Q.: We've noticed that concrete from onsite mixers often stiffens within minutes of pouring, then loosens up later. What causes this?
A.: You're seeing an example of false set--a pronounced loss of plasticity without much heat evolution shortly after mixing. It's mainly a nuisance rather than a major problem because plasticity can be restored by remixing or vibrating the concrete. The concrete will then set normally.
False setting is more likely when onsite mixers are used. That's because ready mixed concrete is usually being mixed or agitated during the period when false setting occurs. But sometimes, when a set-retarding admixture is used with a cement that has false setting properties, false set occurs after ready mixed concrete has been discharged.
Several possible mechanisms of false setting are described by Mindess and Young in their book Concrete, published by Prentice-Hall. One explanation is that false set is caused by dehydration of gypsum added to cement during manufacture to control setting of tricalcium aluminate. At high temperatures during the cement grinding process, water is driven from the gypsum, converting some of it to plaster (calcium sulfate hemihydrate). When water is added during mixing of concrete, the plaster rehydrates back to gypsum, forming a matrix that stiffens the mix. Because there's so little gypsum in the cement, the effect doesn't last long and further mixing destroys the matrix.
According to Mindess and Young, there is also evidence that set-retarding admixtures can accelerate the initial hydration of tricalcium aluminate and thus cause false set by increasing the formation of ettringite. In some high-alkali cements the formation of syngenite may cause false set. But it has also been found that false set sometimes occurs even when the crystallization of gypsum, ettringite, or syngenite can't be implicated.