Be careful, here!! In the building code in your area or region, you may find a set MINIMUM required formula for concrete for homes, i.e. footings, foundations, steps, porches, etc.
There may also be a strength value listed, such as 20Mpa, 30 Mpa, etc.
These listings and formulii are minimum requirements under the building code. Anything less, or different has to be dealt with by the City or Regional engineers, for acceptance. Some mixes can be sand mix, most need a gravel mix.