Because there are no cement doctors it can consult for being hard for more then 4 hours.

It doesn’t? Have I been getting scammed?

The soft melts together to become solid

We say that new cement dries, but what it’s doing is chemically curing. Keeping cement wet actually strengthens that curing process.

- If I take a concrete block and grind it down to a fine powder and then add water, will it re-solidify?..

- Huzzah! In short the answer is no, mostly because the idea that concrete existing as both a solid and a liquid (or a flowable solid) is incorrect. Concrete in reality only exists as a solid. Before mixing, the dry components of concrete are a combination of binder and aggregate. The binder is typically portland cement (limestone, alumino-silicate, and calcium sulfate) while the aggregate is sand or rocks, the size of the aggregate varies depending on the mix and is tightly controlled. When water is added to the mix it reacts with calcium silicate (either tricalcium silicate or dicalcium silicate) and results in calcium silicate hydrate, calcium hydroxide, and heat. In a perfect world all of the calcium silicate would hydrate during the first 28 days, but sometimes a small percentage is left over and reacts slowly over time as the concrete is exposed to to the environment. This can cause expansion and cracking of the concrete. If you took cured concrete and ground it down, that minute portion of unhydrated calcium silicate would react, but it wouldn’t be nearly enough to hold the rest of the mixture together.

tl;dr Concrete is formed through a chemical reaction, and can’t be ground down and used again.

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Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) - plurale tantum: coal combustion residuals (CCRs) - is a coal combustion product that is composed of the particulates that are driven out of coal-fired boilers together with the flue gases…

Owing to its pozzolanic properties, fly ash is used as a replacement for Portland cement in concrete. The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937. Roman structures such as aqueducts or the Pantheon in Rome used volcanic ash or pozzolana (which possesses similar properties to fly ash) as pozzolan in their concrete. As pozzolan greatly improves the strength and durability of concrete, the use of ash is a key factor in their preservation.

Use of fly ash as a partial replacement for Portland cement is particularly suitable but not limited to Class C fly ashes. Class “F” fly ashes can have volatile effects on the entrained air content of concrete, causing reduced resistance to freeze/thaw damage. Fly ash often replaces up to 30% by mass of Portland cement, but can be used in higher dosages in certain applications. In some cases, fly ash can add to the concrete’s final strength and increase its chemical resistance and durability.

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Cement was invented by the early Roman Empire, back when the Romans had a terrible navy because they were terrified of getting wet, to the point that they turned naval battles into land battles to avoid the water.

For an empire that was so terrified of water that they could literally turn the ocean into dry land, preventing cement from reabsorbing water was effectively a child’s play application of their advanced technology. In fact, they hated water so much that Roman cement actually got even harder when it was submerged.

As is well known, however, the secret of Roman cement, like the mysterious Roman language itself, has been lost to time, and so while we can still poorly replicate their technology without fully understanding it, the question of why even inferior modern cement is able to stay hard in the rain is a mystery that will probably never be solved.