Volume 107, Number 2, March–April 2002

Journal of Research of the National Institute of Standards and Technology

[J. Res. Natl. Inst. Stand. Technol. 107, 137–148 (2002)]

The Visible Cement Data Set

Volume 107 Number 2 March–April 2002

Dale P. Bentz, Symoane Mizell,

Steve Satterfield, Judith Devaney,

William George, Peter Ketcham,

James Graham, and James

Porterfield

National Institute of Standards

and Technology,

Gaithersburg, MD 20899-0001,

USA

Daniel Quenard, Franck Vallee,

and Hebert Sallee

Centre Scientifique et Technique

du Batiment

Grenoble and Marne-la-Vallee,

France

Elodie Boller and Jose Baruchel

European Synchrotron Radiation

Facility

Grenoble, France,

dale.bentz@nist.gov

steven.satterfield@nist.gov

judith.devaney@nist.gov

william.george@nist.gov

peter.ketcham@nist.gov

james.graham@nist.gov

james.porterfield@nist.gov

With advances in x-ray microtomography,

it is now possible to obtain threedimensional representations of a material’s

microstructure with a voxel size of less

than one micrometer. The Visible Cement

Data Set represents a collection of 3-D

data sets obtained using the European

Synchrotron Radiation Facility in

Grenoble, France in September 2000. Most

of the images obtained are for hydrating

portland cement pastes, with a few data

sets representing hydrating Plaster of

Paris and a common building brick. All of

these data sets are being made available

on the Visible Cement Data Set website

at http://visiblecement.nist.gov. The

website includes the raw 3-D datafiles, a

description of the material imaged for

each data set, example two-dimensional

images and visualizations for each data

set, and a collection of C language

computer programs that will be of use

in processing and analyzing the 3-D

microstructural images. This paper

provides the details of the experiments

performed at the ESRF, the analysis

procedures utilized in obtaining the data set

files, and a few representative example

images for each of the three materials

investigated.

Key words: cement hydration; image

analysis; microstructure; Plaster of Paris;

visualization; x-ray microtomography.

Accepted: March 14, 2002

Available online: http://www.nist.gov/jres

1. Introduction

To produce materials with acceptable or improved

properties, adequate characterization of their microstructure is critical. While the microstructure can be

viewed in two dimensions at a variety of resolutions

(e.g., optical microscopy, scanning electron microscopy,

and transmission electron microscopy), it is usually the

three-dimensional aspects of the microstructure that

have the largest influence on material performance.

Direct viewing of the three-dimensional microstructure

is a difficult task for most materials. Confocal microscopy and atomic force microscopy can each provide

three-dimensional information on surface topography,

but not a complete representation of the bulk threedimensional microstructure. For this task, the most relevant technique is that of three-dimensional (micro)-

tomography.

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Three-dimensional microtomography has been actively applied to the characterization of materials for

about 15 years [1], with much interest by the oil industry

in the characterization of porous rocks [2, 3]. More

recently, it has been applied to building materials with

applications to cement-based mortars [4, 5], building

bricks [6], and concrete aggregates [7]. With microtomography, a complete three-dimensional image of the

microstructure is obtained. As capabilities of the tomography systems have increased, it has now become possible to obtain such images with a resolution better than

one micrometer per voxel (image element), at facilities

such as the European Synchrotron Radiation Facility

(ESRF) in Grenoble, France [8]. This resolution is of

particular interest for cement-based materials, where

the starting particles are typically micrometers and tens

of micrometers in diameter and many hydration products are also formed at the scale of micrometers [9].

Imaging cement-based materials at a pixel size of 1 m

will allow direct comparison to current digitalimage-based microstructural models which also operate

at this scale [10, 11]. A variety of computational tools

exist to directly calculate the transport and elastic

properties of a three-dimensional image-based

microstructure, when the corresponding properties of

the component phases are known [12].

2. Experimental Procedure

For the experiments at the ESRF, Cement and

Concrete Reference Laboratory (CCRL) cement 133,

issued in June 1999, was used. This cement was selected

because it has been well characterized in the CCRL

proficiency sample testing program [13] and also via

computer modelling [11]. Detailed information on the

particle size distribution and mineralogical phase distribution for this cement can be found in the NIST Cement

Images online database [14].

