In this experiment, according to the water demand ratio index of fly ash, the concrete performance test of fly ash with 6 different water demand ratios collected in Fujian area was carried out, the influence of fly ash water demand on the performance of concrete was studied, and the fineness and ignition loss of fly ash were tested to explore the factors affecting the water demand of fly ash, and the control and improvement measures of fly ash water demand were proposed, so as to provide reference for the production quality control of concrete.
1. Test raw materials and test design.
1.1. Test raw materials.
1) Cement: China Resources brand P O425R ordinary Portland cement, cement performance indicators are shown in Table 1;
2) Fine aggregate: machine-made sand, all indicators meet the requirements of GB T14684-2011 "Construction Sand" standard;
3) Coarse aggregate: 5 25mm continuous graded gravel, all indicators meet the requirements of GB T14685-2011 "Construction Pebbles, Gravel" standard;
4) Fly ash: 6 fly ash with different water demand ratios collected in Fujian region are used, and the performance indicators are shown in Table 2;
5) Mixing water: meet the requirements of JGJ63-2006 "Concrete Water Standard";
6) Superplasticizer: POINT-420HS high-efficiency superplasticizer, in line with GB8076-2008 "Concrete Admixture" standard requirements.
1.2. Experimental design.
In this test, the C30 mix ratio was used to verify the effect of fly ash on the performance of concrete with different water demand ratios, and the design was as follows
1) Mix ratio: C30 mix ratio is adopted, and the aggregates used in the test are all saturated surface dry state, and the mix ratio is shown in Table 3;
2) Mixture preparation and performance test: the test controls the initial slump and expansion of each tray of concrete mixture at (210 10) mm and (550 10) mm by fixing the water consumption unchanged and adjusting the amount of water reducing agent, and the test items are: initial slump, expansion, gas content, apparent density and slump, expansion, compressive strength, and workability after 1h time loss;
3) During the test, the test group with a superplasticizer content higher than the benchmark was then compared by adjusting the water consumption to make the initial mixture performance comparable according to the fixed superplasticizer dosage.
2. Test results and analysis.
2.1. Mixture properties.
The measured performance of concrete mixture is shown in Figure 1 and Figure 2.
It can be seen from Figure 1 that when the initial slump and expansion degree of concrete are prepared and the water consumption is fixed, the water reducer content increases with the increase of the water demand ratio of the fly ash sample, and the loss over time is large. Although the initial slump and expansion degree are increased to meet the requirements, the workability is poor, because the free water is absorbed by the fly ash, the free water in the concrete is insufficient, resulting in the concrete state is more viscous, the flow rate is slow, and the water reducer dosage is too high and easy to leak water, which does not meet the construction requirements.
It can be seen from Figure 2 that the water demand ratio of fly ash has different degrees of influence on the gas content and apparent density of concrete, and with the increase of fly ash water demand ratio, the gas content of concrete gradually increases, and the apparent density gradually decreases.
2.2. Concrete strength.
The concrete strength data is shown in Figure 3 for comparison with the fixed water consumption test.
It can be seen from Figure 3 that with the increase of the water demand ratio of the mixed fly ash, the compressive strength of the test block gradually decreases under the same curing age, and the difference between the highest (f2) and the lowest (f5) strength in 28 days is 104MPa, a difference of 2 strength grades. From the strength benchmark ratio with pure cement, it can be seen that the water demand ratio of fly ash mixed with fly ash is 103The strength reference ratio of the sample below 6% increased with the increase of curing age, and the 28-day strength of the F2 sample reached 105 of the cement benchmark strength5%；The water requirement ratio is higher than 103On the contrary, the 28-day compressive strength benchmark strength ratio was reduced for the 6% sample, and the 28-day strength of the F5 sample was only 80 of the cement benchmark strength8%。This is because the incorporation of high-quality fly ash samples with low water demand ratio into concrete has a promoting effect on the improvement of the later strength of concrete, while the incorporation of fly ash with high water demand ratio will be counterproductive and hinder the growth of concrete in the later stage. Table 4 compares the intensity data of fly ash samples with superplasticizer content higher than the cement benchmark for adjusting water consumption test.
As can be seen from Table 4, the high water demand ratio sample is fixed at a dosage of 2At 0%, the same initial state is reached when the water consumption is increased. At the same age, the intensity of the same sample decreased by 2MPa and 3MPa, and the 28-day intensity of the F5 sample was only 308mpa。The results show that the high water demand ratio of fly ash will hinder the growth of concrete strength in the later stage, and if the water consumption is adjusted by fixed dosage, it will have a great impact on the strength of concrete.
3. Factors affecting the ratio of fly ash water demand and improvement measures.
3.1. Factors affecting the ratio of fly ash water demand.
There are many influencing factors of fly ash water demand ratio, and after literature review and actual performance test comparison, the main factors are fly ash fineness, loss on ignition, particle form, type of coal burned in power plant and whether other components are added to fly ash.
3.1.1 fineness.
The relationship between fineness and water demand ratio is shown in Figure 4.
