NISTIR 7532 

X-ray Microtomography Studies of Air-Void 

Instability and Growth during Drying of Tile Adhesive 

Mortars

90 min

150 min

190 min

330 min

Dale P. Bentz 

Claus-Jochen Haecker 
NISTIR 7532 

X-ray Microtomography Studies of Air-Void 

Instability and Growth during Drying of Tile Adhesive 

Mortars

Dale P. Bentz 

Materials and Construction Research Division 

Building and Fire Research Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-8615 

Claus-Jochen Haecker

SE Tylose GmbH & Co. KG (Shin-Etsu Chemical) 

Rheingaustrasse 190 - 196, Building H346 

65203 Wiesbaden, GERMANY

November 2008 

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Deputy Director 

ii
Abstract 

 A previous X-ray absorption study of tile adhesive mortars [1] identified a new 

phenomenon during drying, namely the movement of small cement particles from the interior of 

the specimen to its surface. In that study, an analysis based on Stokes’ equation indicated that 

particle movement is consistent with the very high viscosity of these tile adhesive mortars. In 

addition to this particle movement, the concurrent formation/growth of large scale flaws (voids) 

within the specimens was observed. The goal of the present study was to observe this air void 

instability and growth in real time using three-dimensional X-ray microtomography. Mitigation 

strategies for avoiding the formation of these flaws were also investigated. Successful mitigation 

strategies included the utilization of a coarser cement (larger particles that do not move during 

drying), the addition of an accelerator (to achieve set before flaw formation can occur), and the 

addition of a co-thickener (to form a polymer network to prevent particle movement). 

Conversely, the addition of an air entrainer was not successful as a mitigation strategy. 

iii
iv
Table of Contents 

Abstract..........................................................................................................................................iii 

List of Figures................................................................................................................................ vi 

List of Tables...............................................................................................................................viii 

1. Introduction................................................................................................................................. 1 

2. Materials and Experimental Methods......................................................................................... 2 

3. Results......................................................................................................................................... 6 

4. Image Gallery from X-ray Microtomography Measurements.................................................... 9 

5. Conclusions............................................................................................................................... 32 

6. Acknowledgements................................................................................................................... 32 

References..................................................................................................................................... 33 

v
List of Figures 

Figure 1. Measured particle size distributions for the two cement powders used in this study. 

The x-axis indicates an equivalent spherical particle diameter and the y-axis the mass % at 

each given diameter. The shown results are the average of six individual measurements and 

the error bars (one standard deviation) would fall within the size of the thicker line. ........... 2 

Figure 2. Measured mass loss during drying for the mortars prepared for this study. ................... 4 

Figure 3. Isothermal calorimetry results for the nine mortar mixtures prepared for this study. 

Top figure shows curves for mixtures with heat flow similar to the control mortar and those 

with acceleration. Bottom figure shows curves for mixtures with heat flow similar to the 

control and those with retardation........................................................................................... 5 

Figure 4. Diagram showing coronal, sagittal, and transaxial views for the cylindrical mortar 

specimens................................................................................................................................ 9 

Figure 5. Coronal views of a cross-section of mortar #1 after various drying times, indicating 

instability and growth of large void near the top surface. Tile adhesive mortar specimens 

are approximately 11 mm in thickness. ................................................................................ 10 

Figure 6. Sagittal views of a cross-section of mortar #1 after various drying times, indicating 

instability and growth of large void near the top surface...................................................... 11 

Figure 7. Transaxial views of a cross-section of mortar #1 after various drying times. Transaxial 

images are 32.5 mm x 32.5 mm............................................................................................ 12 

Figure 8. Coronal, transaxial, and sagittal images of mortar #2 after being exposed to drying for 

30 min. .................................................................................................................................. 14 

Figure 9. Coronal, transaxial, and sagittal images of mortar #2 after being exposed to drying for 

1660 min. .............................................................................................................................. 15 

Figure 10. Coronal, transaxial, and sagittal images of mortar #3 after being exposed to drying for 

30 min. .................................................................................................................................. 16 

