APPLICATION OF 2-DIMENSIONAL XRD FOR THE CHARACTERIZA- TION OF THE MICROSTRUCTURE OF SELF-LEVELING COMPOUNDS (SLC)

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1 1 APPLICATION OF 2-DIMENSIONAL XRD FOR THE CHARACTERIZA- TION OF THE MICROSTRUCTURE OF SELF-LEVELING COMPOUNDS (SLC) Severin Seifert (1), Juergen Neubauer (1), Friedlinde Goetz-Neunhoeffer (1), and Hubert Motzet (2) (1) Mineralogy, University of Erlangen-Nuremberg Schlossgarten 5a, Erlangen, Germany (2) Schoenox GmbH, Alfred-Nobel-Strasse 6, Rosendahl, Germany ABSTRACT The 2-dimensional XRD (GADDS) was used to characterize the microstructure of an applied self-leveling compound (SLC). A calcium aluminate cement based SLC was prepared on two different substrates (water absorbent and non-water absorbent) to determine the vertical distribution of the crystalline phases. The application of the GADDS enables the detection of the phase composition of the hydrating mortar in horizontal slices. Thus the analysis could be carried out in position-sensitive mode at three different areas: near bottom, in the center, and at the top of the mortar. For investigation of SLCs from the very early hydration stage up to 10 hours of hydration, a custom-made in-situ sample holder for the measurements was designed and constructed. The combination of the GADDS and the custom-made in-situ sample holder provides the possibility to characterize additionally the time-dependent phase composition within the SLC. The non-absorbent substrate has no effect on the hydration of the binder phases but the absorbent substrate influences the formation of ettringite. In the top layer of the SLC the ettringite content is reduced during the first hours of hydration. The absorbing forces of the substrate lead to migration of the mix water to the substrate. This lack of water results in reduced formation of ettringite. INTRODUCTION Due to the great number of old buildings that need refurbishment the request of self-leveling compounds (SLC) has strongly increased during the last years. SLC s are dry mortar systems, which are applied on uneven floors to produce a flat and smooth ground surface for final flooring (parquet, carpet, linoleum, tiles, etc.)[1]. The application of the SLC is carried out in thin layers between 1 and 20 mm on different type of substrates (e.g. concrete, screed, wood [2,3]). Furthermore the requirements of such a dry mortar system are very high, concerning the workability as well as the final properties. These properties are self-leveling, low viscosity, fast setting, rapid hardening, rapid strength gain, durability, dimensional stability, rapid drying, surface strength,

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3 2 and adhesion to the substrate [3]. To fulfill these requirements the SLC contains more than 10 different components. Most of the SLC formulations based on a ternary binder system consist of Portland cement (OPC), calcium aluminate cement (CAC), and calcium sulfates (in general bassanite CaSO H 2 O). Additionally, mineral fillers such as limestone flour and quartz sand as well as different organic and inorganic additives are added to the dry mix to provide the desired properties [4,5]. The presence of the ternary binder system leads to the formation of ettringite (Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 32 H 2 O) as major hydrate phase during the hydration process. Since ettringite consists of nearly 50 wt.-% of water, the formation of ettringite strongly depends on the availability of the mix water. The application of the SLC in thin layers ( 1 mm) on an area of several square meters causes a very high ratio of surface to mortar-volume. This may result in rapid desiccation of the mortar due to evaporation of water at the top surface [6]. Otherwise depending on the properties of the substrate, the water may also be absorbed by the ground surface. The lack of water will result in a decrease of the ettringite content, which will affect the early strength of the SLC. With respect to that background a commercial CAC based SLC was chosen, to investigate the time-dependent crystalline phase distribution at the top, in the center, and near the bottom of the applied mortar. For this reason the SLC was measured from the very early hydration stage up to 10 hours of hydration by an in-situ technique. The water-absorbing properties of the substrate are simulated by different materials (PVC and unglazed ceramic tile). METHODS AND MATERIALS The vertical distribution of the hydrated phases in a mortar was already investigated by different analytical methods [5,7]. However the vertical resolution of the mortar was achieved by mechanical ablation of a thin layer from the hardened mortar. Furthermore, for investigations with time resolution, the ongoing hydration reaction has to be stopped. Both mechanical ablation and stopping of the ongoing reaction may destroy the crystal structures of hydrated phases, especially the sensitive ettringite. This may result in the investigation of artifacts. For this reason the investigation by in-situ X-ray diffraction was chosen which is already proven by other research works [8]. The in-situ X-ray diffraction analysis offers the possibility to observe the ongoing formation of ettringite or any other crystalline hydrate phases without interrupting the hydration. For the time-resolved investigation of the SLC combined with a good vertical resolution from top to the bottom, we choose the General Area Detector Diffraction System (GADDS) for the measurement. The GADDS [9] equipped with crossed-coupled Goebel mirrors primary X-ray optics and pinhole collimator [10] enables the detection of small areas of the sample. The 2-dimensional Hi-Star detector covers a wide range of 2-theta which allows reduction of the measurement time. The second dimension of the detector covers the chi angle which benefits the fast detection. Additionally, due to the detection of the chi angle the measurement is independent from a potential preferred orientation of the phases. For sample preparation some require-

