Standard for determining the methylene blue index of aggregates for concrete and mortar in Vietnam

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1. General regulations on the procedure for determining the methylene blue index of aggregates for concrete and mortar

TCVN 7572-21:2018 has the following definition: "A quantity that characterizes the level of methylene blue adsorption of aggregates containing clay and is calculated by the number of grams of methylene blue adsorbed per kg of aggregate."

General regulations on the procedure for determining the methylene blue index of aggregates for concrete and mortar according to TCVN 7572-21:2018 are as follows:

- Water used in analysis according to TCVN 4851 (ISO 3696) or water of equivalent purity (hereinafter referred to as “water”).

- The percentage concentration of a diluted solution is expressed as the number of grams of solute in 100 mL of water. For example, for methylene blue, a 0.5% solution is a solution of 0.5 g of methylene blue dissolved in 100 mL of water.

- Analytical criteria are carried out in parallel on two weighings of the test sample.

The absolute value difference between two parallel determination results must not exceed the allowable limit. If it exceeds the allowable limit, reanalysis must be performed.

- Determine constant volume

Determine the constant mass by: heating or drying the sample to a specified temperature and keeping it at that temperature for 15 minutes; cooling the sample in a desiccator to room temperature; and weighing. The process is repeated until the difference between two consecutive stops does not exceed 0.1% of the mass of the test sample.

- Indicates mass, volume and results

+ Mass in grams (g);

+ Volume in milliliters (mL);

+ The final result in grams per kilogram (g/kg), is the average of two analytical results conducted in parallel, resulting in three significant figures.

2. Standards for chemicals and reagents to determine the methylene blue index of aggregates for concrete and mortar

- Crystalline methylene blue (C16H18CIN3S.nH2O, n = 3, chemical purity ≥ 98.5 %).

- Methylene blue standard solution, 5 g/L.

Dissolve [(100+W)/20] g methylene blue to the nearest 0.0001 g (equivalent to 5 g methylene blue dried at 100 °C ± 5 °C) in 500 mL to 700 mL warm water (water does not exceed 40 °C). Gently stir the methylene blue solution for about 45 minutes until the solution completely dissolves, then cool to room temperature. Transfer to a 1000-mL volumetric flask, add water to the mark, and shake well. The solution can be used for four weeks, stored in a dark glass bottle.

W is the moisture content determined in Appendix A.

- Kaolinite

Kaolinite is finely ground to a particle size that passes through a 0.063 mm sieve, dried at a temperature of (105 ± 5) °C to a constant weight, and cooled in a desiccator to room temperature before weighing for testing.

3. Equipment and tools used to determine the methylene blue index of aggregates for concrete and mortar in Vietnam

Equipment and tools used to determine the methylene blue index of aggregates for concrete and mortar are specified in TCVN 7572-21:2018, including:

Common glassware used in the laboratory and:

- Analytical balance, accurate to 0.0001 g.

- Technical scale, accurate to 0.01g.

- Drying oven, capable of adjusting temperature and controlling temperature within (105 ± 5) °C.

- Sieves of all types, with hole size of 2 mm; 0.25 mm; 0.1 mm and 0.063 mm.

- Agate mortar and pestle.

- Glass cup or polyethylene plastic cup, capacity 1000 mL, 2000 ml.

- Volumetric flask, made of glass, with a capacity of 1000 mL.

- Erlenmeyer flask, made of glass, with a capacity of 500 mL.

- Glass chopstick, length 300 mm, diameter 8 mm.

- Stirrer, capable of controlling the stirring speed to (600 ± 60) r/min with 3 or 4 stirring blades with a diameter of (75 ± 10) mm, magnetic stirrer with a diameter of over 10 mm.

- Stopwatch, accurate to 0.1 s.

- Thermometer monitors temperature, with graduations accurate to 0.1 °C.

- Desiccator, containing silica gel beads.

- Burette, capacity 5, 25, 50 mL.

- Filter paper, medium flow, ash-free quantitative (average pore diameter of about 7 μm).

Best regards!

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