Cement additive adaptability problems are manifested in the following three ways. First, the new concrete slump is small, and the expansion is smaller. But at this time, the amount of water-reducing agents has been quite large. Second, the slump loss is large, and sometimes even a false set. That is, the cement slurry is very thin at the beginning of the mix, but quickly sticky, and dry. The workability of the concrete out of the machine is very poor. Third, although the slump and expansion are not small, the concrete has water secretion problems. Sometimes there is a lag of 1-3 hours and still a large amount of secretion. There are times when the mortar can not be wrapped around the stones, segregation occurs but not with a lot of secretion, and so on and so forth. Sometimes, no obvious maladjustment is observed in fresh concrete, but the strength is low after hardening.
The reasons for the maladaptation of a particular additive to a particular cement may come from three sources.
Cement properties.
Concrete constituent materials, especially the sand and admixtures therein.
Improper matching of the additive itself.
Which is the main reason? It needs to be tested and analyzed. How to adjust to fit. It is necessary to conduct experiments. Here are 7 steps to solve cement additive adaptability problems
Cement PH for Cement Additive Adaptability
It is advisable to start by testing the PH value of the cement you intend to use, that is, the alkalinity of the cement. This can be done with a PH test paper, but of course, it is better to use a PH meter. You can dissolve one part of cement with three parts of water, stir thoroughly and then precipitate to clarify. Take a drop of the clear solution and place it on a wide range of PH test paper and observe the degree of discoloration on the back of the paper to determine the alkalinity of the cement. Generally, the PH value should be above 12, but there is some general silica cement that is only 9-10, and individually lower. The test results allow us to make a preliminary judgment: whether the amount of soluble alkali in the cement is large or small. Whether the mixture in the cement contains acidic materials or inert materials of stone powder type to make the PH value low.
Examination for Cement Additive Adaptability
The first part of the examination is to try to obtain the results of the clinker analysis for the type of cement. The cement plant does a rapid fluorometric analysis of the clinker once a shift. There is an average value for each month. It is not possible to write it on the cement certificate, but it is not confidential information either. It is okay if we can get the clinker analysis results for any recent day. From the data analyzed it is possible to calculate the amount of the four minerals in the cement: tricalcium aluminate C3A, tetra calcium iron aluminate C4AF, tricalcium silicate C3S, and dicalcium silicate C2S. The minerals that affect the cement additive adaptability are tricalcium aluminate, tricalcium silicate, and tetra calcium iron aluminate. These data can help us to choose the type of retarder. In addition, according to the data of alkali and sulfur content in the clinker analysis, we can calculate the plasticity value SD, which can be used as a reference basis to add sulfate or alkali appropriately when compounding additives.
Although the alkali in the clinker analysis sheet is the total amount of alkali rather than just the amount of soluble alkali, it is still an important reference value for us to quickly determine the SD value. And after dissolving the cement in water, the alkali content of the solution is the soluble alkali content including the mix, which may be more meaningful for our test to adjust the cement additive adaptability.
The second part of the examination is to understand how much of what type of blending material is added to the clinker when it is ground into cement. This is useful for analyzing causes such as concrete water retention and abnormal setting times (too long, too short). When grinding clinker with only slag (water slag) or fly ash, the finished cement is well adapted to additives, especially retarders. However, cement with slag as the mixing material sometimes has the problem of water secretion. This is because the water slag hardness is greater than clinker, not easy to grind with clinker as fine as the cause.
When the mixed material is gangue, shale ash, kiln skin, and other volcanic ash material, the finished cement adsorbs high-efficiency water-reducing agent. The amount of water-reducing agent must be increased a lot to get the expected concrete slump, and the expansion is not up to the requirements. When grinding clinker with limestone powder in the mix, the finished cement is easy to have water secretion problems. The addition of old slag that has been stored for a long time in the cement before grinding is also likely to cause water secretion problems. Adding other industrial waste slags, although the amount of admixture may not be large, but after grinding into cement, it will make the cement additive adaptability poor, but also particularly difficult to adjust. The gypsum mixed with the clinker when it is crushed is also very relevant, desulfurization gypsum, anhydrous gypsum, is difficult to dissolve.
The third part of the examination is to grasp the variety and fineness of concrete admixtures. Fully blended mineral powder easy to secrete water, but improved a variety of concrete properties. Full blended ordinary fly ash requires more mixing water, and a long setting time, but it significantly inhibits water secretion. However, high-quality fly ash can reduce the amount of water-reducing agent, if the mix is a unified ash because it contains a part of the three-stage ash, which is often the main reason for the high-efficiency water-reducing agent “failure”.
The fineness of the admixture is very critical, coarse material will not only make the concrete water (such as coarse fly ash), and even make the slump loss becomes larger. The amount of fly ash burn loss and the amount of mineral powder mixed with stone powder (the degree of falsification) is very important, and will directly affect the admixture, especially the high-efficiency water-reducing agent and retarder to the cement additive adaptability.
Saturation Dosing Value for Cement Additive Adaptability
The third step of the cement additive adaptability test is to find out the saturation value of the water-reducing agent used for this cement. If it is a mixture of two or more high-efficiency water-reducing agents, the total amount of the mixture is used to find out the saturation dosing point through the cement net paste test. The closer the amount of high-efficiency water-reducing agent to the saturation of the cement, the easier it is to get good adaptability.