4 3 ments are necessary. To analyze the vertical phase distribution of the hydrating mortar in horizontal slices a view of the cross section of the SLC is needed. Furthermore due to the small incident angle of the X-ray beam we need a smooth surface for the investigation, which should not be achieved by mechanical preparation. The work under in-situ conditions requires a special sample holder for the investigations in order to observe the sample over a longer hydration period. Therefore we designed and constructed a custom-made sample holder which fits the requirements (Fig. 1). The stage of the sample holder is made of brass. All parts, which are in contact with the SLC, are made of Teflon. The base of the sample holder, which replaces the substrate, is exchangeable, so as to be able to simulate different properties of the substrates (absorbent or nonabsorbent). The cross section (50 x 10 mm 2 ) through the SLC is covered by a 7 µm Kapton film that has proven itself in in-situ X-ray investigations [8]. The absorption of the X-ray beam is only weak. Fig. 1: Custom-made in-situ sample holder with X-ray permeable Kapton film and exchangeable substrate. With this custom-made sample holder, the SLC can be mixed outside and introduced into the sample holder by pouring from the top. Consequently, mixing and placing of the SLC can perform in a manner conforming as closely as possible to the technical conditions. For the measurements a conventional CAC based SLC was used. The dry mortar was mixed with a predefined amount of water (w/s-value=0.22) and subsequently poured (10 mm thick) into the in-situ sample holder. After the insertion of the sample holder into the GADDS the measurement was started. To simulate the different properties of the substrate two different materials were chosen. The first set of measurements was taken with a non-absorbent substrate (PVC) and for the second measurements an absorbent substrate (unglazed ceramic tile) was used.

5 4 Generally, the investigations were carried out at the level of three different layers of the SLC (Table 1). The thickness of the layers is determined by the diameter of the pinhole collimator (800 µm). Table 1: Levels of the three measured layers of the SLC Layer Bottom Center Top Distance from bottom mm mm mm Each layer was measured over an hydration period of 10 hours up to 40 times whereas the measurement time of each single scan being 5 minutes. During a single scan of each layer, the sample was moved horizontally so that the SLC was analyzed over the full width of 50 mm. The resulting sequence of the measurement was always: bottom, center, top. Thus the hydration of the SLC was recorded for each layer with a time resolution of 15 min. The parameters used (Table 2) for the investigations are defined by the 2-theta range of the main hydrate phase ettringite. The main reflection of ettringite is located by approx. 9 2-theta. This requires the start of the detection angle range by 7 2-theta and an incident X-ray beam (Omega) of 5 respectively. Table 2: X-ray parameter for the GADDS investigations Parameter Value X-ray source CuK α Voltage 40 kv Current 40 ma Collimator pinhole 800 µm Detection angel (2θ) 7 to 41 Detection angle (Chi) -30 to 30 Incident X-ray beam (Omega) 5 The resulting diffraction pattern (frame) of each single scan was integrated over the chi angle to generate conventional X-ray patterns. For better evaluation of the measurement, the 40 recorded diffraction patterns from 0 h to 10 h hydration are summarized for each layer in level plots (Fig. 2). In these level plots the time of hydration is plotted over 2-theta and the intensities of the diffracted lines are given in color scale. Thus the level plot shows an overview of all 40 X-ray patterns and the consumption as well as the formation of phases is visible as changes in color scale. RESULTS AND DISCUSSION The level plot (Fig. 2) shows clearly the hydration process of the SLC. After addition of the mix water the calcium sulfate phase bassanite starts to dissolve and afterwards the crystallization of the main hydrate phase ettringite begins. Both the reflections of ettringite (100), (110), (104),

6 5 (114), and (216) and the reflection of bassanite (200) are clearly visible. Due to the dominance of the main reflection (104) of calcite (fine aggregate) at theta the reflections of the OPC as well as the CAC phases can hardly be distinguished from the background signal. Only the main reflection (606 ) of the OPC phase C 3 S (Ca 3 SiO 5 ) is observable. From this level plot it is also seen that there are no changes in the intensity of the calcite reflections over the time of measurement. Calcite is a widely inert aggregate which is added to the formulation of SLC as filler material. Fig. 2: Level plot of SLC on non-absorbent ground at the bottom from 0 h to 10 h (the intensity is given in color scale). For better comparison of the X-ray patterns from bottom, center, and top layers of the SLC, the regions of interest of the level plots are enlarged (Fig. 3 and Fig. 4). The comparison of the three layers of the SLC prepared on the non-absorbent substrate shows no significant qualitative difference in the phase composition (Fig. 3). The phase development in each layer over the entire measurement time from 0 h to 10 h is the same. The hydration process starts right after the addition of the mix water with the dissolution of bassanite. After a short time the crystallization of ettringite begins in all layers and bassanite has been entirely used up. The intensity of ettringite reflections still increases up to 3.0 h till 3.4 h after contact with water. Subsequently, the phase content of ettringite shows no further change during the whole remaining period up to the elapse of 10 h of hydration. The comparison of the enlarged level plots (Fig. 4) from bottom, center, and top layers prepared on the absorbent substrate show significant differences in the phase development of the SLC. Mainly, the top layer of the mortar shows differences in the dissolution of bassanite as well as the formation of ettringite.

