NUREG/CR-#### 

NISTIR 7947 

http://dx.doi.org/10.6028/NIST.IR.7947 

Hydrated Phases in Blended Cement Systems and 

Synthetic Saltstone Grouts 

Prepared by: 

Kenneth A. Snyder 

Paul E. Stutzman 

National Institute of Standards and Technology 

Engineering Laboratory 

100 Bureau Drive 

Gaithersburg, MD 20899-8615 

Manuscript Completed: 

Date Published: 

Jacob Philip, NRC Program Manager 

NRC Job Code: XX-##### 

Office of Nuclear Regulatory Research 
 

 

 

 

 

 

NISTIR 7947
 

Hydrated Phases in Blended Cement 

Systems and Synthetic Saltstone Grouts 

Kenneth A. Snyder 

Paul E. Stutzman 

Materials and Structural Systems Division 

Engineering Laboratory 

http://dx.doi.org/10.6028/NIST.IR.7947 

June 2013 

U.S. Department of Commerce 

Cameron F. Kerry, Acting Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 

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ABSTRACT
 

A blended binder system (10 % portland cement, 45 % fly ash, and 45 % slag) is characterized as part of 

a broader study of its long-term effectiveness in stabilizing treated salt waste at the Savannah River Site. 

The binder system is reproduced in the laboratory using a simulated salt solution, and the hydrated 

phases are identified and quantified out through one year of hydration. The phases are identified through 

the use of thermogravimetric analysis and X-ray diffraction. Supporting information is obtained from 

scanning electron microscopy and pore solution analysis by inductively coupled plasma optical 

spectroscopy. The evidence suggests that the phases being formed are the types one would expect, given 

the starting materials. Moreover, even though the salt solution adds considerable sulfate to the system, 

there is no evidence of excess sulfate bearing phases indicative of expansive sulfate attack. Furthermore, 

the data out to one year of hydration suggests that the phases observed are stable. 

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TABLE OF CONTENTS
 

ABSTRACT............................................................................................................................................................................III

LIST OF FIGURES ................................................................................................................................................................VI

LIST OF TABLES...............................................................................................................................................................VIII

EXECUTIVE	SUMMARY ..................................................................................................................................................... XI

ABBREVIATIONS .............................................................................................................................................................XIII

1 INTRODUCTION.............................................................................................................................................................1
 

1.1 EXPERIMENTAL PLAN......................................................................................................................................................................1
 

1.2 CHARACTERIZATION	TECHNIQUES................................................................................................................................................2
 

1.2.1 Isothermal Calorimetry...........................................................................................................................................................2
 

1.2.2 Thermogravimetric Analysis................................................................................................................................................2

1.2.3 Quantitative X-ray Powder	Diffraction............................................................................................................................4

1.2.4 Scanning Electron Microscopy.............................................................................................................................................5
 

1.2.5 Image Segmentation ................................................................................................................................................................6

1.2.6 Inductively-Coupled	Plasma	Spectroscopy..................................................................................................................14

1.2.7 Thermodynamic Modeling..................................................................................................................................................15

2 BLENDED	CEMENT	PASTES .................................................................................................................................... 16

2.1 EXPERIMENTAL PLAN...................................................................................................................................................................17
 

2.2 RESULTS..........................................................................................................................................................................................18

2.3 THERMODYNAMIC CALCULATIONS ............................................................................................................................................21

2.4 IMAGE SEGMENTATION ................................................................................................................................................................23

2.5 DISCUSSION ....................................................................................................................................................................................25

2.6 SUMMARY........................................................................................................................................................................................25
 

3 SALTSTONE GROUT .................................................................................................................................................. 26

3.1 EXPERIMENTAL PLAN...................................................................................................................................................................27
 

3.2 SURROGATE WASTE SOLUTION	(SWS)....................................................................................................................................27

3.3 CEMENTING	MATERIALS..............................................................................................................................................................28

3.3.1 Portland Cement.....................................................................................................................................................................28

3.3.2 Fly Ash .........................................................................................................................................................................................32

3.3.3 Slag ............................................................................................................................................................................................... 33

3.3.4 Previous Reports.....................................................................................................................................................................33

3.4 MIXTURE DESIGNS ........................................................................................................................................................................34

3.5 RESULTS..........................................................................................................................................................................................35

3.5.1 Calorimetry ...............................................................................................................................................................................35

3.5.2 Thermogravimetric Analysis.............................................................................................................................................36

3.5.3 X-ray Powder	Diffraction ....................................................................................................................................................38

3.5.4 Nitratine ..................................................................................................................................................................................... 40

3.5.5 Rietveld Analysis.....................................................................................................................................................................41

3.5.6 Pores Solution	Analysis........................................................................................................................................................43

3.5.7 Total Sulfur................................................................................................................................................................................45

3.6 DISCUSSION ....................................................................................................................................................................................46

3.7 SUMMARY........................................................................................................................................................................................46
 

4 CONCLUSION ............................................................................................................................................................... 48

5 REFERENCES................................................................................................................................................................
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LIST OF FIGURES 

Figure 1. The backscattered electron image and a segmented, color-composite image derived from a set 

of X-ray and the BEI illustrate the chemical and compositional complexity of cement, and use of the 

image set to assign phase classes (lower-right) to a segmented image. ................................................ 6

Figure 2. Backscattered electron image (BEI) of hydrated cement paste after 3 months of hydration. ..... 7

Figure 3. Original elemental maps of the major components for Mix 1 at 3 months hydration. ................ 8

Figure 4. A summary of image processing techniques used in the segmentation. The bottom row 

demonstrates before (left) and after (right) images of the despeckle and median filter algorithms...... 9

Figure 5. Processed element images from Figure 3. ................................................................................... 9

Figure 6. Phase identification color maps; only a portion of the entire image is shown to enhance detail. 

The three element labels that appear over each image represent (from left to right) the red, green, and 

blue channels, respectively. ................................................................................................................. 10

Figure 7. Phase training sets for the cement paste shown in Figure 2. ..................................................... 11

Figure 8. Statistics reported by MultiSpec software after a segmentation analysis. .................................. 11

Figure 9. The segmented image when using processed starting images. .................................................. 12

Figure 10. Segmentation statistics using the same training set locations, and the original (unprocessed) 

images.................................................................................................................................................. 13

Figure 11. Comparison of the segmented image using unprocessed starting images and the original BEI 

image. .................................................................................................................................................. 13

Figure 12. Calorimetry data for Mixes 1-4 (l) and for Mixes 5-8 (r). Inset shows data from Mix 8 until 

800 hours of hydration. ....................................................................................................................... 18

Figure 13: Thermogravimetric data for Mix 1-4 (l) and Mix 5-8 (r) after 12 weeks of hydration. .......... 19

Figure 18. Relative remaining mass μ as a function of temperature for the portland cement. Individual 

peaks are labeled; the mass loss near 100 °C is due to water loss from C-S-H and other hydrated 

phases. ................................................................................................................................................. 30

Figure 19. Analysis of the portlandite peak in the cement: (left) regression to the TGA data, showing the 

background function b(T) and the peak function p(T); (right) the measured data minus the background 

function, compared with the peak function (the two lie nearly on top of one another). ..................... 30

Figure 20. Analysis of the syngenite peak in the cement: (left) regression to the TGA data, showing the 

background function b(T) and the peak function p(T); (right) the measured data minus the background 

function, compared with the peak function (the two lie nearly on top of one another). ..................... 31

Figure 14: XRD scans for all 8 mixes. The consecutive upper curves are displaced from the immediately 

lower curve by a value of 5. ................................................................................................................ 20

Figure 15. Backscattered electron image (BEI) and segmented image of the fly ash cement paste in Mix 1 

after 3 months hydration...................................................................................................................... 23

Figure 16. Backscattered electron image (BEI) and segmented image of the fly ash cement paste in Mix 2 

after 3 months hydration...................................................................................................................... 23

Figure 17. Backscattered electron image (BEI) and segmented image of the slag cement paste in Mix 3 

after 3 months hydration...................................................................................................................... 24

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LIST OF TABLES 

Table 1. The volume fraction (VF) and mass fraction (MF) of the phases identified in the segmented 

images for the processed and unprocessed starting images. ............................................................... 14

Table 2. The composition of the three different standard solutions used to analyze the extracted pore 

solutions from the OPC pastes (Standard A) and the saltstone waste forms (Standard B1 and 

Standard B2). ....................................................................................................................................... 14

Table 3. Mixture mass fractions of the cementitious binders in each mixture. ........................................ 17

Table 4. Oxide mass fractions for the binders. The cement calcite content and the slag sulfide content 

are also given. ...................................................................................................................................... 17

Table 5: Estimated portlandite mass fraction as calculated from TGA data. The parameter coefficient of 

variation reported by the regression software was typically less than 0.3 %, and other comparisons to 

XRD data were generally within 2 %.................................................................................................. 19

Table 6: Hydrated phase mass fractions after 3 months hydration at 25 °C, as determined by Rietveld 

analysis. Mix 1 through Mix 7 use the portlandite content (see Table 5) as the internal standard. 

Mix 8 uses corundum as an internal standard. .................................................................................... 21

Table 7: Estimated phase mass fractions after 3 months hydration as calculated using the 

Lothenbach et al. [15] thermodynamic hydration model. ................................................................... 21

Table 8. Estimated mass fractions from quantitative XRD (QXRD) and imaging................................... 25

Table 9. Binder proportions for mixtures at two water:cementitious material mass ratios (w/cm), 

neglecting the dissolved salts in the surrogate waste solution. The quantity of cement, fly ash, and 

slag are designated by the corresponding mass fraction. .................................................................... 27

Table 10. Composition of the surrogate waste solution; there are 0.8866 kg of water in each liter of 

solution. ............................................................................................................................................... 27

Table 11. The component composition of the surrogate waste solution. The numbers between 

parentheses represent the oxidation state; this is the notation used in a number of thermodynamic 

databases.............................................................................................................................................. 28

Table 12. Oxide composition of the ASTM Type I/II cement from the mill certificate report based on a 

single measurement. ............................................................................................................................ 28

Table 13. Portland cement mineral phase composition as determined by X-ray diffraction with Rietveld 

analysis (ASTM C1365); uncertainty expresses one standard deviation. ........................................... 29

Table 14. Fly ash oxide composition reported in the mill certificate, based on a single measurement. ... 32

Table 15. Fly ash mineral phase composition as determined by X-ray powder diffraction and Rietveld 

analysis. ............................................................................................................................................... 33

Table 16. Oxide percentages reported by Harbour et al. [24 (Table 3-5)] for the portland cement, the fly 

ash, and the slag................................................................................................................................... 33

Table 17. Mixture designs for w/cm=0.40 pastes....................................................................................... 34

Table 18. Mixture designs for w/cm=0.60 pastes...................................................................................... 34

Table 19. Portlandite mass fraction, as determined by TGA analysis, for the four portland cement 

mixtures. The uncertainty represents the measurement uncertainty having a coverage factor of one 

standard deviation................................................................................................................................ 37

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Table 20. Results of Rietveld analysis for the six mixtures. The values reported are the mass fractions, 

and the numbers in parentheses represent the uncertainty (one standard deviation) in the last two 

digits. ................................................................................................................................................... 42

Table 21. Pore solution composition of the 0.40 w/cm paste made with only portland cement; numbers in 

parentheses indicate one standard deviation of uncertainty. ............................................................... 44

Table 22. Pore solution composition of the 0.60 w/cm paste made with only portland cement; numbers in 

parentheses indicate one standard deviation of uncertainty. ............................................................... 44

Table 23. Pore solution composition of the 0.40 w/cm paste made with fly ash, slag, and surrogate waste 

solution; numbers in parentheses indicate one standard deviation of uncertainty. ............................. 44

Table 24. Pore solution composition of the 0.60 w/cm paste made with fly ash, slag, and surrogate waste 

solution; numbers in parentheses indicate one standard deviation of uncertainty. ............................. 45

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EXECUTIVE SUMMARY
 

A novel cementitious blend of portland cement, fly ash, and slag (mass ratio – 10:45:45) has been 

developed for stabilizing a salt waste for disposal at Savannah River Site (SRS), and the mixture of 

binder and treated salt solution is referred to as a saltstone grout. The blend exploits the reduced state of 

sulfur (sulfide) in the slag to help immobilize radionuclides, the high alkalinity of the treated salt solution 

to help activate the hydration reaction, and the lower heat of hydration of fly ash to achieve construction 

requirements. Because the formulation and the application are novel, studies are needed to forecast the 

long-term performance of these engineered materials. A key parameter in the assessment of the longterm performance of these materials is the identification and quantification of the hydrated phases. This 

information serves as the initial conditions to computer models that can simulate the effects of different 

environments on the long-term performance of cementitious materials. 

