Engineering
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Detecting the Water-soluble Chloride Distribution of Cement Paste in a High-precision Way
Chapters
Summary November 21st, 2017
A protocol for obtaining a water-soluble chloride profile by using a high precision milling method is presented.
Transcript
The overall goal of this procedure is to use an advanced grinding method to accurately obtain the chloride distribution along the depth of cement paste under cyclic wet dry conditions. This method can help answer key questions in the chloride transport field about how to obtain high accuracy chloride profiles. The main advantage of this technique is that the accuracy of the chloride distribution along the depth can be greatly improved.
To begin the sample preparation, use a brush to remove all contaminants and dust from a 70 millimeter by 70 millimeter by 70 millimeter mold. Coat the inner surfaces of the mold with mold release diesel oil. Then, sequentially place 1, 000 grams of deionized water and 2, 000 grams of cement in a five liter mixing pot.
Secure the pot on a cement mixer and mix at 65 rpm for 90 seconds. Let the mixture sit for 30 seconds while scraping paste from the inner walls and returning the paste to the bulk mixture. Then, mix at 130 rpm for 60 seconds.
Remove the mixing pot from the cement mixer and pour the cement paste into the mold. Shovel the remaining paste into the mold with a scraper knife. Place the filled mold on a vibrating table and vibrate the mold for 10 seconds to compact the paste.
Then, seal the mold with cling film to prevent water loss to evaporation. Allow the cement to sit at room temperature for 24 hours. Then, remove the hardened cement paste specimen from the mold.
Cure the specimen at 23 degrees Celsius in 95%relative humidity for 60 days. Next, fix the cured specimen on a high precision cutting machine. Cut off a 20 millimeter thick slab from the surface that did not contact the mold to form a 70 millimeter by 70 millimeter by 50 millimeter specimen.
The freshly cut face will be the exposure surface. Seal the other five sides of the specimen with liquid epoxy resin. Allow the resin to harden in air for 24 hours.
Place the specimen on one centimeter spacers in a plastic box with the exposure surface facing down. Add a 3.5%by mass solution of sodium chloride and deionized water to the box until the solution level is about one centimeter above the exposure surface. Note the time at which the sodium chloride solution was poured into the box.
Seal the box with 0.25 millimeter thick plastic film to prevent concentration changes from water evaporation. Store the box at a constant temperature of 23 degrees Celsius in a relative humidity of 65%for 24 hours from the time the sodium chloride solution was added. Then, remove the specimen from the box and gently wipe off the solution with a paper towel.
Leave the specimen at 23 degrees Celsius in 65%relative humidity for six days to dry. Repeat the 24 hour immersion of the exposure surface in a 3.5%sodium chloride solution and the six day drying period 11 more times for a total of 12 cycles. After the 12th wet dry cycle, fix the specimen on the base of a high precision CNC grinding machine fitted with a titanium alloy cutter.
Place powder collecting paper around the specimen. Set up an automated grinding protocol to grind the sample in a series of layers with a 60 second pause between each layer. Then, start the grinding process.
Recover the powder during each 60 second break and replace the collecting paper for the next layer. Store the powder samples in separate containers. Recover the powder during each 60 second break and replace the collecting paper for the next layer.
Make sure that you can finish collecting the powder within that 60 second break before creating the next layer. Once the process is finished, grind each powder sample with a mortar and pestle until the sample can be sifted through an 80 micron sieve. Dry the sieved samples at 105 degrees Celsius for two hours.
Then, place two grams of a dry sample in a 100 milliliter plastic bottle. Add 50 milliliters of deionized water and cap the bottle. Shake the bottle vigorously to ensure that the sample and the water are thoroughly mixed.
Place the bottle on an automatic vibrator and vibrate the bottle for 24 hours to dissolve water soluble chlorides. Filter the solution through filter paper and place two 10 milliliter portions of the filtrate in conical flasks. Add two drops of phenolphthalein solution to each conical flask.
Add dilute sulfuric acid to each solution until the solutions become colorless. Then, add 10 drops of a potassium chromate indicator to one flask and immediately titrate with a silver nitrate solution. Manually shake the conical flask during titration to ensure that the silver nitrate quickly reacts with the chloride ions.
Stop the titration when the solution appears reddish and the color does not fade. Record the volume of consumed silver nitrate solution. Make sure to stop the titration when the solution becomes reddish and the color does not fade.
Repeat the titration process with the second flask. Calculate the water soluble chloride content for each flask and average the results. Water soluble chloride profiles were created for two cement paste specimens sampled at intervals of 0.5 millimeters and two millimeters.
The curves were fitted with the error function of Fick's second law and the chloride diffusion coefficient was calculated for each profile. When sampled at two millimeter intervals, the chloride content in the cement paste specimen appeared to decrease monotonically with depth. However, when sampled at 0.5 millimeter intervals, the chloride content was revealed to reach a maximum before decreasing.
The significant difference in chloride diffusion coefficients and in the shapes of the curves suggested that two millimeter intervals were too large for an accurate chloride profile for this paste, demonstrating the importance of small, precise intervals. After watching this video, you should have a good understanding about how to obtain high precision chloride profiles using this grinding method. It should be helpful for both research and fieldwork addressing the durability of cement-based materials.
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