (a-b); d) Slice of the dry sample; e) Slice after 8 weeks of exposure; f) New volume resulted

after the subtraction procedure of (d-e);

Other findings:

Figure 3.25 - a) Acid infiltration in pores after 8 weeks; b) Zoom on filled pore

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GLF recipe:

Figure 3.26 - a) 3D rendering; b) 3d rendering sectioned

After the 3 week marker the corrosion was of about 0.13 mm (Fig. 3.27 a, b, c).

After the 8 week marker the corrosion was of about 0.2 mm (Fig. 3.27 d, e, f).

Figure 3.27 - a) Slice of the dry sample; b) Slice after 3 weeks of exposure; c) New volume resulted after the subtraction procedure of (a-b); d) Slice of the dry sample; e) Slice after 8 weeks of exposure; f) New volume resulted

after the subtraction procedure of (d-e);

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Figure 3.28 - a) Acid infiltration in pores after 8 weeks; b) Zoom on filled pore

3.1.5 Conclusions

Using the X.C.M.T. technique at different intervals of exposure it was able to observe the degradation evolution on the exterior part of the samples. There were measured erosions of about 500 μm after 2 month of exposure, a degradation which couldn’t have been

spotted by eye. Hence, observing the phenomena from its early stages represents a consistent tool in assessing and predicting this kind of processes.

We could also observe the solution infiltration into the samples pores. Depending of the liquid particle shape we can determine its aggregation state. In our case, of acid attacks, the meniscus shape of the infiltrated solution can be concluded as a liquid form.

In the case of very aggressive degradation it could have been noticed the micro cracks network inside the cement paste. Also, it was able to see the affected zones in comparison with the unaffected ones.

From the mortar recipes analyzed, REF proved to be the weakest and less durable to acid attacks, this being observed with Avizo 8.1 Fire software in the subtraction operation and also in accordance with the mass loss charts.

The highest degradation was done by the acetic acid with pH 3.0. The solutions had a stable behavior on the 3-5 interval purposed by the current test. On the other hand, the nitric acid had an unstable behavior on the same interval, the pH 4.5 solution experiment being even interrupted. We can conclude from the above, that the weakest acid had a more aggressive behavior.

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The X.C.M.T. analysis performed revealed gradually the degradation evolution from an exposure period to another. In the case of nitric acid, after 2 weeks of exposure the degradation was about 100 μm, after 2 month 500 μm and after 3 month the degradation

reached 600 μm. Depending of the amount of time assigned for data interpretation, the results

and the observation could become more rigorous.

SEM analysis were not performed in order to see the chemical interaction between the acids and the cement paste compounds; the degradation occurred was represented by the cement paste erosion on the exterior part of the samples and in the interior, losing the bond between the cement matrix and the aggregates; this type of degradation reached the highest level on GC sample in pH 3.0 acetic acid who completely lost its mechanical properties (the sample shattered).

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3.2. Sulfate attack

3.2.1 Introduction

The current study serves as a test of ettringite reformation in toxic soils for the mortar compositions. To achieve this, the samples are exposed to a concentrated sodium sulfate solution so that the medium facilitates chemical reactions with mortars.

Ettringite is an expansive product of cement based mortars. It forms naturally during the hardening phase and is usually associated with the formation of gypsum. Although gypsum has not been proven to be an expansive product.

The initial ettringite formed during the hardening phase doesn’t influence in a negative way the mortar’s mechanical proprieties. This is due to the fresh mortar’s plasticity. Ettringite

also helps, by its expansive nature, counter the contraction tendency of cement based mortars.

The ettringite formation becomes a hazard for the hardened concrete which does not benefit from the same plasticity as the fresh one. Ettringite starts to crystalize (grow) in the material’s pores creating small pressure points which build up in time.

3.2.2 The experiment

For these experiments, the ASTM (American Society for Testing and Materials) C1012-04 standard was followed as closely as possible. This standard refers to size modification measurements due to expansive product (ettringite) formation in sulfate solutions.