7 6 Fig. 3: Details of the level plots of bottom, center, and top section of a SLC on non-absorbent ground. Fig. 4: Details of the level plot of bottom, center, and top section of a SLC on absorbent ground. Also in this case the hydration process starts in all three layers right after the addition of water. However in contrast to the preparation of the SLC on the non-absorbent substrate, the crystallization of ettringite is stopped after a short time in the top layer. But at the bottom and in the center layer the crystallization of ettringite is going on. Generally, the intensity of ettringite in the top layer is very low, whereas the intensity of calcite is similar to that of the other two layers. Compared to the top layer, the intensities of the ettringite reflections in the center and the bottom layer

8 7 are higher at any given time in the measured period of hydration. The diffraction patterns show no preferred orientation of any phase. Therefore intensities of the reflections could be used for phase quantification purposes. These results lead us to assume that after 10 h of hydration the content of ettringite in the top layer is lower than in the other two layers. Furthermore the dissolution of bassanite is different in bottom, center, and top layer. In the bottom layer bassanite is completely dissolved 1.9 h after contact with water. But in the center and in the top layer the dissolution of bassanite ceases after 1.5 h. In both layers small residues of bassanite continue to be present during the whole period of measurement time. The presented results indicate some differences in the phase development of SLC s for different substrates. From comparison of the level plots, it is evident that there are no differences in the phase content measured by the GADDS technique within the SLC on the non-absorbent substrate after 10 h. The dissolution time of bassanite and the formation time of ettringite are the same in each layer. This indicates that the hydration of the clinker phases proceeds homogeneously throughout the SLC on non-absorbent substrate. However the application of the SLC on an absorbent substrate strongly effects the hydration. The formation of ettringite in the top layer compared to the bottom and center layer is clearly reduced. Also the ongoing dissolution of bassanite is different in each layer. With these results a conception of the hydration of an applied SLC is possible. The application of SLC on a non-absorbent substrate has no influence on the hydration of the SLC. Due to the use of appropriate water-retaining additives, the effect of evaporation of mix water from the top surface of the SLC is not significant. Thus hydration of the applied mortar proceeds homogeneously. But on an absorbent substrate there is a strong effect on the hydration of a SLC. The absorbing forces of the absorbent substrate withdraw the water from the SLC, resulting in water migration from the top to the bottom of the applied mortar (Fig. 5). Fig. 5: Sketch of a cross section of a self-leveling compound (SLC) on absorbent ground.

9 8 The reduced water content especially in the top layer causes a decrease of the formation of ettringite. The resulting deficiency of ettringite in the top surface region of the mortar may then decrease surface strength of the SLC during the first hours of hydration. The final strength might not be reduced, because later calcium silicate hydration may overcome this weakness. CONCLUSION The accomplished investigations show that the GADDS will be an appropriate method to characterize the microstructure of a SLC. The combination of the GADDS with a custom-made in-situ sample holder provides the possibility to determine a time-resolved phase development within an applied mortar. Additionally, the nondestructive preparation of the SLC prevented the investigation of artifacts. Finally, through the application of the GADDS technique new findings about the hydration of a SLC on different substrates could be obtained. In future, the presented method will be worked out with a real quantitative approach. REFERENCES [1] Winter, C., Plank, J., ZKG International, 2007, 6, [2] Harbron, R., Proc. of International Conference on Calcium Aluminate Cements 2001, Edinburgh, 2001, [3] Motzet, H., Proc. of First International Drymix Conference idmmc one, Nuremberg, 2007, [4] Anderberg, A., Doctoral Dissertation, Division of Building Materials, Lund Institute of Technology, Lund University, Lund, [5] De Gasparo, A., Unpubl. PhD Thesis, University of Bern, [6] Herwegh, M., Zurbriggen, R., Scrivener, K., De Gasparo, A., Kighelman, J., Jenni, A., Proc. of 16 th IBAUSIL, Weimar, 2006, [7] Jenni, A., Holzer, R., Zurbriggen, R., Herwegh, M., Cem. Concr. Res., 2005, 35, [8] Neubauer, J., Goetz-Neunhoeffer, F., Holland, U., Schmitt, D., Proc. of the 26 th International Conference on Cement Microscopy, San Antonio, 2004, on CD Rom. [9] He, B.B., Preckwinkel, U., Smith, K.L., Advances in X-ray Analysis, 2000, 43, [10] He, B.B., Preckwinkel, U., Advances in X-ray Analysis, 2002, 45,