But the saturation point of early-strength cement is generally high so the traditional high-efficiency water-reducing agent is more than 1% or 0.8% (calculated by the active ingredient). This is usually not more than 2.5% of the pumping agent to achieve saturation dosing is not realistic, even “close” is not possible. It mainly needs to rely on different high-efficiency water-reducing agent compounding, as well as with a variety of slow coagulant compounding to achieve the purpose of adjusting the adaptability. According to experience, the amount of water-reducing agent is less than 70% of the saturated mixture, that is, the use of a variety of retardants to adjust the adaptability at the same time, but also can not get satisfactory results.
Cement net slurry test can be done with pure cement, but also the total amount of 300 grams of cementitious materials in accordance with the proportion of cementitious materials in the concrete ratio distribution. First with the former program and then the latter program to do, it is often easier to find the correct results.
Clinker Plasticization for Cement Additive Adaptability
The fourth step is to try to adjust the degree of alkali sulfation in the cement and the clinker plasticization to a suitable range. The value of clinker plasticity SD is calculated as
SD=SO3/(1.292Na2O+0.85K2O)
The SD value range is 40% to 200%. The low SD value means that the sulfur trioxide is less. It should be added a small amount of sulfur salt such as sodium sulfate in the additive. The high SD value means that the molecule is large, which means more sulfur trioxide. The additive PH value should be slightly adjusted with alkali, for example, with sodium carbonate.
Suitable Coagulant for Cement Additive Adaptability
The fifth step is to test the compound additive and find out the suitable species of coagulant and the appropriate amount of mixing through the net slurry test. To refer to the results of the second step of the investigation, choose the most effective conditioning agent possible.
Tricalcium silicate mineral cement is suitable for the use of sodium gluconate and other hydroxy carboxylates, the use of polyphosphate, especially sodium hexametaphosphate and sodium tripolyphosphate, etc. However, the interaction between sodium gluconate and sodium hexametaphosphate is easy and it is recommended not to use them at the same time. In addition to choosing sodium gluconate for cement with many aluminate minerals, it is also necessary to compound appropriate auxiliary partners such as sodium tripolyphosphate, modified starch, and dextrin. Monosaccharides have a synergistic effect on sodium gluconate, so they can be used when the temperature is high.
Therefore, good quality liquid sodium gluconate is sometimes better for this reason. However, when the total amount of retarder exceeds 0.07% of the amount of gelling material, retardation will become the main role. There is a view that at this time should be used instead of polycarboxylic acid water-reducing agent because the aluminate minerals adsorbed naphthalene sulfonate and other sulfonic acid-based high-efficiency water-reducing agent but not adsorbed polycarboxylic acid-based high-efficiency water reducing agent.
A commercial concrete company with normal mud content of sand, but the loss of concrete slump has been very large. Later, after testing, all change to using polycarboxylic acid water reducing agent, so that the slump loss of large expansion of the problem of small more satisfactorily solved. If you do not know the chemical and mineral composition of cement clinker and do not know the type of cement mix, additive compounding can only use sodium gluconate, sodium citrate, or sodium hexametaphosphate in any one of the first to figure out, add alkali or sulfate also have to take turns to try.
When the sand quality is poor, such as containing more mud, or all with artificial sand, special fine sand mixing concrete, after the net slurry test to get satisfactory results, it is appropriate to continue to make sand test and further adjust the cement additive adaptability. Because of the poor sensitivity of the sand test, when the material is not sufficient, the net mortar test is directly after the concrete test.
Concrete Test for Cement Additive Adaptability
The sixth step of the cement additive adaptability test is the concrete test. It should not be less than 10 liters at least. Due to the addition of sand, stone, water, and other materials, the amount of cementitious materials is also more than 10 times larger than the net slurry test, and testers operating practices and other factors, the net slurry, and concrete test results are often contrary to each other. Although may have to make large adjustments, but do not have to push back.
Most of the admixture components used in the previous test can continue to be used, but the amount will be different. Some components to add or delete, increase the amount of high-efficiency water-reducing agent is sometimes necessary. In short, the net pulp may still not be well adjusted in the concrete, the net pulp is not well adjusted then the concrete is more not.
The quality of local building materials is getting worse and worse is a common phenomenon, sand mud content is too high in many areas and has become very “normal”. This makes adaptability adjustment becomes difficult. Because of the high mud content in the aggregate and the mix becoming too sticky, at this time the use of the viscosity reduction agent effect is not obvious. Often amino sulfonate high-efficiency water-reducing agent and naphthalene high-efficiency water-reducing agent compound and appropriate mix with easy to lyse water retarder may get better results.
After a successful small amount of test, sometimes need to put a lot of repeats once, such as 25-45 liters, because the results may have some difference.
Modify the Mix Ratio Appropriately
Only a certain number of concrete test successes are considered to have completed the cement additive adaptability adjustment. However, in many cases, only with the adjustment of the additives approach always fails to achieve satisfactory results, so that the adaptability can pass, sometimes the product cost is too high and the user is not acceptable.
Therefore, the seventh step of the adaptation test is to properly modify the concrete ratio: depending on the different performance of non-adaptation, you can appropriately increase or reduce the number of mineral admixtures, and a single material into double admixture, that is, while using two different materials. Double admixture than single admixture is undoubtedly superior. Increasing or reducing the amount of cement, can solve the concrete sticky, slump loss fast and concrete water, especially the surface of the sand defects. Slightly increase or decrease the amount of water used. Increase or decrease the sand rate, or even partially switch the sand species, such as coarse and fine sand, natural sand and artificial sand, etc.
These small modifications will cost basically the same, the operation is also simple, but the effect is sometimes very obvious. Of course, if necessary, changing the cement of a production plant will also solve the long-standing problems.