CHARACTERIZATION OF THE HYDRATION PRODUCTS OF AN ANTIFREEZE TREATED CEMENT PASTE

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1 CHARACTERIZATION OF THE HYDRATION PRODUCTS OF AN ANTIFREEZE TREATED CEMENT PASTE Ouafi Saha 1, Mohamed Boulfiza 2, and Leon D. Wegner 3 1 Ph.D. Candidate, Department of Civil Engineering, University of Saskatchewan. ouafi.saha@usask.ca 2 Associate Professor, Department of Civil Engineering, University of Saskatchewan. moh.boulfiza@usask.ca 3 Professor, Department of Civil Engineering, University of Saskatchewan. leon.wegner@usask.ca 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 ouafi.saha@usask.ca Tel:

2 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).

3 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 (Al 2 O 3 ) powder was chosen as an internal standard. It was added and homogeneously mixed with the cement powder at the ratio of Al 2 O 3 /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.

4 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 -10 C -15 C Lable -7d+2h -14d+2h -28d+2h +6h-7d+2h +6h-14d+2h +6h-28d+2h MNC-C 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 Al 2 O 3 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 Figure 1. Optical view of the cement grains (b) Control cement grain

5 (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)

6 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 Control Reflective Element Region 1 Region 2 Avg Region 1 Region 2 Avg wt% wt% C O Na Mg Al Si S K Ca Fe 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

7 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. REFERENCES [1] American Concrete Institute. (2002). Cold weather concreting. (No. 306R-88). ACI. [2] Kurtz, F. S. (1997). Practitioner's guide to cold weather concreting. ACI International. [3] Ratinov, V. B., & Rozenberg, T. I. (1995). Antifreezing admixtures. In V. S. Ramachandran (Ed.), Concrete admixtures handbook, prop, sci, and technol (pp ) Noyes Publication. [4] Korhonen, C. J. (1990). Antifreeze admixtures for cold regions concreting: A literature review. (No. SR 90-32).U.S. Army Cold Regions Research and Engineering Laboratory. [5] Brook, J. W., Factor, D. F., Kinney, F. D., & Sarkar, A. K. (1988). Cold weather admixtures. Concrete International, 10(10), [6] Karagöl, F., Demirboğa, R., Kaygusuz, M. A., Yadollahi, M. M., & Polat, R. (2013). The influence of calcium nitrate as antifreeze admixture on the compressive strength of concrete exposed to low temperatures. Cold Regions Science and Technology, 89, [7] Arslan, M., Çullu, M., & Durmuş, G. (2011). The effect of antifreeze admixtures on compressive strength of concretes subjected to frost action. Gazi University Journal of Science, 24(2), [8] Vasović, D., Folić, R. J., & Vasović, T. (2008). Antifreeze water additives applied to fresh concrete in winter conditions. Materijali i Konstrukcije, 51(3), [9] Bullard, J. W., Jennings, H. M., Livingston, R. A., Nonat, A., Scherer, G. W., Schweitzer, J. S., Thomas, J. J. (2011). Mechanisms of cement hydration. Cement and Concrete Research, 41(12), [10] Barnes, P., & Bensted, J. (2002). Structure and performance of cements. CRC Press. [11] Aïtcin, P. (2000). Cements of yesterday and today: Concrete of tomorrow. Cement and Concrete Research, 30(9), [12] Cheung, J., Jeknavorian, A., Roberts, L., & Silva, D. (2011). Impact of admixtures on the hydration kinetics of Portland cement. Cement and Concrete Research, 41(12), [13] Ramachandran, V. S., & Beaudoin, J. J. (2000). Handbook of analytical techniques in concrete science and technology: Principles, techniques and applications. Elsevier. [14] Saha, O., Boulfiza, M., & Wegner, L. D. (accepted 2015) Developing antifreeze admixtures for mortar from available off-the-shelf admixtures. 12th North American Masonry Conference. Denver, Colorado, USA.