After cementing a wall, water is applied on the surface to keep it moist to maintain its strength for the next few days. Why is this necessary for maintaining the strength of the concrete?
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$\begingroup$ If the surface dries prematurely, you get cracks. Then the inside of the crack dries, and it becomes deeper. $\endgroup$– KarlCommented Aug 4, 2019 at 11:46
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2 Answers
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I'm not sure what you mean by "cementing" a wall. However, cement (typically Portland cement) is the paste that binds together the aggregate (gravel and sand) in concrete. The better the bond between the cement and the aggregate, the higher the resultant compressive strength of the concrete.
When cement is mixed with water in concrete it binds to the aggregate through a chemical reaction known as "hydration."
http://en.wikipedia.org/wiki/Hydration_reaction
A hydration reaction consumes water as a reactant, which is why you must keep concrete wet for several days after it is placed. If the concrete dries out then it has no water to continue the hydration reaction. The end result then is that you're left with cement that is not fully adhered to the aggregate (less compressive strength).
Careful though - you only need to keep the concrete wet after it has set up and is curing. Adding too much water to the mix (giving it a high water to cement ratio, w/c) actually drastically reduces the compressive strength of the concrete.
Curing compounds are especially helpful for properly curing concrete. (speaking from a heavy civil general contractor's point of view) We use this product all all the time:
http://www.wrmeadows.com/1100-clear-resin-water-based-concrete-curing-compound/
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1$\begingroup$ It's okay that you mentioned a product, but if you happen to have an affiliation with it, please disclose that in the answer. $\endgroup$– jonscaCommented Aug 3, 2013 at 3:47
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$\begingroup$ I have no affiliation with any products related to this post. $\endgroup$ Commented Aug 5, 2013 at 16:55
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$\begingroup$ Awesome, thanks for clarifying and most of all for lending us your expertise! $\endgroup$– jonscaCommented Aug 5, 2013 at 18:47
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V.shortly. Cement is a mixture of $\ce{CaO}$, $\ce{MgO}$ and $\ce{SiO2}$ (sand). The 4th component comes from air - $\ce{CO2}$. You need water for two main reasons. 1st is that you want metal oxides to react with $\ce{SiO2}$. They do it better if they are in form of hydroxide. Also some of the salts that are creted are hydrates (i.e. they have crystaline water). The 2nd reason is that hydroxides of both these metals easier react with $\ce{CO2}$ than oxides. One of the reasons is solubiltiy of $\ce{CO2}$ in water, the other, bigger renwewable reaction surface (reaction between solids are usually slower than between solid and liquid).
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$\begingroup$ Thanks, but how could the reaction of metal oxides with SiO2 gives strength? $\endgroup$– AbdullahCommented Jul 6, 2013 at 15:03