Mix your own concrete from fine sand

[Nilzie]

I am going to pour a foundation for a wall, which will be part of a greenhouse we are going to build. As we prepare the ground now, I see that it, under a thin layer of topsoil, consists of tightly packed sand. And then it suddenly feels a bit unnecessary to go and buy a lot of coarse concrete where essentially half or more of both the weight and money for the concrete is just sand. So, I thought of mixing my own concrete with the sand I have on site, cement, and some coarser fractions of gravel which I’ve understood concrete should contain. But - to those of you who know about this - how important is it for the sand to be completely clean, and how fine can it be? Where I'm digging now, the sand is very fine-grained, between 0-1mm, sometimes almost like flour in consistency. And it may happen that now and then a small piece of a tree root slips by that I miss when digging. Does this - potential organic elements, and the fineness of the sand, make the concrete less durable?

 

[fribygg]

And it may happen that now and then a small piece of a tree root slips in that I miss when digging. Does this - potential organic elements and the sand's fineness, make the concrete less durable?

Probably, but it probably won't matter much in your small garden project.

 

[Nilzie]

Likely, but it probably doesn't matter much in your small garden project.

You are absolutely right that it's a small garden project. No dangerous weights it needs to hold. But still, one doesn't want the foundation to crack because I've made poor concrete. But if I study up and mix well, the fine sand fractions shouldn't pose much risk, right?

 

[fribygg]

If you reinforce properly, the risk of it cracking is minimal.

 

[hapazard]

You need relatively more cement to bind the fine sand than if you mixed in coarser fractions, so it's not necessarily economical to use the sand you have - at least not exclusively. But it might be worth doing anyway to avoid hauling gravel...