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I know that ideal thing is to use concrete, a mix of cement and aggregate (usually sand and gravel/rocks. However I have a bag of cement that's not going to go to any use, and my aim is just to cover a hole or channel that exists between concrete ground and a fence. The channel is only about 15 cm (6 inches) wide and will most likely not have any weight placed on it. The channel is about half a foot deep, so what I thought of doing is to fill the channel as high as possible with dirt and rocks, and then mix up cement with water just to smoothly cover the surface.

My main aim here is to basically plug the gap were things can fall between the fence and concrete ground, such as rubbish. Will cement and water be satisfactory for this job? I don't have any sand, but I can (if it's advisable) try to mix up some small rocks with the cement, so that the mix will basically be 1 part gravel to one part cement. Or do I even need the gravel? As I said, I'm just trying to plug a hole/channel in the ground, and I intend to fill most of it with medium sized rocks before I apply the cement anyway.

I don't know much about concrete/cement/aggregate. I only know that the cement by itself is useless in most applications because the strength comes from the aggregate, not the cement, which I understand is just a binder. However in this case where I'm just plugging a gap I'm wondering if this is OK.

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Portland cement mixed alone (called "slurry") will result in a brittle and soft compound. It may fill the void you're seeking to eliminate, but not with ideal results. It may break up and wear away faster than actual concrete. A 6" channel is more than "just a gap". It's a substantial structure in the end. I wouldn't pour less than 3" thick for strength or you'll have a broken surface in no time.

A typical concrete mix contains 60 to 80 percent sand and gravel, also known as an "aggregate." This aggregate compound is more than filler. It plays an important part in the concrete's composition. The amount of sand and gravel in a bag of concrete determines the mixture's strength and texture. In fact, when you remove sand and gravel from the concrete mixture, it becomes a completely different product.

Sand and gravel in concrete serve several purposes. Because they act as a filler, they also add more volume to the concrete. More volume means less air and a stronger product. The size of the gravel also helps to determine the concrete's strength. Though larger pieces of gravel produce more friction and make it harder to mix, they also make a stronger concrete.

source

The bottom line: Find some sand or gravel. It can't be that rare almost anywhere.

  • Gravel I can find around my house, sand not so easily. I can easily some bags of pre-mixed concrete, that's not a problem. The reason I ask is because I have a full bag of cement and I don't know what to do with it. By the way, is cement and gravel better than just cement? Even without the sand? I just watched a video that described the sand as being the "fine" aggregate and the rocks/gravel as being the "course" aggregate. – Zebrafish Sep 27 at 16:01
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    Depends on what "gravel" means in your case. In my experience, gravel contains sand. I'd use your cement up by blending it with your concrete mix in minor proportion. – isherwood Sep 27 at 16:31
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You could fill the gap with gravel and then just cement over it. This will be less durable than properly mixed concrete, but probably hold up better than just filling it up with cement on its own.

  • That sounds like an OK idea. I guess that goes to my question of whether cement and gravel mix would be better than just cement, even if I don't have sand, that sounds right, doesn't it? – Zebrafish Sep 27 at 17:35
  • take a bucket to the landscape supplies place and buy 50c worth of sand. – Jasen Sep 27 at 23:17
  • Zebrafish, sand is also an important part of what makes concrete so strong, so gravel + cement would not be stronger than a more common blend of concrete. Portland cement by itself is merely a binder, and cracks very easily. My understanding is that the finer aggregate (sand) reinforces the space between the larger aggregate (stone/gravel), giving the mix much greater durability under certain stresses. Of course, a sand-only mix is essentially mortar, grout, and parge (depending on the ratio of sand to cement), all of which lack the impact strength of concrete. – Nate Sep 28 at 0:49
  • For your application I don't think normal concrete mix offers much benefit over just mixing cement and larger aggregate. For building vertical structures, though, I would not recommend forgoing the sand. – Nate Sep 28 at 0:51

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