The chosen sulfate solution was sodium sulfate (Na2SO4).

Gypsum and ettringite formation according to the equations (1) & (2):

Ca(OH)2 + 2Na+ + SO42- CaSO4x2H2O (gypsum) + 2Na+ + 2OH- (1)

2SO42- + Ca4Al2(OH)12xSO4.H2O + 2Ca2+

Ca6Al2(OH)12(SO4)3.26H2O (ettringite) (2)

These experiments did not need pre-testing as a standard procedure was followed.

Two solutions were prepared for these experiments, one of 1 Mol/L and one of 0.35 Mol/L. In these, 6 big samples (~2.1cm diameter, 3-5cm height) (Fig. 3.29) of each mortar recipe were introduced. In the 1 Mol/L solution three small samples were also introduced (~1cm diameter, 2-3 cm height), that were previously X.C.M.T. scanned.

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Figure 3.29 - Samples before experiment launch

The solutions were maintained at a pH between 6 and 8 with the aid of an automatic pumping set-up (Fig. 3.30). The system pumps sulfuric acid (H2SO4) each time the pH electrode reads pH values over the set limit. In order for the solution to be homogenous, a small pump recirculates the solution inside the tank. Cement mortars are naturally alkaline, so the solution’s pH is expected to rise.

Figure 3.30 - a) Sulfate attack set-up tank; b) Inside set-up; c) pH electrode; d) Acid pumping set-up

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In conformity with ASTM – C 1012 – 04, the experiments were kept in normal conditions (temperature between 20oC and 22oC) and the liquid to sample ratio was over 5. The sample weight was taken twice every week for the first month and once a week after this period.

At the 4 and 8 week markers, the samples were taken out and disks were cut out of them. These disks were put in 4 cm diameter molds and resin poured over them immediately, so that ettringite will not form during the evaporation process. After the resin hardened these were prepared for SEM analysis.

3.2.3 Results

The mass variation over time is presented in Figure 3.31 and Figure 3.32:

Figure 3.31 - a) Mass evolution over time in grams c=0.35 Mol/L; b) Mass evolution over time in percentages c=0.35 Mol/L

The mass increase for these samples is relatively constant. In the first 3 days of the experiment, GLF reached the highest saturation rate of 9.60%, followed by GC with 7.91% and REF with 7.68%. After this threshold the mass increase was fairly stable, the highest mass increase was for GC with 1.95% (Fig. 3.31).

Figure 3.32 - a) Mass evolution over time in grams c=1 Mol/L; b) Mass evolution over time in percentages c=1 Mol/L

The second experiment shows the same stable mass increase, similarly to the first one. GLF had the largest mass gain of 8.69% in the first three days, and over the whole experiment

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(70 days) a total of 10.76%. This mass gain was expected, GLF presenting the highest porosity of the three (Fig. 3.32).

3.2.4 Micro – tomography

For the sulfate attack tests, scans over the smallest diameter core samples were performed. The sample diameter was of ~1cm and height between 2 and 3cm for each of the composition.

The scans were performed at the following markers:

Initially, in dry form, (a comparison basis can be established). During the experiment, in wet form (wrapped in plastic film, at the 9 week

mark)

When the samples were taken out of the tanks, their surfaces were grainy. After the scans within the REF composition fluid infiltrations were found as well as crystals in the pores. The identification of the crystals can be attained by SEM analysis. They are expected to be gypsum or ettringite.

X.C.M.T. REF:

Figure 3.33 - a) Slice dry state; b) Slice after 9 weeks of exposure; c) Slice dry form; d) Slice 9 weeks of exposure;

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In the 9 weeks of exposure some of the pores were filled with liquid (Fig. 3.33 a, b). On further inspection, in some of them, foreign bodies were found (Fig 3.33 c, d). It seems that the structures grew only in the pores that were in contact with the liquid.