Understanding the long-term performance of cementitious engineered barriers is an important challenge 

because these materials account for much of the components within disposal facilities. To help meet this 

challenge, the Department of Energy (DOE), the National Institute of Standards and Technology (NIST), 

and the U.S. Nuclear Regulatory Commission (NRC) formed the Cementitious Barriers Partnership 

(CBP) to develop tools that facilitate performance assessment of cementitious engineered structures. A 

requirement of these tools is to predict the chemical and physical state of the hydrated binder over time; 

the chemical state can change due to both the hydration processes (the chemical reaction between the 

liquid and powder) and the chemical degradation processes that arise from interactions with the 

environment. An important challenge for these tools is predicting the initial chemical state of the 

material: i.e., the type and quantity of mineral phases present and the chemical composition of the pore 

solution that remains behind in the microscopic pores after the hydration reaction. 

Although the cement and concrete industries have considerable experience working with blended 

binders, the proposed saltstone mixture is considerably different than those used in the concrete 

construction industry. In construction, binary mixtures of portland cement and either fly ash or slag are 

common practice, and ternary mixtures (that may alternately contain silica fume or limestone) are also 

used, but to a lesser extent. As a rule, however, the portland cement component of the cementitious 

binder in these commercial applications is almost always 50 % or more, by mass. Moreover, the 

mixtures used in the concrete construction industry are almost always made with potable water. By 

comparison, the treated salt solution is a highly concentrated (more than 5 mol/L) alkaline solution 

containing, among other species, sulfate ions, which are associated with a concrete degradation 

mechanism known as sulfate attack that leads to expansion and the loss of material. 

To aid in the development of these tools to accurately predict the properties and performance of saltstone 

grouts, the NIST used a number of techniques for characterizing the hydrated phases in the saltstone. 

The characterization techniques included quantitative X-ray powder diffraction, thermogravimetric 

analysis, scanning electron microscope imaging coupled with scanning electron micro-probe elemental 

analysis imaging, and inductively-coupled plasma elemental analysis of the pore solution. 

After one year of hydration the mineral phases present and the composition of the pore solution are 

generally consistent with the expectations based on thermodynamic modeling. The phases that are 

formed appear to be stable after one year. Also, even though the salt solution includes sulfates, there do 

not appear to be sufficient expansive sulfate phases present to cause degradation. The reason for this is 

because the high alkalinity of the salt solution appears to suppress the formation of these sulfate phases. 

Based on this assumption, as long as the alkalinity is maintained in these systems, one would not expect 

expansion over the long-term. 

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ABBREVIATIONS
 

BEI Backscattered Electron Image 

CBP Cementitious Barriers Partnership (http://www.cementbarriers.org) 

C-S-H Calcium Silicate Hydrate (gel) 

DOE Department of Energy 

GGBFS Ground Granulated Blast Furnace Slag 

HDPE High-Density Polyethylene 

ICP-OES Inductively-Coupled Plasma Optical Emission Spectroscopy 

NIST National Institute of Standards and Technology 

NRC U.S. Nuclear Regulatory Commission 

OPC Ordinary Portland Cement 

QXRD Quantitative X-ray Powder Diffraction 

PA Performance Assessment 

SEM Scanning Electron Microscope 

SRM (NIST) Standard Reference Material® 

SRNL Savannah River National Laboratory 

SWS Surrogate Waste Solution 

TGA Thermogravimetric Analysis 

w/cm water:cementitious material (mass ratio) 

XR X-ray Microanalysis 

XRD X-ray Diffraction 

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1 INTRODUCTION
 

The various infrastructure elements of future nuclear facilities may be composed of the broad range of 

cementitious binder proportions: structural elements may be composed entirely of portland cement 

concrete; massive concrete elements may contain large quantities of fly ash and/or ground granulated 

blast furnace slag (GGBFS); and saltstone grouts [1] may contain less than 10 percent portland cement, 

with fly ash and slag making up the remainder. Improved performance assessment (PA) tools are needed 

for predicting the performance of all these systems because existing tools were developed for systems 

composed mostly of portland cement. An important aspect of these tools is the ability to estimate the 

type and quantity of hydrated phases present, and the pore solution composition, as these factors impact 

overall performance because they control, among other things, the buffering capacity of the material, the 

mobility of certain radionuclides, and the chemical composition of any effluent. 

Sophisticated computer tools for predicting transport and reaction in cementitious systems incorporate 

thermodynamic data for the mineral phases present. Transport of ions from the external ground water 

changes the chemical equilibrium, and the thermodynamic model adjusts the quantity of phases to 

maintain equilibrium. Therefore, an accurate assessment/prediction of the initial hydrated phases and 

pore solution composition is vital component to an accurate PA tool. 

Standard test methods exist for characterizing the raw materials used to make mortars and concretes (e.g., 

ASTM C1365 – X-ray diffraction analysis of portland cement paste). By comparison, there are few 

standardized techniques for quantifying the hydrated phases. There are, however, a number of 

characterization techniques that could be applicable to these blended cement systems. The principle 

objective when considering these techniques is the ability to identify and quantify the hydration phases or 

the composition of the pore solution. Although these techniques have been used to identify and quantify 

phases and solutions for other applications, they have not been evaluated for use with these unique 

blended cement systems. 

1.1 Experimental Plan 

The experimental plan was composed of two parts. The first part investigated blended cements mixed 

with water, and the second part replicated the saltstone mixture using binder materials obtained from 

Savannah River Site (SRS), and approximated the treated salt solution using previously published 

formulations. The objective of the first part of the plan was to use familiar materials to develop 

characterization techniques for hydrated systems composed of large quantities of glassy binders. 

Because no single characterization technique could provide a complete description, a combination of 

techniques was needed. 

The specific techniques that were used differed between the two parts of the plan. The first part, which 

characterized blended cements mixed with water, used calorimetry, quantitative X-ray powder 

diffraction, thermogravimetric analysis, and electron micro-probe X-ray image analysis. The second 

part, which characterized the saltstone binder mixed with surrogate waste solution (SWS), used 

calorimetry, quantitative X-ray powder diffraction, thermogravimetric analysis, and inductively-coupled 

plasma – optical emission spectroscopy to characterize the hydrated paste and the pore solution. 

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1.2.3 Quantitative X-ray Powder Diffraction 

X-ray powder diffraction (XRD) measures the angular-dependent X-ray reflection from crystalline 

phases within a powder sample. If there are crystallographic structural models for the crystalline 

materials being analyzed, one can identify the component mass ratios that would best agree with the 

measured reflections. This process of estimating mass fractions using the crystallographic structural 

models is part of the Rietveld analysis that is the basis of quantitative X-ray powder diffraction (QXRD). 

If there are only crystalline phases present in the sample, and there are structural models for each phase, 

the mass ratios obtained by the Rietveld analysis can be used to determine the mass fractions of each 

phase. 

If there are amorphous phases present, an internal standard must be used to determine the mass fractions. 

Amorphous components do not exhibit distinguishable peaks in the XRD data. Rather, they typically 

contribute a broad background “hump” to the data. Although a quantitative Rietveld analysis can still be 

used to estimate the mass ratios of the remaining crystalline phases, the total mass fraction of the 

amorphous phase is still unknown. To overcome this challenge, one can add an internal standard, which 

is a unique crystalline phase that is added to the sample at a known mass fraction. Using the known mass 

fraction of the internal standard, QXRD can be used to determine the total mass fraction of the crystalline 

phases, and to determine the mass fraction of total amorphous content by subtraction. 

For analysis of the portland cement, a 5 g powder sample of material was wet-ground in 20 mL of 

ethanol to a mean particle size of about 7 μm. Vacuum filtration removes the ethanol, then the powder is 

dried at 60 °C. Analysis by XRD was performed on three sub-samples run in triplicate: a potassium 

hydroxide-sucrose extraction residue composed essentially of the silicates and periclase; a salicylic acidmethanol extraction residue containing the interstitial phases periclase and alkali sulfates; and a bulk 

(untreated) sample. Selective extractions aid the phase identification: 1) there are fewer phases due to the 

selective dissolution, making identification of the remaining phases simpler, 2) concentrating the 

remaining phases makes low-abundance phases easier to identify and include in the subsequent 

quantitative analysis, and 3) quantitative chemical extraction provides additional data for the final 

quantitative analysis estimates. This XRD characterization technique has been standardized in the 

ASTM 1365 [4] standard test method for quantitative phase abundance analysis of portland cement. 

By contrast, identifying and quantifying the hydrated phases using X-ray diffraction techniques requires 

special considerations. Portland cement contains virtually no amorphous material, obviating the need for 

an internal crystalline standard. Hydrated cementitious systems, by contrast, can contain a majority mass 

fraction of amorphous material. The approach of adding an inert crystalline material at the time of 

mixing may have an unanticipated effect on the rate of hydration. Alternatively, adding a known amount 

of an internal standard to a sample of hydrated paste requires moisture stabilization to reach equilibrium 

with the laboratory atmosphere, and this can affect the structure of those phases that are sensitive to 

changes in temperature and humidity (e.g., ettringite, monosulfate, etc.). 

In these studies, these challenges were overcome in two ways. For mixtures that would create portlandite 

from the hydration reaction, TGA was used to determine the mass fraction of portlandite. For the other 

mixtures in which the portlandite was consumed by the fly ash or slag hydration, an internal standard 

(corundum) was added at the time of mixing. To avoid moisture loss due to grinding a sample to a 

powder, the XRD specimen was prepared by cutting a thin (2 mm) specimen and polishing the cut 

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through a process whereby the images are merged into a red-green-blue composite image. This process 

will be useful in reconciling the QXRD data with fly ash and slag image sets. 

Figure 1. The backscattered electron image and a segmented, color-composite image derived from a set 

of X-ray and the BEI illustrate the chemical and compositional complexity of cement, and use of the 

image set to assign phase classes (lower-right) to a segmented image. 

1.2.5 Image Segmentation 

Image segmentation is a process by which common elements within an image can be identified, and the 

area fractions quantified. A common use for image segmentation is the study of satellite images of 

farmland whereby different crop types (corn, wheat, oats, etc.) are identified and the relative areas 

quantified. In general, information is gathered using multiple “filters,” and the combined information is 

sufficient to ensure identification. For example, the satellite images can be taken with red, green, blue, 

and infrared filters, from which the composite data are used to identify the different crops in an image. 

