SULFATE ATTACK OF PORTLAND CEMENT STUDIED BY X-RAY MICROTOMOGRAPHY (MICRO-CT)

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1 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume SULFATE ATTACK OF PORTLAND CEMENT STUDIED BY X-RAY MICROTOMOGRAPHY (MICRO-CT) S.R. Stock 1,*, K. Ignatiev 1, A.P. Wilkinson 2, N. Naik 3 and K.E. Kurtis 3 Schools of 1 Materials Sci. & Eng., 2 Chemistry & Biochemistry and 3 Civil & Environmental Eng., Georgia Inst. of Technology, Atlanta, GA, USA * On leave at Inst. for Bioengineering & Nanoscience in Advanced Medicine, Northwestern Univ., Chicago, IL, USA ABSTRACT Attack of Portland cement by ambient sulfates presents a considerable risk to structures in some environments. In this study, x-ray microtomography, a high resolution variant of medical CT, noninvasively mapped the progression of damage in Portland cement resulting from exposure to an aqueous solution of Na 2 SO 4 (sodium ion concentration of 10,000 ppm). Cement paste samples from water-to-cement ratios w/c = 0.45, 0.50 and 0.60 and exposures between 0 and 120 days were examined. Microtomography revealed cracking that would have been attributed to sample preparation during sectioning of the friable material and allowed damage to be followed in individual specimens. The greater w/c, the more rapidly damage progressed, and spalling originated at the corners of the 10 mm diameter, 40 mm long cylindrical samples. INTRODUCTION Sulfate ions present in soil, groundwater, seawater, decaying organic matter, acid rain and industrial effluent adversely affect the long-term durability of concrete, and severe sulfate exposure conditions are not uncommon in North America [1] and worldwide [2]. Evaporationrelated concentration of sulfate ions can produce damage unexpectedly [3]. Sulfate attack on the hardened cement paste in concrete manifests itself in the form of cracking, spalling, increased permeability and strength loss. Limited understanding of sulfate attack reaction kinetics and damage mechanisms in Portland cement [4] hinders rational design for long-term durability and affects safety (concrete containment vessels) as well as economics/environmental sustainability. Concrete (as well as cement paste) specimens vary greatly, even within the volume of a single specimen. Enormous advantages accrue if individual samples can be examined nondestructively multiple times during the course of environmental attack. Very high resolution computed tomography (i.e., microtomography) can image the internal structure of optically opaque samples with spatial resolution akin to that of optical microscopy. In brief, a series of views through the sample (i.e., radiographs taken along different directions) are recombined mathematically into a cross-sectional map of the specimen s x-ray absorptivity; microtomography instrumentation and earlier work are reviewed elsewhere [5]. This paper describes microtomography data from a preliminary study of Type I cement paste samples subjected to sulfate attack.

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3 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume MATERIALS AND METHODS Cylindrical samples (10 mm diameter, 40 mm length) were prepared with water-to-cement ratios w/c = 0.45, 0.50 and 0.60 (by weight) of Type I cement (ASTM C ). Chemical analysis revealed that the samples consisted of (in wt %): 65.6 CaO, 19.8 SiO 2, 5.4 Al 2 O 3, 3.0 SO 3, 2.7 Fe 2 O 3, 1.3 volatile components, other constituents each less than 1.0 [6]. The cement pastes were cast in a plastic mold for 24 h and, after removal from the mold, were cured in a lime bath for 3 d or longer (up to 8 d in some cases). After rinsing, the samples were placed in a ppm sodium ion solution (Na 2 SO 4 ), and, after each increment of exposure, the progress of sulfate attack was monitored visually and by microtomography using a Scanco MicroCT-20 or a MicroCT-40 system. Sampling was with 500 projections of 0.35 s integration time, 1024 samples/projection, 70 kvp x-rays, a 20 µm in-plane voxel (volume element) size and a 30 µm slice thickness. RESULTS AND DISCUSSION Four reconstructed slices are shown in Fig. 1, with the darker pixels representing lower values of the attenuation coefficient, i.e., lower absorptivity. The small dark circular images are casting porosity, and the thin concentric circles in each slice are reconstruction artefacts typical of many microtomography systems and are the result of slight, uncorrected variations in the response of the different detector elements. The upper left image of Fig. 1 is of a 0.50 w/c sample containing chunks of quartz crystal (dark block-like images) which serve as an idealized aggregate. In Fig. 1, upper left, the slightly darker ring just inside the edge sample may be related to the sulfate attack; such contrast is observed only after sulfate exposure, and cracks formed through sulfate exposure often follow this ring. Whether the circumferential crack at the bottom of Fig. 1, upper left, is related to sulfate attack or to casting cannot be established definitively because data was not collected from this portion of the sample prior to sulfate exposure. It is interesting to note the presence of radial cracks extending from the exterior edge of the sample through the paste and along the quartzpaste interface. Such interfaces are regions of relative weakness in concrete, and these cracks appear to originate due to the shrinkage of the cement paste adjacent to the rigid quartz blocks. The other three images of Fig. 1 (lower left and both right-hand) are from different positions of the same 0.60 w/c sample and show the gradient of sulfate damage which is greatest near the ends of the cylindrical samples. Several concentric circumferential cracks have been produced, and severe spalling has occurred near the end of the cylinder (upper right) while no cracks are visible midway between the ends of the cylinder (lower left). Figure 2 shows 3-D renderings of two samples after different sulfate exposures (left, w/c = 0.50 after 78 days; center and right, top and middle of a w/c = 0.6 cylindrical sample after 45 days). The 0.6 w/c samples is the same as that shown in Fig. 1. Cracking is much more severe for the w/c = 0.6 sample, even for exposures times which are only 60% of that of the w/c = 0.5 specimen. Cracking led to severe spalling at the ends of the 0.6 w/c sample, and several layers of material appear to been removed, rounding the end of the cylinder. The surface midway between the cylinder ends shows severe cracking, but significant spalling has not yet resulted.

