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The adaptability of concrete admixture? Seven-step solution!

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The adaptability of concrete admixture? Seven-step solution!

Issue Time:2020-04-23
The unsuitable performance is firstly that the slump of fresh concrete is small and the expansion is small, but the amount of water reducing agent is already quite large at this time. The popular saying is "unopenable"; secondly, the slump loss is large, Sometimes there is even false setting, that is, the cement slurry is very thin at the beginning of the mixing, but it quickly sticks and dries, and the workability of the concrete after the machine is very poor; the third is that although the slump and expansion are not small, (Sichuan Water reducer) But the concrete bleeds, sometimes lags 1-3 hours and bleeds in a large amount; sometimes the mortar can't wrap the stones, and the segregation occurs but it is not accompanied by a lot of bleeds. In some cases, no obvious incompatibility is observed in fresh concrete, and the strength after hardening is low. The reasons why the specific admixture and the specific cement are not suitable may come from three aspects: caused by the characteristics of the cement; caused by the concrete constituent materials, especially the sand and admixture therein; caused by the mismatch of the admixture itself. (Quick setting agent) Which one is the main reason needs to be tested and analyzed, and how to adjust it to suit it must be experimented. So, where to start the experiment, it was before us.
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It is better to start by testing the PH value of the cement to be used, that is, the alkalinity of the cement. PH test paper can be used to complete this work, of course, PH meter is better. Three parts of water can be used to dissolve one part of the cement. After fully stirring, the precipitate is clarified. One drop of the clear solution is placed on a wide pH test paper to observe the degree of discoloration on the back of the test paper to determine the alkalinity of the cement. Generally, the PH value should be above 12, but some ordinary silica cements are only 9-10, and some are even 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 mixed material in the cement contains acidic materials or stone powder inert materials makes the PH value low. (Efficient release agent)
The part of the investigation is to try to obtain the clinker analysis results of this kind of cement. The cement plant does a quick fluorescent analysis of the clinker every shift, and there is an average value every month. Although it is impossible to write on the cement certificate, it is not a confidential information. If we can get the clinker analysis results of any recent day. According to the analyzed data, the number of four minerals in cement: tricalcium aluminate C3A, tetracalcium iron aluminate C4AF, tricalcium silicate C3S and dicalcium silicate C2S can be calculated. The minerals that affect cement adaptability are tricalcium aluminate, tricalcium silicate, and tetracalcium iron aluminate. These data can help us choose the type of retarder. In addition, based on the alkali and sulfur content data in the clinker analysis, we can calculate the plasticization value SD as a reference basis for whether to add sulfate or alkali when mixing the admixture.

Although the alkali in the clinker analysis sheet is the total alkali quantity rather than the pure soluble alkali quantity, it still has important reference value for us to quickly determine the SD value. After dissolving the cement in water, the alkali content of the solution is the soluble alkali content including the mixed material, which may be more meaningful for our test of adaptability.

The second part of the investigation is to understand how much and what kind of mixed material is added when the clinker is ground into cement. This is very helpful to analyze the causes such as concrete bleeding and abnormal setting time (too long and too short). When grinding the clinker, the mixed material is only slag (water slag) or fly ash, and the finished cement that comes out has a good adaptability to the admixture, especially the retarder. The reason is that the hardness of water slag is greater than that of clinker, and (polycarboxylic acid water reducing agent) is not easy to be ground as fine as clinker.
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