Figure 11. Coronal, transaxial, and sagittal images of mortar #3 after being exposed to drying for 

2475 min. .............................................................................................................................. 17 

Figure 12. Coronal views of a cross-section of mortar #4 after various drying times, indicating 

instability and growth of large void near the right side. ....................................................... 18 

Figure 13. Sagittal views of a cross-section of mortar #4 after various drying times. ................. 19 

Figure 14. Transaxial views of a cross-section of mortar #4 after various drying times.............. 20 

Figure 15. Coronal, transaxial, and sagittal images of mortar #5 after being exposed to drying for 

30 min. .................................................................................................................................. 22 

Figure 16. Coronal, transaxial, and sagittal images of mortar #5 after being exposed to drying for 

950 min. ................................................................................................................................ 23 

Figure 17. Coronal, transaxial, and sagittal images of mortar #6 after being exposed to drying for 

30 min. .................................................................................................................................. 24 

Figure 18. Coronal, transaxial, and sagittal images of mortar #6 after being exposed to drying for 

270 min. ................................................................................................................................ 25 

Figure 19. Coronal, transaxial, and sagittal images of mortar #7 after being exposed to drying for 

30 min. .................................................................................................................................. 26 

Figure 20. Coronal, transaxial, and sagittal images of mortar #7 after being exposed to drying for 

1040 min. .............................................................................................................................. 27 

Figure 21. Coronal, transaxial, and sagittal images of mortar #8 after being exposed to drying for 

30 min. .................................................................................................................................. 28 

vi
Figure 22. Coronal, transaxial, and sagittal images of mortar #8 after being exposed to drying for 

260 min. ................................................................................................................................ 29 

Figure 23. Coronal, transaxial, and sagittal images of mortar #9 after being exposed to drying for 

30 min. .................................................................................................................................. 30 

Figure 24. Coronal, transaxial, and sagittal images of mortar #9 after being exposed to drying for 

1370 min. .............................................................................................................................. 31 

vii
List of Tables 

Table 1. Mortar mixture proportions and measured air contents and open times........................... 3 

viii
- 1 -

1. Introduction 

 Specialty thin layer mortars are often formulated for use as tile adhesives or renderings 

for external and internal walls. Two common additives to such mortars are cellulose ethers (CE) 

and redispersible polymer powders (RPP) [2, 3]. CE retains water in the mortar for a sufficient 

time to allow proper cement hydration and is also employed to control the workability and 

increase the volumetric yield of the fresh mortars, first by acting as a thickener and second by 

entraining a considerable amount of air voids (about 25 % by volume). RPP are often added to 

improve mechanical properties of the hardened mortar. A key property of these mortars is their 

open time, defined as the time during which tiles may be applied to the mortar surface and 

achieve adequate adhesion (i.e., the time during which the mortar surface remains “tacky” to the 

touch) [3]. Ultimately, this tackiness is lost due to the formation of a film at the top surface of 

the mortars. The film may be composed of both polymeric materials and (carbonated) hydration 

products. Jenni et al. [2] used a variety of analytical techniques to study the polymer-mortar 

interactions in these materials. They observed, for example, that both the CE and the RPP are 

dissolved/redispersed in the pore solution and can form isolated films at interfaces during drying. 

They also identified a fractionation process in which polymer components (and calcium 

hydroxide) were enriched at the top drying surface of the mortars. Recently [1], X-ray 

absorption measurements have indicated that in addition to the polymer components and calcium 

hydroxide “concentrating” at the top surface during drying, small cement particles do likewise, 

due to the extremely high viscosity of the pore solution. Due to the additives present in these tile 

adhesive mortars, the pore solution viscosity becomes high enough to carry smaller particles 

along with the fluid during fluid movement from the interior of the specimen to the top (exposed) 

surface during drying. 