4 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Figure 3 shows renderings of a 0.5 and a 0.6 w/c specimen cut by sampling planes parallel to and perpendicular to the cylinders axes. The upper left image of Fig. 3 shows renderings of the same 0.5 w/c sample shown in Fig. 2. The other three images of Fig. 3 are of the same 0.6 w/c sample (different from that shown in Fig. 1 and 2) cut along different planes to show the extent and geometry of subsurface cracking. Detailed interrogation of the entire 3-D data sets revealed that the subsurface cracks are not linked to the surface. Microtomography data collected from these specimens at shorter exposure times show no evidence of these cracks, implying that the cracks originate with the sulfate damage and not from other sources such as shrinkage during Fig. 1. Reconstructed microtomography slices of the 10 mm diameter cement paste samples. Upper left, 0.50 w/c sample containing chunks of quartz crystal, 74 days sulfate exposure. The other three slices are from the same 0.60 w/c sample, 45 days sulfate exposure. Upper right, slice near one end of the cylinder. Lower right slice 3.25 mm from the upper right slice. Lower left, slice mm from the upper right slice.

5 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Fig. 2. Microtomography-derived 3-D rendering of cement paste samples after sulfate exposure. Left, w/c = 0.50 after 78 days; center and right, top and middle of a w/c = 0.6 cylindrical sample after 45 days exposure. Fig. 3. Top left, the same w/c 0.5 sample shown in Fig. 2 (78 days exposure). Spherical casting pores are prominent in the numerical section through the cylinder. The other three images show different sectioning planes through a single 0.6 w/c sample (45 days exposure); this specimen is different than that shown in Fig. 2. Several ring-like reconstruction artefacts are visible on the top surface of the lower left image.

6 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume curing. The data at earlier times, however, is less complete one would like, and definitive attribution of the subsurface cracks to sulfate damage must await results of further testing. It is always possible that the cracks were already present, albeit too tightly closed to be seen with microtomography. If the detection limit for a crack is taken at an opening one-quarter the voxel size, a value dictated by previous experience with high contrast features such as cracks, openings smaller than about 5 µm in the plane of reconstruction and 8 µm perpendicular to this plane would result in the crack being invisible. Without microtomographic imaging, however, no information on subsurface cracking would be obtained until the specimen was sacrificed. It is interesting to note that the cracks leading from the top of the cylinders follow different paths for the 0.5 and 0.6 w/c samples shown in Fig. 3. In the former case, the crack grew inwards from points near the edge of the top surface while the latter crack grew toward the outside of the cylinder from points closer to the cylinder s axis. CONCLUSIONS For cement paste samples with 0.45 = w/c = 0.60, x-ray microtomography revealed the greater w/c, the more rapidly damage progressed in a solution of 10,000 ppm sodium ions (Na 2 SO 4 ). Damage began with cracking and progressed to severe spalling. The cracks appeared to initiate most rapidly at the corners of the 10 mm diameter, 40 mm long cylindrical samples, and the spalling proceeded most rapidly near the outer surfaces at the end of the cylinders. ACKNOWLEDGMENTS The research was supported by NSF CMS grant , and data was collected at the Georgia Tech Microtomography Facility supported by NSF BES grant REFERENCES 1. F. Lea, Lea's Chemistry of Cement and Concrete (John Wiley & Sons, New York, 1998). 2. Y-S. Park, J-K. Suh, J-H. Lee et al., Cem Concr Res 29 (1999) M. D. Cohen, B. Mather, ACI J Mater 88 (1991) P. K. Mehta, in Mater Sci Concr III (Amer Cer Soc, 1992) pp S.R. Stock, Int Mater Rev 44 (1999) Report , Wyoming Analytical Lab Inc., Golden CO.