The most straightforward way of segmenting the image is to use a training set of data from which to 

classify the remaining portion of the image. For example, a few of the crop types in a satellite image that 

can be identified with near certainty are “outlined” and classified. The relative contributions from each 

filter (red, green, blue, and infrared) are characterized statistically, and all the pixels in the image are 

classified depending upon the degree to which its filter components agree with the classification from the 

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training set; it is possible that pixels from a training set will be re-classified, and the degree to which this 

occurs is an indication of the robustness of the training set. 

For cementitious systems, image segmentation is performed on images created on an SEM. The “filters” 

include the BEI, where the brightness is proportional to electron density, and the different elemental 

maps that are created using electron microprobe analysis. The combination of the BEI and the chemical 

information gleaned from the elemental maps provide very effective means of identifying certain phases 

with a high degree of confidence. 

The entire image segmentation process is performed with the assistance of publicly available software. 

Because the elemental maps have relatively few counts, phase identification for developing a training set 

is facilitated by enhancing the starting images through eliminating noise and increasing the contrast. 

This was accomplished using the ImageJ software [12] from the National Institutes of Health. The 

segmentation was performed using the MultiSpec software [13] from Purdue University. 

Figure 2 is a BEI of a polished section taken from cement paste that had hydrated for 3 months. In the 

image, one can see the features of individual phases, identifiable (primarily) from the greyscale. 

Segmenting by greyscale alone, however, is not reliable, and additional information is helpful. 

Figure 2. Backscattered electron image (BEI) of hydrated cement paste after 3 months of hydration. 

The polished section in Figure 2 is also analyzed with electron microprobe analysis. For identifying 

phases commonly found in hydrated pastes, there are six primary elements of interest: calcium (Ca), 

silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), and sulfur (S). These “elemental maps” are 

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given in Figure 3, and correspond to the entire field of view in Figure 2. From these elemental maps, one 

can observe that some elements are distributed throughout the paste (Ca, Si, and S), and a number of 

elements appear to be localized in certain phases (Al, Fe, and Mg). 

Ca Si Al 

Fe Mg S 

Figure 3. Original elemental maps of the major components for Mix 1 at 3 months hydration. 

The original elemental map images in Figure 3 have relative low intensity counts, making phase 

identification difficult. To reduce noise and improve contrast, the following procedure, summarized in 

Figure 4, was used to enhance each elemental map. First, the contrast was enhanced by expanding the 

recorded counts to extend throughout the 0 to 255 dynamic range, and then by eliminating the pixels 

having an intensity less than the modal value. Then, noise was removed using the ImageJ despeckle filter 

to eliminate isolated pixels. Finally, to enhance the perception of uniform phases, a 3x3 median filter 

was used to smooth out neighboring pixels. 

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Original Autoscale Signal-to-Noise 

Despeckle 3x3 Median Filter 

Figure 4. A summary of image processing techniques used in the segmentation. The bottom row 

demonstrates before (left) and after (right) images of the despeckle and median filter algorithms 

The resulting enhanced elemental images are shown below in Figure 5. The enhanced images exhibit 

more distinct regions, which facilitates individual phase identification. These resulting images are then 

combined with the BEI image in Figure 2 to identify individual phases. 

Ca Si Al 

Fe Mg S 

Figure 5. Processed element images from Figure 3. 

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BEI Ca Si Al Al Ca Si 

Ca Ca Mg Ca Al Fe Al Ca S 

Figure 6. Phase identification color maps; only a portion of the entire image is shown to enhance detail. 

The three element labels that appear over each image represent (from left to right) the red, green, and 

blue channels, respectively. 

Using the enhanced elemental maps and the BEI, MultiSpec is used to identify distinct phases and create 

training sets. MultiSpec allows the user to overlay different combinations of three images, using the red, 

green, and blue channels to represent three different images. Useful combinations of elements are used, 

like those shown in Figure 6, to ensure accurate phase identification; the BEI can also be used as one of 

the color channels. 

For each three-color combination, different phases are identified to create a training set for segmentation. 

The best examples found for each phase are outlined and labeled in the image, and multiple regions can 

be used as part of the training set for a particular phase. The training set for the paste is shown in Figure 

7. 

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Figure 7. Phase training sets for the cement paste shown in Figure 2. 

Once the training set has been established, the software classifies all the pixels based on the training set. 

Each pixel is characterized by the contributions from all seven input images: the BEI and the six 

elemental maps. At each pixel, the relative contribution from each image forms a type of “fingerprint.” 

A global “fingerprint” is created from all the pixels in a phase training set. The segmentation software 

then classifies all the pixels in the image, including the pixels in training sets, based on how “close” a 

pixels resembles the global fingerprint of the a training set. All pixels are assigned to a phase, and some 

pixels from a training set may be “reassigned” to a different phase. The degree to which this occurs is a 

measure of the accuracy of the training set. 

Phase Assignment Sample 1 2 3 4 5 6 7 8 9 10 

1 C2S/C3S 0.996 447 445 0 0 2 0 0 0 0 0 0 

2 C4AF 1.000 81 0 81 0 0 0 0 0 0 0 0 

3 C-S-H 1.000 131 0 0 131 0 0 0 0 0 0 0 

4 CH 1.000 297 0 0 0 297 0 0 0 0 0 0 

5 Periclase 1.000 80 0 0 0 0 80 0 0 0 0 0 

6 Limestone 1.000 254 0 0 0 0 0 254 0 0 0 0 

7 AFt 1.000 40 0 0 0 0 0 0 40 0 0 0 

8 AFm 1.000 88 0 0 0 0 0 0 0 88 0 0 

9 Hydrotalcite 1.000 29 0 0 0 0 0 0 0 0 29 0 

10 Void 1.000 203 0 0 0 0 0 0 0 0 0 203 

Totals: 1650 445 81 131 299 80 254 40 88 29 203 

Accuracy: 1.000 1.000 1.000 0.993 1.000 1.000 1.000 1.000 1.000 1.000 

Figure 8. Statistics reported by MultiSpec software after a segmentation analysis. 

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The segmentation analysis results based on the training set in Figure 7 are reported in Figure 8. The 

numbers (“1” through “10”) along the top row correspond to the numbered phases in the leftmost 

column. The second column from the left is an indication of what fraction of the training set for that 

phase was ultimately classified in that phase (i.e., the “purity” of the training set). The “Accuracy” along 

the bottom row is an indication of the certainty to which phases are being classified. In general, 

enhanced images result in relatively high purity and accuracy. 

The resulting segmented image is shown below in Figure 9. The image shows well-defined component 

phases. By comparison to the original BEI, the segmentation appears to be consistent with what one 

would arrive at if using only a BEI. 

Figure 9. The segmented image when using processed starting images. 

Upon closer inspection, one realizes that the image processing has eliminated a lot of fine detail 

associated with the sulfur-bearing phases such as monosulfate (AFm) and ettringite (AFt). These phases 

tend to be finely divided, and uniformly distributed throughout the cement paste, which is consistent with 

being formed through precipitation of diffusing sulfur and aluminum species. 

To remedy this, the segmentation was repeated using the same locations of the training set of pixels, but 

using the original (unprocessed) images. The segmentation statistics given below in Figure 10 indicate a 

lower purity of the training set, and a lower overall accuracy in the segmentation. 

12
 
 

 

Figure 10. Segmentation statistics using the same training set locations, and the original (unprocessed) 

images. 

The resulting segmented image shown in Figure 11, however, has identified far more AFm and AFt, and 

these are more finely distributed throughout the paste. 

Figure 11. Comparison of the segmented image using unprocessed starting images and the original BEI 

image. 

The area fractions of the segmented phases are an estimate for the phase volume fraction. For 

comparison to XRD analysis, the volume fractions are converted to mass fractions using the density of 

the phase. The volume and mass fractions for both segmented images, using either the processed or the 

unprocessed starting images, are shown in Table 1. The data in the table suggest that using the 

unprocessed images (based on training sets created from processed images) increases the detection limit 

of the more finely distributed, lower volume fraction phases. For the combined mass fraction of AFt and 

AFm, the segmentation performed using the original images doubled the quantity detected. 

13
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 1. The volume fraction (VF) and mass fraction (MF) of the phases identified in the segmented 

images for the processed and unprocessed starting images. 

PROCESSED UNPROCESSED 

VF MF VF MF 

C2S/C3S 0.022 0.028 0.020 0.027

C4AF 0.015 0.023 0.021 0.033

C-S-H 0.648 0.705 0.520 0.572

CH 0.137 0.130 0.138 0.133

Periclase 0.007 0.006 0.006 0.005

Limestone 0.014 0.016 0.079 0.091

AFt 0.004 0.003 0.075 0.052

AFm 0.066 0.086 0.063 0.084

Hydrotalcite 0.002 0.002 0.004 0.004

Therefore, it appears that using processed starting images to identify a training set, and then obtaining 

segmentation statistics using the original image is a viable means of identifying hydrated phases, 

especially those that are finely distributed throughout the cement paste, like AFm and AFt. 

1.2.6 Inductively-Coupled Plasma Spectroscopy 

Inductively-coupled plasma optical emission spectroscopy (ICP-OES) was used to characterize the 

elemental composition of the saltstone grout pore solution. This method is coupled with pore expression 

[14], whereby the material is cast into a 50 mm (dia) x 100 mm (long) sealable plastic cylindrical molds 

that were stored in re-sealable plastic bags that were kept in a walk-in chamber maintained at 25°C. At 

the desired age, the specimen was removed from the plastic mold and pressed in a die, using a 

compressive testing machine, to expel the liquid remaining in the pore space; the press is flushed with 

nitrogen to prevent carbonation during the process. The captured liquid was filtered (less than 0.5 μm) to 

remove suspended particles, and then diluted with water and 5 % (by mass) nitric acid (HNO3). 

The solutions used to standardize the ICP analysis were made with the same atomic elements that were 

expected in the extracted samples. In addition to the elements supplied by the SWS, the hydrating 

cementitious materials will contribute potassium, calcium, and silicon. The Standard A solution was 

developed for analyzing mixtures that were made with water. A separate standard was developed for 

systems made with the SWS. Initially, the expectation was that the high pH would reduce the silicon 

concentration to very low values. Subsequent thermodynamic modeling indicated the opposite. As a 

result, there were two standard solutions developed for systems made with the SWS: the initial Standard 

B1 (without silicon), and the subsequent Standard B2 (with silicon). In addition, assuming that a 

significant fraction of the nitrate and nitrite concentrations of the “mix water” would remain in the pore 

solution, nitrates were added to Standard B. The composition of both of these standard solutions appears 

below in Table 2. 

Table 2. The composition of the three different standard solutions used to analyze the extracted pore 

solutions from the OPC pastes (Standard A) and the saltstone waste forms (Standard B1 and 

Standard B2). 

14
 
Element Standard A 

(mol/kgw) 

Standard B1 

(mol/kgw) 

Standard B2 

(mol/kgw) 

Na 0.04360 0.7329 0.7482 

K 0.02597 0.09304 0.1871 

S 0.03119 0.1054 0.08563 

Ca 0.02451 0.00018 0.00018 

Al 0.03719 0.00008 

Mg 0.04115 

Si 0.00408 

N 0.3195 

The solutions used to standardize the ICP were made using de-ionized water (18 MΩ-cm) and by diluting 

the standard solution by factors of 10, 20, and 40 using a 5 % (by mass) nitric acid (HNO3) solution. The 

unknown sample measurements were repeated 3, 4, or 5 times, depending on the volume of sample 

obtained. 

