Concrete curing

From what I've learned in school, mixing ingredients is generally used to speed up any chemical reaction.
However, people are mixing concrete for it **not** to cure.


1. How is that possible?
2. Is concrete curing a different kind of reaction? If yes, what is it called?
3. Are there other examples of reactions that are slowed down by mixing ingredients?




Concrete is a mixture of aggregate (stones of varying sizes, including sand), which is (almost) completely inert (usually), and cement and water. The cement is a mixture of various oxides and silicates which are somewhat reactive toward water, especially when the cement is finely ground.


And, yes, concrete setting and curing are different kinds of reactions in that the chemistry (the hydration of metallic oxides and silicates) and the physics (the development of compressive strength) have different time scales and different complexities. The first figure shows the typical strength gain of concrete for a month or so. (Ref 1)


[![enter image description here](https://i.stack.imgur.com/agGny.png)](https://i.stack.imgur.com/agGny.png)


The mixing processes which can delay the setting process take place in the first few hours of hydration. (Curing is a slower process of hydration which fills voids and provides strength.) The chemistry of hydration can be monitored most easily by heat evolution: it reaches a peak within a few minutes (from the aluminates), and then slowly falls over the next few hours and peaks again as the calcium silicates come out of dormancy; then it becomes difficult to track after a day or so. The next figure "Figure 4." shows the heat evolution during the first day or so of cement paste (cement + water, no aggregate) hydration. (Ref 2.) After about 6 hours or so, the concrete will not be remixable and will be gaining strength as a whole.


[![enter image description here](https://i.stack.imgur.com/b5sI5.png)](https://i.stack.imgur.com/b5sI5.png)


When the cement particles are first contacted by water, the calcium aluminates tend to react within minutes, and could stiffen the mix so much that it becomes unworkable. Calcium sulfate hemihydrate (added at the cement mill) diverts the aluminum hydroxide flocs to ettringite crystals. But these can also stiffen the mix because they are needles. Mixing can break these stiff needles and fluidify the mix, or at least keep it workable.


Even so, the calcium silicates will hydrate and stiffen the mix by adhering particles together. Mixing the fresh (i.e., wet) concrete can break up large agglomerations of particles and prevent them from becoming larger. But eventually, the cement will become unmixable.


Vibrators can fluidify fresh concrete as if tiny stirring undoes some of the setting - but as soon as the vibration stops, the mix becomes viscous again.


The paradox here is that mixing/stirring might be expected to grind the cement particles, exposing more surface area, permitting more reaction and stiffening the mix even faster. In practice, the slow rotation of a concrete mix in a truck is not energetic enough to cause more rapid stiffening, but rather keeps cement particles from clumping together with aggregate to form large masses of stiff semi-set concrete.


Mixing can be a complicated process. A situation where mixing could slow down a process might be where there is a difference whether you pour A into B or B into A.


A longer description can be found at <https://en.wikipedia.org/wiki/Concrete>.


Ref 1. <http://www.ce.memphis.edu/1101/notes/concrete/section_3_properties.html>


Ref 2. <http://matse1.matse.illinois.edu/concrete/prin.html>


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