It can be seen from Figure 4 that the water demand ratio and fineness of fly ash are not linear, and when the sieve allowance is around 20%, the water demand ratio has an inflection point, and the relationship trend is roughly the same as the research results of Wu Bin and Guo Hui on the relationship between fly ash fineness and water demand ratio, but different from Yang Hongwei's research results, he believes that the water demand ratio of fly ash has a linear relationship with fineness. This may be related to the different composition of the samples obtained in different regions, and also indicates that although fineness affects the water requirement ratio, it is not the decisive factor.
3.1.2. Loss on ignition.
The relationship between the ratio of water demand and the loss on ignition of fly ash is shown in Figure 5.
As can be seen from Figure 5. The ratio of water demand of fly ash is linearly related to the loss on ignition, which is the same as the results of Wu Bin, Guo Hui and Yang Hongwei. Therefore, it can be inferred that the water requirement ratio of fly ash is significantly affected by the loss on ignition.
3.1.3 Particle morphology.
The water requirement ratio of fly ash has a lot to do with the particle morphology of fly ash. High-quality fly ash is generally dominated by spherical particles, with a smooth outer surface and few porous components. In general, the more spherical particles with smooth surface in fly ash, the lower the corresponding fly ash water demand ratio, and the more porous particles, the fly ash water demand ratio will also increase.
3.1.4. Incorporation of other ingredients.
Now the fly ash on the market contains a large number of coal slag that is not fully combusted, which is finely ground twice, and will be mixed with stone powder, coal gangue powder and other substances, which greatly reduces the activity and strength of fly ash, and also affects the water demand ratio of fly ash.
3.2. Improvement or preventive measures.
3.2.1. The quality of fly ash manufacturers has been improved.
1) From the above analysis, it can be seen that the fineness of fly ash and the loss on ignition have a great influence on the water demand ratio of fly ash. The fineness of the fly ash can be achieved by improving the grinding process. Hefei Cement Design and Research Institute has developed a semi-final grinding process, which can not only effectively reduce the water demand ratio of fly ash, but also help improve the grinding efficiency of the system and reduce power consumption.
2) Fly ash loss on ignition is an important indicator to characterize the amount of unburned organic matter in fly ash, including carbon particles. It is generally believed that the greater the loss on ignition of fly ash, the greater the amount of unburned carbon in it, resulting in an increase in water demand. To reduce the loss on ignition of fly ash is to make fly ash burn more fully and reduce its carbon content. However, for power plants, this will lead to higher costs.
3.2.2. The mixing plant is inspected in the factory.
1) The water demand ratio of fly ash is closely related to the fineness and loss on ignition of fly ash, and the mixing plant can test its fineness index and loss on ignition index when fly ash enters the plant. Because the loss on ignition test has high accuracy and takes a long time, it can be detected by sampling at regular intervals. It can also be done a simple water demand ratio test: take a certain amount of fly ash and a certain amount of water, stir evenly for cement slurry expansion, if the expansion is greater than the specified value, the preliminary judgment of the water demand ratio meets the requirements, if the expansion is less than the specified value, it is deemed that the fly ash water demand ratio does not meet the requirements.
2) For the fly ash mixed with stone powder, coal gangue and other substances, it can be distinguished from the color and feel, the color of high-quality fly ash varies from light yellow to gray, and it is more flexible to grind by hand, and the color of inferior fly ash is black, and there will be oil stains after blistering and stirring.
3.2.3. High water demand ratio fly ash application measures.
In the face of the current situation that the industry is reducing costs, many mixing plants are trapped by economic and supply pressures, and can only use fly ash with a high water demand ratio, which can be adjusted to minimize the risk caused by such fly ash. For example, reduce the amount of fly ash in the mineral admixture, appropriately incorporate or increase the amount of mineral powder, and verify it in the laboratory; Or appropriately increase the water consumption in the mix ratio, and test the performance of the concrete after increasing the water consumption in the laboratory; Increasing the water consumption in the mix ratio is more controllable than adding water at will in production or on-site workers, avoiding serious over-water addition, and can also reduce the impact of the application of high water demand ratio to fly ash to a certain extent.
4 Conclusion. Through the performance test and concrete test of fly ash with different water demand ratios collected in Fujian area, the following conclusions are obtained:
1) The water demand ratio of fly ash has a great impact on the performance of concrete. With the increase of the water demand ratio, the amount of superplasticizer required for concrete is increased, which leads to poor workability of concrete and increases the production cost of concrete. At the same time, the water demand ratio is too high, which also adversely affects the strength of concrete.
2) There are many factors affecting the water demand of fly ash, among which the fineness of fly ash itself and the loss on ignition have a great impact, especially the fly ash with too high loss on ignition, which will significantly increase the water demand of fly ash.
3) Now the quality of fly ash on the market is uneven, and the concrete batching plant needs to carry out strict inspection of the fly ash entering the plant to avoid hidden dangers such as abnormal production and even insufficient strength caused by the use of inferior fly ash in production.