1.2.7 Thermodynamic Modeling 

Thermodynamic modeling has been developing as a tool for analyzing both hydration reactions and 

degradation reactions. One of the most advanced thermodynamic models is a combination of 

thermodynamic data for the phases that may occur during hydration [15] and a Gibbs free energy 

minimization solver (GEMS) [16]. The analyst provides these models with a type and quantity of 

components present (each element, and if appropriate, the various oxidation states of these elements), and 

the model determines the equilibrium distribution of species (in solution) and precipitated mineral 

phases. Because thermodynamic models are equilibrium calculations, they cannot model hydration 

without additional information regarding the rate that mineral phases react. For a hydrating system like 

portland cement, the analyst would supplement the thermodynamic model with a kinetic model for rate 

of hydration for each of the phases in a cement (i.e., alite, belite, aluminate, ferrite). In addition, 

cementitious systems typically incorporate dissolved species into the hydration products, so the 

thermodynamic model must also have a model (e.g., partitioning coefficients, surface complexation 

models) to account for these species. 

The thermodynamic hydration model of Lothenbach et al. [17] was used to estimate the type and quantity 

of phases present after 3 months hydration of cementitious pastes made with water. The model has 

equations characterizing the rate of portland cement hydration. By contrast, there are no established rate 

equations for fly ash or slag, and a complementary experimental program would be required to identify 

the relevant material properties for establishing a model. 

15
 
 

 

 

 

2 BLENDED CEMENT PASTES
 

The various infrastructure elements of future nuclear facilities may be composed of the broad range of 

cementitious binder proportions: structural elements may be composed entirely of portland cement 

concrete; massive concrete elements may contain large quantities of fly ash and/or ground granulated 

blast furnace slag (GGBFS); and saltstone grouts [1] may contain less than 10 percent portland cement, 

with fly ash and slag making up the remainder. Improved performance assessment (PA) tools are needed 

for predicting the performance of all these systems because existing tools were developed for systems 

composed mostly of portland cement. An important component of these tools is the ability to estimate 

the type and quantity of hydrated phases present, and the pore solution composition, as these factors 

impact overall performance because they control, among other things, the buffering capacity of the 

material, the mobility of certain radionuclides, and the chemical composition of the effluent. 

Sophisticated computer tools for predicting transport and reaction in cementitious systems incorporate 

thermodynamic data for the mineral phases present. Transport of ions from the external ground water 

changes the chemical equilibrium, and the thermodynamic model adjusts the quantity of phases to 

maintain equilibrium. Therefore, an accurate assessment/prediction of the initial hydrated phases and 

pore solution composition is vital component to an accurate PA tool. 

To validate the hydrated phases prediction component of PA tools, standardized materials 

characterization techniques are needed to identify the phases in existing materials. In addition, system 

characterization data are required for model validation. Two very useful characterization tools are X-ray 

powder diffraction (XRD) and thermogravimetric analysis (TGA). 

X-ray diffraction characterization techniques for quantifying hydrated cementitious phases differ from 

that for quantifying portland cement. Portland cement contains virtually no amorphous material, 

obviating the need for an internal crystalline standard. Also, a standardized method for portland cement 

characterization has been documented [4]. Hydrated cementitious systems, by contrast, contain a 

considerable mass fraction of amorphous material. The approach of adding an inert crystalline powder at 

the time of mixing may have an undesirable effect on the rate of hydration. Alternatively, adding a 

known amount of an internal standard to a sample of hydrated paste requires moisture stabilization to 

reach equilibrium with the laboratory atmosphere, and this can affect the structure of those phases that 

are sensitive to changes in temperature and humidity (e.g., ettringite, monosulfate, etc.). 

As an alternative to adding an internal standard, TGA is used to quantify the calcium hydroxide 

(portlandite) present in the system. The water loss during conversion of portlandite to calcium oxide 

(lime) happens at a distinct and identifiable temperature. Because other hydrated phases are losing water 

over the same temperature range (but at a much slower rate), the analysis of the portlandite peak must 

account for the background mass loss. 

The type and quantity of hydrated phases in paste samples are estimated by combining XRD and TGA 

data for systems made with blended cementitious powders for which the portland cement mass fraction 

varied from 100 % to 10 %. The XRD data are used to identify the mineral phases present, and the 

relative mass fractions of each phase. TGA is used to estimate the portlandite content, from which the 

absolute mass fraction of each phase is estimated. The results are compared to estimates from a 

thermodynamic hydration model. 

Image segmentation is also considered as an alternative approach to estimating the type and quantity of 

hydrated phases present. Backscattered electron imaging is used to indicate the phase density, and 

16
 
 

 

 

 

 

electron probe microanalysis images represent the relative concentration of elements. The various 

images are used to identify individual phases within the field of view, and a training set is used to 

classify all the pixels within the image. The results are compared to phase estimates from QXRD. 

2.1 Experimental Plan 

Eight cement paste mixtures were developed to span the range of cementitious binder mixtures 

anticipated for a saltstone grout. The mass fractions of cement, fly ash, ground granulated blast furnace 

slag (GGBFS), and silica fume for each of the eight mixtures are given in Table 3. The 

water:cementitious materials (w/cm) mass ratio for all mixtures was 0.40. 

Table 3. Mixture mass fractions of the cementitious binders in each mixture. 

Mix # Cement Fly Ash GGBFS Silica Fume 

1 1.00 

2 0.70 0.30 

3 0.70 0.30 

4 0.70 0.20 0.10 

5 0.50 0.50 

6 0.50 0.50 

7 0.30 0.35 0.35 

8 0.10 0.45 0.45 

All the materials are commercial products. The ASTM Type I/II portland cement contained 3.5 % 

limestone by mass and had a Blaine fineness of 372 m2/kg. The fly ash is an ASTM Type C, and the 

GGBFS and the silica fume were commercial products. The oxide mass fractions were measured at a 

commercial laboratory using X-ray fluorescence, and the values are given in Table 4. The slag contained 

no crystalline phases, and the fly ash contained 7 % quartz, 5 % mullite, and 2 % hematite by mass. Also 

given in the table are the cement calcite content and the slag sulfide content. 

Table 4. Oxide mass fractions for the binders. The cement calcite content and the slag sulfide content 

are also given. 

Phase Cement Fly Ash GGBFS Silica Fume 

CaO 0.605 0.246 0.371 0.005 

SiO2 0.191 0.370 0.373 0.962 

Al2O3 0.050 0.200 0.104 0.003 

Fe2O3 0.033 0.053 0.005 0.000 

MgO 0.041 0.048 0.116 0.001 

SO3 0.031 0.014 0.022 0.000 

Na2O 0.001 0.015 0.002 0.001 

K2O 0.007 0.006 0.003 0.005 

TiO2 0.003 0.016 0.005 0.000 

CaCO3 0.034 

Sulfide 0.010 

17
 

 

 

 

 

 

 

Figure 13: Thermogravimetric data for Mix 1-4 (l) and Mix 5-8 (r) after 12 weeks of hydration. 

The peak at approximately 425 °C is due to loss of water in the conversion of portlandite to lime. The 

portlandite peak prominence and separation is exploited to obtain an estimate for the portlandite content. 

This value is then used to establish the quantity of portlandite for the XRD Rietveld analysis. The 

portlandite mass fractions for the mixtures are reported in Table 5. 

Table 5: Estimated portlandite mass fraction as calculated from TGA data. The parameter coefficient of 

variation reported by the regression software was typically less than 0.3 %, and other comparisons to 

XRD data were generally within 2 %. 

Mix Portlandite Mass Fraction 

1 0.119 

2 0.064 

3 0.063 

4 0.049 

Mix Portlandite Mass Fraction 

5 0.031 

6 0.039 

7 0.019 

8 -

No portlandite was detected in Mix 8. Therefore, corundum (87 % crystalline) was added as an internal 

standard. To stabilize the powder mass prior to adding the corundum, the sample was dried briefly in a 

60 °C oven. Prior to adding the corundum, a small portion was taken and analyzed by TGA to estimate 

the adjusted water content so that the corundum mass fraction could be expressed as a mass fraction of 

the original sample. 

The XRD data from all 8 mixes are shown in Figure 14 after hydration for 12 weeks. Sequential 

repetitions on a single sample showed no measurable difference in the XRD data, so only a single scan is 

shown. 

19
 

 

 

 

 

 

€

€

Table 6: Hydrated phase mass fractions after 3 months hydration at 25 °C, as determined by Rietveld 

analysis. Mix 1 through Mix 7 use the portlandite content (see Table 5) as the internal standard. Mix 8 

uses corundum as an internal standard. 

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Mix 7 Mix 8 

Amorphous 0.600 0.691 0.674 0.749 0.730 0.785 0.751 0.816 

C3S 0.013 0.014 0.041 0.023 0.033 0.021 0.017 0.014 

β -C2S 0.060 0.037 0.037 0.027 0.040 0.014 0.028 0.016 

C3A 0.002 0.001 0.003 0.001 0.008 

C4AF 0.027 0.017 0.019 0.011 0.009 0.006 0.013 0.013 

Portlandite 0.119 0.064 0.063 0.049 0.031 0.039 0.019 

Ettringite 0.075 0.041 0.052 0.052 0.015 0.019 0.034 0.016 

Calcite 0.014 0.014 0.018 0.019 0.011 0.024 0.016 0.013 

Monocarbonate 0.058 0.042 0.040 0.028 0.051 0.030 0.038 0.028 

Periclase 0.012 0.009 0.008 0.007 0.006 0.003 0.006 0.006 

Hemicarbonate 0.015 0.032 0.015 0.016 0.016 0.008 0.006 0.007 

Quartz 0.015 0.003 0.010 0.025 0.002 0.017 0.026 

Monosulfate 0.007 0.018 0.008 0.011 0.029 0.018 0.028 0.018 

Hydrotalcite 0.004 0.024 0.032 0.026 0.019 

Strätlingite 0.009 

2.3 Thermodynamic Calculations 

The thermodynamic calculations require an assumption for the rate of hydration for the slag and the fly 

ash. To achieve this, the fraction of each supplemental material that was “consumed” was the value that 

resulted in the best agreement with the measured quantities of the hydration products. Results from the 

calculations are given in Table 7. 

Table 7: Estimated phase mass fractions after 3 months hydration as calculated using the 

Lothenbach et al. [17] thermodynamic hydration model. 

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Mix 7 Mix 8 

Amorphous 0.362 0.544 0.531 0.605 0.673 0.652 0.796 0.886 

C3S 0.026 0.016 0.016 0.016 0.012 0.012 0.007 0.002 

β -C2S 0.069 0.049 0.049 0.049 0.035 0.035 0.021 0.007 

C3A 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.000 

C4AF 0.017 0.011 0.011 0.011 0.008 0.008 0.005 0.002 

Portlandite 0.127 0.054 0.064 0.013 0.030 0.027 0.006 

Ettringite 0.144 0.080 0.109 0.100 0.041 0.079 0.045 0.017 

Calcite 0.008 0.003 0.002 

Monocarbonate 0.102 0.101 0.085 0.092 0.072 0.072 0.015 0.014 

Periclase 

Hemicarbonate 0.056 

Quartz 

Monosulfate 0.036 0.051 0.008 

21
 
 

Hydrotalcite 0.020 0.009 0.039 0.006 0.002 

Strätlingite 0.037 

There is general agreement between the observed and the predicted phases present. The thermodynamic 

model assumed a greater degree of reactivity for the periclase and the calcite than otherwise expected, 

and the quantity of quartz could have been approximated by assuming zero reactivity. Although the 

quantity of each phase differs between the measured and predicted values, this is due, in large part, to 

uncertainty in estimating the degree of reactivity for each of the supplementary materials. 

