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CHARACTERIZATION OF THE HYDRATION
PRODUCTS OF AN ANTIFREEZE TREATED CEMENT
PASTE
Ouafi Saha1, Mohamed Boulfiza2, and Leon D. Wegner3
1
2
3
Ph.D. Candidate, Department of Civil Engineering, University of Saskatchewan. [email protected]
Associate Professor, Department of Civil Engineering, University of Saskatchewan. [email protected]
Professor, Department of Civil Engineering, University of Saskatchewan. [email protected]
ABSTRACT
The cold season in many northern regions of the globe, with temperatures below the water freezing point,
results in unfavourable curing conditions for cementitious materials. The solution given by the current
regulations and codes is to provide heat to the raw ingredients and thermal protection to the finished
product. These measures range from heating water alone at moderately low temperatures to the use of a
heated enclosure to protect the whole structure in extreme cases. Another solution to the problem of cold
weather, which has been the subject of many recent research studies, is the use of chemical additives to
improve the fresh and hardened state of the cementitious materials cured at low temperatures. Several
encouraging results have been obtained by different research groups using different types of chemical
compounds. However, the question of how these compounds act on the hydration reaction products has
not been adequately addressed so far. In this research, a proprietary antifreeze compound was tested and
showed promising results. A tentative characterization using SEM-EDS of the hydration products of a
cement paste treated with this antifreeze is presented in this paper. The results show that there is a
formation of a reflective phase of the admixture-added cement paste when cured at low temperatures. The
elemental composition of the reflective grains showed the presence of a higher sulfur and sodium content
and lower oxygen content compared to control cement paste.
Keywords: Cement paste, Hydration products, Characterization, Antifreeze admixtures, Cold weather
Ouafi Saha, Ph.D. Candidate
University of Saskatchewan
57 Campus Drive
Saskatoon, SK S7N 5A9
Canada
Email: [email protected]
Tel: 306-966-8614
1.
INTRODUCTION
Cement is one of the most, if not the most, widely used construction materials in the world, since it is the
main constituent of concrete and masonry construction materials. This is because of its many
advantageous properties. When mixed with water and aggregates, the resulting material is highly
workable, that is, plastic in its fresh state so it can fit almost any shape, and hard like a rock after a curing
period. It is readily available almost anywhere on earth, the basic raw materials being limestone, clay,
iron oxide, and sand. The final product is relatively cheap, because the cement paste plays the role of a
binder between the aggregates which constitute the largest part of the volume of the final product. It is a
durable material that requires very little maintenance and can last easily for many decades. Finally, one of
this material’s key features is its simplicity and ease of use; it does not require highly skilled workers to
handle, and it consists of just mixing the cement powder with aggregates and water.
In contrast to its simplicity is cement paste’s high complexity, considering the chemical reactions that
occur. The contact between water and cement, which itself is a combination of at least five compounds,
initiates a very complex series of reactions, most commonly referred to as “the hydration reaction,” which
evolves over time. The hydration reaction produces many new phases, including calcium silicates hydrate
and calcium hydroxide, among others, responsible for the micro- and macro-structure development of the
final product.
Similar to the majority of chemical reactions, the kinetics of the hydration reaction depend highly on the
ambient temperature; the higher the temperature, within a reasonable range, the faster the reaction. On the
other hand, when the temperature drops, the hydration reaction becomes slower, stopping completely
when no water is left in liquid form. This is what happens in many cold regions of the world. The most
common solution for this problem, if the work has to carry on, is to provide heat and protection to the
freshly cast cementitious material [1, 2]. Another possible solution proposed by many authors [3, 4, 5] is
to use chemical admixtures to allow the reaction to proceed even below the normal freezing point of
water. Various chemicals, some publicly known and others proprietary, have been tested by various
researchers, mostly on concrete, and many have shown acceptable results under several curing conditions
[6, 7, 8].
The hydration reaction itself is not simple and is still the subject of numerous research projects [9, 10].
