Watching Concrete Cure
The concrete on the left in the photo above is three days old. The concrete on the right is two days old. Notice the color difference – the sample on the right looks greenish. The younger concrete is referred to as green concrete which is to say it is not as cured and not as hard. In fact, it is still soft enough that I can score it with my finger nail – that’s a 2.5 on the Mohs Hardness Scale.
Concrete is supposed to cure, not dry. The hardening process is a chemical reaction which requires water. Thus we keep the concrete wet for about a month after pouring. The easiest way to do this is to keep it covered, and ideally insulated. The next easiest way is to have it rain every day. So far that has been working most of the time. When the weather’s too dry we run a hose down the hill for periodic spraying. Interestingly, concrete will cure underwater.
Concrete also needs to be kept warm. Burlap bags, insulation, the earth and wooden forms all help. The concrete curing process is exothermic so it generates some heat in the process of curing. When you pour in cool weather, as we often end up doing, it is good to add something like Calcium Nitrate (Polarset) to the mix which causes the concrete to harden faster. It costs a little more but lets us even pour in very cold weather. This allowed us to do the greenhouse foundation and our tiny cottage in December and the military to pour air craft runways when invading Russia in the winter – that was why they developed the technique.
As a test I did a sample of concrete with 4% Polarset in the mix which I then left outdoors over night in 0°F weather. Next spring I dug it out of the receding snow. It was rock hard. The sample without the Polarset was crumbly due to freezing during the curing process. Good to know, not that I’m planning to invade any northern countries.
Outdoors: 43°F/26°F Sunny
Tiny Cottage: 58°F/53°F Kids window glazing in place