Methylene Blue

The Methylene Blue Value (MBV) is a widely used method for detecting the presence of clay minerals in aggregates. This involves reacting harmful clay fines with a blue dye and measuring dye uptake, which results in a color change that can be used to estimate clay contamination. The literature describes several variations of this test, which utilize an end-point titration technique. However, these tests typically require a time-consuming sieving process to obtain a sample size of -75 microns. Additionally, the titration process is slow, and the visual determination of a blue "halo" on filter paper is dependent on the operator's judgment. Despite these limitations, the test provides a direct measurement of clay contamination, and the results can be highly useful in predicting concrete performance.

Apparatus

The equipment required for performing the Methylene Blue test is as follows:

• A high-speed agitator motor with a speed of 400/600 r.p.m

• A stirring propeller with a diameter of 70mm and 4 flanks

• Filter papers with a diameter of 125mm, a weight of 95g/m2, and a thickness of 0.20mm

• Methylene Blue

• Kaolinite

• 50ml glass burette with 0.1ml increments

• Burette holder and stand

• 3000ml plastic beaker

• 8x300mm glass rod

• Balance

• Oven

Test Method

To conduct the Methylene Blue experiment, follow these steps:

• Step 1: Prepare a methylene blue solution by adding 10g of methylene blue powder to 1000g of distilled water. Mix the solution at 600 rpm for 45 minutes and let it sit for 24 hours. The solution must be used within 28 days.

• Step 2: Prepare a 200g aggregate mixture that passes through a 2mm (No. 10) sieve.

• Step 3: Add the prepared aggregate mixture to 500g of distilled water and mix it at 600 rpm for 45 minutes.

• Step 4: Add 5ml of the methylene blue solution to the aggregate-water mixture obtained in Step 3. Mix it for 1 minute at 400 rpm.

• Step 5: Dip a glass burette into the resulting mixture and drop a drop onto the filter paper.

• Step 6: Add 5ml of methylene blue solution to the same aggregate-water-methylene blue solution and mix it for 1 minute at 400 rpm.

• Step 7: Dip the glass burette into the resulting mixture again and drop a drop onto the filter paper.

• Step 8: Repeat Steps 4 and 5 until the halo image with a dark blue center is observed on the filter paper, as shown in Figure 1.

• Step 9: When the experiment is finished, calculate the amount of methylene blue (MB) using the following formula: MB = V1/M1*10 - where V1 is the amount of methylene blue used (in ml) and M1 is the aggregate amount (in grams).

Methylene Blue - Method Development

A recent update to the conventional MBV test has been developed to address the limitations of the titration method. The revised technique employs a colorimeter, a device that measures the absorption of a given solution at a particular light wavelength.

By removing the subjective interpretation of the blue halo, the colorimeter significantly enhances the reproducibility of the results. Moreover, the new test allows for the entire sand sample to be used, rather than just the 75-micron fraction. The updated test takes roughly 10 minutes to execute, offering a swift and reliable MBV test that can be conducted both in the laboratory and in the field. Figures 1a to 1e depict the equipment and testing procedure.

Compared to standard MBV tests, such as AASHTO T330-07, ISSA 145, or EN-933-9, the improved MBV test demonstrates an excellent correlation.