To prepare a sample cement paste for viewing in the

microtomography unit at the ESRF, the appropriate

masses of cement (typically 50 g) and water were added

to a small plastic beaker. The pastes were first mixed by

hand for 1 min in the beaker, followed by 1 min of

mixing using a “drill” mixer (a mixing blade attached to

a regular electric drill). The sides of the beaker were

then scraped and one final minute of drill mixing employed. Small pats of the paste were then carefully

“extruded” into the sample tube molds, shown in Fig. 1.

The molds were either capped or left open depending on

the desired curing conditions (sealed, open to drying, or

open with water periodically added on top to maintain

saturation). In general, pastes with water-to-cement

mass ratios (w/c) of 0.3 to 0.45 were prepared and

viewed after various periods of hydration. The specific

gravity of cement is typically about 3.2, so that a 50:50

volumetric proportion of cement and water would correspond to a w/c of about 0.31. In addition, samples of a

packed dry cement powder were prepared and viewed

using the microtomography unit. In some cases, after

viewing the original dry packed powder, water was

added to the sample tube mold and the hydrated

“packed” cement paste was viewed after various hydration times. For most pastes, it was not possible to obtain

a stable microtomography image prior to the setting of

the cement paste (at about 4 h), due to local motion of

cement particles within the pore solution. The only

exception to this was the sample of cement paste that

was first prepared as a dry packed powder in the tube

mold, with the subsequent addition of water. For this

specimen, we were able to obtain a reasonable

image immediately after addition of the water, as the

particles basically remained in their original packed

configuration.

For the Plaster of Paris, a commercial locallyavailable product was utilized and the paste was prepared at a water-to-solids mass ratio (w/s) of 1.0. The

low viscosity of these pastes allowed the tube molds to

be easily filled. Plaster of Paris hydration was viewed

after 4 h, 7 h, 15.5 h, and 5 d. In addition, a sample

of the dry Plaster of Paris powder was placed in a

separate tube mold and viewed using the microtomography setup.

The clinker brick was a hard-burned clay brick

that had been examined previously using the ESRF

microtomography unit [6] at a resolution of 6.67 m per

pixel. For this experiment, a small 2mm2mm2mm

cube of the brick was carefully sawn from a larger

specimen and mounted for viewing with the microtomography unit.

Fig. 1. Schematic of plexiglass tubes for holding cement paste specimens.

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All samples were imaged on the 3-D microtomography (CT) unit developed on beamline ID 19 at the

ESRF. The system utilizes a large monochromatic

parallel beam and a 2-D area detector. The specimen to

be imaged is mounted on a translation/rotation stage

allowing precise alignment in the beam. To compile a

3-D image set for a material, a series of over 1000

radiographic images are recorded at different angular

positions from 0 to 180. After conversion to light by a

flourescent scintillator screen, the radiographic images

are digitized using a Frelon camera [15], which consists

of a 2-D charge coupled device (CCD) array with

1024  1024 elements, covering an area of 19.5 mm

by 19.5 mm. The distance from the sample to the

scintillator is 8 mm. The Frelon camera has a resolution

of 2 m full width-half maximum. A 3-D filtered backprojection algorithm is then used to reconstruct a 3-D

image of the specimen from the series of 2-D projections [16]. For the operating conditions and optical setup

employed in this study, it was possible to acquire a

1024  1024  1024 image with a voxel dimension of

0.95 m in just over 20 minutes (1.2 s per radiographic

image). Including sample preparation, adjustment, and

equipment setup, each data set required about 1 h of

total clock time.

The (greylevel) intensity at each voxel in the final 3-D

image corresponds to the linear attenuation coefficient

of the material contained in that voxel. Thus, cement

particles will appear much brighter than hydration

products, which will appear brighter than water-filled or

air-filled porosity.