22
 
 

 

 

 

 

 

2.4 Image Segmentation 

The approach of developing training sets using processed images and performing the segmentation using 

the original images was repeated for Mix 1 (OPC paste), Mix 2 (30 % fly ash), and Mix 3 (30 % slag). 

The BEI image, and the segmented image (using non-processed images for the training set) for all three 

mixtures are shown in Figure 15, Figure 16, and Figure 17. For all three mixtures, the segmentation 

process is able to identify the more finely distributed phases within the hardened cement paste. 

Figure 15. Backscattered electron image (BEI) and segmented image of the fly ash cement paste in Mix 1 

after 3 months hydration. 

Figure 16. Backscattered electron image (BEI) and segmented image of the fly ash cement paste in Mix 2 

after 3 months hydration. 

23
 
 

 

 

Figure 17. Backscattered electron image (BEI) and segmented image of the slag cement paste in Mix 3 

after 3 months hydration. 

A comparison of the result for the mass fraction as determined by XRD to the mass fraction estimated 

from image segmentation is given in Table 8. There is general agreement between the two methods. For 

the sulfate-bearing phases, however, the difference between the estimated values is nearly a factor of ten 

for Mix 1 and Mix 2. 

24
 
 

 

 

 

 

 

 

 

Table 8. Estimated mass fractions from quantitative XRD (QXRD) and imaging. 

QXRD Imaging 

Mix 1 Mix 2 Mix 3 Mix 1 Mix 2 Mix 3 

2.5 Discussion 

The methods used here for characterizing hydrated phases will require further development and 

refinement. Using amorphous silica to approximate the amorphous phases is useful in representing the 

XRD data background, and the results generally agree with SEM image segmentation. Also, using TGA 

to estimate portlandite content will have limited applicability in the study of blended systems with very 

little portland cement because hydration processes in these mixes will eventually consume all the 

portlandite. 

The advantage of the image segmentation procedure is the ability to classify a number of amorphous 

phases. By comparison, the XRD process estimates total amorphous content by subtraction of the 

crystalline phases. More specifically, XRD can only lump together all amorphous phases, where as 

image segmentation can classify cementing glassy phases (fly ash or slag) differently from the calcium 

silicate hydrate gel (C-S-H). However, because the effort required for SEM image segmentation is very 

large in relation to the information obtained, this technique will not be used to characterize the saltstone 

mixtures. 

2.6 Summary 

Combining thermogravimetric data with X-ray diffraction data shows promise for characterizing the type 

and quantity of hydrated phases in blended cement systems. The phases identified are consistent with 

current thermodynamic models, but further research is needed to quantify the rate of reaction of fly ash 

and slag. Further experimentation is needed, however, to independently validate the quantity of phases 

observed through XRD and TGA. Image segmentation shows promise in being able to quantity the 

degree of fly ash and slag hydration, but independent chemical analysis is needed to validate the results. 

25
 
 

 

 

 

3 SALTSTONE GROUT
 

There exist a number of computer tools that can be used to assess the performance of a cementitious 

barrier. The most useful models are those like 4sight [22] that incorporate the relevant physics and 

chemistry of the interactions between the material and its environment. These processes include both 

transport (through a non-ideal aqueous pore solution) and reaction (with ion-exchange, surface 

complexation, etc.). For these types of models, the material composition (type and quantity of phases) 

constitutes the initial condition, and the environment (temperature, humidity, dissolved species in the 

groundwater, etc.) constitutes the (potentially time-dependent) boundary condition. 

An important part of the predictive nature of these computer tools is the ability to predict the initial 

material composition due to hydration. The thermodynamic “hydration models” convert the mineral 

phase composition of the binding phases and, along with an additional model for the rate of hydration for 

each phase, predict the type and quantity of phases produced; this differs from a hydration model that can 

also predict the microstructure. There exist models for the rate that cement phases hydrate [23] and for 

the rate of slag hydration [17], but there are no reliable models for the rate of fly ash hydration. These 

models have been validated for mixtures composed mostly of portland cement. 

Some engineered barrier applications, however, use cementitious blends composed of very little portland 

cement, and may use a treated salt solution as the “mix water”. The concentrated salt supernate and 

crystallized saltcake are to be treated and then stabilized as a cementitious saltstone grout [1] formulated 

by the Savannah River National Laboratory (SRNL) for final disposal in vaults at the Saltstone Disposal 

Facility at the Savannah River Site (SRS). These grout mixtures may contain as little as 5 % portland 

cement, with the remainder of the binder being equal portions of (ASTM Class F) fly ash and slag. 

Moreover, the supernate contains, among other salts, sodium hydroxide, sodium nitrate, and sodium 

nitrite [24]. Therefore, not only is this solution highly alkaline, it contains nitrates and nitrites, which are 

typically not found in thermodynamic databases for cementitious systems. 

In anticipation of these challenges, the NRC has become a member of the Cementitious Barriers 

Partnership (CBP) [25]. The CBP works to improve the understanding, and develop tools for predicting 

the long-term structural, hydraulic, and chemical performance of cementitious materials and waste forms 

used primarily in nuclear waste disposal. The CBP objective is to develop a set of computational 

software tools that are expected to reduce the uncertainties of current methodologies for performance 

assessment. As part of the CBP program there are software tools for estimating the mineral phase 

composition of the starting material, but none of these tools have been validated on mixtures having such 

low portland cement contribution to the total binder. 

In support of the CBP program, the National Institute of Standards and Technology (NIST) characterized 

the mineral composition of a set of pastes that mimic the type of saltstone grout being developed for 

SRS. These pastes contained the same cement, slag, and fly ash used at SRNL, and the “mix water” was 

an approximation to the SRNL surrogate waste for physical testing [24] that contains no radionuclides. 

The raw materials and the hydrated pastes were characterized using quantitative X-ray powder diffraction 

(QXRD) and thermogravimetric analysis (TGA) to provide direct estimates of the bulk mineralogical 

composition and texture of the materials. The pore solution composition was characterized by elemental 

analysis using inductively-coupled plasma optical emission spectroscopy. The results represent a 

validation data set for the THAMES [26] microstructural development model that is used to compute 

realistic transport and mechanical properties for cement paste microstructures as a function of time. 

26
 
 

 

 

 

 

 

3.1 Experimental Plan 

The portland cement, fly ash, and slag, were obtained from SRNL, and the composition of the surrogate 

waste solution was based on a SRNL report on the properties of the saltstone grouts [24]. These mixtures 

are different from typical portland cement concrete formulations in two important ways: very low cement 

content, and a “mix water” that is a concentrated solution. Anticipating the effects due to either of these 

differences is a formidable challenge to the computer models. To bridge the gap, paste mixtures were 

developed having two levels of cement content and two levels of “mix water” composition (water or 

SWS). 

In addition, two series of pastes were made having different water:cementitious mass ratio (w/cm). The 

grout formulated by SRNL [24] had a w/cm (neglecting the dissolved solids in the waste solution) of 

0.60. At this water content, there can be considerable sedimentation and bleed water. If sedimentation 

and bleeding do occur, there will be a gradient of properties through the paste, as the effective w/cm 

decreases with depth. Therefore, mixtures were also made with a 0.40 w/cm ratio so that comparisons 

could be made to systems for where there should have been relatively little bleeding. The binder 

compositions of all mixture designs considered are given in Table 9, and the composition of the SWS is 

described below. 

Table 9. Binder proportions for mixtures at two water:cementitious material mass ratios (w/cm), 

neglecting the dissolved salts in the surrogate waste solution. The quantity of cement, fly ash, and slag 

are designated by the corresponding mass fraction. 

w/cm Water / SWS Cement Fly Ash Slag Sample 

Name 

0.40 Water 1.00 40-0-100 

SWS 1.00 40-1-100 

SWS 0.10 0.45 0.45 40-1-055 

0.60 Water 1.00 60-0-100 

SWS 1.00 60-1-100 

SWS 0.10 0.45 0.45 60-1-055 

3.2 Surrogate Waste Solution (SWS) 

The composition of the SWS was based on that used by Dixon et al. [24]. The salts used, and the 

corresponding concentrations, are given in Table 10. Using the reported solution specific gravity of 1.248 

[24], the molar (mol/L) concentrations were converted to molal (mol/kgw) units to facilitate 

thermodynamic calculations. 

Table 10. Composition of the surrogate waste solution; there are 0.8866 kg of water in each liter of 

solution. 

Salt mol/L mol/kgw 

NaOH 2.866 3.233 

NaNO3 1.973 2.225 

NaNO2 0.485 0.547 

27
 
 

 

 

 

 

Na2CO3 0.118 0.133 

Na2SO4 0.055 0.062 

Al(NO3)3 ● 9H2O 0.114 0.129 

Na2H(PO4) ● 12H2O 0.007 0.008 

The component composition of the SWS is given in Table 11. The component concentrations are 

provided to facilitate thermodynamic modeling because the calculations are performed on a component 

basis. 

Table 11. The component composition of the surrogate waste solution. The numbers between 

parentheses represent the oxidation state; this is the notation used in a number of thermodynamic 

databases. 

Component Molarity (mol/L) Molality (mol/kgw) 

Na 5.684 6.411 

N(5) 2.315 2.611 

N(3) 0.485 0.547 

C 0.118 0.133 

S(6) 0.055 0.062 

Al 0.114 0.129 

P(5) 0.007 0.008 

H 0.007 0.008 

3.3 Cementing Materials 

The portland cement, the fly ash, and the slag obtained from SRNL included the certification report that 

the material producer supplies with the material. The materials were the same as those used to develop 

the saltstone mixture, and they have been characterized by SRNL [27]. 

The primary means of characterizing the cementing materials are the oxide contents and the crystalline 

mineral content. The oxide content can be determined from X-ray fluorescence measurement, and the 

crystalline phase identification and quantification can be determined from X-ray diffraction coupled with 

Rietveld analysis. 

3.3.1 Portland Cement 

The portland cement was an ASTM Type I/II containing 1.8 % limestone (93 % as CaCO3). The Blaine 

fineness was 393 m2/kg, and the loss on ignition was 1.8 %. The chemical composition provided in the 

mill certificate is given in Table 12 below. 

Table 12. Oxide composition of the ASTM Type I/II cement from the mill certificate report based on a 

single measurement. 

Oxide Mass Fraction (%), 

CaO 64.3 

SiO2 20.3 

28
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Al2O3 5.0 

Fe2O3 3.8 

SO3 3.0 

MgO 1.2 

CO2 0.7 

Na2Oe 0.48 

The cement was analyzed to determine the mineral phases present. The analysis was conducted using Xray powder diffraction, according to the standardized procedure described in ASTM C1365 [4]. The 

results of the analysis are shown in Table 13 below. The detection of syngenite was confirmed by the 

thermogravimetric analysis (discussed below). 

Table 13. Portland cement mineral phase composition as determined by X-ray diffraction with Rietveld 

analysis (ASTM C1365); uncertainty expresses one standard deviation. 

Phase (XRD) Mass Fraction (%) 

C3S 56.2 ± 2.4 

C2S 15.1 ± 1.6 

C3A 3.4 ± 0.5 

C4AF 13.1 ± 0.6 

Gypsum 1.9 ± 0.3 

Bassanite 1.0 ± 0.5 

Calcite 2.7 ± 0.4 

Syngenite 1.7 ± 0.1 

Portlandite 1.0 ± 0.8 

Amorphous 4.9 ± 1.4 

Additional analysis of the portland cement was conducted using the TGA data shown in Figure 18. In 

the figure, the distinguishable peaks for syngenite, portlandite, and limestone are denoted by arrows. For 

the analysis, the portlandite and limestone peaks are modeled using the symmetrical error function, and 

the syngenite peak is modeled using the asymmetrical logistic function. 