 

 The X-ray absorption measurements also indicated that particle movement during drying 

could be accompanied by the formation/growth of large scale (several mm in size) voids within 

the mortar microstructure. The formation of these large void regions could be partially 

responsible for the unusually low late age tensile strength sometimes exhibited by these materials, 

especially if the voids are formed at or near the interface between mortar and substrate. Voids of 

several mm in size could surely result in substantial strength reductions. Within the thin-set 

mortar industry, such voids have also been observed to produce anomalous needle penetration 

(setting) readings. In terms of open time, the densification of the top surface by the smaller 

particles in the system likely contributes to a reduction in the open time by further decreasing the 

“tackiness” of the top surface. Furthermore, their movement results in a concentration of the 

most reactive (smallest) cement particles at the top surface of the mortar where their more rapid 

hydration could further contribute to a reduction in open time both by reducing free water 

content and by stiffening the paste. Thus, from a practical standpoint, it would seem to be 

beneficial to formulate a mortar where this particle movement is minimized and air void 

instability and growth does not occur. In the present study, three-dimensional X-ray 

microtomography is employed to view in situ air void instability and growth in these tile 

adhesive mortars. A variety of mitigation strategies are evaluated on the basis of their ability to 

create a stable microstructure where air void instability is avoided. 
2. Materials and Experimental Methods 

Mortars were prepared in a 1.5 L plastic container using a single blade rotary mixer. All 

dry ingredients (cement, sand, CE, and other additives) were first homogenized in a sealed 

plastic bag. The dry ingredients were then placed in the mixing bowl and the appropriate mass 

of water added. Mixing proceeded for 1 min, followed by a rest period of 3 min, then 30 s of 

final mixing. The mixture proportions and air contents for the nine mortars are provided in 

Table 1. The mortars were proportioned on the basis of either 300 g or 150 g of dry ingredients 

(solids). 

 

 Two different binder powders were utilized in the experiments. For eight of the mortars, 

a European CEM I 42.5 N cement was used. For one of the experiments, a coarse cement based 

on a previous sieving (air classification) of Cement and Concrete Reference Laboratory 

proficiency sample 135 [4] was employed. The particle size distributions of the starting binder 

powders were measured using a laser diffraction method (Figure 1). 

0

2

4

6

8

10

0.1 1 10 100 1000

Diameter [μm]

Differential [%]

Cement 1

Coarse cement

Figure 1. Measured particle size distributions for the two cement powders used in this 

study. The x-axis indicates an equivalent spherical particle diameter and the y-axis the 

mass % at each given diameter. The shown results are the average of six individual 

measurements and the error bars (one standard deviation) would fall within the size of the 

thicker line.

 The prepared mortars were immediately placed into two round pre-weighed cylindrical 

plastic cups with a volume of 9130 mm3

 (inner diameter of 32 mm and height of about 11.4 mm) 

for the X-ray micro-tomography experiments and concurrent mass loss measurements. Initial 

masses of the filled cups, along with the volume of each, were used to estimate the air contents 

of the fresh mortars (Table 1). The average standard deviation in calculated air content between 

duplicate specimens was 0.9 %. Mass loss measurements were generally performed over the 

course of 1 d and were used to validate that the various prepared mortars were all drying at 

approximately the same rate (Figure 2). The temperature (nominally 23 oC) and relative humidity 

- 2 -
3

Table 1. Mortar mixture proportions and measured air contents, open times, and peak heat flow times. 

Mortar 1 Mortar 2* Mortar 3 Mortar 4 Mortar 5 Mortar 6 Mortar 7 M

Sand 191.7 g+ 95.85 g 191.7 g 191.7 g 191.7 g 186.7 g 187.8 g 1

Cement 107.4 g 53.7 g* 107.4 g 107.4 g 107.4 g 104.6 g 105.3 g 1

Water 78 g 33 g 78 g 78 g 78 g 78 g 78 g 

CE& 0.9 g 0.45 g 0.9 g 0.9 g 0.9 g 0.9 g 0.9 g 

Co-thickener 1 0.18 g Co-thickener 2 0.18 g 

Air entrainer 0.24 g 

CaCl2 7.8 g 

Polyvinyl 

alcohol (PVA) 