The incorporation of chemical and mineral admixtures with the cement mix to improve its overall
properties, which is a common practice nowadays, will only increase the level of complexity of the
reactions. Understanding the active mechanisms of these admixtures is the subject of multiple debates and
speculations [11, 12]. A variety of tools and techniques can be used, depending on the targeted objective,
to explore the cementitious material and its products [13].
In this project, a commercial antifreeze admixture for concrete was tested on masonry mortar and showed
satisfactory compressive strength gain at temperatures on the order of -10 to -15°C with very little precuring [14], compressive strength development of masonry mortar samples will be shown in the result
section of this paper. Investigation of the hydration reaction was performed by exploring the phase
constituents of the cement paste treated with the antifreeze admixture at low temperatures. Results of an
X-ray powder diffraction (XRPD) analysis are not yet fully processed, hence it will not be discussed in
this paper. However, during sample preparation for the XRPD analysis, consisting of grinding the cured
cement paste to powder, an interesting observation was made. A highly reflective phase was present on
some grains of the antifreeze-added cement paste cured under low temperature, but not in the control
cement paste or in the antifreeze-added cement paste cured at room temperature. It is believed that this
phase could be one of the reaction products responsible for the strength gain achieved at below freezing
temperatures. This paper presents preliminary results of the investigation made to compare the reflective
grains to control and non-reflective ones, using optical microscope and scanning electron microscope in
the energy dispersive spectroscopy mode (SEM-EDS).
2.
MATERIALS AND METHODS
The cement used in this experimental program was the Type GU cement, which is similar to ASTM Type
I. For the XRPD analysis, corundum (Al2O3) powder was chosen as an internal standard. It was added and
homogeneously mixed with the cement powder at the ratio of Al2O3/Cement = 10%. The antifreeze
admixture used was a commercial product referred to here as MNC-C15, which comes in powder form. It
was incorporated into the mix at the maximum manufacturer recommended dosage of 4% by weight of
cement. The commercial pamphlet claims that the antifreeze admixture is based on naphthalene, but the
elemental and XRPD analysis showed that it is rather based on sodium nitrite. Tap water was used at the
ratio of w/c = 0.5. This choice was guided by the uniformity between the control and the admixture-added
pastes. Whereas the control cement paste had just an acceptable workability, the admixture-added paste
was very sloppy, probably because the admixture contains some plasticizer.
The cement, corundum, and eventually the admixture were mixed first in their powder form at the
specified proportions by shaking them in a closed container for three minutes. The mixed powder was
then transferred to a bowl and mixed again with a hand beater for 2 more minutes. Finally, the water was
added in two stages at ¾ and ¼ quantity while mixing continued for about 3 minutes. A small quantity of
the paste, about 10 grams, was then poured into a capped plastic cylindrical vial of dimensions 27 x 80
mm, and transferred to the appropriate curing space. Two curing temperatures were chosen, room
temperature, and deep freeze temperature of -10°C. Four samples were prepared for each age: two types
of pastes (control and antifreeze added) combined with two temperatures (+22° and -10°). Although
samples were left to cure for 1, 3, 7, 14, and 28 days, only two ages are presented and discussed in this
paper: the control sample at 3 days and the admixture added sample at 28 days.
At the specified ages, the samples were removed from their curing space, and a procedure to stop the
hydration was applied, depending on the sample’s age. The freeze drying technique was used for samples
of ages less than 3 days, and the solvent exchange technique for samples older than 3 days. A VirTis
Genesis 35 EL model freeze dryer was used in automated mode with a procedure that cycles through
thermal treatment, vacuum and condenser cooling, and the sublimation drying cycle. The principle of the
freeze drying technique is to cool the material to a very low temperature, then to vacuum the air to a very
low pressure. This will put the water below the triple point of the phase diagram. Then, when the
temperature is raised, the frozen water turns into vapor by sublimation without passing through the liquid
phase. The solvent exchange technique consisted of immersing the thin layer (wafer) of cement paste into
the isopropyl alcohol for two hours and exchanging the alcohol every two hours three times. The samples
were then wet crushed (while immersed in isopropyl) with a ceramic mortar and pestle to the desired
fineness. It is during this grinding process that the reflective crystals were observed. The powder was then
washed twice with diethyl ether before putting it in sealed vials in a desiccator for conservation. The
desiccator contained silica gel and soda lime to reduce the humidity and carbon dioxide. It is important to
note that the grinding of the freeze dried samples was not done in wet conditions, and some coarse grains
(on the order of 1 mm) were conserved in isopropyl alcohol for the SEM analysis.
