There is generally quite a bit of confusion surrounding the terms related to concrete. Consequently many people misuse these terms, which makes learning about the subject quite difficult, especially in relation to concrete crafting, where there is also limited specific information for a U.K. based crafter. So to briefly clarify:
Cement is an ingredient of concrete, which binds the other ingredients together. It is never normally used alone (although could be for small craft projects). Cement is made of clay, limestone and sand which are combined with other materials and heated to high temperature before grinding down to a powder.
Concrete is made of cement, sand and aggregate/gravel/stones. Other ingredients such as plasticisers and fibres are also added to give additional properties. Colour can also be added.
I should also mention mortar which is made of cement and builders sand and used primarily in brick-laying.
In the U.K. concrete is not widely used as a crafting material, unlike in the United States and across other parts of Europe. We only have a relatively small selection of concrete products to choose from here which makes using it for craft much more difficult. There are only very few (and therefore expensive) specific craft concretes available. These generally come in small amounts and are usually sourced from abroad, but they are really great in terms of the quality of the finished product and ease of use, such as this one: Viva Decor Concrete for Creatives, Grey, 1.5 kg. The remaining concrete market is geared towards building work and therefore it usually contains large aggregate (stones) which is not so good for crafting with.
There are a few ways to get round this if you don’t fancy forking out for the craft concrete from abroad. The first, and probably best option is to make your own concrete as you can then design it for your project. Alternatively you can buy ready mixed concrete and sift out the larger lumps (but this will be dusty). Another thought I had but haven’t tried is using a ready mixed mortar as this will not have stones in it but it might not be as strong because of this.
I make my own concrete mix which I am constantly tweaking as I learn more about the stuff. I’d love to hear any insider tips if you have discovered something interesting in your experiments. 😀