6 g 

Starch ether 

Imbentin$ 

w/c 0.726 0.615 0.726 0.726 0.726 0.746 0.741 

Air content 31.2 % 34.1 % 30.1 % 31.1 % 33.1 % 21.6 % 37.0 % 2

Open time 25 min 5 min# 10 min 25 min 25 min 80 min 15 min 3

Peak heat flow 

time 12 h 20.7 h 22.3 h 13 h 12.7 h 2.8 h 11.9 h 

Stability Unstable Stable Stable Unstable Stable Stable Stable 

*Coarse cement used in mortar mixture #2 only. + The mass of each ingredient was measured to the nearest 0.01 g. & Unmodified cellulose ether; methylhydroxyethylcellulose; high degree of etherification. $ Imbentin (Imbentin-AGS/35, 25 % in water) is an alcohol ethoxylate. # Measured in a previous experiment [1], not as part of the current study.
4

(nominally 50 %) inside the X-ray microtomography chamber and in the laboratory where the 

drying specimens were stored were monitored using a USB-compatible data logger throughout 

the course of the experiment. The mass loss results in Figure 2 indicate that within experimental 

variability, the nine mortar mixtures exhibited similar mass loss curves during drying. 

0.0

0.5

1.0

1.5

2.0

2.5

0 60 80

Mass loss (g water)

0 20 4

Time (h)

Mortar1

Mortar2

Mortar3

Mortar4

Mortar5

Mortar6

Mortar7

Mortar8

Mortar9

Figure 2. Measured mass loss during drying for the mortars prepared for this study. 

 Hydration rates were monitored using isothermal calorimetry on two replicate mortar 

specimens, each with a mass of 5.9 g. The heat flow curves for the two replicates generally 

overlapped one another so that only a single curve for each mixture is presented in the results in 

Figure 3. For each curve in Figure 3, the time of peak (maximum) heat flow is included in 

Table 1 to provide an indication of any acceleration or retardation produced by the additives 

relative to the “control” mortar (#1). Based on these values, the heat flow curves in Figure 3 

clearly indicate the successful acceleration of the hydration by the addition of calcium chloride 

(mortars #6 and #8), the retardation of hydration by various additives (starch ether in mortar #9 

and co-thickener in mortar #3), and the very low hydration rates exhibited by the coarse cement 

(mortar #2). 

 X-ray microtomography was conducted using a Skyscan 1172 system. 1 Operating 

parameters consisted of a voltage of 90 keV, a current of 112 μA, the utilization of aluminum 

and copper filters, and a resolution (voxel size) of 32 μm. These operating conditions were held 

constant for all of the three-dimensional image sets that were acquired during this study. Once 

the specimen for a given mortar was placed in the X-ray microtomography chamber, images 

were then acquired at multiple times up to 24 h without any movement of the specimen or 

opening/closing of the chamber door. A single complete three-dimensional scan took 10 min. 

Three-dimensional images were reconstructed from the raw scan data using the system software. 

Representative images obtained at the same (x,y,z) location but after different drying times were 

 

1

 Certain commercial products are identified in this paper to specify the materials used and procedures employed. In 

no case does such identification imply endorsement by the National Institute of Standards and Technology, nor does 

it indicate that the products are necessarily the best available for the purpose. 
then compared to determine the stability/instability of the air void system in each mortar during 

drying. 

 In separate experiments, the open times of the various mortars were measured according 

to European Standard EN 1347 (Determination of the wetting capability) [5]. Using this test 

method, a layer of a tile adhesive is combed onto a concrete slab using a notched trowel. Glass 

plates (100 mm × 100 mm × 6 mm) are embedded in the fresh mortar after 5 min, 10 min, 

15 min, etc., from which the ability of the applied material to wet the glass plates is assessed. 

Only samples with a wetted area > 50 % pass the test. The open time results are included in 

Table 1. 

