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emperorsGlory wrote:
We're more like 60% water. Let's assume Terra has 900 billion people, average weight is 70kg, meaning each person has around 42kg of water in them. If we multiply 42kg by 900 billion we get 37 800 000 000 000 kg of water. The mass of earths hydrosphere today is about 1 400 000 000 000 000 000 kg, meaning the population would only hold about 2.7% of all the water.

Fresh water, which is what we use to survive, is not the same as salt water, which is what the vast majority of the planet's water is. In fact, fresh water only makes up 3% of the current hydrosphere, so water recycling is a lot more feasible than you think, especially with future tech, even with a population nearing a trillion.

 Exergy wrote:
If earth built a huge galaxy spanning empire and used a lot of plastic to construct their materials, a lot of water would be consumed in building all that material.

I think if you were capable of building a huge, Galaxy spanning empire, you'd also be able to capture comets and mine them for water, or have the common sense to use local resources to build things on site rather than import everything from a planet light years away.

The oceans are gone through a combination of exports (fuel, plastic, etc.), Industrial repurposing (underground reservoirs in order to have more living and industrial space), and global warming (most water poils would evaporate too quickly to form something like a sea or an ocean.

So, time for XKCD style approximations!

Water on earth = 1.4E18kg
Water:cement ratio to get pretty much full hydration = 1:4
Cement required to use up all of earths water = 5.6E18kg
Minimum cement content in concrete is about 360kg/cu.m
Total volume of concrete that all of earth's water can create = 1.56E16cu.m

Surface of the earth = 5.1E12sq.m

Thickness of total concrete volume over whole of earth surface = 30.6m

So if this concrete was only to be used to make floor slabs of about 200mm thickness, then the whole of the earth would be covered by a building with 153 storeys or so, or the same number of storeys as the Burj Kalifa.

Now allowing for walls and columns and foundations, then the number of storeys comes down, but basically I think this calc shows that if the whole world was covered by a city made of concrete, then it could conceivably use up all the world's water supply without having to come up with unsatisfying "boiled off into space" type theories. The water is chemically bound into the building materials used to make the earth-spanning city.

Happy for someone to check my calcs for errors

As far as I could find, the water doesn't remain in the concrete. It simply makes its way through the chemical process and then gets spit back out.

CaCO3 → CaO + CO2

CaO + H2O → Ca(OH)2

Ca(OH)2 + CO2 → CaCO3 + H2O

The calcium carbonate reaction is only one of the things going on. The water gets absorvbed into the calcium silicon hydrate crystals (or so I understand).