What is the difference between Concrete, Cement and Mortar? Why do I have to use one and not the other for certain projects? Can they be interchanged?
Cartooniverse
Cement refers to Portland cement, a binder formed by heating limestone. Concrete is Portland cement with sand and/or crushed stone mixed in. Mortar is a mixture of Portland cement and sand (but the consistency is thicker than concrete).
One uses mortar.
Incidentally, Portland cement on its own is a lousy structrual material. A lot of times people will refer to “a cement structure” when they really mean concrete. Mortar is really a type of concrete with properties suitable for brick and stone work.
So is Portland the only type of cement?
In the terms of the OP, I think so. I’ve seen several types of adhesives referred to as “cement”, but AFAIK, Portland cement is the only structural “cement”. Can anyone confirm or not?
The term “cement” in the broadest sense means adhesive. The vast majority of the time, we mean the structural material. Most of the time, what is used is Portland cement, but there are combinations that some people don’t consider classic Portland cement.
ASTM spec C150 defines 5 types of Portland Cement with varying properties. In addition there are slag cements with blast furnace slag added, pozzolan cements (ASTM C340), masonry cement (ASTM C91), plasticized cements, waterproof cements, etc. that are not classically Portland cement.
And for historical reference terra pozzolana was the naturally occurring cement that the Romans used. Last time I looked the Pantheon was still standing.
To simplify the above, cement is the binder. Add sand to the cement and a little water, and you get mortar. Add rocks, gravel and more water and you get concrete.
Cement is a substance that, when wetted, grows out into a (microscopic) spikey crystalline sort of structure. The crystals interlock and bind to one another, forming a sort of stone.
Cement, by itself, would have only itself to bind to- like fiberglass without the epoxy, it would be pretty weak. The sand, gravel and stone give something for the cement to bind together. In other words, it’s better to glue pieces of wood together to make a panel, than it is to try and make the same panel out of just glue.
Thank you. Ach, I love this place !!!
Small wonder they call it The Straight Dope.
Okay, so what happens if you mix up Portland Cement with sand ( Mortar- tres cheap ), then add some more crushed stone. Would it be not as strong as concrete?
I am painfully aware that cement isn’t just Portland Cement. I have had injected directly into my L-3 vertibrae, a sterile exothermic cement that is more or less surgical-grade Crazy Glue. Cryanoacrylate somesuch, IIRC. Yep, got REALLY hot as it set. It was called cement by the sturgeon, who surely knew.
In that case, I wonder if my bone and marrow matter itself was a part of the process, or if the cement in of itself would have set up like stone, if just mixed and gushed out onto a plate.
If I mix cement from Home Depot too dryly, it just stays chunky and doesn’t “set up” right. If I mix it too wet ( thin ), it can of course be poured too easily. Given more time, will it set up properly anyway, or will be appear to be dried and set but in fact be a very weak material? I know, there’s QuikCrete that is idiot ( and Cartooniverse) proof. Still, are there rules governing how one mixes concrete opposed to mortar?
And, if Portland Cement is formed by heating limestone, what part of this process creates a material that is exothermic in it’s setting? ( Color me curious but chemistry-ignorant in the extreme ).
Well, if you mixed Portland cement with sand and added stone, it would not only be as strong as concrete, it WOULD BE concrete. If you have mortar mix, it may contain lime and the sand is carefully selected for optimum mortar properties. Also, there is too much sand to be optimum for concrete. However, if you add rocks it is concrete.
Concrete comes in many different forms depending on the amount of cement, size and amount of aggregate, etc. The components and curing conditions affect strength with concrete rated by compressive strength (in psi). Plain old ready mix is probably about 4000 psi but 5000 psi is readily available. Special mixes can go up to about 7000 psi. The best thing going now with ready mix is adding fiberglass to have built in reinforcement for tensile strength.
Water is critical in concrete strength. It needs just enough to be fully wetted and flowable. Any excess water reduces strength. This is tested with a “slump test” that tests how high a 12" high cone is after a form is removed. Slabs or roads might have a slump of 1 to 4 inches but columns or walls might have slumps of as high as 7 inches. Less water increasaes strength as does slower drying. After it is poured, it should be kept covered and damp to the extent possible for a week or so.
There are standards on concrete and mortar content. The “classic” concrete mix we used to use when we had a small portable concrete mixer was one shovel of cement, 2 shovels of sand and 4 shovels of gravel. Then add water until it looked right. There are much more scientific methods.
Mortar - at least as far as the term seems to be used in the trades - is intentionally made softer than concrete. So you obviously don’t want to pour your driveway with mortar. Neither would you want to lay bricks with concrete. Concrete is extremely hard relative to most bricks, and over time the movement from hot/cold and dry/wet conditions will wear away at the brick. One of the biggest mistakes that people make in historic restoration projects is to repoint (redo the mortar) with concrete.
And the lime in mortar is what differentiates it from the other types of cement mixes.It makes the mix more elastic for proper positioning of the brick/block to maintain the uniform appearance of the design (plumb and square.)
Stone masons (that is field stone),ITOH use a sand/cement mixture for the support it gives when laying the stone.Mortar,because of it’s consistency couldn’t support those stones because they use much bigger joints between their stones than a brick/ block wall construction.
Also the 2 mixes use different types of sand,with mortar having a finer type than the stone mixes
Ah, yes! Lime. In my own historic restoration project (shameless plug) I create my own mortar mix by using one part lime, 3 parts sand and 1/16th part Portland cement.
bnorton - nice plug. And nice building…
From bnorton’s link
How TOTALLY cool. I never knew why some bricks, especially older ones, seemed to have an uneven spotty glassy glazing to them. And, that bit with the horsehair- that’s the old day equivalent of mixing in fiberglas for strength. Thank you for sharing that link, the building is amazing.
I can see it now. " Yes, I’d like the Torture Cell Weekend Special please? "