3. Results

3.1 Data Content

The Visible Cement Data Set is organized into distinct

subsets for each experiment performed at the ESRF in

September 2000. Each distinct data set is comprised of

a set of files that provide the raw data, a text-based

description, 2-D and 3-D images of the microstructure,

and movie files containing animated views of the

microstructure. Details on each of these are as follows:

Raw data files—each raw datafile contains a

512  1024  1024 volume of data comprising

either the top or bottom half of the complete

microstructure. These files have been rescaled with

a data resolution of one byte per data element, each

element thus having a value (greylevel) between

0 and 255. The initial data provided by the ESRF

was at a data resolution of four bytes (floatingpoint) per data element, with a proportionately

higher greylevel resolution. All initial indications

are that the reduced 0–255 greylevel range is

adequate for reliably distinguishing features

(phases) in the data sets. In both the raw data files

available on the web site and the initial data files

supplied by the ESRF, the right-most index varies

the fastest in the file. That is the first 1024 data

elements in the file correspond to the first row (left

to right) of 3-D data and the first 1024  1024 data

elements correspond to the first slice (back to

front) of the 512 slices comprising the (half, top to

bottom) data set.

Description—a text-based description provides details on the material being observed for each data

set, including the date and hour that the sample was

actually imaged at the ESRF facility.

2-D slice images (JPEG format)—generally, three

or more complete 1024  1024 2-D images are

produced representing the top, middle, and bottom

slices of a 300 pixel thick subsection of the

3-D microstructure (see Fig. 2 for an example).

These images were used to determine the best

region from which to extract the 300 voxel by

300 voxel by 300 voxel subvolumes used in the

fly-through animations, generally avoiding regions

that contained processing flaws or imaging

artifacts.

3-D image subvolume—a 300 voxel by 300 voxel

by 300 voxel subvolume was selected from

each data set and stored in a 1 byte per voxel data

file.

Movie files (QuickTime compatible format)—for

each 3003 subvolume, a fly-through animation has

been created. For this, each of the 300 2-D slices

are viewed in sequence, giving the illusion that one

is flying through the microstructure.

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3.2 Computer Programs

The Visible Cement Data Set website also contains a

variety of computer programs, written in the C programming language, to aid in the processing of three-dimensional data sets. The programs may be downloaded (via

FTP), compiled, and applied to the data sets also

available from the website. The following computer

programs are currently available:

extract—program to extract an arbitrary subvolume of data from the 512  1024  1024

volumes;

makehist—program to produce a greylevel

histogram file (named makehist.out) for a 3-D

image;

segment—program to produce a segmented

(phase) image from an initial greylevel image by

applying a multi-level thresholding operation [17];

median and mediangrey—programs to apply a median filter [17] to either a segmented (median) 3-D

image or to an initial (mediangrey) greylevel

format image;

clusterid—program to apply a “burning” algorithm

[18] to identify individual particle clusters in a 3-D

image;

percolate—program to assess the percolation or

connectivity of a “phase” in a 3-D segmented

image [18];

3.3 Example Images, Visualizations, and

Observations

3.3.1 Cement Pastes

Figure 2 provides a complete 2-D (slice) image for

the data set for the dry cement powder with no addition

of water. A dense packing of bright cement particles

Fig. 2. 2-D image (1024 pixels by 1024 pixels) of slice 151 for unhydrated cement powder. Bright grey

particles are unhydrated cement and dark grey is air-filled porosity. Greylevel bar at bottom of image

indicates greylevel intensity variation from 0 (left) to 255 (right). The diameter of all slices is approximately 1 mm.

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surrounded by darker air space is clearly visible. In the

two-dimensional image, particle sizes are observed to

range between several micrometers and about 100 m,

as is typical of most portland cements [9]. There is some

indication of variable levels of brightness within individual cement particles, corresponding to the different

cement minerals (silicates and aluminates). Additionally, gypsum particles present in the cement powder

appear darker and more uniform than the cement

particles. There is some evidence of slight experimental/

imaging artifacts, as evidenced by the bright ring surrounding the exterior of the sample. For this reason, a

300 voxel by 300 voxel by 300 voxel subvolume was

selected from the exact center of the data set in the

xy plane. Based on analysis of the greylevel histogram

obtained for this subvolume and a subsequent segmentation of the subvolume into particles and air (porosity),

the initial packing of cement particles would correspond

to a w/c of about 0.28 if the air in the tube mold were

replaced by water. Individual cement particles could

be extracted from this data set and characterized

with respect to size and shape, as has been performed

previously for aggregate particles [7].