For this exterior pore (Fig. 3.34 a, b, c) it was unclear if in the pore there is a crystal or a piece of debris. In order to clarify, the 3D rendering (Fig. 3.34 d, e) was done for this pore. This rendering showed that it is an actual crystal, judging by its shape.

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Figure 3.34 - a) Vertical slice dry state; b) Vertical slice 9 week of exposure; c) Vertical angled slice 9 week exposure; d) 3D rendering, angled, of structure 9 week exposure; e) 3D rendering frontal pore view 9 week exposure; f) 3D

vertical section dry form; g) 3D vertical section 9 week of exposure; h) 3D rendering section dry state; i) 3D rendering section 9 week exposure;

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3.2.5 Conclusions

Although it didn’t allow observing resolutions lower than 5 μm the X.C.M.T. technique

revealed the sulfate solution infiltration into the samples pores and its local crystallization. The analysis was performed at the end of the exposure period of 70 days. We could have noticed a higher solution infiltration than in the case of acid attacks.

We could not establish for sure the time and the manner of sulfate recrystallization. In order to not interfere and facilitate this phenomena, the samples were transported towards X.C.M.T. scans wrapped into plastic foil; this way their state was conserved as much as it could have been possible. Also, the samples were scanned into these foils.

For the real case of soil migration of sulfate solution who flood the C.M.C and then recede, in the next chapter we have analyzed the efflorescence phenomena who may appear as a habitat, temperature and humidity changing of the columns surface.

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CHAPTER 4 – EFFLORESCENCE AND CAPILARITY RISE

4.1 Internal efflorescence

4.1.1 Introduction

The object of the study is to observe the internal recrystallization phenomena of a sodium sulfate solution (Na2SO4) which enters into the samples by the capillarity effect. After recrystallization salt crystals appear and expand their volume. When the crystallization forces overcome the internal strength of the mortar, micro cracks form and expand until the mechanical properties of the sample is lost.

For better control of the internal crystallization phenomenon, the external part of the samples was covered with see-through resin and the ensemble was put into ovens with various temperatures.

4.1.2 The experiment

After checking the specialty literature (Noiriel et. al CG 2010) we chose to follow the experiment using 3 ovens with 3 different temperatures: 1 oven with a 30oC temperature and a controlled humidity of 20%, 1 oven with 45oC and 1 oven with 60oC with uncontrolled humidity.

The set-up was chosen as such the sample will be submerged into solution only 3 mm from the bottom, the solution level remaining constant throughout the experiment. An evaporation front, was left on the top part of the samples, ~2 centimeters from the top.

Figure 4.1 - a) Silicon seal at the base of the sample; b) Sample covered with resin

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Round boxes with a diameter of ~10 cm were used and height of ~3 cm with a round lid. In the lid a hole of roughly the sample’s diameter was drilled. The sample was introduced trough this hole and it was sealed with silicon to the lid (Figure 4.1 a, b). This set-up creating the perfect environment for solution to enter the samples by the capillarity effect and recrystallize inside.

Samples with a diameter of ~2.1 cm and height of ~8 cm were used. Two of each formulation were put into the ovens with 30o and 45o. Into the 60o oven only 1 sample of each formulation was used, considering that this experiment is far away of the real case; this experiment was done to establish a high reference point and to observe the aggressiveness of the phenomenon.

Figure 4.2 - Samples ready to be put into the oven

Daily mass readings were done for each sample to see the evaporation rate of the sulfate solution. The samples in the 30o oven were scanned with X.C.M.T. before launching the experiment with a reference purpose. In accordance with the evolution of the experiment, samples were chosen to be scanned.

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4.1.3 Results

The samples mass evolution:

Figure 4.3 - Mass loss variation in 30o oven

Figure 1.4 - Mass loss variation in 45o oven

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Figure 4.5 - Mass loss variation in 60o oven

In the 30oC oven, the evaporation was of 18 g (REF1) to 35 g (GLF1) of sulfate solution (Figure 4.3). The exposure period was 76 days as well as for the 45oC oven.

