Why Concrete Doesn't Actually Dry Out
Concrete is everywhere. You can't go many places anymore without seeing it. Most of the concrete we see is in the form of sidewalks or buildings, but some artists even use it to build odd concrete structures, like the ones at the Garden of Eden in Kansas:
[Image Source: Kohler Foundation]
But have you ever wondered why concrete does not reabsorb water once it has dried out? After all, many materials like pasta and sponges do reabsorb water after they have dried out. Let's first explore what concrete is and how it's made.
Concrete is made by the combination of cement, water, and aggregates such as sand and/or gravel which makes a slurry that has the consistency of thick cake batter. The most common type of concrete is made from Portland cement which was developed from natural cements made in Britain beginning in the middle of the 18th century. Portland cement is a mixture of compounds made by burning limestone and clay together at very high temperatures. But what specific compounds are contained in Portland cement? It contains tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite and gypsum.
When water is added to the cement and aggregates, various chemical reactions occur which create a hard matrix that binds all the materials together. When these chemical reactions are going on, it's better to think of the concrete as curing instead of drying out. But what exactly is going on in the concrete?
[Image Source: Maria Amenta Wikimedia]
When water is added, each of the compounds undergoes hydration and contributes to the final form. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength, in the first seven days. Dicalcium silicate, which reacts more slowly, contributes to the strength at later times.
These chemical reactions form calcium silicate hydrate which then grow into crystalline fibers which bind to each other and quite literally, cement the entire mixture together. These fibers bind to each other and also to the inert materials, like sand and gravel.
The reason that concrete doesn't dry out and then reabsorb water later is because water isn't leaving the concrete at all. It's reacting with it and becoming part of the concrete form. Concrete continues to set and cure for months and years, increasing its strength over time. It's a long-lived giant chemical reaction that continues on long after it has set.
This video explains this process flawlessly:
It's a good thing concrete doesn't reabsorb water after it has cured. Can you imagine what a soggy bridge would be like to drive on?
For further reading and studying on the chemical reaction specifics of concrete, go here.