29
 
 

 

 

0.985 

0.987 

0.988 

Figure 18. Relative remaining mass μ as a function of temperature for the portland cement. Individual 

peaks are labeled; the mass loss near 100 °C is due to water loss from C-S-H and other hydrated phases. 

The presence of syngenite (K2Ca(SO4)2·(H2O)) and portlandite (Ca(OH)2) suggest that the cement has 

been exposed to a humid environment for an extended period. The extent to which this has occurred, 

however, was characterized through a TGA regression analysis. The results of the regression analysis for 

each peak are shown below in Figure 19 through Figure 21 below. 

0.003 

0.002 

μ-b(T), 

p(T) 

350 400 450 500 

Temperature (oC) 

b(T) 

p(T) 

b(T)+B0 

350 400 450 500 

Temperature (oC) 

μ0.986

0.001 

0.000 

Figure 19. Analysis of the portlandite peak in the cement: (left) regression to the TGA data, showing the 

background function b(T) and the peak function p(T); (right) the measured data minus the background 

function, compared with the peak function (the two lie nearly on top of one another). 

30
 

0.984 

 

 

 

 

 

 

 

 

 

 

0.975 

0.980 

0.985 

μ-b(T), 

p1

(T)+p2

(T) 

500 600 700 

Temperature (oC) 

μ

b(T) 

p1

(T) p2

(T) 

b(T) + B0 

0.015 

500 600 700 

Temperature (oC) 

0.010 

0.005 

0.970 

0.000 

Figure 21. Analysis of the limestone peak in the cement: (left) regression to the TGA data, showing the 

background function b(T) and the two peak functions p1(T) and p2(T); (right) the measured data minus the 

background function, compared with the sum of the two peak functions (the two lie nearly on top of one 

another). 

The result of the regression of the limestone peak is shown in Figure 21. The mass loss due to the 

decomposition of the limestone was resolved into two distinct peak functions, both regressed against 

individual complementary error functions. This double-peak mass loss is attributed to the presence of a 

dolomitic limestone. The estimated mass fraction of limestone (as calcite) was (0.0315 ± 0.0005), and the 

peak location parameters were approximately 605 °C and 663 °C. For comparison purposes, the 

estimated calcite mass fraction determined by XRD (Table 13) was (0.027 ± 0.004). 

3.3.2 Fly Ash 

The fly ash was an ASTM Class F with a specific gravity of 2.38, and 17.8 % (by mass) was retained on 

a #325 (45 μm) sieve. The chemical composition data provided by the material supplier are provided in 

Table 14 below. According to these values, more than 90 % of the total mass is composed of silicon, 

aluminum, and iron oxides. 

Table 14. Fly ash oxide composition reported in the mill certificate, based on a single measurement. 

Oxide Mass Fraction (%) 

SiO2 53.09 

Al2O3 27.97 

Fe2O3 10.12 

CaO 1.39 

MgO 1.63 

SO3 0.10 

Na2Oe 0.59 

32
 
 

 

 

 

 

 

 

 

 

 

 

The fly ash was also analyzed at NIST using X-ray powder diffraction to identify and quantify the 

crystalline mineral composition, and the results are shown in Table 15 below. The major crystalline 

minerals are mullite and quartz. 

Table 15. Fly ash mineral phase composition as determined by X-ray powder diffraction and Rietveld 

analysis. 

Phase (XRD) Mass Fraction 

(%) 

Assumed Chemical 

Composition 

Mullite 18.3 ± 0.4 (A2O3)2(SiO2) 

Quartz 7.3 ± 0.4 SiO2 

Hematite 2.2 ± 0.1 Fe2O3 

Magnetite 1.0 ± 0.0 (Fe3O4) 

Amorphous 71.2 ± 0.8 

3.3.3 Slag 

The slag certification report did not include the oxide content, and a lack of any quantifiable peaks in the 

XRD data suggested that the material lacked crystalline phases. 

3.3.4 Previous Reports 

For comparison purposes, and to provide supplementary information about the slag, the oxide analysis of 

Harbour et al. [27] for all three materials is repeated in Table 16 below. The values for portland cement 

and fly ash are in agreement with the mill certificates. 

Table 16. Oxide percentages reported by Harbour et al. [27 (Table 3-5)] for the portland cement, the fly 

ash, and the slag. 

Oxide 

Portland Cement Fly Ash Slag 

Batch 1 Batch 2 Batch 1 Batch 2 Batch 1 Batch 2 

Al2O3 5.4 5.2 28.6 28.6 8.4 6.6 

CaO 64.9 63.0 0.7 0.6 38.5 35.0 

Fe2O3 3.7 3.8 6.0 5.6 0.4 0.3 

K2O 0.5 0.7 2.6 2.5 0.3 0.5 

MgO 1.2 1.2 0.9 0.8 12.9 13.1 

Na2O 0.1 0.2 0.3 0.4 0.3 0.3 

SO3 3.2 3.3 0.1 0.3 1.0 2.5 

SiO2 20.5 20.3 54.2 56.8 37.9 40.4 

TiO2 0.3 0.3 1.6 1.4 0.4 0.3 

Volatiles 0.0 0.0 2.5 1.4 0.0 0.0 

Total 99.8 98.0 97.5 98.4 100.1 99.0 

33
 
 

 

 

 

3.4 Mixture Designs 

The saltstone grout to be used is a combination of cement, fly ash, slag, and an alkaline solution for 

which there is relatively little engineering experience in the concrete construction industry. As a result, 

hydrated phase models would have to make a dramatic leap between typical mixes used today and the 

proposed mixture. To help span this conceptual gap, a progressive series of mixture designs was chosen, 

starting from mixtures that existing models could use as a frame of reference. 

The mixture designs were divided between two water:binder (mass basis) ratios. The reference mixtures 

were ordinary portland cement (OPC) and water. The three spanning mixtures were OPC and synthetic 

waste solution (SWS), a ternary blend and water, and a ternary blend and SWS. The mixture designs are 

shown in Table 17 and Table 18, and are proportioned to make approximately 500 mL of paste. 

Table 17. Mixture designs for w/cm=0.40 pastes. 

Mass (g) 

40-0-100 40-1-100 40-0-055 40-1-055 

Water 280.7 256.0 263.3 241.1 

NaOH 0.0 33.9 0.0 31.9 

NaNO3 0.0 49.5 0.0 46.7 

NaNO2 0.0 9.9 0.0 9.3 

Na2CO3 0.0 3.7 0.0 3.5 

Na2SO4 0.0 2.3 0.0 2.2 

Al(NO3)3 ● 9H2O 0.0 12.6 0.0 11.9 

Na2H(PO4) ● 12H2O 0.0 0.7 0.0 0.7 

Cement 701.8 654.7 65.8 61.7 

Fly Ash 0 0.0 296.2 277.6 

Slag 0 0.0 296.2 277.6 

Table 18. Mixture designs for w/cm=0.60 pastes. 

Mass (g) 

60-0-100 60-1-100 60-0-055 60-1-055 

Water 328.8 296.4 312.7 283.0 

NaOH 0.0 39.2 0.0 37.4 

NaNO3 0.0 57.4 0.0 54.8 

NaNO2 0.0 11.4 0.0 10.9 

Na2CO3 0.0 4.3 0.0 4.1 

Na2SO4 0.0 2.7 0.0 2.6 

Al(NO3)3 ● 9H2O 0.0 14.6 0.0 14.0 

Na2H(PO4) ● 12H2O 0.0 0.9 0.0 0.8 

Cement 547.9 505.4 52.1 48.3 

Fly Ash 0.0 0.0 234.5 217.1 

Slag 0.0 0.0 234.5 217.1 

34
 
 

 

 

 

 

 

 

 

 

The fresh pastes hydrated under sealed curing conditions, to the extent possible. Each mixture was cast 

into 25 mm diameter, 25 mm tall, cylindrical plastic molds. The molds were sealed in plastic bags, and 

the samples were demolded after 24 hours, placed into airtight plastic jars, and stored at 25 °C until the 

time of testing. The w/cm=0.60 pastes were rotated (at approximately 1 rotation every 10 seconds) 

during the first 24 hours, and then demolded. The demolded samples were placed into sealed highdensity polyethylene (HDPE) jars and stored in a walk-in environmental chamber maintained at 25 °C. 

3.5 Results 

3.5.1 Calorimetry 

The calorimetry results for these mixtures are shown in Figure 22 and are similar to the results from the 

previous study shown in Figure 12. The ordinary portland cement (OPC) systems (0-100) displayed the 

common overlapping peaks. The OPC pastes containing SWS (1-100) were accelerated by the alkalinity 

of the SWS, as one would expect. The saltstone mixtures (1-055) exhibited a heat peak at approximately 

the same time as the OPC peak, but the peak was much lower and much wider. The water and ternary 

blend is only included for the 0.40 w/cm mixture because the corresponding 0.60 w/cm mixture did not 

react appreciably during the same time frame (as might be expected). The 0.40 w/cm water and ternary 

blend paste exhibits a significantly delayed peak, suggesting that the material would remain in a liquid 

state for a very long time. 

Figure 22. Heat generation rates (W/g) as a function of time (hours) for the 0.40 w/cm pastes (left) and 

the 0.60 w/cm pastes (right). 

Because the blended systems made with water (0-055) were so significantly delayed in the hydration, 

data from these mixtures is omitted from the subsequent results. The very low portland cement content 

contributed to a very low alkalinity that delayed the hydration of the Class F fly ash and the slag. 

Therefore, there is very little to glean from any subsequent analysis of these systems. 

35
 
 

 

 

 

 

 

 

 

 

3.5.2 Thermogravimetric Analysis 

The hydrated pastes were cut on a diamond saw, using ethanol as a lubricant, to create discs that were 

1 mm to 2 mm thick. These discs were gently broken into pieces using a mortar and pestle. Two of 

these pieces were selected (total mass approximately 50 mg) and placed into the TGA sample holder. 

The TGA data for the mixtures are shown in Figure 23 through Figure 25. Each figure shows a series of 

TGA curves for a particular mixture, with each curve representing a different age. The large peak below 

250 °C is the loss of free water and the decomposition of most of the hydration products. The peak 

between 400 °C and 500 °C is due to portlandite (Ca(OH)2) decomposition. The barely perceptible peak 

at 650 °C in Figure 23 is due to limestone calcination. 

0.40 0.40 

40-0-100 60-0-100 

0.30 0.30 

-d

μ/dT (1/ o C) 

0.20 

-d

μ/dT (1/ o C) 

0.10 

0.20 

0.10 

0.00 0.00

0 200 400 600 800 1000 0 200 400 600 800 1000 

Temperature Temperature 

Figure 23. Differential relative mass loss (dμ/dT) as a function of temperature (°C) for pastes made with 

portland cement and water: w/cm=0.4 (left); w/cm=0.60 (right). Each line corresponds to a different age: 

7 d, 14 d, 28 d, and 56 d. 

0.40 0.40 

40-1-100 60-1-100 

0.30 0.30 

-d

μ/dT (1/ o C) 

-d

μ/dT (1/ o C) 

0.20 

0.10 

0.20 

0.10 

0.00 0.00

0 200 400 600 800 1000 0 200 400 600 800 1000 

Temperature Temperature 

Figure 24. Differential relative mass loss (dμ/dT) as a function of temperature (°C) for pastes made with 

portland cement and surrogate waste solution: w/cm=0.4 (left); w/cm=0.60 (right). Each line corresponds 

to a different age: 7 d, 14 d, and 28 d. 