Three types of grains were selected: control cement paste grains cured at room temperature for three days,
and two subsets of grains (high reflectivity and low reflectivity) from the antifreeze-added cement paste
cured at -10°C for 28 days. The choice of control sample at three days was justified by the relative ease of
crushing the samples at that age, and the admixture-added at 28 days because the rate of observed
reflective grains was much higher than at earlier ages. No specific preparation was applied to the grains
before inspecting them under the optical or electron microscopes. The only preparation was to separate
the high-reflective from low-reflective grains. The samples were mounted on a double sided adhesive and
conductive tape for the SEM analysis. A Reichert Stereo Star Zoom optical microscope model 570 with a
magnification factor reaching up to 126 times, and a Hitachi electron microscope model SU6600
controlled by AztecEnergy EDS software were used. Secondary electron pictures showing the topography
of randomly selected regions of the grain surfaces were taken before performing EDS scans, referred to as
maps, on smaller areas within each region.
3.
RESULTS AND DISCUSSION
In a previous work [14], a masonry mortar treated with the commercial admixture MNC-C15 was
optimized and tested for its compressive strength after being cured at two low temperatures of -10°C and
-15°C. For this particular test the mortar was prepared with a Type S mortar cement, where w/c=0.45 and
sand/cement=2.75. The mixed mortar was cast in the small sized (5 x 10 cm) plastic cylinders and
transferred immediately to the freezing chamber for the temperature of -10°C and after 6 hours for the
temperature of -15°C. The samples were extracted from the freezing chamber two hours before the
strength test is carried out to allow them to thaw. Table 1 shows an excerpt of the compressive strength
results obtained for both cases. It shows a clear strength gain with time, and an acceptable compressive
strength at 28 days.
Table 1. Compressive strength (MPa) of masonry mortar at -10 and -15°C
Lable
MNC-C15
-7d+2h
5.83
-10°C
-14d+2h
8.56
-28d+2h
9.00
+6h-7d+2h
6.15
-15°C
+6h-14d+2h
+6h-28d+2h
8.23
8.99
To reduce the number of parameters that may have been responsible for the creation of the reflecting
phase, a few batches were mixed without the Al2O3 and at both temperatures. The only case that produced
the reflecting surface was when the cement was mixed with the antifreeze admixture and cured at a low
temperature. This eliminated the possible effect of a reaction between the aluminum oxide and the
admixture, and also showed that the shiny surface was promoted at low temperature. This last observation
needs more investigation to explain why the reaction did not produce the reflective phase at room
temperature.
As seen in Figure 1(a) the size of selected grains was not greater than 1 mm. Here, C refers to the control
cement grain, NR to the non-reflective grain, and R to the reflective grain. The three other figures show
the same grains individually with a magnification factor of approximately 100 times. Figure 1(b) shows
the magnified control cement grain cured at room temperature for three days; it appears completely
opaque. Figure 1(c) shows a grain from the admixture-treated cement cured at low temperature for 28
days but displaying only low reflectivity. Finally, Figure 1(d) shows a high-reflectivity grain selected
from the same batch as the previous grain. The optical micrographs confirm the naked eye observation of
a high reflectivity of a certain portion of the antifreeze-added cement paste cured at -10°C.