0.000

0.002

0.004

0.006

0.008

0.010

0 24 48 7

Time (h)

Heat flow (W/g)

2

Mortar1-Control Mortar4-AE

Mortar5-Co-thickI Mortar6-accel

Mortar7-PVA Mortar8-accel/Imbentin

0.000

0.001

0.002

0.003

0.004

0 24 48 7

Time (h)

Heat flow (W/g)

2

Mortar1-Control Mortar2-Coarse

Mortar3-Co-thickII Mortar4-AE

Mortar5-Co-thickI Mortar7-PVA

Mortar9-Starch ether

Figure 3. Isothermal calorimetry results for the nine mortar mixtures prepared for this 

study. Top figure shows curves for mixtures with heat flow similar to the control mortar 

and those with acceleration. Bottom figure shows curves for mixtures with heat flow 

similar to the control and those with retardation. 

5
3. Results 

 The goals of the present study were twofold: 1) to observe air void instability and growth 

during the drying of tile adhesive mortars to verify that previously observed large scale flaws had 

not been present immediately after mixing and casting, and 2) to determine the effectiveness of 

various mitigation strategies in preventing this instability and growth. With this in mind, the 

control mortar (#1) was selected as a likely candidate in which to observe this instability, based 

on the results of the previous X-ray absorption experiments [1]. Once the complete threedimensional data sets were obtained for a given experiment, data viewing software was 

employed to assess the overall stability of each mortar mixture exposed to drying by viewing the 

coronal, sagittal, and transaxial two-dimensional images, as diagrammed in Figure 4. In the 

image gallery that follows, when a system was unstable, a time progression of the coronal, 

sagittal, and transaxial images representing one of the instabilities is presented. For a stable 

system, representative images in these three planes at the beginning and end of the experiment 

are presented so that the reader may assess that the microstructure (or at least air void structure) 

is indeed stable. 

 As an example of an unstable mortar formulation, in Figures 5 to 7, the instability and 

growth of an air void near the top surface of the exposed specimen of mortar #1 is clearly 

observed. From 30 min to 90 min, the drying initially induces a meniscus (curvature) at the top 

surface as some sedimentation occurs, and the air voids near the surface are seen to deform from 

their initially spherical shape to a more ellipsoidal one. By 150 min, however, the large air void 

on the top left of the coronal images of the specimen has become unstable and begins to grow. 

This growth continues over the course of about 3 h, until set occurs, ultimately resulting in a flaw 

(void) that has more than double the volume of the original air void. 

 Each air void in the tile adhesive mortar microstructure is surrounded by an interface that 

consists of pore solution and solid (cement and perhaps sand) particles. When the smallest of 

these particles are mobile during drying, it is conjectured that their movement destabilizes this 

interface and allows the expansion/coalescence process to initiate. As the drying continues, so 

does the growth of the instability. Eventually, after about 6 h for this mortar, initial set is 

achieved and the physical microstructure of the specimen is stabilized, preventing further growth 

of these instabilities. This hypothesis suggests three possible approaches to stabilizing the 

system: 1) preventing particle movement, 2) stabilizing the air voids via the addition of an air 

entrainer, or 3) accelerating the cement hydration so that set is achieved prior to the initiation of 

the instabilities. All three of these mitigation strategies were investigated in the eight subsequent 

mortar mixtures. 

 In the 2nd experiment, the control cement was replaced by a much coarser cement that had 

been obtained by classifying an ordinary portland cement in a previous study [4]. The previous 

X-ray absorption study [1] had indicated that this was a viable approach to minimizing particle 

movement during drying. Indeed, in this study, no evidence of instabilities could be observed in 

the X-ray microtomography data sets. Figures 8 and 9 provide representative images at the 

beginning (30 min) and end of the experiment (1660 min); any differences between the two sets 

of images are quite minimal. Unfortunately, previous results [1] have indicated a very short 

6
open time of only 5 min for this system (there was not enough cement remaining to measure the 

open time in this study) and as shown in Figure 3, hydration achieved during the first 72 h is 

minimal in comparison to that achieved by the control cement. The minimal hydration indicated 

for this mortar is due to both the large size of the cement particles and the fact that this separated 

coarse cement had been stored in a closed container for over 5 years. 

