Brazil, the world's largest coffee bean producer, recently announced that it is facing the worst drought in 91 years, affecting local hydropower generation and agricultural concrete water reducer keeps going up.
The water-reducing agent or concrete water reducer is a kind of concrete admixture that can reduce the water consumption under the condition of keeping the slump of concrete basically unchanged. It has a dispersive effect on cement particles, can improve its workability, reduce water consumption per unit, improve the fluidity of concrete mixture, or reduce the amount of cement per unit, saving cement.
Because different cement has different mineralized compositions, in addition to the variety of mixed materials, the effect of admixtures for different types of cement is very different.
In this article, we will talk about the problems when water reducer meets different kinds of cement and solutions.
1. High-alkali Cement
Soluble alkali in cement is usually expressed as Na2O equivalent, which mainly comes from the clay and mixture of cement production. An appropriate amount of soluble alkali is conducive to promoting cement hydration and is more conducive to the early strength development of concrete. The results show that the fluidity of cement concrete increases with the increase of alkali content. However, when a certain amount of cement is reached, the cement will hydrate rapidly and the fluidity of cement slurry decreases greatly. Plasticizing effect decreased obviously after adding water reducing agent. When the water reducing agent is used in the construction of concrete and pumped concrete, the loss rate of slump increases.
It is generally believed that alkali in cement promotes the dissolution of tricalcium aluminate (C3A). At this time, cement forms certain AFt crystals quickly with the help of coagulant CaSO4, which wraps on the surface of C3A, inhibits the direct hydration of C3A to form calcium aluminate, and improves the fluidity of cement slurry. However, if the alkali content in cement is too high, due to the formation of a large number of AFt crystals at the beginning, the fluidity will decrease, and the adaptability of water reducing agent to the above cement will inevitably decrease, for example, the water reduction rate is not enough, the plasticizing effect is poor, and the slump loss rate is high.
For high alkali cement, the effect of the water reducing agent with low sulfate content is poor, while the use effect of higher sulfate content of water reducing agent (sodium sulfate content of more than 20%) will be significantly improved. Primarily, low water reducing agent CaSO4 is contained in the synthesis and production, it has good properties of water-soluble. Gypsum in cement before it a large number of soluble in water, dissolved when higher alkali to speed up the C3A dissolution, because the water has a large number of SO3, reaction with C3A, form the AFt, to prevent the liquidity caused by the formation of calcium aluminate. And the slump loss is reduced. It is not difficult to see, that the water reducing agent with high content of sodium sulfate can adapt to high alkali cement more.
The PH value of many polycarboxylate water reducing agents is low, such as with citric acid and other acid retarder combined with high alkali cement is difficult to adapt. It is mainly because when acid admixture is mixed with cement of high alkali, it can produce acid alkali quickly neutralize exothermic reaction, temperature rises sharply, which not only make cement hydrate rapidly, heat of a large number of hydrate is put more can produce vicious circle, the concrete that place makes up not only fluidity is poor, slump falls to spend very likely disappear inside a very short time. Using other kinds of alkaline retarders can avoid these problems.
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