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Classification and Identification Methods of Alien Fly Ash

Release time:

2022-08-12

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The abnormal fly ash mainly includes: original coarse fly ash, ground fly ash, desulfurization fly ash, denitration fly ash, floating black fly ash, adulterated fly ash, etc. After use, it is easy to lead to insufficient 28d strength of concrete and low strength growth in the later period, resulting in unqualified concrete engineering quality. The following is an introduction to these 6 kinds of fly ash.

01 Raw ash

Raw ash, that is, undisturbed coarse ash, is mainly the type of fly ash that is not sorted by thermal power plants but is discharged to dry ash warehouse or wet ash and then stored in ash sink tank. Its surface smoothness is poor, with fineness usually jumping 45%, loss on ignition jumping 15%, and activity index about 60%. Nowadays, this is often used as a cement mixed building material. The residual amount of the original ash screen is relatively large, contains more impurities, and the loss on ignition is large, so the actual effect cannot be well reflected. In view of this, the concrete made of it does not have ideal fluidity, and the water retention and cohesion do not meet the requirements, which will greatly affect the quality of fresh concrete; due to the large water demand, it will lead to the actual unit volume of concrete The water consumption exceeds the expected water consumption, the water-binder ratio increases, the compressive strength level decreases, and the durability of hardened concrete is seriously weakened.

02 ground fly ash

Grinding fine fly ash is to obtain recycled fly ash by means of waste original coarse ash and selecting grinding processing method. Compared with the original coarse ash, the fine fly ash 45 m sieve balance is less, and the activity is relatively poor. In terms of chemical composition, there is no obvious difference between ground fly ash and high-quality fly ash, but the physical properties of the two are very different, which will greatly change all the properties of concrete. Compared with high-quality fly ash, because the carbon content and impurity content of ground fly ash are relatively large, it can better absorb water and various admixtures. Most of its particles are hemispherical particle fragments with poor surface smoothness, and the lubricity of spherical microspheres is not ideal. In view of this, if the doping amount is consistent, the fluidity of fresh concrete will be relatively low, and there will be very serious loss over time, thus seriously affecting concrete pouring construction.

03 desulfurization fly ash

The key reason is actually caused by desulfurization technology. Now the country has high environmental requirements for thermal power plants. In order to meet the emission standards of sulfur dioxide in coal-fired thermal power plants, desulfurization measures must be taken. The key component of desulfurization fly ash is calcium sulfite, which can retard cement, which will affect the setting time of concrete and increase the difficulty of concrete setting.

Identification method of desulfurized fly ash: weigh 50g of fly ash into a beaker, add 100mL of distilled water and fully stir for more than 1min, drop a few drops of 1% phenolphthalein alcohol reagent, the solution is red, use pH test paper to check that the pH value is> 10, I .e. the solution is moderately alkaline, indicating that the fly ash may be desulfurized fly ash, and further use the national standard GB/T176-2017 "Cement Chemical Analysis Method" to determine the content of sulfur trioxide and free calcium oxide, if one or two of these two indicators seriously exceed the requirements of the national standard GB/T1596-2017 "fly ash used in cement and concrete", it is confirmed that the fly ash is desulfurized fly ash.

Another more direct identification method of desulfurized fly ash: put the fly ash into a shallow tray and expose it to humid air for 2~3 days. if the fly ash is not wet but hard, it is basically confirmed to be desulfurized fly ash. the reason is that the desulfurized fly ash contains dehydrated gypsum (I .e. anhydrite), which reacts with water and becomes strong dihydrate gypsum (CaSO₄·2H₂O).

04 Denitration Fly Ash

Because of the strict needs of environmental protection, coal-fired thermal power plants in order to meet the emission standards of nitrogen dioxide, must take the nitrogen removal process, nitrogen removal process often residual excess ammonia (NH4) in the fly ash, in the process of concrete mixing and vibration NH₄⁺Will hydrate with cement to produce OH^-The reaction generates ammonia gas, which needs a period of time to release in the concrete. If it is released after the initial setting of the concrete or after plastering, it will eventually stay on the surface of the concrete and form a large number of bubbles or pores or holes, which will seriously affect the appearance quality and physical quality of the concrete, as well as the mechanical properties and durability,

The method for identifying denitration fly ash is relatively simple, fast and convenient: first weigh 200g of fly ash and 100g of cement into a 500mL conical flask respectively, add 200g of distilled water and fully stir, immediately put wet test paper (impregnated with 1% phenolphthalein alcohol solution) on the cap of the conical flask, if the test paper turns red (ammonia is ammonia water when it meets water, which is medium-strong alkaline), and can smell pungent ammonia smell, it can be determined that the sample is denitration fly ash.