In the 45oC oven the highest evaporation occurred for REF2 sample, 27 g; at the same time, from REF1 18 g of solution were lost (Figure 4.4).

From the 3 ovens it can be seen in all mass variations, the REF sample has consumed the least, in accordance with it having the lowest porosity from the formulations.

For 60oC oven, the highest evaporation was for GC sample (Figure 4.5) but in the 45 day of exposure sealing deterioration was found which lasted until day 49, when the sealing was redone, the set-up lost 7.47% of its mass. After day 49 the evaporation rate for GC continued in the same trend.

The 60oC oven experiment was 68 days long, during which it has consumed between 45 g (REF) and 90 g (GC) of sodium sulfate (Figure 4.5).

Regarding the mass lost evolution of the set-ups, a conclusion or a prediction about the behavior of the materials cannot be made. It can be said that the sulfate evaporation may be influenced, besides the distribution of the materials porous network, by the quality of the set-ups seal as well as by the resin adherence to the surface of the samples.

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Any X.C.M.T. findings can be assessed by comparison to the mass loss aspect to base or improve a conclusion.

Except for the behavior of REF1 – 45oC oven and GC – 60oC oven, the general trend of evaporation was in close relation with the samples porosity, from lowest (REF) to highest (GLF).

4.1.4 Micro – tomography

The first scan was done for REF – 60oC oven sample after an exposure period of 12 days and REF – 45oC oven after an exposure period of 21 days. There porous network didn’t

suffer any modification nor alteration, the sulfate solution didn’t reach the middle of the samples.

Further the 3D volume view of the REF – 45oC sample (Figure 4.6 b), c), d)) is presented with the untouched core. The scan was performed approximately 1 cm above their bottom part (Figure 4.6, a).

Figure 4.6 - REF 21 day exposure- a) 3d scanned area; b) 45o sample 3D volume; c) 50% sample 3D volume d) 25 % sectioned

Next were analyzed REF - 45o, GC – 60o, GLF – 60o samples after an exposure period of 2 months. For REF and GLF there were no findings, for REF no changes were found by comparison with the previous scan and for GLF the acquisition was too noisy in order to conclude that what is seen is efflorescence.

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In the case of GC – 60oC the findings show the evolution of efflorescence as follows:

Figure 4.7 -vertical sections GC 60o oven after 2 month exposure- a) vertical slice; b)vertical slice several slices over from a); c) zoom in on fracture with crystal formations; d) zoom in on fracture with crystal formations several slices

over from c);

In the images above the sulfate solution breaks the resin film and recrystallizes on the exterior of the sample (Figure 4.7 a, b, c, d). The solution reaches a height of ~3 cm above the bottom.

Figure 4.8 - Pores filled with sodium sulfate solution

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In Figure 4.8 a network of filled pores can be distinguished. We can make this observation by comparing the darker grey color of the pores located into the center of the sample with the brighter grey color of the pores on the right side (filled pores).

In the case of GLF – 45oC, a vertical section through the samples is shown. In this section it can be observed metallic fiber contained by the mortar. Due to its high X-Ray absorption, it came up with a bright white color.

Another observation can be made, the very porous micro-structure of the clay aggregate. Except for the metallic fibers, the GLF mortar is the REF mortar with 30% of the sand replaced with this clay aggregate; this replacement gives GLF a porosity with 10% higher.

Figure 4.9 - Resin breaching into a pore

Figure 4.10 –Shadows appeared due to the acquisition

In Figure 4.9 it can be seen the resin that got into the pore after the 3 step process of covering the exterior part of the sample, for sealing purposes.

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The Figure 4.10 reveals a lower grade acquisition, the shadows don’t allow proper interpretation.

4.1.6 Conclusions

Realistically, the efflorescence phenomenon is the result of many factors and may occur under many different circumstances. Although, this study represents a small simulation of a single phenomenon with pre-established parameters, the results obtained are relevant in understanding the materials behavior submitted to harsh conditions.

