I am often asked how I test the quality of cement. I do not test it myself because I do not want to know. The better the cement, the more expensive it is. If I had to pay for the time it took me to measure the properties of my own concrete, I would never use that cement. This is why good quality cement is prohibitively expensive compared to cheap stuff.
I usually respond that there are several ways to test quality, including smell, feel and toxicity. But at some point somebody will say, “But what about your blog? Aren’t you just saying that with every post?”
At that point I have no choice but to admit that yes, in a very real sense, I am saying that with every post. The whole point of this blog is to help people run their own tests on cement and other materials that they are considering buying.
Cement is a gray substance that can be used to fill holes. (It is not, as recently suggested by a lawyer representing the city of New York, the best option for filling holes caused by serious crimes such as murder.) It has one property that is worth noting: it does not absorb water.
One way to test the quality of cement is to put it in an oven and measure how much it changes from moment to moment. Doppler radar technology is a closely related tool. A Doppler radar measures how fast sound moves through a particular volume of space. In principle the speed could be measured directly by the radar; but instead the volume of space being swept is chosen so that the speed will first increase, then decrease. The rate at which it decreases shows how much water is in the volume.
The same goes for other materials, such as concrete: placing it in an oven and measuring how much it expands and contracts while baking tells you how much water there is inside.
This is a very good question. If you’re building a house and you want to know whether the cement has the right qualities, it’s hard to test the concrete yourself. It’s easy to get the cement from a big concrete plant, and then it’s hard to tell if it’s good or not. There are no independent tests for concrete: no laboratory that can do chemical analysis on each and every batch.
Most of us decide on the quality of cement by asking what other people say. You see a lot of buildings with crumbling walls, and you remember that this is what people usually say about concrete. Or you see a building with strong walls, and people say that they were reinforced by steel. We’ve been pretty bad at deciding how to test the quality of cement; there is no place where we can go and get real information about concrete.
Consulting engineers who think they can tell you what cement to use is like consulting astrologers who think they can tell you where the planets are going.
A cement test, by contrast, is a simple and reliable way to figure out whether your cement is good or not.
The test is easy and cheap. To do it right, you have to have the right equipment; but if you don’t have that, you can still make a qualitative assessment using a couple of grams of sample and a scale.
Suppose you want to know if your cement has been properly mixed. The test involves adding water to a small amount of cement in one hand and comparing its consistency with a pure sample in another hand. If the two are similar, the cement is mixed properly; if the two are different, it’s been mixed improperly.
My interest in cement began when I was a child, but I didn’t learn about how to test it until later. I started out wanting to build a house; the only thing standing in my way was the cost of the concrete and the lack of information on how to choose the right kind of cement.
I looked around, and people were recommending various kinds of concrete. The one they were always talking about was hydraulic cement, which is what you buy at your local hardware store. But that wasn’t what I wanted. The problem with hydraulic cement is that it doesn’t set up all that well. You mix it, and then three hours later you have to pour it again because it hasn’t hardened yet. Not good for building a house.
To make a good foundation for a building, the concrete should have four things: 1) high strength; 2) slow setting time; 3) long-term resistance to cracking; and 4) low cost. Hydraulic cement scores high in two of those categories, but not so much on the others. So we needed something else–something with all four qualities.
Some people recommended Portland cement, which is strong and cheap, but it has an unpleasant smell, so we don’t use that one at our house anymore;
When you walk into a building, you often notice its quality before you see its exterior. The quality of the cement is probably the most important factor in a building’s function. Cement is more than just what you mix with water to make concrete; it’s also what makes concrete work.
Cement is the hardest substance known to man, and you can grind it into powder to make concrete. But there is no way to predict whether your concrete will be strong enough or even work at all.
In fact, concrete is a fairly complex material, so no one knows exactly how its properties depend on what you add to it or how you mix it. But there are reasonable approximations that come pretty close.