Figure 3 shows a 2-D slice image for the w/c = 0.35

cement paste hydrated for 16 h. Many unhydrated

cement particles are still visible in this image. Different

types of hydration products are seen to fill the initial

water-filled space between the particles. Because the

sample was cured under sealed conditions, some of the

original water-filled pores have been converted to air

(water vapor)-filled pores due to the chemical shrinkage

and self-desiccation that occur during the hydration

reactions [19].

As shown in Fig. 4, it is informative to view the

greylevel histograms for the subvolumes as a function of

hydration time. In Fig. 4, the disappearance of cement

Fig. 3. 2-D image (1024 pixels by 1024 pixels) of slice 151 for w/c = 0.35 cement paste hydrated for 16 h.

Bright grey particles are unhydrated cement, medium grey is cement hydration products, and dark grey

are water-filled or air-filled pores. Some indication of ettringite needles can be seen in the upper right

portion of the image. The large void region in the top central region of the image is a processing flaw due

to improper compaction in the tube mold.

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(high greylevels) and water (low greylevels) over time to

create hydration products (middle greylevels) can be

clearly distinguished. Based on these histograms, a

segmentation of the subvolumes into water/air, hydration products, and unhydrated cement particles was

attempted. Water and air-filled pores were selected as all

voxels with a greylevel below 46. Hydration products

were selected as having a greylevel between 46 and the

local minimum in the range of (99, 120) shown for each

curve in Fig. 4. Unhydrated cement was identified as all

voxels having a greylevel greater than this local minima.

Based on this segmentation, the starting w/c and degree

of hydration of each data set could be estimated. The

following estimates of w/c were thus obtained for

300 voxel by 300 voxel by 300 voxel portions of the top

and bottom halves of each data set, respectively: 0.28

and 0.285 for the 4 h data set, 0.29 and 0.27 for the 12 h,

and 0.335 and 0.317 for the 40 h. All of these are lower

than the nominal w/c = 0.35 used to prepare the cement

paste, suggesting that extrusion into the very small tube

molds may have lowered the paste w/c by expelling

water and densifying the cement pastes. It is interesting

that the lowest obtained average value of 0.28 is very

close to the value obtained for the data set based on a

packing of dry cement particles directly into the tube.

The determined degree of hydrations (volume fraction

of the cement which has reacted) for the subvolumes

from the top and bottom halves of each data set are then:

0.19 and 0.21 at 4 h, 0.36 and 0.36 at 12 h, and 0.47 and

0.46 at 40 h. These values can be contrasted against

previously measured values (based on loss on ignition

measurements) for a w/c = 0.3 cement paste of: 0.19 at

8 h, 0.40 at 24 h, and 0.49 at 72 h [11]. The comparison

is reasonable, with some suggestion that the x rays may

be accelerating the hydration at early times. To assess

the possibility of an acceleration due to local heating of

the specimen within the x-ray beam, a thermocouple

was inserted into one of the specimens during an image

acquisition. A slight temperature rise on the order of

2 C was measured during the image acquisition,

suggesting minimal acceleration due to thermal effects.

Figure 5 shows a 2-D slice for hydrated cement paste

prepared at a w/c of 0.45 and hydrated for 137 h while

Fig. 6 shows a 3-D image of a small subvolume of the

same paste with only the unhydrated cement particle

“cores” shown. Due to the higher w/c and the longer

hydration time, there are fewer unhydrated cement

particles in this image than in that for the w/c = 0.35

paste shown in Fig. 3. Upon careful examination of the

image, individual needles and crystals of hydration

products can be clearly observed. Shells of hydration

product are visible around each of the larger unhydrated

cement particles. Analyzing the greylevel histogram

in a manner similar to that described above for the

w/c = 0.35 pastes, estimates of w/c and the degree of

hydration of 0.47 and 0.62, respectively, are obtained.

This degree of hydration value compares favorably to

the values previously determined by loss on ignition for

a w/c = 0.45 paste [11]: 0.60 after 72 h and 0.70 after

168 h. Thus, the microtomography-determined values

Fig. 4. Greylevel histograms for 3-D (300 voxel by 300 voxel by 300 voxel) subvolumes of

w/c = 0.35 cement pastes hydrated for various times.

