The preparation of early-strength concrete no longer requires high C3S, C3A content and high fineness, but only reduces the water-cement ratio or water-binder ratio. The compressive strength of concrete is closely related to the fineness and dosage of cement particles. In fact, the 200MPa reactive powder concrete (RPC) cement is coarser, and the content of C3S and C3A is also lower. It is easier to control the rheology with such cement. When the strength of concrete is 20MPa or 25MPa, the current cement standards are very safe; but when high performance concrete is mixed with superplasticizers, those standards are not so suitable. Moreover, at present, we emphasize more on 28d compressive strength, while not paying enough attention to durability. Under special circumstances, it is very important that the compound concrete structure can maintain 28d compressive strength during the entire service cycle. Finally, cement and concrete will continue to develop to adapt to various environments to meet the requirements of sustainable development. This means that more mineral materials will be added to the cement clinker, and the water-binder ratio will be reduced, thereby increasing the service cycle of concrete and extending the service cycle of cementing materials and aggregates as much as possible.
According to the reports of concrete manufacturers and contractors, the compatibility problem between water reducer (lignin sulfonated salt water reducer) and cement has a long history, although this problem does not often occur with current superplasticizers.
However, it is in contrast to the above point of view. Scientific data on compatibility issues are very scarce. It does not seem to have attracted the attention of admixture and cement manufacturers. Moreover, a solution to this problem can be found every time: replace the admixture or not mix it, no one cares about how the problem occurs. In rare cases found in the literature (Ranc [14], Dodson and Hayden [15]), the most common cause seems to be the higher content of anhydrous calcium sulfate in gypsum. Of course, cement manufacturers can add gypsum or gypsum and anhydrous calcium sulfate to make the SO3 content meet the current standard requirements, but when the content of anhydrous calcium sulfate is too high and lignin sulfonated salt is used, sulfate The solubility of ions will be greatly reduced (Ranc [14]). High-performance concrete mixed with superplasticizers makes the water-cement ratio or water-cement ratio very low, and the probability of incompatibility is greatly increased; in other words, the water-cement ratio is much lower than the standard test water-cement ratio. In this case, it is impossible to remove the superplasticizer to solve the problem, and the problem needs to be studied in more detail. Of course, this issue is still not fully addressed in the cement industry. The admixture industry should pay more attention to this problem, because high-performance concrete is a very promising market: the amount of superplasticizer per square of high-performance concrete is as much as several liters. The incompatibility problem does not appear in every cement and admixture, but it still attracts the attention of admixture companies, and many university researchers are also trying their best to analyze the problem from a basic perspective.
As far as I know, the strong desire to understand and solve the compatibility problem is one of the reasons for the development of admixture science.
Although we have not yet fully understood how superplasticizers react with all cements and sulfates in Portland cement, we have found some effective methods to solve the incompatibility problem from many cases. The double-lead method is one of them.
The high-efficiency water reducing agent is added in two times: the first time is at the beginning of the mixing, the second time is at the end of the mixing or before the concrete is poured. A small amount of retarder or sodium sulfate can also be mixed. However, the binding problem of cement superplasticizer has not been solved yet.
In fact, if the SO3 content of the current cement and the previous cement has not changed,
Then the SO3 content in the clinker is not true. Not long ago, the SO3 content in clinker was usually on the order of 0.5%, but sometimes it was as high as 1.5% or even higher (relevant reports indicate that it can be as high as 2.5%). The maximum SO3 content allowed by cement standards is still 3.5%, and cement companies sometimes need to limit the amount of calcium sulfate added to the clinker. From a purely chemical point of view, the SO3 content of cement is the same now; but from the perspective of the solubility of sulfate ions, the SO3 measured by chemical analysis mainly comes from alkaline sulfate, either dissolved in the clinker C2S, or in the During grinding, it is added to the clinker in the form of gypsum.
A recent study by Jiang et al. of the University of Sherbrooke showed that from a rheological point of view, many cements have an optimal soluble alkali content. However, in order to please the merchants and meet the limit requirements of the alkali content, it is unnecessary to reduce the alkali content to avoid potential or imaginary alkali-aggregate reactions. Therefore, the alkali content of most modern cements does not reach the ideal value.