The soluble alkali in cement is usually expressed as Na2O equivalent, which mainly comes from the clay and mixture used in the production of cement. An appropriate amount of soluble alkali is beneficial for promoting cement hydration and promoting the early strength development of concrete. Experiments have shown that the flowability of cement concrete increases with the increase of alkali content. But when a certain amount is reached, the cement will rapidly hydrate and the fluidity of the cement slurry will decrease significantly. The plasticizing effect is also significantly reduced after adding water reducing agents. The use of water reducing agents in the construction of commercial concrete and pumped concrete increases the rate of slump loss over time.
Many polycarboxylate superplasticizers have low pH values, and when used in combination with acidic retarders such as citric acid, they are difficult to adapt to high alkali cement. The main reason is that when acidic admixtures are added to high alkali cement, they will quickly produce an acid-base neutralization exothermic reaction, causing a sharp increase in temperature. This not only promotes rapid hydration of the cement, but also generates a vicious cycle due to a large amount of hydration heat release. The prepared concrete not only has poor fluidity, but also the slump is likely to disappear in a very short time. But if other alkaline retarders are used, the above phenomenon can be avoided.
The optimal content of soluble alkali in cement is generally considered to be 0.4% -0.6%. Cement with an alkali content below 0.4% is usually referred to as low alkali cement. However, water-soluble alkalis often exist in the form of alkaline sulfates, so low alkali cement is also referred to as sulfur deficient or sulfur deficient cement.