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for w/c and degree of hydration for this data set both

appear reasonable. For the 3003 subvolume, the segmented capillary pores (31.7 % of the overall volume)

were evaluated using the program percolate.c. Using a

burning algorithm, it was determined that about 98 % of

the pores are part of a percolated (connected) pathway

across the microstructure, indicating a highly connected

“pore network.” In future microtomography experiments, it would be of interest to view well-hydrated

(e.g., several months) specimens to investigate the

de-percolation of the capillary pores that is expected to

occur around 20 % porosity [20, 21].

3.3.2 Plaster of Paris

Figure 7 shows a 2-D image for the dry Plaster of

Paris powder. A wide variety of different size and shape

particles are present in this material. Some of the particles have a very dense structure while others clearly

exhibit some sort of microporosity and internal flaws,

which could have a large influence on their reactivity

with water. In this two-dimensional image, particle

sizes are seen to range from a few micrometers up

to about 300 m (a somewhat wider range than that

observed for the cement particles).

Fig. 5. 2-D image (1024 pixels by 1024 pixels) of slice 151 for w/c = 0.45 cement paste hydrated for

137 h. Bright grey particles are unhydrated cement, medium greys are cement hydration products, and

dark grey are water-filled or air-filled pores. Fewer cement particles are present than in the previous

image, due to the higher w/c and the longer hydration time.

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Figure 8 provides a 2-D image of the Plaster of Paris

microstructure after 15.5 h of hydration time while

Fig. 9 provides a 3-D image of a portion of the microstructure after 4 h of hydration. These images are vastly

different from that of the dry powder in Fig. 7. Numerous needles of calcium sulfate dihydrate reaction

product are present in the hydrated systems. Partially

reacted calcium sulfate hemihydrate particles and

completely reacted particle “shells” can also be clearly

observed in the 2-D image. In some cases, reaction

products appear to have deposited within a portion of a

partially reacted calcium sulfate hemihydrate particle.

Because the morphology of calcium sulfate dihydrate

crystals is strongly dependent on chemical admixtures

added to the mixture and in turn has a large influence

on mechanical and other physical properties, microtomography may provide a powerful tool for quantitatively evaluating the influence of these chemical admixtures on microstructure and performance. Because the

elastic properties of both forms of calcium sulfate are

known, these 3-D images could be used as input into a

finite element software package [12] to compute the

elastic properties of the material as a function of degree

of hydration, w/s, chemical additives, etc., as has been

done previously for computer model microstructures

designed to mimic the Plaster of Paris system [22].

3.3.3 Brick

The clinker brick had been imaged previously at the

microtomography facility in 1996, at a voxel size of

6.67 m per pixel [6]. At this resolution, it was difficult

to properly resolve many of the slit-like pores present in

the material. However, as shown in Fig. 10, at a resolution of 0.95 m, these (dark) pores are clearly visible.

A simple thresholding (segmentation) could be used to

isolate the porosity in the three-dimensional microstructure and examine percolation and transport properties such as conductivity [12] and permeability, as was

performed previously for the microstructures obtained at

the lower resolution [6]. According to physical measurements, the clinker brick has an average porosity of 20 %

[6], so in this case, the threshold greylevel value used to

separate solids from pores could be selected so as to

Fig. 6. 3-D image (64 voxels by 64 voxels by 64 voxels) for w/c = 0.45 cement paste hydrated

for 137 h. Only the unhydrated cement particles are shown for clarity.

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closely match this porosity value in the segmented

image. While the porosity appears as discontinuous

isolated pores in two dimensions (Fig. 10), in three

dimensions it is percolated. This serves to illustrate

the importance of obtaining and analyzing threedimensional images of a material’s microstructure, as

opposed to attempting to infer three-dimensional information from two-dimensional images, in agreement

with previous conclusions [22].

4. Summary and Prospectus

The Visible Cement Data Set website has been

created and is currently being populated with the various

data sets collected during the experiments conducted at

the ESRF facilities. The three-dimensional data sets

available from the site are presently unique in the world

due to their high spatial resolution (voxel size less than

one micrometer). The data sets should find usage in a

variety of research and educational environments as the

web site has been created with the primary objective of

increasing the accessibility of these unique data sets.

While a few example applications (determination of w/c

and degree of hydration, assessment of percolation,

computation of physical properties) have been discussed

and presented in this paper, it is likely that other uses

for this three-dimensional data will be discovered and

implemented over time.