36
 
 

 

 

 

 

 

The portlandite content of the OPC mixtures were estimated using the process described previously, and 

the results are given in Table 19. The regression process reports an uncertainty for the mass loss that is 

propagated to the estimated portlandite content. After 8 weeks of hydration at 25 °C, the mass fraction of 

portlandite is relatively constant. 

Table 19. Portlandite mass fraction, as determined by TGA analysis, for the four portland cement 

mixtures. The uncertainty represents the measurement uncertainty having a coverage factor of one 

standard deviation. 

Age 40-0-100 60-0-100 40-1-100 60-1-100 

7d 0.1321 ± 0.0003 0.1350 ± 0.0004 0.1050 ± 0.0028 0.1146 ± 0.0000 

14d 0.1375 ± 0.0005 0.1502 ± 0.0007 0.1081 ± 0.0016 0.1224 ± 0.0006 

28d 0.1350 ± 0.0003 0.1568 ± 0.0006 0.1248 ± 0.0013 0.1416 ± 0.0009 

56d 0.1433 ± 0.0004 0.1502 ± 0.0004 0.1370 ± 0.0009 0.1458 ± 0.0008 

The TGA data for the mixtures that were composed of the ternary mixtures are shown in Figure 25. 

Because of the supplemental mineral admixtures present, there are no peaks corresponding to portlandite. 

Moreover, there would be far too little limestone present to use it as an internal standard. 

0.00 

0.10 

0.20 

0.30 

0.40 

-d

μ/dT (1/ o C) 

40-1-055 

0.00 

0.10 

0.20 

0.30 

0.40 

-d

μ/dT (1/ o C) 

60-1-055 

0 200 400 600 800 1000 0 200 400 600 800 1000 

Temperature Temperature 

Figure 25. Differential relative mass loss (-dμ/dT) as a function of temperature (°C) for pastes made with 

portland cement paste, fly ash, slag, and surrogate waste solution: w/cm=0.4 (left); w/cm=0.60 (right). 

Each line corresponds to a different age: 7 d, 14 d, and 28 d. 

For the samples containing the SWS, there is a significant peak located near 600 °C that does not appear 

in samples without the SWS. Upon studying the corresponding X-ray diffraction data for these samples, 

it was determined that these peaks correspond to the decomposition of nitratine (sodium nitrate). The 

temperature at which this mass loss occurred was consistent with reported decomposition temperatures 

[28]. 

37
 
 

 

3.5.3 X-ray Powder Diffraction 

The X-ray powder diffraction samples were cut from the smaller cylindrical samples using a diamond 

saw with ethanol as the lubricant. A circular disc approximately 2 mm thick was cut from the specimen, 

and the tested surface was gently polished with a 600-grit abrasive paper using ethanol as the lubricant. 

The polished sample was mounted onto a sample holder, and the sample was rotated to maximize the 

sampling area. The X-ray diffraction data for the different mixtures, with the data from each age offset 

from one another, are shown in Figure 26 through Figure 28 below. 

38
 



Table 20. Results of Rietveld analysis for the six mixtures. The values reported are the mass fractions, 

and the numbers in parentheses represent the uncertainty (one standard deviation) in the last two digits. 

40-0-100 7d 14d 28d 56d 12w 

Alite 0.0494 (34) 0.0499 (32) 0.0369 (31) 0.0291 (29) 0.0303 (28) 

Belite 0.0886 (60) 0.0588 (48) 0.0623 (53) 0.0443 (46) 0.0298 (39) 

Aluminate 

Ferrite 0.0844 (61) 0.0835 (65) 0.0835 (61) 0.0826 (59) 0.0690 (54) 

Portlandite 0.1321 (32) 0.1375 (43) 0.1350 (40) 0.1433 (34) 0.1390 (33) 

Ettringite 0.0679 (30) 0.0812 (36) 0.0647 (34) 0.0719 (32) 0.0712 (31) 

Hemicarbonate 0.0148 (12) 0.0171 (12) 0.0160 (14) 0.0161 (14) 0.0107 (13) 

Monocarbonate 0.0384 (59) 0.0365 (45) 0.0458 (47) 0.0582 (45) 0.0502 (41) 

Amorphous 0.5244 (118) 0.5357 (113) 0.5559 (112) 0.5546 (104) 0.5998 (96) 

60-0-100c 7d 14d 28d 56d 12w 

Alite 0.0366 (32) 0.0354 (46) 0.0184 (48) 

Belite 0.0740 (65) 0.0347 (65) 0.0411 (78) 

Aluminate 0.0028 (12) 0.0029 (12) 0.0024 (11) 

Ferrite 0.0764 (57) 0.0778 (62) 0.0727 (57) 

Portlandite 0.1406 (29) 0.1441 (40) 0.1448 (32) 

Ettringite 0.0788 (30) 0.0839 (35) 0.0909 (34) 

Hemicarbonate 0.0207 (11) 0.0237 (12) 0.0176 (11) 

Monocarbonate 0.0521 (42) 0.0664 (44) 0.0705 (39) 

Corundum 0.0579 (19) 0.0520 (21) 0.0562 (20) 

Amorphous 0.4600 (112) 0.4792 (125) 0.4853 (126) 

40-1-100 7d 14d 28d 56d 12w 

Alite 0.1275 (43) 0.1206 (41) 0.0896 (36) 0.0735 (30) 0.0576 (27) 

Belite 0.0604 (65) 0.0443 (55) 0.0605 (51) 0.0385 (41) 0.0285 (32) 

Aluminate 0.0029 (11) 0.0039 (10) 0.0046 (10) 0.0039 (99) 

Ferrite 0.0569 (59) 0.0576 (57) 0.0593 (55) 0.0603 (54) 0.0532 (49) 

Portlandite 0.1050 (33) 0.1081 (32) 0.1248 (39) 0.1370 (37) 0.1222 (33) 

Ettringite 0.0044 (26) 0.0038 (20) 0.0096 (35) 0.0032 (28) 0.0117 (36) 

Hemicarbonate 0.0053 (12) 0.0065 (13) 0.0027 (13) 

Monocarbonate 0.0199 (38) 0.0189 (45) 0.0109 (44) 

Amorphous 0.6429 (107) 0.6617 (97) 0.6517 (107) 0.6836 (140) 0.7268 (93) 

60-1-100c 7d 14d 28d 56d 16w 

Alite 0.1010 (40) 0.0639 (34) 0.0499 (31) 0.0375 (66) 

Belite 0.0551 (67) 0.0556 (75) 0.0427 (75) 0.0060 (23) 

Aluminate 

Ferrite 0.0687 (60) 0.0449 (69) 0.0441 (58) 0.0435 (59) 

Portlandite 0.1085 (34) 0.1122 (37) 0.1184 (36) 0.1258 (38) 

Ettringite 0.0043 (29) 0.0108 (39) 0.0023 (18) 0.0043 (31) 

42
 
 

 

 

 

 

Hemicarbonate 0.0109 (15) 0.0071 (14) 0.0084 (14) 0.0057 (13) 

Monocarbonate 0.0568 (44) 0.0530 (49) 0.0543 (47) 0.0492 (41) 

Corundum 0.0513 (22) 0.0431 (22) 0.0407 (19) 0.0421 (19) 

Amorphous 0.5364 (130) 0.6084 (143) 0.6324 (128) 0.6942 (124) 

40-1-055c 7d 14d 28d 56d 16w 

Alite 0.0216 (19) 0.0216 (24) 0.0209 (26) 0.0170 (56) 

Belite 0.0069 (24) 0.0041 (25) 0.0026 (27) 0.0059 (36) 

Aluminate 

Ferrite 0.0174 (36) 0.0103 (28) 0.0105 (31) 0.0136 (32) 

Portlandite 

Ettringite 

Hemicarbonate 0.0077 (12) 0.0094 (13) 0.0091 (14) 0.0092 (16) 

Monocarbonate 0.0208 (36) 0.0164 (36) 0.0203 (38) 0.0165 (35) 

Corundum 0.0449 (21) 0.0449 (15) 0.0449 (18) 0.0449 (61) 

Quartz 0.0289 (15) 0.0216 (11) 0.0234 (12) 0.0232 (32) 

Mullite 2:1 0.0638 (32) 0.0509 (23) 0.0543 (27) 0.0541 (74) 

Amorphous 0.8250 (81) 0.8594 (69) 0.8522 (78) 0.8539 (142) 

60-1-055c 7d 14d 28d 56d 16w 

Alite 0.0261 (30) 0.0200 (34) 0.0259 (30) 0.0192 (18) 

Belite 0.0090 (99) 0.0072 (99) 0.0072 0.0052 (99) 

Aluminate 

Ferrite 0.0056 (30) 0.0060 (26) 0.0067 (24) 0.0048 (31) 

Portlandite 

Ettringite 

Hemicarbonate 0.0081 (14) 0.0086 (17) 0.0079 (13) 0.0141 (18) 

Monocarbonate 0.0143 (41) 0.0108 (35) 0.0148 (38) 0.0134 (45) 

Corundum 0.0890 (46) 0.0890 (14) 0.0890 (34) 0.0890 (44) 

Quartz 0.0164 (13) 0.0173 (28) 0.0173 (15) 0.0178 (12) 

Mullite 2:1 0.0378 (27) 0.0423 (67) 0.0385 (35) 0.0413 (27) 

Amorphous 0.8238 (128) 0.6685 (136) 0.5447 (118) 0.8255 (127) 

3.5.6 Pores Solution Analysis 

The results from the pore solution analyses of the OPC pastes appear in Table 21 and Table 22. The 

limited number of observations from the 0.40 w/cm OPC paste is due, in part, to the limited ability to 

extract useable samples from these relatively low w/cm systems. By comparison, the 0.60 w/cm OPC 

pastes yielded useable samples after 113 days of hydration. 

43
 
 

 

 

 

 

 

 

 

Table 21. Pore solution composition of the 0.40 w/cm paste made with only portland cement; numbers in 

parentheses indicate one standard deviation of uncertainty. 

40-0-100 

(mmol/kgw) 

Element 7 days 16 days 

K 463.3 (2.8) 407.2 (1.7) 

Na 141.6 (1.3) 167.35 (0.86) 

S 12.48 (0.42) 14.41 (0.04) 

Ca 3.63 (0.11) 1.924 (0.023) 

Al 0.079 (0.005) 0.0910 (0.0015) 

Table 22. Pore solution composition of the 0.60 w/cm paste made with only portland cement; numbers in 

parentheses indicate one standard deviation of uncertainty. 

60-0-100 

(mmol/kgw) 

Element 8 days 16 days 28 days 113 days 360 days 

K 262.1 (3.0) 230.1 (1.8) 250.6 (1.5) 281.1 (1.2) 

Na 73.74 (0.47) 98.05 (0.92) 169.32 (0.99) 101.77 (0.14) 

S 3.24 (0.11) 4.327 (0.078) 8.11 (0.21) 4.194 (0.057) 

Ca 3.112 (0.026) 2.396 (0.026) 2.761 (0.027) 2.765 (0.041) 

Al 0.069 (0.002) 0.0637 (0.0003) 0.0141 (0.0019) 0.0752 (0.0018) 

Mg 0.0085 (.0001) 

The results from the pore solution analyses of the saltstone waste forms appear in Table 23 and Table 24. 

