The "Heat of Hydration."

     To begin, the ferries were canceled on Tuesday, so we did not pour. Staying late on Monday and getting that ready to pour was important for PVC. A month or two months from now, when the job is behind, and the steel erection date gets changed, and other milestone dates begin to change, it is important that it was the weather and ferries, not PVC. The good news is, the island is going to make a few special ferry trips for us on Monday, so we will be able pour. That also means there is not much more to write about, so this would be a good time to address Angel's questions about heat and concrete. Angel understands, but is being an instigator, which is fine, but that means my answer also needs to teach him something. So, get ready to learn.

     What is concrete? The four main ingredients are stone, sand, Portland cement and water, there are many variations to this, but for this, Portland cement and water are all we need to know. Angel's question was, why does concrete give off heat, and why is it important to keep it warm?

     The water needs to dissolve, or change the molecular structure or the crystal lattice of the Portland cement into a new, stronger crystal lattice. In order for water to dissolve an ionic solid (an ionic solid is by definition a solid composed of oppositely charged ions) the water molecules must break up the interactions between the all of ions in that solid, the Portland cement being that solid. To do this, they orient themselves such that they effectively reduce the localized charge on each ion. This is called hydration. Hydration of ions is a thermodynamically favorable process, and as such, can produce heat. This is why it is called the, "heat of hydration." So, now we know why concrete produces heat as it cures. Let's move on to why it is important to keep the heat in, while it is curing.

     Concrete's strength depends on the growth of crystal within the matrix or the lattice of the concrete. These crystals grow from the reaction between Portland cement and water, or hydration. If there isn't enough water, the crystals can't grow and the concrete doesn't develop the strength it should. If there is enough water, the crystals grow out like tiny rock hard fingers, wrapping themselves around the gravel and sand mix, intertwining with one another.

     The other aspect of curing is temperature, concrete cannot get to hot or to cold, or it will not cure properly. As fresh concrete cools, the hydration reaction slows down. The temperature of the concrete is what is important here, not necessarily the air temperature. Below fifty degrees, hydration slows down a lot. Below forty degrees and hydration virtually stops.

     Hot concrete has the opposite problem: the reaction goes to fast, and since the reaction is exothermic (produces heat) it can quickly cause temperature differentials, and the concrete begins to crack. And cement that reacts to quickly doesn't have time for the crystals to grow properly so the concrete doesn't gain the strength it should.

     So Angel, how's that for an answer? Next we will tackle Anna's question about rebar. And try to explain compressive strength and tensile strength.