Acknowledgments

The authors would like to thank Stefania Nuzzo,

Abdelmajid Elmoutaouakkil, and all the staff from the

Materials Division at CSTB for their valuable assistance

and important contributions during the preparation

for and the execution of the experiments at the ESRF

facility. They would also like to thank Dr. Richard Spal

of NIST for a detailed review of the manuscript.

Symoane Mizell would like to thank the SURF summer

student program at NIST for funding her efforts on this

project. Funding for the creation of the Visible Cement

Data Set website was provided by the Advanced

Technology Program at the National Institute of

Standards and Technology.

Fig. 7. 2-D image (1024 pixels by 1024 pixels) of slice 151 for dry Plaster of Paris powder.

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5. References

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Fig. 8. 2-D image (1024 pixels by 1024 pixels) of slice 251 for w/s = 1.0 Plaster of Paris hydrated for

15.5 h. Large grey particles are unhydrated calcium sulfate hemihydrate. Grey needles (typically about

50 m in length) are calcium sulfate dihydrate product. “Pore” impressions of totally and partially reacted

starting particles are easily visible.

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Fig. 9. 3-D image (100 voxels by 100 voxels by 100 voxels) of a portion of the w/s = 1.0

Plaster of Paris hydrated for 4 h. A variety of needle-like and flat plate crystals of calcium

sulfate dihydrate can be seen.

Fig. 10. 2-D image (512 pixels by 512 pixels) of the clinker brick microstructure.

Slit-like pores are dark grey and solid brick material is lighter grey. The image is

approximately 0.5 mm by 0.5 mm.

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[12] E. J. Garboczi, Finite Element and Finite Difference Programs

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Importance in Concrete Technology, Lund Institute of Technology, Lund Sweden (1997).

[20] D. P. Bentz and E. J. Garboczi, Percolation of phases in a threedimensional cement paste microstructure model, Cem. Concr.

Res. 21, 325-344 (1991).

[21] E. J. Garboczi and D. P. Bentz, The effect of statistical fluctuation, finite size error, and digital resolution on the phase percolation and transport properties of the NIST cement hydration

model, Cem. Concr. Res. 31, 1501-1514 (2001).

[22] S. Meille and E. J. Garboczi, Linear elastic properties of 2-D and

3-D models of porous materials made from elongated objects,

Mod. Sim. Mater. Sci. 9, 1-20 (2001).

About the authors: Dale P. Bentz and Symoane Mizell

are members of the Building Materials Division of the

NIST Building and Fire Research Laboratory. Dale

Bentz’s research interests include experimental studies

and computer modeling of the microstructure and

performance of cement-based materials. Symoane

Mizell is currently an undergraduate in the Civil

Engineering program at Southern University, and

participated in this project as part of the SURF summer

student program at NIST. Judith Devaney, Steve

Satterfield, Peter Ketcham, and William George are all

members of the Scientific Applications and Visualization group of the NIST Information Technology Laboratory. Judith Devaney is group leader of this research

team, whose research interests include machine discovery in scientific data, scientific visualization, and

parallelization of complex computational codes.

William George’s research interests include parallel and

distributed algorithms for scientific applications and

environments for these applications. Steve Satterfield’s

research interests include immersive visualization for

scientific applications. Peter Ketcham’s research interests include the visualization of both measured and

simulated scientific data. James Graham and James

Porterfield are members of the Information Processing

Support group of the NIST Information Technology

Laboratory. This group is responsible for designing,

implementing, and maintaining NIST’s World Wide Web

presence. Daniel Quenard, Franck Vallee, and Hebert

Sallee are members of the Centre Scientifique et

Technique du Batiment, France. Daniel Quenard and

Hebert Sallee are members of the Materials Division at

the CSTB in Grenoble, France. Their research interests

include heat and moisture transport in building materials and relating material microstructure to performance. Franck Vallee is a research engineer at the

CSTB in Marne-La-Vallee, France. His research interests include the characterization of polymer-modified

mortars, including their degradation during weathering.

Elodie Boller and Jose Baruchel are staff members of

the ID19 beamline at the European Synchrotron

Radiation Facility in Grenoble, France. This beamline is

used for x-ray microtomography, x-ray topography

diffraction, and high resolution x-ray diffraction studies.

The National Institute of Standards and Technology is

an agency of the Technology Administration, U.S.

Department of Commerce.

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