(a) Actual size of the grains on a 50 mm wood block
(b) Control cement grain
Figure 1. Optical view of the cement grains
(c) Low-reflection admixture-added grain
(d) High-reflection admixture-added grain
Figure 1. Optical view of the cement grains
Figure 2 shows the secondary electron image of two regions, one from the control grain and the other
from the highly reflective grain with a magnification factor of 2000 times. Along with the secondary
electron images, three random sub-regions were selected and probed for their elemental composition
using the EDS capability of the SEM equipment (Figure 3). From the secondary electron 3D rendered
images, not much can be inferred, except a slightly larger and smoother particle size of the highly
reflective region (b) compared to the control specimen (a). Micrographs from a larger number of regions
in each case is required before drawing any final conclusion.
(a) Control
(b) high reflectivity
Figure 2. Secondary electron image of random regions with EDS scanned maps
Figure 3. Sample of the EDS spectra of a sub-region (map)
The elemental weight percent results collected from the EDS spectra were averaged and are compiled in
Table 2 for each region and type of sample. It is clearly seen that sodium is present in the reflective
sample, especially Region 2, and practically absent from the control. This corresponds to the main
constituent of the admixture, which is thought to be based on sodium nitrite. The other observation is that
oxygen seems to be lower in the admixture-treated sample, whereas calcium is slightly higher for the
same sample compared to the control one. Another element of note is the sulfur, which was three times
higher for Region 2 of the reflective grain than for any other region. These results will be used to inform a
more precise study and in combination with other characterization techniques, in an attempt to identify
the reaction triggered by the incorporation of this admixture.
Table 2. EDS results of four regions
Element
C
O
Na
Mg
Al
Si
S
K
Ca
Fe
4.
Region 1
Control
Region 2
17.85
45.95
0.05
0.85
0.95
4.80
0.70
0.35
27.90
0.55
17.80
42.43
0.00
1.93
2.60
4.60
1.00
0.40
26.40
2.83
Avg
wt%
17.83
44.19
0.03
1.39
1.78
4.70
0.85
0.38
27.15
1.69
Region 1
Reflective
Region 2
18.90
40.63
0.57
0.93
1.50
2.07
0.93
0.10
33.83
0.53
18.80
35.80
1.03
1.00
2.53
4.57
3.10
0.70
30.73
1.67
Avg
wt%
18.85
38.22
0.80
0.97
2.02
3.32
2.02
0.40
32.28
1.10
CONCLUSION
The use of antifreeze admixtures to partially overcome the adverse effect of low temperatures on cement
hydration was satisfactorily tested in a previous work [14]. An attempt to understand part of the antifreeze
admixture action on the cement paste was initiated in this current work. Two techniques were used to
explore the hydration results, optical microscope and scanning electron microscope in the energy
dispersive spectroscopy mode.
The tests revealed the formation of a reflective phase in the antifreeze-added cement paste when cured at
a temperature of -10C. The reflective phase did not appear in the same antifreeze-added mix when cured
at room temperature, suggesting the occurrence of a specific, yet unknown, reaction at low temperatures.
Secondary electron imaging showed relatively larger and smoother particles in the antifreeze-added
cement paste compared to the control one. No conclusion can be drawn from this, but more imaging at
lower magnifications has yet to be conducted
The elemental composition of the grain surfaces using the SEM-EDS technique did not show a clear
trend, except that sulfur and sodium contents were higher in the admixture reflective grain case. One
possible reason for this uncertainty could be the high magnification, which led to a probed area of 10 x 10
microns. Lower magnifications will be used for future experiments. A second reason could be that a nonreflective spot was probed, since it is not possible to “see” the reflective surface with the SEM. Scans of a
larger number of regions, the use of polished samples, and Electron Probe Micro Analysis (EPMA) for
higher accuracy may provide more definitive results.
ACKNOWLEDGMENTS
This research was funded by the Saskatchewan Masonry Institute through the Saskatchewan Centre for
Masonry Design, by the Natural Sciences and Engineering Research Council of Canada, and by the
Canada Masonry Design Centre.
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