If you think about it, fluid concrete is pretty amazing (yes, we just said that concrete is amazing). It appears wetter and soupier than standard concrete, even though it has the same amount of water in it. How's this possible? Give a round of applause to a surprisingly simple principle you probably learned about in science class -- electromagnetism.
To understand how fluid concrete works, you have to understand how superplasticizers work, since this chemical additive is essentially what differentiates fluid concrete from its standard counterpart. Superplasticizers are made from either sodium salts or calcium salts and are classified in one of four ways: sulfonated melamine-formaldehyde condensates, sulfonated naphthalene-formaldehyde condensates, modified lignosulfonates, and others such as sulfonic-acid esters and carbohydrate esters. Those terms are definitely a mouthful, but the way they work is pretty simple. When concrete is first mixed, the presence of water causes cement particles to be drawn to one another, which thickens the batch. Superplasticizer molecules, which are made of long chains and links, attach themselves to the cement particles and give them a highly negative charge. As a result, they repel each other, the same way that the negative pole of one magnet will push against the negative pole of another. Because the cement particles are no longer attracted to the others, the concrete remains fluid.
However, the effect of the superplasticizers is only temporary. The cement particles, though repulsed by one another, slowly become hydrated as they react with the water in the concrete. This process causes crystals made of calcium hydroxide and calcium silicate hydrate to form on the round surface of the cement, creating a spiky ball shaped kind of like a sweetgum fruit. The crystals slowly engulf the superplasticizers so that they're no longer able to function. Without the negative charge to keep the cement particles apart, the concrete thickens again, usually after about 30 to 45 minutes. More superplasticizers can be added to thin the concrete again, but as the crystals grow and begin to interlock, increasingly higher doses are needed to keep the cement particles apart.
Clearly fluid concrete is a pretty magical concoction. But how is it put to work at construction sites?