Concrete Aggregates
Aggregates are the bulk of the material that makes up concrete. This would be the sand, gravel or other space filler that you are using of which the cement will bond together like glue. In that way the aggregates that you choose for your concrete mix will heavily influence the overall strength, workability, and weight of your concrete.
Rounder aggregates tend to bind with less overall strength than aggregates with sharper edges. This sharp aspect is what landscaping supply stores refer to when they advertise "sharp sand" for masonry products. This would indicate a suitable sand for high strength mortar applications.
It is possible for smaller and round aggregates to create a strong concrete mix but the amount of cementitious mortar that is in the mix will be critical to the final strength. If too little is used the concrete can crack on the thin mortar joints between the aggregates.
Concrete Weight VS Strength
By using three parts sand mixed with one part cement you would achieve a solid, heavy and strong finished concrete. If you were to replace the sand in part with a suitable lightweight concrete additive the weight of the concrete would decrease. However in decreasing the density and weight of the concrete you also decrease the strength at an advanced rate.
Since concrete is prone to failure from cracking having a strong concrete is always desirable however weight can become an issue for some concrete projects where portability is required.
Aggregate Shape
Of significant importance to concrete strength is the shape of the aggregates that you use. Since cement is similar to a glue that bonds together aggregates is it helpful to image the difference between trying to glue round objects together and trying to glue square objects together.
For this reason you can not simply mix gravel and cement together without seriously undermining the strength of the finished concrete. Another aggregate, sand most commonly, which is much smaller is used to span the gaps between the pieces of gravel. Cement itself is not strong. It is strongest when used in combination with a proper ratio of correctly sized and shaped aggregates.
Mortar mixes
You can use sand and cement powder only instead of gravel and achieve a strong finished product. This is referred to as a mortar mix. However it is important to choose the correct type of sand to avoid a potential problem. Terminology for sand grades varies depending on region, but also varies in relation to quality control of the supplier which can make it difficult to be sure that you are getting the best sand for your concrete.
Proceed to the next section on concrete aggregates - sand to learn which types of sand to use and which never to use for making concrete or mortar.