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Though it isn a hugely reliable source, this article suggests that the rumors are just that; rumors. More likely, and confirmed, is the use of concrete to weight down a corpse after killing them a different way. be immobilized. That a lot of work to go to just for a creative murder. Concrete is what is used in construction and consists of large and small aggregate, water, and cement. Cement, most commonly Portland Cement, is powder mostly consisting of silicates that reacts with water to form a strong crystalline structure that holds all of the rocks, gravel, and sand in your concrete together. The former implies that the water is evaporating off the top, this happens a little, but it is usually negligible and when it isn steps are taken to avoid it happening. What actually happens is that silicates are reacting with the water in a process called hydration; the water added to the mix remains there once the concrete has set and forms a crystalline structure with the cement that holds all of the aggregate in place.
After concrete is mixed it immediately starts to cure and the strength grows logarithmically. Generally it will reach it design strength (the strength they been using when analysing the building to make sure it won fall down) after 28 days. You could let it cure indefinitely, but the diminishing returns on strength and the fact that you can do anymore work on the building while the concrete is still surrounded by casts means that going past 28 days is very rare. There are admixtures you can use to speed or slow the process and it also depends on the heat of the surrounding area. But 28 days is standard. no inert rocks to absorb the heat. The strength will continue to increase toward a theoretical maximum. You can see in this diagram that after 28 days very little strength is gained. It simply isn worth it to ensure that the concrete will continue to cure beyond that point because it costs a lot of money to just have a construction site. An owner wants the job completed as fast as possible so he can start leasing the property.
Depending on the conditions in which you are pouring, you may do a variety of things to prevent evaporation, keep the concrete from freezing, keep the concrete from getting to hot (this was a problem in building the Hoover Dam because of the size and the whole thing took many separate pours to complete). These can all interfere with the curing process and weaken the resulting concrete. If I remember correctly the molds are usually taken off after 14 days and work can resume around them and after 28 days the concrete has reached it design strength and you don have to worry about loading it with the rest of the structure.
These are all just common practice based on concrete. There are all kinds of chemicals that can be added to the mix that will change the timing. For example, if you need to take the molds down after seven days so finish work can start then you could use high early strength concrete which follows a slightly different curve.
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