Experimental Procedure

Details of the X-ray environmental chamber are provided in [4]. The X-ray source consists of an X-ray tube, requiring an energy level between 20 keV and 125 keV, with a maximum current of 0.5 mA. One of five different filters may be used to separate the generated X-ray beam into two energy levels, to allow for the simultaneous assessment of density and moisture content, for example. The filtered beam is collimated and the resultant 1 mm diameter beam is passed through the specimen to be evaluated. A detector, with an NaI crystal, measures the photon count for each of 256 discrete energy channels. A 1.0 mm diameter pinhole collimator is placed directly in front of the detector. The X-ray source and detector are mounted on an xyz positioning table within the environmental chamber. The resolution of the movement along each axis is + 0.1 mm. Thus, while data can be obtained with a spatial resolution of 0.1 mm, it must be kept in mind that the "spot" size at each location is on the order of 1.0 mm. The entire system is computer controlled, so that the user may set up a grid of specimen points to be evaluated at periodic intervals.

For this preliminary study, a Type I/II ordinary portland cement, issued as Cement 133 in June 1999 by the Cement and Concrete Reference Laboratory at NIST, was used. The cement has a potential Bogue composition of 58.6 % C₃S, 14.8 % C₂S, 10.6 % C₃A, and 7.5 % C₄AF, with a Blaine fineness of about 350 m²/kg. Small (50 g to 100 g) samples of cement paste were prepared in glass beakers and mixed by hand using a spatula for two minutes. Cement pastes of w/c=0.3 and 0.45 were prepared.

The fresh cement pastes were placed in small inverted Lego² blocks (a common children's toy building block), which were either left open or sealed with a second Lego block cap. The Lego blocks were chosen as sample holders due to: 1) their low absorption of X-rays, 2) their inherent stackability (allows repeatable placement of the block within the X-ray chamber), 3) the ease with which they can be sealed by adding a cap, and 4) the ease with which they can be filled with a level volume of the viscous cement paste. The basic experimental setup, illustrated for the composite (layered) cement paste specimens to be described below in the Results and Discussion section, is shown in Fig. 1.

Figure 1: Experimental setup for the layered cement pastes experiment. Image on the left shows a horizontal view of the setup and image on the right is a vertical view.

Each block was numbered and weighed (to + 0.01 g) before the cement paste was added. The masses of the cement paste-filled blocks were determined initially and periodically throughout the exposure period. The blocks were located sequentially on a holder (an inverted Lego base element) placed at a fixed location within the X-ray chamber. After specific periods of exposure, the X-ray system was used to scan vertically (in 0.2 mm increments) a distance of 8 mm along the central y-axis of each block. In system coordinates, the bottom of the cement paste in each block is located at a y-coordinate of 28 mm so that each scan was conducted from system y-coordinates of 28 to 36. The internal dimensions of a Lego block are approximately: length = 12.5 mm (X-ray beam direction), width = 4.7 mm, and height = 8.4 mm. Including the thickness of the side walls, the center to center distance for adjacent blocks (x-direction) is 8 mm. A five second count time was used at each location to improve the signal to noise ratio. Assuming a Poisson process, the relative standard uncertainty in the sum of the counts obtained for channels 50 to 150 should be on the order of 0.4 % or 300 counts (for a sum of 70,000 counts, a typical value for the cement paste specimens).

Examples of the measured spectra for the two fresh cement pastes are provided in Fig. 2. The small peak at channel 200 is from an internal Cobalt source used for system calibration. Based on the pattern observed in Fig. 2, the sum of counts between channels 50 and 150 was selected as the dependent variable to be analyzed as being representative of the density (and water content) of the cement paste. This value was normalized by dividing it by the ratio of the counts achieved in free air at each measuring time to the counts achieved in free air for the first measuring time (to account for variability in the X-ray source). In the graphs which follow, these normalized counts have been divided by one thousand producing values generally between 70 and 100. For certain cases, the data have been further processed by subtracting the values obtained at an early age (e.g., 3 h to 5 h) from all subsequent measurements to obtain the differential density profile and highlight the changes due to drying. In Fig. 2, the denser w/c=0.3 cement paste is seen to absorb more of the X-rays as indicated by its lower count values at all channels between 50 and 150.

Figure 2: Measured spectra for fresh cement pastes.