This investigation outlined the resilience of the analyzed mortars to fluid capillarity rise into their cores. Although, subjected to harsh conditions, the porous network of the samples proved to be hardly breached by the solution and by that revealing the calcium carbonate’s function of capillarity breaker and filler into the mortars.

In accordance with the bibliography, the internal recrystallization forces way overcome the strength of the cement paste thus, after the phenomenon occurred, the micro-cracks network expand exponentially. It is entitled to assume that, for a longer exposure period, the second the recrystallization phase of the experiment could have been reached and the results would had permitted considerable predictions.

The special aggregates diminished the adherence of the resin to the samples exterior surface, the solution infiltrated into their pores managing to breach through the weakest spots. By this, the capillarity rise phenomenon was slower.

The X.C.M.T. analysis performed, revealed the applicability of the technique in assessing the evolution by observing the phenomenon from its early stages.

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4.2 External efflorescence

4.2.1 Introduction

In order to have an overview about the behavior of C.M.C. to sulfate attacks as well to add another perspective to the previous investigation, in this study the aim was to observe the damage inflicted by the recrystallization of the sodium sulfate solution on the exterior side of the samples. The height of the crystallization front will be measured with the beam compass.

4.2.2 The experiment

One core of each formulation was prepared with a diameter of ~2.1 cm and height of ~8 cm. The samples were immersed with their bottom part, 3 mm into sodium sulfate (Na2SO4) of concentration 0.35 M (Figure 4.11). To prevent them from sliding and facilitate there contact with the solution they were placed on a thin layer of small capillarity gravel. The tray was kept at normal environmental temperature.

Measurements of the capillarity fluid rise and of the ulterior efflorescence were done when considerable progress was observed for the phenomenon. The tray was refilled with sulfate when a drop of the fluid level under the 3 mm was noticed, as initially established.

4.2.3 Results

After 9 days of exposure it was observed that all the exterior part of the samples was shrouded with efflorescence (Figure 4.12), due to this measurements were no longer necessary.

Figure 4.11 - Samples immersed into sodium sulfate

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

47 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.12 - Samples covered with efflorescence

The experiment continued until 30 days of exposure were reached after which the samples were pulled from the tray. The efflorescence was carefully removed in order to observe the cores surface. Also, the efflorescence powder was stored for further analysis (Figure 4.13).

4.13 - Samples after efflorescence removal

The REF sample had strong degradations, its top part splintering (Figure 4.14). X.C.M.T. and SEM analysis were performed in order to have a more complete vision about the phenomenon occurred.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

48 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.14 – REF – strong degradation inflicted to its top part

Next, the results for each mortar are presented, the efflorescence evolution chart (for the first 9 days) and pictures for the all exposure evolution phases.

1. REF mortar:

Figure 4.15 - Efflorescence evolution chart for REF sample

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6 hours 24 h 3 days 6 days 7 days 9 days

hei

gh

t (c

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Evolution of efflorescence for REF mortar

sain sample

efflorescence

sulfat rise

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

49 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.16 - a) Efflorescence stages at different points in time; b) Initial/after and after removing the efflorescence

2. GC mortar:

Figure 4.17 - Efflorescence evolution chart for GC sample

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6 hours 24 h 3 days 6 days 7 days 9 days

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Evolution of efflorescence for GC mortar

sain sample

efflorescence

sulfat rise

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

50 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.18 - a) Efflorescence stages at different points in time; b) Initial/after and after removing the efflorescence

3. GLF mortar

Figure 4.19 - Efflorescence evolution chart for GLF sample

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6 hours 24 h 3 days 6 days 7 days 9 days

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Evolution of efflorescence for GLF mortar

sain sample

efflorescence

sulfat rise

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

51 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.20 - a) Efflorescence stages at different points in time; b) Initial/after and after removing the efflorescence

The charts shown in the Figure 4.15, 4.17, 4.19 are a schematic representation of the exterior efflorescence evolution, the 9 day reading meaning the maximum level reached. As seen, the bottom part of the samples was not comprised by the efflorescence; this section is in contact with the solution, the efflorescence occurring just above the convergence of the two zones: the wet and the dry one.