05 floating black fly ash

At present, many thermal power plants use inferior coal-fired power generation in order to save costs and resources. This kind of coal contains many coal gangue, with low calorific value and low ignition point. Generally, diesel oil or heavy oil or wood are added as combustion improver in the process of coal combustion. These combustion improver generally cannot be completely burned, and residual organic matter is also collected in fly ash. When mixing and vibrating this concrete mixed with "floating black" fly ash, due to the small proportion of these oil and carbon powder, it will float to the concrete surface (large slump concrete is more obvious), forming a floating layer of black material, resulting in solid color difference of concrete, seriously affecting the appearance of concrete structure. Especially the use of polycarboxylic acid admixture mixing station, the production of concrete floating black ash situation is relatively serious. In addition, due to the high carbon content of fly ash will increase the water demand of concrete, improve the water-binder ratio, will eventually reduce the strength of concrete, but also affect the durability of concrete.

"Floating black" fly ash can be identified by a simple method: add about 200mL distilled water into a 500mL beaker or conical flask, weigh 100g of sample, stir with a glass rod while adding the beaker, and the carbon content in the fly ash can be roughly determined according to the amount of black substances floating on the water surface. If the fly ash is black in color and has abnormal smell and oil beads on the water surface, the fly ash should be floating oil ash. If a further loss test is carried out, the result is generally one or two percentage points higher than that of ordinary fly ash. Therefore, it is recommended to use it carefully or to leave the field.

06 Adulterated Fly Ash

Adulterated fly ash because the composition is very much, and the mixing uniformity is very poor, the impact on the properties of fresh concrete is very unstable, which will lead to the quality control of fresh concrete is very difficult, and the work of evaluating the performance of hardened concrete is also very difficult to carry out.

In order to improve economic benefits, fly ash manufacturers install fly ash that meets the quality conditions on the upper part of the tanker, and the middle and bottom of the tanker are filled with fly ash of relatively poor quality. Generally, the merchants add fly ash after grinding the slag or coal gangue or limestone from thermal power plants with a mill. This adulterated fly ash has no obvious difference from the real fly ash in appearance and color, and its chemical composition (CaO, SO₃Content, loss on ignition, etc.) and physical indicators (volume stability, moisture content, water demand ratio and activity index, etc.) are not much different, but under the microscope of more than 100 times, the core substance of fly ash, namely glass sphere, is basically invisible. The activity index of adulterated fly ash is extremely low or has no activity at all. In the mixing concrete, the water demand is large, the loss is large, and the fluidity and pumping effect are not as good as real fly ash, it also has a great adverse effect on the strength and durability of concrete, and the surface of the concrete entity is not shiny and the appearance quality is poor.

For fly ash mixed with limestone or dolomite, dilute hydrochloric acid can also be used for testing. The method is simple, fast and intuitive. The specific method is: weigh about 10g of dried sample and put it into a clean beaker, add 50mL of distilled water and fully stir, and then add about 10mL of dilute hydrochloric acid. If a large amount of bubbles (I .e. carbon dioxide gas) are generated, it is proved that limestone materials are mixed in the fly ash. This kind of fly ash mixed with limestone or dolomite is often white in color, and the loss on ignition is very large, generally above 25%, and the activity index is obviously lower than that of real fly ash or has no activity at all. The adulterated fly ash can be inspected and identified by chemical analysis. This method is complicated and time-consuming. Specifically, GB/T176-2017 "Chemical Analysis Method for Cement" can be used. If the calcium oxide content in the sample is 25% ~ 40%, it is proved to be fly ash mixed with limestone. If the total content of calcium oxide and magnesium oxide in the sample is 25% and 40%, and the content of magnesium oxide is above 8%, which proves to be fly ash mixed with dolomite. If the content of aluminum oxide jumps 25%, it is proved to be fly ash mixed with coal gangue or cinder. If the content of silica jumps 65%, it is proved to be fly ash mixed with shale or quartz sand. The higher the test results of these chemical indicators, the more serious the degree of adulteration and the greater the impact on the quality of concrete.

To sum up, there are many kinds of fly ash in the construction market. In order to ensure the workability of concrete mixture and the quality of concrete entities, test and inspection personnel need to have relatively comprehensive theoretical knowledge and identification ability. At the same time, they should have strong sense of responsibility, dedication and professional ethics. First of all, they should carry out preliminary screening and identification of fly ash with their own professional ability and experience accumulation, then, in strict accordance with the relevant national standards, each batch of materials will be inspected by batch, and unqualified or inferior fly ash will be rejected from the door, so that lawbreakers can take advantage of it, so as to achieve the purpose of purifying the construction market.

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