Fly ash is one of the main raw materials of ready-mixed concrete, and its quality directly affects the quality and performance of concrete. This paper summarizes the rapid identification method of fly ash quality. Details are as follows:

Fly ash is a powder collected from the flue gas of pulverized coal furnaces. Incorporating into concrete, its particle shape effect can generate water-reducing potential energy, energy; thereby reducing water demand, improving durability and impermeability, micro-aggregate effect generating compacting potential energy, pozzolanic effect generating activation potential to reduce shrinkage, It can reduce internal temperature rise, improve tensile strength, resist sulfate corrosion, reduce bleeding and inhibit alkali-aggregate reaction. Mixing with fly ash can not only save the cost of concrete production, but also improve and improve the performance of concrete, so it is widely used in concrete production.
With the continuous development of clean energy such as hydropower, wind power, and nuclear power, the proportion of thermal power plants is getting lower and lower, and the amount of fly ash produced is also becoming less and less, which cannot meet the market demand. Therefore, researching and determining the rapid identification method of fly ash quality can effectively control the quality of incoming fly ash and ensure that it meets the requirements of concrete quality control.

1. Change the sampling method
The traditional sampling of fly ash is to open the roof cover of the powder tanker, and use a shovel or a sample scoop to take surface samples. In order to obtain greater profits, some suppliers first put poor-quality or inferior fly ash into the middle and lower parts, and finally put better-quality fly ash on the face, so that the receiving personnel can get better samples every time, So as to muddle through.
In order to prevent the supplier from cheating, we ordered a stainless steel sampler with a length of 2.5m, a diameter of 40mm for the large tube, and a diameter of 36mm for the small tube. The sampling tube can be rotated to close or open the sampling port. Through the actual sampling and use, it is found that after opening the sampling port, the sample will drop to the lower end, and there is a phenomenon of mixing samples of different layers, and the situation of each layered sample cannot be observed. After improvement, partitions are set on the upper, middle and lower parts, so that the samples of different layers will not be mixed, and the purpose of layered sampling and observation is basically realized.

2. Color identification quality
The color of fly ash of the same quality in the same power plant is basically the same. If the color changes, although the quality cannot be judged, it at least indicates that the quality of fly ash may change. There are several possibilities for the color change of fly ash: first, the coal quality or calcination process of the power plant has changed; second, the source of the fly ash is not the same manufacturer; third, the raw material or formula of the finely ground fly ash has changed; fourth, the supplier has falsified, etc. .
The color change of fly ash is very obvious and can be seen at a glance, but when the color change of fly ash is not obvious, it is difficult to find the color change. Comparing the incoming fly ash with the previous batch (or normal color) samples, it can be clearly observed whether the color has changed. The comparison method is to install the sample entering the field on the sample plate, press the bottom of the sampling spoon into a concave shape, and then take 2-5g of the reserved sample and place it in the middle of the concave surface, and then use the bottom of the sample spoon to flatten it. It is obvious to judge whether there is a color difference between the two batches of samples.

3. Density identification quality
The apparent density of fly ash is generally 2100-2400Kg/m3, while the apparent density of stone powder, slag and other materials is generally 2700-2900Kg/m3. If the density of fly ash is high, it means that there may be problems with the quality of fly ash. However, the determination of the apparent density of fly ash is cumbersome and takes a long time, which is inconvenient for timely detection of the quality of fly ash entering the site. The bulk density of fly ash is generally 700-900Kg/m3. If the bulk density of fly ash is too high, it can also be judged that its apparent density is too high. The determination of the bulk density of fly ash is simple and fast, and fewer instruments are required for detection. The detection method can refer to the sand bulk density detection method. Put the fly ash through the funnel into the capacity cylinder for measuring the loose bulk density of the sand, freely fill it with 1L, use a ruler to level it from the middle to both sides, and weigh and tare for calculation. When the weight of fly ash entering the site deviates greatly from the normal situation, it should also be suspected that there is a problem with its quality.

4. Identification of stone powder mixed in fly ash
Some suppliers reduce costs and obtain greater profits by incorporating stone powder. The addition of stone powder will reduce the activity of fly ash and affect the construction performance of concrete. In order to prevent suppliers from mixing stone powder with fly ash, acid solutions such as oxalic acid and hydrochloric acid can be used to detect the content of stone powder. The principle is that when calcium carbonate in stone powder encounters acid, it will decompose carbon dioxide and generate bubbles. The test method is to dilute hydrochloric acid or oxalic acid to 10-20%, put about 1g of fly ash sample into a beaker containing about 200ml of diluted acid, observe whether there are bubbles or produce bubble sound, and determine the stone powder content according to the number of bubbles.

5. Quality of Microscopic Identification

Because the air-selected fly ash is mostly spherical in vitreous body (commonly known as microbeads), transparent spherical vitreous body can be clearly observed under the microscope; many hemispherical vitreous bodies and a small amount of spherical vitreous body can be observed under the microscope by grinding finely ground fly ash.

Because of the fineness of fly ash, the shape of fly ash cannot be observed with the naked eye, and can be observed with a high-power electron microscope. The particle size of unscreened fly ash is different, and it is difficult to observe clearly under the same magnification and focal length under the microscope. Through the test, use a 45um square-hole sieve to observe under a microscope, and the effect is the best.

6. Water requirement ratio identification
The water demand ratio is an important indicator of the quality of fly ash, which seriously affects the water consumption and construction performance of concrete. However, the standard method requires equipment such as a mortar mixer and a jumping table. The operation is complicated and requires high operating skills. It is easy to fail in operation and needs to be re-tested. Through experiments, a simple and quick test method is concluded. Directly put 100g of fly ash into a 400ml beaker, add 50ml of water, stir evenly with a glass cup or a spatula, and determine the water demand ratio of fly ash by observing the consistency of fly ash, as shown in Figure 8. The water demand ratio can also be determined by the amount of water required to achieve a certain consistency.

7. Determination of sulfur trioxide
In order to reduce SO3 emissions, power plants often need to take desulfurization measures, and the generated fly ash is CFB desulfurized fly ash. It contains a lot of sulfide or sulfate, which can easily cause concrete to crack. In an aqueous medium, the calcium sulfate in the fly ash is statically exchanged twice with a hydrogen-type cation exchange resin to generate hydrogen ions of the same substance. Using phenolphthalein as an indicator, it is titrated with a standard titration solution of sodium hydroxide, which can be quickly detected. Sulfur trioxide content in fly ash.

8. Ammonia gas identification
In order to reduce the emission of NOx in the coal combustion process, the power plant needs to carry out “denitrification” treatment during the coal combustion process. Improper denitration process may result in a part of NH4+ remaining in the fly ash. When the fly ash is mixed with cement, it encounters alkali. NH3 (ammonia) will be released in the sexual environment, and a large amount of gas will be generated in the plastic stage of concrete, which will affect the quality of concrete.