Determination of Aggregate Surface Morphology at the Interfacial Transition Zone (ITZ)

We present a protocol to investigate the effect of aggregate surface morphology on the formation of individual transitions zones in cement-based materials. It combines the experimental method with data processing method to illustrate the effect of aggregate surface roughness on ITZ formation. Begin by molding the model concrete.

Weigh 1, 000 grams of cement and 350 grams of water with an electronic balance, and wipe the five liter mixing pot with a wet towel to moisten it. Add the water and cement to the pot, place it on the mixer and raise it to the stirring position. Mix at 65 rpm for 90 seconds and let the mixture sit still for 30 seconds.

Scrape in the paste of the inner wall of the pot. Then mix at 130 rpm for another 60 seconds. Remove the pot from the mixer and put the ceramic particle into the paste, then thoroughly mix it with the cement paste by hand.

Half-fill the mold with the fresh cement paste, place the ceramic particle on top of the paste and fill the rest of the mold with the paste. Wipe off excess cement paste with a scraper knife and vibrate the mold on a vibrating table for one minute. Seal the mold surface with cling film to prevent moisture evaporation.

Cure the specimen in a curing room for 24 hours then remove the specimen from the mold and cure for 28 days under the same environmental conditions. Scan the specimen with x-ray computed tomography to obtain a stack of slices and roughly choose a slice where the ceramic particle appears to be largest. Fit the boundary of the ceramic particle with the circle and determine the center of the circle as the geometric center of the particle.

Use a cutting machine to cut the specimen into two parts through the geometric center of the ceramic particle. Then immerse the two parts into isopropyl alcohol for three days to remove unbounded water and terminate internal hydration. Making sure to replace the isopropyl solution every 24 hours.

Dry the two parts in a vacuum drying oven for seven days at a temperature of 40 degrees Celsius. To solidify the microstructure, use a finger to smear the inner surface of two cylindrical plastic molds with demolding paste. Place a piece of the sample into each mold with the surface to be examined facing downward.

In a paper cup, weigh 50 grams of low-viscosity epoxy resin, add five grams of hardener, and manually stir the mixture for two minutes. Place the mold into the cold mounting machine along with the paper cup. Start the vacuum on the machine and pour the epoxy resin into the mold until it merges with each sample.

Keep the mold in the machine for 24 hours to harder the epoxy resin. On the next day, remove the bottom of each mold and squeeze out the sample. Store it in a vacuum drying oven.

When ready, grind the sample with silicon carbide paper and alcohol on an automatic polishing machine as described in the text manuscript. Then attach the flannelette to the turn table of the machine and polish the sample with diamond paste of three, one and 0.25 micrometers at the speed of 150 rpm. Remove the debris in an ultrasonic cleaner with alcohol after each grinding and polishing step.

When finished, store each sample in a plastic box with the surface to be examined facing up and keep the boxes in a vacuum dry oven. In a vacuum environment, spray a thin layer of gold foil on the surface to be examined with an automatic sputter coater. Place a strip of adhesive tape on the side of the sample to connect the testing surface and opposite surface, then position the sample on the test bench with the testing surface facing upward.

Move the sample to focus on region one then vacuum the SEM and change to backscattered electron mode. Set the magnification to 1000X and carefully adjust the brightness and contrast. Move the lens along the direction of the aggregate boundary to another position and take another image.

Obtain at least 15 images for statistical analysis. Then repeat the imaging process on regions two and three. After imaging, use Image J to pretreat the images with a best fit and three by three median filter three times to reduce the noise and enhance the boundary of different phases.

Manually capture the boundary of the ceramic particle and cut this part from the original image. Roughly determine the upper threshold value of pore phases by setting different threshold values and segmenting the image to compare with the original one. Obtain the gray-scale distribution of the remaining part of the image.

Choose two approximately linear parts of the curve, just above the predetermined upper threshold value and fit these two parts with the linear curve. The intersection point will be set at the exact upper threshold value of this image. Use this value to perform segmentation and compare the binary image with the original gray-scale image for final threshold value determination.

Then convert the gray-scale image to a binary image with white representing pore phase and black representing solid phase. The porosity distribution of ITZ regions above, on the side of and below the aggregate were compared. The porosity above the upper surface was smaller than that on the side or above the aggregate, while the ITZ below the aggregate was the most porous due to microbleeding.

The aggregate surface morphology was investigated by fitting the manually captured, irregular boundary with the straight line and a circular arc. The straight line appeared to be a better fit for the chosen boundary. The defined surface roughness and porosity gradient parameters were calculated and the K-means clustering algorithm was applied to subdivide the scattering points into two groups, a rough group and a smooth group.

As porosity gradient decreases, surface roughness increases. The porosity distributions of the ITZ's in the rough and smooth group were averaged and compared at nearly every distance, the porosity of ITZ around the smooth surfaces was significantly lower than the porosity around rough surfaces, proving that surface morphology indeed plays an important role in ITZ formation. Proper grinding and polishing process should be chosen to obtain a smooth enough surface for BSE examination.

Based on the quantitative analysis the BSE measures with Gaussian mixture model, the volume fractions of different phases in cement-based materials could also be determined. The BSE is a powerful technique in quantitative analysis of composition of multi-phases materials.