Because of its highest degradation, from the REF sample a disc of ~1 cm height was cut. The disc was introduced in a round mold in which see-through resin was poured. After unmolding, the sample was prepared for SEM analysis. For a better electron penetration, after polishing, the disc was metallized with a carbon film.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

52 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

The results are as follows:

Figure 4.21: a) Damaged area (SEM; b) Framing the damaged area (SEM)

Analyzing on with X100 magnification, the damaged zone can be distinguished, on the exterior part of the sample, with a darker grey (Figure 4.21 a, b). The cement paste interacted with the sulfate solution.

Furthering the inquiry, after zooming up to X300 it was observed the debut forming of the expansive products: gypsum (Figure 4.22 a & b) and ettringite (Figure 4.23 a & b).

Figure 4.22 - Ettringite formation (a & b)

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

53 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.23 - Gypsum formation (a & b)

Expansive products appear as a result of chemical interaction between cement and calcium trialuminate with the sulfate solution. In the Figure 4.2.22 the needle shape of the ettringite can be observed. The crystal shape of the gypsum can be seen in the Figure 4.2.23

4.2.4 Micro – tomography

Scanning of the REF sample was performed on its top part were the degradation was visually identified as the most aggressive. The Figure 4.24 represents a vertical section of the sample. The resolution was 1 voxel = 12,5 µm, the time for acquiring the 1441 radiographs was about 1 hour and 20 minutes and 1 acquisition was taken in 750 ms.

Figure 4.24 - REF- Vertical section of the scanned zone

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

54 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

Figure 4.25 – REF- 3D reconstructed volume (a & b)

4.2.5 Conclusions

Solution infiltration wasn’t found in the sample’s interior pores, the degradation being the result of an aggression inflicted from the exterior to the interior. Along with the chemical interaction between calcium tri-aluminate with the sodium sulfate, the expansive products formed expanded their volume until the strength of cement paste was lost. With it, the aggregates lost their adherence and got expulsed, the result being of crumbling little by little. Once started, the degradation reduces the sample’s diameter.

The aggression of eroding the sample, was spotted for the REF mortar best, on GC and GLF the phenomenon wasn’t observed to be as aggressive. It can be concluded that the special aggregates (rubber, clay and metallic fiber) gave resilience to the mortars, the crumbling effect being much attenuated. For REF, the experiment disclosed a 1 mm circular degradation in 30 days.

Regarding the real scenario, we conclude that the solution migration through the soils are very dangerous for the C.M.C. By the recrystallization on the exterior surface of the columns the cement paste is damaged until it loses its cohesive properties. With it, the aggregates dislocate leading to erosion and at the end, the mechanical properties of the material are lost. This phenomenon is very fast comparing with the life expectancy of the C.M.C.

The X.C.M.T. analysis allowed us to observe the interior of the cores and to measure within microns the eroded zones. The technique proved to be helpful in achieving a general perspective of the degradation regarding the mechanical side of the phenomena.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

55 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

CHAPTER 5 – FINAL CONCLUSIONS

Working with the L.M.D.C. team on this very interesting and exciting project was a chance for us to improve our knowledge regarding the civil engineering applications, becoming more skillful and learning new abilities. During the time of the internship we attended to the lab’s weekly academic sessions as well to final PhD defenses, all of these giving us another perspective on the intricacy of our field.

The usage of the X.C.M.T. method for studying the cimentoid materials used in civil engineering is in its early stages. More and more research projects began to use this technique in order to observe the evolution of experiments from there early stages, the results being considerable.

