Water trapped in concrete

[Sam t]

Hi,
I have a question I have been thinkimg about.
How much water is "lost" in the chemical reaction of concrete globally per year? And can that amount have an affect on global weather? Drought?
Cheers,
Sam.

Edited January 13, 2020 by Sam t
Spelling/clarity

[Sensei]

Concrete does not trap much of water *). But in urban areas it causes water to quickly flow to rivers disallowing to be absorbed by soil. Accelerated rate of flow of water might cause flooding even in places hundred kilometers away, as river water level is unnaturally raised up. Simply saying water is too quickly finding path to rivers. In older times the rivers used to meander. But it was taking large of areas of countries, so people started "straightening" rivers, to gain fertile areas around rivers, which used to be muds and swamps, for agricultural production.

https://www.google.com/search?q=straightening+rivers

"This straightening causes the streams to flow more rapidly, which can, in some instances, vastly increase soil erosion. It can also increase flooding"

https://en.wikipedia.org/wiki/River_engineering

Quote

Water trapped in concrete

Read instruction of making concrete (there is mentioned how much of water you have to add to cement), and multiply by worldwide concrete production (search net for both information).

https://en.wikipedia.org/wiki/Water–cement_ratio

*) ~35% according to the above article.

 

Edited January 13, 2020 by Sensei

[LaurieAG]

4 hours ago, Sam t said:

Hi,
I have a question I have been thinkimg about.
How much water is "lost" in the chemical reaction of concrete globally per year? And can that amount have an affect on global weather? Drought?
Cheers,
Sam.

The water isn't exactly trapped during hydration as water in excess to the chemical reactions forms voids in the concrete during curing and over time. These voids can allow external water to corrode the reinforcing in the concrete, often called concrete cancer, leading to structural problems. The answer to your second question is no.

http://matse1.matse.illinois.edu/concrete/prin.html

Water is the key ingredient, which when mixed with cement, forms a paste that binds the aggregate together. The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products. Details of the hydration process are explored in the next section. The water needs to be pure in order to prevent side reactions from occurring which may weaken the concrete or otherwise interfere with the hydration process. The role of water is important because the water to cement ratio is the most critical factor in the production of "perfect" concrete. Too much water reduces concrete strength, while too little will make the concrete unworkable. Concrete needs to be workable so that it may be consolidated and shaped into different forms (i.e.. walls, domes, etc.). Because concrete must be both strong and workable, a careful balance of the cement to water ratio is required when making concrete.

http://www.remedial.com.au/structural-repairs/concrete-cancer

[studiot]

8 hours ago, Sam t said:

Hi,
I have a question I have been thinkimg about.
How much water is "lost" in the chemical reaction of concrete globally per year? And can that amount have an affect on global weather? Drought?
Cheers,
Sam.

Good morning Sam and welcome.

Was this a class question to think about?

The short answer is that very little if any water is lost.

Concrete is made from cement, aggregate (largers stones and sand) and added water.

The aggregate usually already contains some water spread over (adsobed onto) the surface of the stones and sand grains, and less water is added to compensate for for this.
The mosisture content of the aggregate is continually measured for this purpose.

The cement is very dry (anhydrous) but it is made from crystalline rocks (eg limestone) which contain water of crystallisation.
Water of crystallisation is water that is incorporated into crystals as they form a solid.
This is driven off (by heat) in the manufacturing process of the cement and escapes to the atmousphere.

The chemical reactions of the cement and the added water are very complicated but essentially the dry cement powder is recombined with water as water of crystallisation in the new artificial rock that is formed.

Excess water percolates to the surface of the freshly mixed concrete as it is worked into place. (Water is of course much less dense than concrete)
You can often see pools of water on the top of a concrete 'pour'.

Further drying out occurs during the month or so after casting the concrete.
The chemical reactions generate heat which increase the drying effect, so much that sometimes the concrete is kept moist on the surface to prevent cracking.

So the bottom line is that substantial amounts of excess water is used in the process, but is eventually returned to the environment.
The water that is chemically combined with the cement more or less balances the water that was removed in the manufacture of that cement.