 For the 3rd mortar mixture, a co-thickener was utilized along with the cellulose ether. As 

shown in Figures 10 and 11, this produced a stable microstructure. In viewing the transaxial 

images for these two figures, some changes are observed, but these are due to settlement of the 

specimen (as noted in the corresponding coronal and sagittal images). Viewing the isothermal 

calorimetry results in Figure 3, it is observed that this co-thickener caused a measurable 

retardation of the cement hydration reactions. This would likely correspond to an increase in the 

setting time for this particular mortar, which could possibly be an advantage in increasing the 

open time or a disadvantage in slowing down strength development. However, separate 

measurements of the open time for this mortar instead indicated a reduction from the 25 min 

measured on the control mortar #1 to only 10 min, perhaps due to enhanced film formation at the 

top surface due to the addition of the co-thickener. 

 For the 4th mortar mixture, an air entrainer was added in an attempt to (chemically) 

stabilize the air voids without eliminating the particle movement. As observed in the coronal, 

sagittal, and transaxial images in Figures 12, 13, and 14, respectively, this system exhibited the 

instability and growth of a large air void near the right edge (coronal view) of the specimen. For 

this study, stabilizing the air voids using an air entrainer was thus an unsuccessful mitigation 

strategy. This mortar exhibited the same open time as the control (Table 1). 

 For the 5th mortar mixture, a different co-thickener was employed, one that has minimal 

effects on the cement hydration reactions (see Figure 3). As observed in Figures 15 and 16, this 

also proved to be a successful mitigation strategy to avoid the formation and growth of 

instabilities, and provided the additional advantage of an open time equal to that of the control. 

 In the 6th mortar mixture, an accelerator (calcium chloride) was added at a fairly high 

dosage (7.5 % by mass of cement) to decrease the setting time and try to mitigate instabilities by 

achieving set prior to their formation. As indicated in Figure 3, the calcium chloride greatly 

accelerated the hydration reactions and the peak hydration rate was indeed achieved prior to 3 h. 

Figures 17 and 18 indicate that this strategy was also successful in avoiding instability formation 

and growth. However, it can be observed in Table 1 that the calcium chloride functioned 

somewhat as an air detrainer and reduced the air content of mortar #6 down to about 22 %, about 

10 % less than that of the control mortar. The potential to offset this air content reduction by the 

incorporation of an additional air entrainer was investigated in the 8th mortar mixture to follow. 

Even though the calcium chloride produced a considerable acceleration of the hydration 

reactions, the measured open time for mortar #6 was 80 min (Table 1), much greater than that of 

the control mortar. The reason for this increased open time is unknown, but is perhaps due to 

favorable interactions between the Ca++ and Cl-

 ions and the cellulose ether. 

 In the 7th mortar mixture, a polyvinyl alcohol was added to the control mortar in hopes of 

forming a polymeric network that would prevent the movement of the smaller cement particles 

7
during drying. This mortar exhibited a minor decrease in heat release (Figure 3), but was stable 

from a microstructural viewpoint (Figures 19 and 20), even though it contained some quite large 

(several mm in size) air voids initially. This mortar also exhibited the highest air content 

(greatest yield) of any of the mixtures investigated in this study (Table 1). The polyvinyl alcohol 

addition reduced the open time of the mortar from 25 min to only 15 min. 

 In the 8th mortar mixture, the calcium chloride accelerator and an air entrainer (Imbentin) 

were both added to the control mortar. This increased the air content from about 22 % to 27 %, 

still somewhat less than that of the control mortar. The heat release curves for mortars #6 and #8 

(Figure 3) were quite similar and the stability of the mortar was verified, as shown in Figures 21 

and 22. The measured open time, 35 min, exceeded that of the control mortar #1 by 10 min. 

 In the 9th mortar mixture, a starch ether was added to attempt to stabilize the 

microstructure by preventing particle movement during drying. As can be seen in Figure 3, this 

produced extensive (greater than 24 h) retardation of the cement hydration and as shown in 

Figures 23 and 24, resulted in a highly unstable microstructure in which void instability and 

growth (and even subsurface cracking) were clearly present. The measured open time of this 

mortar was equivalent to that of the control mortar #1. 