The most important and consistent part of this technique is represented by the “data

processing”. The present processing power permits a heavy handling and manipulation of the data and results in interpretation. After obtaining the 2D radiographs, the actual results are proportional with the amount of time assigned to actually process the data.

Regarding acid attacks, the samples were submerged into nitric and acetic acids, both with 2 concentration: pH 3.0 and pH 4.5. The comparative analysis performed revealed the aggressiveness of the acetic acid which shattered the samples (68 days) while the nitric acid only did a degradation on the exterior part of the samples of ~0.5 mm after 105 days of exposure. Although the nitric acid is more aggressive, in the case of interacting with cement, it only attacks the calcium compound. The acetic acid attacks the cement silicates (C2S and C3), destroying its binding property and in the end, leading to total destruction of the material.

For sulfate attack, the solution used was sodium sulfate (Na2SO4) in two concentrations of 0.35 M and 1 M. After an exposure period of 70 days the sulfate infiltration into the samples pores was observed. SEM analysis was not performed in order to see the chemical interaction between sulfate and the cement compounds.

The efflorescence experiment highlights the phenomenon of recrystallization, firstly in the interior of the cores due to capillarity rise of the solution and secondly on the exterior part of the samples. It was concluded that the solution recrystallized on the exterior surface was of a high degradation. Regarding internal efflorescence, the mortars were resilient to fluid capillarity rise; only some solution infiltrated into the pores after 68 days of exposure in the 60o oven without the recrystallization process starting.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

56 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

In the real conditions, both the phenomenon of internal and external efflorescence appear almost simultaneously. The internal efflorescence evolves slowly but as soon as the recrystallization of the solution begins, the degradation rapidly leads to mechanical failure of the material. The external efflorescence has a constant evolution, eroding the sample surface little by little once the recrystallization process begins. Regarding our experiment, for the considered mortars, it can be concluded that the external efflorescence is way more dangerous.

The current study results revealed that the weakest mortar from a durability stand point is REF. it can be concluded that the GC and GLF samples proved to be more resilient to chemical attacks as well to efflorescence phenomena due to their special aggregates: rubber, clay and metallic fibers.

Concerning research in the field of civil engineering materials, the X.C.M.T. technique can bring a significant contribution to the developing and implementing of high durability materials. Being a nondestructive technique, its applicability is limited only by the materials dimension and not by their functionality and usage.

The present work represents a path change in analyzing the durability of underground used mortars to various attacks that may occur. In this study only the surface was scratched towards a proper analysis of the degradations. Through a more detailed investigation the final and the most important objective will be achieved: predicting the materials behavior.

Licsandru Glad Calin ,,Multiscale study of cementitious materials by Popa Ioan Alexandru X-ray Computed Micro – Tomography ”

57 Institut National des Sciences Appliquees, Toulouse Technical University of Civil Engineering, Bucharest

BIBLIOGRAPHY

[1] Pavlik 1994-1 – Corossion of hardened cement paste by acetic and nitric acids, dd Bratislava, Slovacia 1994

[2] ASTM – C1012-04 Standard test method for length change of hydraulic-cement Mortars exposed to a sulfate solution

[3] Noiriel-et-all GC 2010 - Intense fracturing and fracture sealing induced by mineral growth in porous rocks

[4] Burlion-CCR-2006 - X-ray microtomography: Application to microstructure analysis of a cementitious material during leaching process

[5] Peysonn-EP-2011 - Permeability alteration due to salt precipitation driven by drying in the context of CO2 injection

[6] Rougelot – C&CR – 2010 - About microcracking due to leaching in cementitious composites: X-ray microtomography description and numerical approach

[7] Matthieu Angeli - Multiscale study of stone decay by salt crystallization in porous networks - Université de Cergy-Pontoise, France 2007

[8] Francois Duplan - Composites Cimentaires à Module d’Elasticité Contrôlée :

conception, caractérisation et modélisation micromécanique – L.M.D.C. Toulouse 2014