By far one of the most common questions any concreter is asked is, what is the difference between concrete and cement. Well, to be honest, the short answer is quite easy and a very quick explanation, however we would discredit the subject and our role as a concrete contractor id we didn’t provide an in-depth explanation of each individual difference, and their base compositions.
So just tell us already….
Well, no need to rush, this isn’t something that every concreter knows in depth. In short, cement is a limestone-based material, or a compound based on a limestone-based material that is created from raw ingredients mined out of the earth. It is important to understand that it is a base component in itself however when talking in terms of concrete. You see, cement, or cement powder as we buy it commercially, then makes up one component of the substance we call concrete. So you see, the cement is the raw, basic material, and it becomes concrete only when you mix with other ingredients, mainly water and an aggregate (whether it be sand, soil, pebbles or rocks). As soon as this is completely mixed, even when it’s in it’s wet, slurry form, it becomes concrete.
SO the true final answer is – cement is the fine, raw, powdery material you buy in the shops, where as the concrete is the cement powder mixed with other substances to form a final product that can’t be achieved when the cement is sitting by itself.
Huh, we didn’t know that. So what exactly is cement then?
Well, we’re glad you asked your local concreter. The term cement can actually, and historically, have a very broad meaning. Some may argue that it should include any limestone based building material, including early mortar formulas. However, in terms of what we understand as cement today (ie, the substance that you can easily purchase from a hardware shop in brown paper bags that fall apart and rip easily so you lose have of the fine powdery substance inside…yeah that stuff), it is a finally attuned formula that we call Portland Cement. Other materials such as ash and charcoal may also be added.
So what exactly is the fine powder cement that we know and love, and where does it come from? The three main ingredients are all very abundant naturally occurring resources – limestone, sand and clay. First all three materials, especially the harder limestone, are crushed down from various machines called crushers until they are manageable sizes for mixing. After being crushed (normally to a size of less than three inches or seven and a half millimetres) they are then dried to remove excess moisture before being ground down into a fine powder, but not the powder you see in today’s concrete stores. No, this material is than sent to a kiln where it is fired at a ridiculously hot temperature, 1400 degrees Celsius (I told you, ridiculously).
This process is called sintering and quite important to get the concrete material we know today. Basically, what happens is it forces certain chemical process to be undertaken, chemical bonds are formed as material bonds itself forming small masses. These masses are known as clinkers. It is these clinkers which are then sorted again, that are ground down into the fine cement powder that will form the basis of our concrete.
That’s very sciency of you. Then you probably know how the cement becomes hard and forms concrete then….?
Are you asking about the reaction that occurs when cement powder is mixed with water and sand or pebbles to form the hard substance known as concrete?
Well, I’m glad you asked. And no, it is not something that every concreter knows, it just so happens that we are interested in this sort of thing.
To understand the exact process, you need to get very scientific, we’re talking chemistry, chemical reactions and chemical equations here, but we will try to dole it down for you. If you believe it’s to basic, well Doctor Google can help you further in your search.
The first basic principle is Hydration. Sounds simple right, hydration is the addition of water which is something we all know is required when you mix cement powder to form concrete. As a side note, the better quality of water for this reaction, the better quality concrete as an end result, as this presents unwanted compound making their own reactions and forming unwanted by products (kind of like when you make your own beer).
This basic process allows chemical reactions to undertake with in the cement formula, and this is where it gets science related. First you must understand that whenever something is strong or durable, it is due to it’s chemical formula and the bonds that are formed between compound.
In concrete, this is caused by a substance called calcium silicate that, yep, you guessed it, is formed by two main components – calcium and silicon. The hydration of this substance undergoes various changes and forms various ionisations of calcium and silica compounds that due to their very nature, stabilise with time and are structurally strong and durable.
Some further points to understand is that the strength of the concrete is equally determined by the amount ow water added, which affects the hydration process, as the raw basic materials involved (limestone/clay clinkers etc).
Then what about the other ingredients?
Many a concreter will change the aggregate, whether it be sand or pebbles or rocks, dependant on the situation. While the aggregate used can determine the overall strength and quality of the final hardened concrete product, it does not from any part of the equation. This is why various different aggregates can be used. What it does affect is the overall texture and form of the concrete, which can thereby affect the tensile strength and durability of the end result. A good concreter will know when to use a different aggregate, or when structural support such as metal bracing is required. IF you need a good concreter, well, just give us a call.
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