 In the unstable mortars (#1, #4, and #9) observed in this study, the instabilities, once 

initiated, were observed to generally grow in a downward direction. This could be influenced by 

the differential stresses/strains that are established through the thickness of the specimen due to 

water content, particle density, and degree of hydration all being a function of height within the 

specimen. The top of the specimen where the smaller particles concentrate may well have a 

higher local density [1] and a greater degree of cement hydration (since smaller particles have 

more reactive surface area for early-age hydration), both of which could contribute to it being 

better able to resist the stresses and strains causing the formation and growth of the instability 

than the material present at a lower depth within the specimen. 

8
4. Image Gallery from X-ray Microtomography Measurements 

 In the images that follow, the views are labeled as coronal, sagittal, or transaxial, 

respectively, in agreement with the nomenclature advocated by the equipment manufacturer. As 

shown in Figure 4, the coronal view is defined as the plane perpendicular to the axis running 

from the X-ray source to the camera detector, with reference to the rotating table being at its 

initial position of 0 degrees. The sagittal view is at 90 o relative to the coronal view, while the 

transaxial view is effectively looking directly down from the top into the specimen. 

Coronal

Transaxial

Sagittal

Figure 4. Diagram showing coronal, sagittal, and transaxial views for the cylindrical 

mortar specimens.

9
Mortar #1 (Control) – Coronal Views 

 40 min 90 min 

 150 min 190 min 

 330 min 1140 min 

Figure 5. Coronal views of a cross-section of mortar #1 after various drying times, 

indicating instability and growth of large void near the top surface. Tile adhesive mortar 

specimens are approximately 11 mm in thickness. 

10
Mortar #1 (Control) – Sagittal Views 

 40 min 90 min 

 150 min 190 min 

 330 min 1140 min 

Figure 6. Sagittal views of a cross-section of mortar #1 after various drying times, 

indicating instability and growth of large void near the top surface. 

11
Mortar #1 (Control) – Transaxial Views 

 40 min 90 min 

 150 min 190 min 

 330 min 1140 min 

Figure 7. Transaxial views of a cross-section of mortar #1 after various drying times. 

Transaxial images are 32.5 mm x 32.5 mm. 

12
13
Mortar #2 (Coarse cement) at 30 min

Figure 8. Coronal, transaxial, and sagittal images of mortar #2 after being exposed to 

drying for 30 min. 

14
Mortar #2 (Coarse cement) at 1660 min

Figure 9. Coronal, transaxial, and sagittal images of mortar #2 after being exposed to 

drying for 1660 min. 

15
Mortar #3 (Co-thickener II) at 30 min 

Figure 10. Coronal, transaxial, and sagittal images of mortar #3 after being exposed to 

drying for 30 min. 

16
Mortar #3 (Co-thickener II) at 2475 min 

Figure 11. Coronal, transaxial, and sagittal images of mortar #3 after being exposed to 

drying for 2475 min. 

17
Mortar #4 (Air entrainer) – Coronal Views 

 30 min 90 min 

 180 min 210 min 

 300 min 390 min 

Figure 12. Coronal views of a cross-section of mortar #4 after various drying times, 

indicating instability and growth of large void near the right side. 

18
Mortar #4 (Air entrainer) – Sagittal Views 

 30 min 90 min 

 180 min 210 min 

 300 min 390 min 

Figure 13. Sagittal views of a cross-section of mortar #4 after various drying times. 

19
Mortar #4 (Air entrainer)– Transaxial Views 

 30 min 90 min 

 180 min 210 min 

 300 min 390 min 

Figure 14. Transaxial views of a cross-section of mortar #4 after various drying times. 

20
21
Mortar #5 (Co-thickener I) at 30 min 

Figure 15. Coronal, transaxial, and sagittal images of mortar #5 after being exposed to 

drying for 30 min. 

22
Mortar #5 (Co-thickener I) at 950 min

Figure 16. Coronal, transaxial, and sagittal images of mortar #5 after being exposed to 

drying for 950 min. 