For saltstone waste form samples, the challenge in collecting usable samples was the converse of the 

OPC pastes. The solid microstructure developed sufficiently slowly that useable samples could not be 

obtained until there was sufficient mechanical strength to resist the entire sample from flowing through 

the outlet tube on the die. For the 0.40 w/cm waste forms, usable samples were not attainable until after 

14 d hydration, and the 0.60 w/cm waste forms did not yield usable samples until after 90 d hydration. 

The advantage of this slower microstructural development was that samples could be obtained at later 

ages than for the OPC pastes. In the case of the 0.6 w/cm waste form, samples were obtained out past 

one year of hydration. 

Table 23. Pore solution composition of the 0.40 w/cm paste made with fly ash, slag, and surrogate waste 

solution; numbers in parentheses indicate one standard deviation of uncertainty. 

40-1-055 

(mmol/kgw) 

Element 14 days 34 days 

(HNO3) 

34 days (DI 

Water) 

86 days 

K 186.1 (2.5) 211.4 (3.8) 198.5 (1.9) 238.9 (4.9) 

Na 2482. (46.) 3380. (48.) 3478. (63.) 3284. (16.) 

S 167.7 (1.3) 181.0 (2.0) 186.5 (3.9) 197.4 (6.4) 

Ca 0.335 (0.006) 0.151 (.002) 0.179 (.005) 0.205 (0.002) 

Al - -

Si 88.7 (1.5) 90.78 (0.37) 

44
 
 

 

 

 

 

 

 

 

 

 

Table 24. Pore solution composition of the 0.60 w/cm paste made with fly ash, slag, and surrogate waste 

solution; numbers in parentheses indicate one standard deviation of uncertainty. 

60-1-055 

(mmol/kgw) 

Element 91 days 126 days 

(HNO3) 

126 days (DI 

Water) 

391 days 

(HNO3) 

391 days (DI 

Water) 

K 152.4 (6.3) 199.4 (1.7) 198.13 (.82) 235.88 (.81) 237.66 (.58) 

Na 3267. (30.) 3260. (40.) 3311. (44.) 3287. (45.) 3381. (79.) 

S 172.0 (7.0) 186.2 (1.4) 201.8 (1.3) 226.6 (2.1) 245.4 (1.6) 

Ca 1.154 (0.009) 0.546 (0) 0.262 (.003) 1.67 (.11) 0.195 (.003) 

Al -

Si 128.90 (.35) 139.11 (.22) 78.73 (.29) 85.31 (.23) 

There were two notable observations from the elemental concentration data. The sodium concentration in 

pore solution of all the mixtures made with SWS was considerably lower than the more than 5 mol/kgw 

sodium concentration in the SWS solution. There are a number of plausible explanations for this. The 

very high concentration is a challenging measurement challenge; one must reproduce the background 

accurately. The lower than expected concentration may also be due to a greater than expected amount of 

sodium binding in the calcium silicate hydrate (C-S-H). This is plausible because the alkali binding 

increases with increasing cement replacement (resulting in lower Ca/Si molar ratios in the C-S-H) [30], 

and there are few data on binding at these sodium concentrations. By contrast, the silicon concentration 

was consistent with thermodynamic modeling prediction of a relatively significant silicon concentration 

in the pore solution. 

3.5.7 Total Sulfur 

Determining the total sulfur content of the pore solution may be complicated by the two different forms 

in which it may be present. Sulfates are commonly available in portland cement; it is often added as 

gypsum during grinding. It has been known for some time that blast furnace slag contributes sulfides to 

the pore solution of cementitious systems. In principle, the ICP-OES method will detect both forms of 

sulfur. In practice, however, diluting the pore solution with nitric acid could convert some or all the 

sulfide to hydrogen sulfide (HS), which could be released as a gas. If this were to occur, the total 

quantity of sulfur detected by ICP would be less than the true value due to some of the sulfur being lost 

as a gas before the ICP measurement took place. 

On a few of the samples taken from mixtures made using surrogate waste solution, two different dilution 

protocols were used. The collected pore solution was filtered and divided into two sub-samples. The 

first sample was diluted with nitric acid, as described above. The second sample was diluted using only 

water, following the practice described by Lothenbach et al. [31]. These results have been included in 

the data in Table 23 and Table 24 with the dilution solution shown in parentheses. From the data given 

in Table 23 and Table 24, there is no evidence that acid dilution removed much, if any, sulfur from the 

sample. 

45
 
 

 

 

 

 

 

 

One possible explanation for observing the same sulfur, regardless of dilution type, is that the sulfide had 

already oxidized prior to the pore expression, resulting in all the sulfur being present as a sulfate. This 

explanation, however, would be inconsistent with the samples being stored in air-tight plastic jars, the 

observation of a dark green interior of the paste sample (the color saturation diminished upon exposure to 

air), and is inconsistent with detecting hydrogen sulfide gas (“rotten eggs”) when cleaning a mortar and 

pestle with acid after the mortar and pestle had been used to crush small saltstone waste form samples. 

3.6 Discussion 

As expected, the blended binder systems (40-1-055 and 60-1-055) contained no measureable portlandite. 

This is to be expected because, in the absence of a source of additional calcium, the alumina and silica in 

the fly ash and slag will consume the calcium in the portlandite (created by the portland cement 

hydration) to create additional C-S-H. 

There are two important points of comparison between the ordinary portland cement paste mixtures and 

the blended binders with SWS. In the 0.40 w/cm ordinary portland cement paste (40-0-100), the 

combined mass fraction of the hemicarbonate, monocarbonate, and ettringite phases is approximately 

12.5 % after 12 weeks of hydration. In the 0.40 w/cm pastes containing both blended binders and SWS 

(40-1-055), the same total was less then 2.5 %, and there was no ettringite detected. In the 0.60 w/cm 

ordinary portland paste (60-0-100), the same sum was more than 17 %, while in the 0.60 w/cm paste 

containing both blended binders and SWS (60-1-055) the sum was below 2 %, and there was no ettringite 

detected. Therefore, although the SWS was contributing considerable sulfate to the mixture, the overall 

chemistry was suppressing the formation of sulfate-bearing phases, compared to the amount generated 

during the hydration of OPC pastes made with water. 

The pore solution analysis bears this out. The sulfur content of the pore solution of the ordinary portland 

cement 40-0-100 specimen was approximately 15 mmol/kgw, and the sulfur content of the blended binder 

and SWS 40-1-055 specimen pore solution was nearly 200 mmol/kgw. This increase is consistent with 

having additional sulfur in the SWS and the simultaneous absence of sulfur-bearing minerals being 

formed during hydration. For the 0.60 w/cm specimens, the corresponding difference was more 

dramatic. The sulfur content of the pore solution of the ordinary portland cement 60-0-100 specimen was 

less than 10 mmol/kgw, and the sulfur content of the blended binder and SWS 60-1-055 specimen pore 

solution reached nearly 250 mmol/kgw. This greater difference between the ordinary portland cement 

system and the blended system is due largely to the greater “mix water” fraction of the 0.60 w/cm 

mixture designs. 

After three months of hydration, the type and quantity of mineral phases appear to be stable. No 

unexpected phases are forming, and the quantity of existing minerals is changing gradually. Moreover, 

the types of mineral phases observed are also commonly found in typical blended cement systems using 

in construction. 

3.7 Summary 

In general, the hydrated phases and pores solution composition of saltstone grouts is similar to what one 

might expect for these systems. Despite the uniqueness of the binder proportions and the concentrated 

solution used as the mix water, the observed phases, and their proportions, were not significantly 

different from what is observed in an analysis of an ordinary portland cement paste made with water. 

46
 
The most notable difference between the ordinary portland cement pastes and the synthetic saltstone was 

the absence of ettringite, and the decreased production of hemicarbonate and monocarbonate, even after 

one year of hydration. The pore solution chemistry of the saltstone, which is dominated by the 

contribution of the SWS, appears to suppress the formation of sulfate-bearing mineral phases, despite the 

contribution of additional sulfate in the SWS. Moreover, although solid sodium nitrate (nitratine) was 

detected in both the XRD and the TGA measurements, it was determined that these observations were the 

result of an experimental artifact. 

The sodium concentration in the pore solution was less than expected, most probably because the degree 

of sodium binding (into the hydration phases) has never been quantified at these very high 

concentrations, and for the type of C-S-H formed by these blended systems. Moreover, after one year of 

hydration, the mineral phases that were detected are found in blended cement concretes and these mineral 

phases appear to be relatively stable. 

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4 CONCLUSION
 

Methods have been developed for characterizing the hydration phases and the pore solution composition 

of cementitious systems containing very little portland cement, large quantities of fly ash and slag, and a 

concentrated salt solution. The material was characterized using isothermal calorimetry, 

thermogravimetric analysis (TGA), quantitative X-ray powder diffraction (QXRD), image segmentation 

of elemental maps created by electron probe microanalysis performed on a scanning electron microscope 

(SEM), and aqueous phase analysis by inductively coupled plasma – optical emission spectroscopy (ICPOES). In nearly all cases, existing methodologies had to be modified or improved to work with these 

materials. 

The hydrated phase analysis by QXRD was facilitated, to a degree, by TGA. For the mixtures having 

little or no fly ash or slag, TGA analysis of the portlandite content was a useful means of establishing an 

internal standard for the QXRD analysis. For the anticipated saltstone formulation, however, all of the 

portlandite is consumed by the fly ash and slag hydration, so an internal crystalline standard (corundum) 

is required. 

A method using image segmentation was developed and shown to be a useful method of hydrated phase 

identification and quantification. The image segmentation process may have its greatest strength in 

classifying the glassy phases of these blended systems because the relevant glassy phases (fly ash, slag, 

and C-S-H) appear differently. By comparison, quantitative X-ray diffraction can only determine total 

mass fraction of glassy phases by using an internal standard and subtracting out the mass fraction of the 

crystalline phases. Before image segmentation can be used reliably, however, independent chemical 

analysis experiments are needed to validate the method. 

The hydrated samples were squeezed in a press to extract the pore solution, which was then analyzed 

using ICP-OES to estimate the concentration of the atomic elements present. The observed sodium 

concentration in the pore solution was approximately one-half the sodium concentration in the salt 

solution, which was attributed, in part, to absorption of the sodium by the C-S-H. The total sulfur 

concentration in the saltstone was approximately twenty times greater than the sulfur concentration in the 

portland cement pastes mixed with water, and two different dilution methods were used to confirm that 

reduced sulfur (sulfide) was not being lost as hydrogen sulfide gas during sample preparation. The 

higher alkalinity in the saltstone pore solution resulted in a factor of ten lower calcium concentration in 

the saltstone waste. This fact was associated with a decrease in the quantity of sulfate phases being 

formed in the saltstone. From this, it was concluded that, as long as the saltstone pore solution remains 

alkaline, there is a low expectation of finding the types of sulfate phases that one associates with 

expansive sulfate attack. 

Ultimately, these data and techniques will support the development of new tools for performance 

assessments of cementitious materials used in the nuclear infrastructure. These tools will need to 

accurately predict the mineralogy and pore solution composition for a wide range of cementitious 

systems, including saltstone waste forms. Because these mixtures, with their very low portland cement 

content and concentrated wastes solutions, are not common practice within the concrete construction 

industry, these data will help to develop a basis of understanding for these systems. Specifically, they 

will support models to accurately predict the type and quantity of hydrated phases, and the equilibrium 

pore solution, of blended cement systems. This information, coupled with detailed information about the 

environment, can then be used to predict the long-term performance of these materials. 

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