23
Mortar #6 (Accelerator) at 30 min

Figure 17. Coronal, transaxial, and sagittal images of mortar #6 after being exposed to 

drying for 30 min. 

24
Mortar #6 (Accelerator) at 270 min

Figure 18. Coronal, transaxial, and sagittal images of mortar #6 after being exposed to 

drying for 270 min. 

25
Mortar #7 (Polyvinyl alcohol) at 30 min

Figure 19. Coronal, transaxial, and sagittal images of mortar #7 after being exposed to 

drying for 30 min. 

26
Mortar #7 (Polyvinyl alcohol) at 1040 min

Figure 20. Coronal, transaxial, and sagittal images of mortar #7 after being exposed to 

drying for 1040 min. 

27
Mortar #8 (Accelerator/Imbentin) at 30 min 

Figure 21. Coronal, transaxial, and sagittal images of mortar #8 after being exposed to 

drying for 30 min. 

28
Mortar #8 (Accelerator/Imbentin) at 260 min 

Figure 22. Coronal, transaxial, and sagittal images of mortar #8 after being exposed to 

drying for 260 min. 

29
Mortar #9 (Starch Ether) at 30 min 

Figure 23. Coronal, transaxial, and sagittal images of mortar #9 after being exposed to 

drying for 30 min. 

30
Mortar #9 (Starch Ether) at 1370 min 

Figure 24. Coronal, transaxial, and sagittal images of mortar #9 after being exposed to 

drying for 1370 min. 

31
5. Conclusions 

X-ray microtomography measurements have verified the occurrence of air void instability 

and growth during the drying/curing of tile adhesive mortars. Time progression images have 

clearly indicated that initially stable air voids can destabilize during the drying/particle 

movement process and can grow to several times their initial volumes. The X-ray 

microtomography measurements were used (in conjunction with isothermal calorimetry, air 

content, and open time measurements) to evaluate various mitigation strategies. Three 

potentially successful mitigation strategies were identified: 1) using a coarser cement to remove 

the mobile smaller particles, 2) incorporating an accelerator to achieve setting of the mortar prior 

to the formation of any instabilities, and 3) using a co-thickener or a polyvinyl alcohol to form a 

polymer network that prevents particle movement. Conversely, attempting to chemically 

stabilize the air void system by incorporating an air entrainer into the control mixture was not 

successful in avoiding the formation and growth of unstable voids. X-ray microtomography has 

demonstrated itself to be a valuable tool for the in situ observation of dynamic processes within 

hardening cement-based material microstructures. 

6. Acknowledgements 

 C.J. Haecker would like to thank the staff of the Inorganic Materials Group at the 

National Institute of Standards and Technology (NIST) for hosting a visit during which these 

experiments were completed. A thorough review of the report by Mr. Duane Emmett and his 

colleagues at Bostik, Inc. is also greatly appreciated. 

32
33

References 

1) Bentz, D.P., Haecker, C.-J., Peltz, M.A., and Snyder, K.A., “X-ray Absorption Studies of 

Drying of Cementitious Tile Adhesive Mortars,” Cement and Concrete Composites, 30 (5), 

361-373, 2008. 

2) Jenni, A., Holzer, L., Zurbriggen, R., and Herwegh, M., “Influence of Polymers on 

Microstructure and Adhesive Strength of Cementitious Tile Adhesive Mortars,” Cement and 

Concrete Research, 35 (1), 35-50, 2005. 

3) Oberste-Padtberg, R., and Sieksmeier, J., “Factors Influencing the Open Time of Building 

Mortars,” Drymix Mortar Yearbook, 44-49, 2007. 

4) Bentz, D.P., “Replacement of “Coarse” Cement Particles by Inert Fillers in Low w/c Ratio 

Concretes II: Experimental Validation,” Cement and Concrete Research, 35 (1), 185-188, 

2005. 

5) EN 1347, “Adhesives for tiles - Determination of wetting capability,” Comité Européen de 

Normalisation Technical Committee 67, 1999.